原文 I - Ⅲ -Ⅵ 2 族半导体太阳能电池/CuInSe2 的薄膜工艺 自 20 世纪 50 年代以来 ,I - Ⅲ -Ⅵ 2 族化合物开始被研究以了解其结晶相和材 料性质 ,Shay 和Wernick 在 1975 年将这些研究成果结集出版。以 CulnSe2 搭配 CdS 所制作的 PN 结太阳能电池的器件特性也随后在 1976 年发表 。之后 CIS 太阳能电地效率随着持续地改良工艺与搭配的材料而逐步提高,目前最高效率的I - Ⅲ -Ⅵ 2 薄膜太阳能电池 ,其主吸收层,其 Ga/ In 的比值必须维 持在0.3以内。若高 于 0.3 ,则导致 太阳能电池效率 大幅降低 。以三元化合 物 CulnSe2 制成的太阳能电池效率稍低 ,这是因为虽然该材料拥有相当高的光吸收 系数,但是 CulnSe2 的能隙仅约 1.O eV 。Ga 的加入使材料成为四元化合物 ,可 增 大CulnSe2 的能隙 ,也使得电池的开路电压( open circuit voltage ) 得以提升并降低 电流减少电阻损失 ,进而获得更高的电池效率 。之前我无意中浏览到一篇文章,作者在对电池效率影响的各个因素写的很细,很全面。如果您感兴趣,请访问tycorun.com。 CulnSe2是所有的 I - Ⅲ -Ⅵ 2 族化合物当中被研究得最多的 ,当然也获得很好 的成果。然而即使加入 Ga 成为来增大能隙以提升电池效率 ,但如前所述,当 x 值超过 0. 3 时就无法得 到 预期的效果 ,而 :r = 0. 3 时,其能隙仅 达 1. 15 eV 。曾经有一些努力 试图借助四元化合物,甚至五元化合物的组成调配来改变能隙 ,以尝试提升电池效率 ,但结果并非 如此 ,反而使得效率降低 L 叫 。似乎长时间以来 ,在CulnSe2 和Se2 所获 得的材料匹配和改良的成功经验无法直接被应用到其他I - Ⅲ -Ⅵ 2 系列的化合物 上,尤其是CulnSe2 ,它是除了CulnSe2 之外 ,另外通过组成调配可得到N 型和 P 型 的 I - Ⅲ -Ⅵ 2 族化合物 ,见表 1 ,它的能隙大约为 1. 5 eV ,如前所述 ,它是一种具有适当能隙值的太阳能电池材料 。往往这些材料特性的变化包括界面和内在缺陷等。 加总之后的影响足以导致可观的负面结果,显然针对这些 I - Ⅲ -Ⅵ 2 系列的化合 物仍需要投入足够的研究力量,以呈现其该有的器件表现 。因此以下即针对高效率太阳能电池的器件结构以及工艺等进行介绍 。 1. CuInSe2 的薄膜工艺 高效率 CIS 太阳能电池都是使用共蒸发 ( co-evaporation ) 或晒化 ( selenization) 反应法镀制 CIS 薄膜。其他如共溅射方式则存在因较高能量的薄膜 表面撞击而产生缺陷 ,以及In 排斥现象造成对薄膜组成的控制有所不足等缺憾 ,目前尚无法以此工艺产出高效率太阳能 电池;另外,如低成本的电化学沉积法 (electro­ deposition ) 所镀制的薄膜则因质量不佳也不适用于高效率太阳能电池的制作。 三源( CIS) 或四源 ( CIGS) 的共蒸发工艺是使用元素态蒸发源 ,在450 ~600℃的基板温 度下蒸发 。因为 Se 元素的蒸 气压高 ,所以常 在 Se 维持过量 ( over pressure) 的情况下来调整 Cu 与第三族元素 1) 时,晶粒约有几百甚至上千纳米的大小 ,薄膜呈现粗糙的 表面 ;而富 In 薄膜的晶粒小于几十纳米 ,故薄膜表面平滑光亮。CIS 薄膜蒸镀的 过程中 ,先形成 Cu2Se 和 In2Se3 两个二元相 ( binary phase ) ,再进一步反应生成 CuInSe2 三元相 ( ternary phase) 。 在 20 世纪 80 年代 ,美国波音公司实验室所制作的 CIS 太阳能电池突破 10 % 的能量转换效率 ,达到12% ,其 CIS 薄膜是先后分别在富 Cu 和富 In 的蒸发条 件下制备的 。在富 Cu 的条件下可得到大晶粒的 CJS 薄膜但却含有 Cu2Se 二次 相 ,在多晶薄膜的生 长过程中 ,Cu2Se 为 液 态并 呈 现 在 表面 和晶界 面 ( grain boundary ) ,此有助于晶粒长大 ;紧接着富In 的生长条件将可让多余的 In2Se3 二 元相跟富 Cu CIS 膜中的 Cu2Se 二次相完全反应而被消除 ,最后得到的是大晶粒且单一相的 CIS 薄膜 ,当然薄膜中先形成的组分含稍多的铜而呈现 P 型导电特 性 ,而稍后形成的部分则为In 偏多的组分,因而具高电阻率N 型接近本征 ( in­ trinsic) 半导体的导电特性 。 同时期 ,ARCO solar 公司亦发展 CIS 的晒化工艺 ,以该工艺制作的 CIS 太 阳能电池,其效率虽略逊于以蒸发工艺所制作的太阳能电池,但紧随其后。晒化 工艺是先分别镀制特定厚度的 Cu 和 In 金属膜以达成特定的原子数配比 ,再置于 H2Se 气体或 Se 蒸气之中 ,以 400 ℃以上的温度让其反应成 Cu2Se 。以晒化法 制备的薄膜 ,其组成均匀度略逊于蒸发者,但仍能满足高效率太阳能电池的要 求 ,其好处在于可使用大面积溅射工艺,甚至低成本的墨印( ink printing ) 工 艺 ,适合量产的规划 。另外 ,新式的晒化法采用快速退火 ( rapid thermal ling ,升温速率至少10℃/s 以上),将萃板上 Cu/ In/Se 三元素预镀层 ( precursor film ) 以 400~500℃的温度在极短时间 (1~5 min ) 内完成晒化反应显然此工艺更具产量大与戚本低的优势 。 晒化法若以逐渐升温的方式为之其成膜过程是一系列的反应先后在进行 ,亦 即先有铜化耐和锢化晒等多神二元中间相的形成 ,最后合成三元的 CIS 单一相。 如果使用快速升温即可跳过中间相的形成 .直接合成CIS 化合物 ,以此方式 所制备的材料并不会造成电池效率减损 ,至于薄膜形成的机制则因合成速度太快 而难以得知 ,但仍可利用间接的方式寻求更好的材质 。 目前以 CIGS 为主要吸收层的太阳能电池 ,其最高转换效率已达19. 2 % ,是 由美国 NREL 于 2003 年提出 。NREL 使用改良式的蒸发 法 称 为 三 阶段工艺 ( three-stage process ) ,顾名思义,此法是将整个工艺分为三个阶段来调变基板温 度与控制不同的元素源及其蒸发温度。 第一阶段是将已 镀上 铝 ( Mo ) 金属的铀玻璃加热至 260℃,井同时提供元素 In 、Ga 和 Se 生长 On ,Ga)先驱物 ;第二阶段则关闭元素 In 和 Ga ,改提供元素Cu 和 缸,井加热 基板至 560°c ,此时四元化合物Cu On , Ga)开始形成 ,并伴随产生二次相于薄膜表面 ,该二次相呈液态,有助于产生大晶粒且致密的柱状结晶;第三阶段即关闭元素 Cu 并保持基板温度在 560℃,再继续提供In 、Ga 和 Se 一小段时间后也关闭 In 和 Ga ,使足够的元素 In 和 Ga 与二次相再次反应 ,在表面形成Cu2(In,Ga)4 Se7或Cu1(In,Ga)3 Se5 薄膜 ,故最后会 形成铜稍微不足 ( Cu-poor ) 的 Cu(In,Ga)4 Se2薄膜 ,其组成比值为o. 93< Cu/ ( In十Ga)≤O. 97 ,最后在元素 Se 气氛下 ,做 5~10 min 的高温退火进行再结晶 。 以三阶段蒸镀工艺生长的 CIGS 薄膜的剖面品粒结构 以此法制备的 CIGS 薄膜,其组成元素纵深分布如图1 所示 0 借着加 入 Ga 形成四元化合物 ,可增大主 吸收层的能隙值 ,如此能使开路电压增加 20~30 mV 。至于 Ga 的浓度分布梯度则运用了 a-SiGe 太阳能电池的 能带设计概念 ,使导带形成V 形双斜率,此设计带有背向电场用来推动电子向 PN 结移动 ,有助于减少在背面金属接触界面处复合 ( recombination ) 的机会并 加强电荷的收集 ,而光入射方向则得益于能隙渐减的斜率设计,扩大光吸收波段 的涵盖植围 。整体而言,可增加短路电流 。 上述的材料 和器件设计理念的成功运用是 将 CIGS 太阳能电池效率推向15 % 以上 的主要原因 。事实上 ,以硫取代部分晒成 为四元化合物也可 以使能隙增加而得到像 CIGS 一样的效果 ,只是硫的蒸气压比晒高许多,若使用蒸发法在控制上需使用特殊的蒸发 源设计以特制阀门调节硫蒸气的输出。目前在 CIG 晒化量产工艺中 ,也可以同时置入晒 和硫合成 CIGSS 五元化合物。 阅读更多:你对多晶硅太阳能电池了解多少? I - Ⅲ -Ⅵ 2 zú bàndǎotǐ tàiyángnéng diànchí/CuInSe2 de bómó gōngyì zì 20 shìjì 50 niándài yǐlái,I - Ⅲ -Ⅵ 2 zú huàhéwù kāishǐ bèi yánjiū yǐ liǎojiě qí jiéjīng xiāng hé cáiliào xìngzhì,Shay hé Wernick zài 1975 nián jiāng zhèxiē yánjiū chéngguǒ jiéjí chūbǎn. Yǐ CulnSe2 dāpèi CdS suǒ zhìzuò de PN jié tàiyángnéng diànchí de qìjiàn tèxìng yě suíhòu zài 1976 nián fābiǎo. Zhīhòu CIS tàiyángnéng diàn dì xiàolǜ suízhe chíxù dì gǎiliáng gōngyì yǔ dāpèi de cáiliào ér zhúbù tígāo, mùqián zuìgāo xiàolǜ de I - Ⅲ -Ⅵ 2 bómó tàiyángnéng diànchí, qí zhǔ xīshōu céng, qí Ga/ In de bǐzhí bìxū wéichí zài 0.3 Yǐnèi. Ruò gāo yú 0.3, Zé dǎozhì tàiyángnéng diànchí xiàolǜ dàfú jiàngdī. Yǐ sān yuán huàhéwù CulnSe2 zhì chéng de tàiyángnéng diànchí xiàolǜ shāo dī, zhè shì yīnwèi suīrán gāi cáiliào yǒngyǒu xiāngdāng gāo de guāng xīshōu xìshù, dànshì CulnSe2 de néng xì jǐn yuē 1.O eV.Ga de jiārù shǐ cáiliào chéngwéi sì yuán huàhéwù, kě zēng dà CulnSe2 de néng xì, yě shǐdé diànchí de kāilù diànyā (open circuit voltage) déyǐ tíshēng bìng jiàngdī diànliú jiǎnshǎo diànzǔ sǔnshī, jìn'ér huòdé gèng gāo de diànchí xiàolǜ. Zhīqián wǒ wúyì zhōng liúlǎn dào yī piān wénzhāng, zuòzhě zài duì diànchí xiàolǜ yǐngxiǎng de gège yīnsù xiě de hěn xì, hěn quánmiàn. Rúguǒ nín gǎn xìngqù, qǐng fǎngwèn tycorun.Com. CulnSe2 shì suǒyǒu de I - Ⅲ -Ⅵ 2 zú huàhéwù dāngzhōng bèi yánjiū dé zuìduō de, dāngrán yě huòdé hěn hǎo de chéngguǒ. Rán'ér jíshǐ jiārù Ga chéngwéi lái zēng dà néng xì yǐ tíshēng diànchí xiàolǜ, dàn rú qián suǒ shù, dāng x zhí chāoguò 0. 3 Shí jiù wúfǎ dédào yùqí de xiàoguǒ, ér:R = 0. 3 Shí, qí néng xì jǐn dá 1. 15 EV. Céngjīng yǒu yīxiē nǔlì shìtú jièzhù sì yuán huàhéwù, shènzhì wǔ yuán huàhéwù de zǔchéng diàopèi lái gǎibiàn néng xì, yǐ chángshì tíshēng diànchí xiàolǜ, dàn jiéguǒ bìngfēi rúcǐ, fǎn'ér shǐdé xiàolǜ jiàngdī L jiào. Sìhū cháng shíjiān yǐlái, zài CulnSe2 hé Se2 suǒ huòdé de cáiliào pǐpèi hé gǎiliáng de chénggōng jīngyàn wúfǎ zhíjiē bèi yìngyòng dào qítā I - Ⅲ -Ⅵ 2 xìliè de huàhéwù shàng, yóuqí shì CulnSe2, tā shì chúle CulnSe2 zhī wài, lìngwài tōngguò zǔchéng diàopèi kě dédào N xíng hé P xíng de I - Ⅲ -Ⅵ 2 zú huàhéwù, jiàn biǎo 1, tā de néng xì dàyuē wèi 1. 5 EV, rú qián suǒ shù, tā shì yī zhǒng jùyǒu shìdàng néng xì zhí de tàiyángnéng diànchí cáiliào. Wǎngwǎng zhèxiē cáiliào tèxìng de biànhuà bāokuò jièmiàn hé nèizài quēxiàn děng. Jiā zǒng zhīhòu de yǐngxiǎng zúyǐ dǎozhì kěguān de fùmiàn jiéguǒ, xiǎnrán zhēnduì zhèxiē I - Ⅲ -Ⅵ 2 xìliè de huàhéwù réng xūyào tóurù zúgòu de yánjiū lìliàng, yǐ chéngxiàn qí gāi yǒu de qìjiàn biǎoxiàn. Yīncǐ yǐxià jí zhēnduì gāo xiàolǜ tàiyángnéng diànchí de qìjiàn jiégòu yǐjí gōngyì děng jìnxíng jièshào. 1. CuInSe2 de bómó gōngyì gāo xiàolǜ CIS tàiyángnéng diànchí dōu shì shǐyòng gòng zhēngfā (co-evaporation) huò shài huà (selenization) fǎnyìng fǎ dù zhì CIS bómó. Qítā rú gòng jiàn shè fāngshì zé cúnzài yīn jiào gāo néngliàng de bómó biǎomiàn zhuàngjí ér chǎnshēng quēxiàn, yǐjí In páichì xiànxiàng zàochéng duì bómó zǔchéng de kòngzhì yǒu suǒ bùzú děng quēhàn, mùqián shàng wúfǎ yǐ cǐ gōngyì chǎn chū gāo xiàolǜ tàiyángnéng diànchí; lìngwài, rú dī chéngběn de diàn huàxué chénjī fǎ (electro­ deposition) suǒ dù zhì de bómó zé yīn zhìliàng bù jiā yě bù shìyòng yú gāo xiàolǜ tàiyángnéng diànchí de zhìzuò. Sān yuán (CIS) huò sì yuán (CIGS) de gòng zhēngfā gōngyì shì shǐyòng yuánsù tài zhēngfā yuán, zài 450 ~600℃de jībǎn wēndù xià zhēngfā. Yīnwèi Se yuánsù de zhēngqì yā gāo, suǒyǐ cháng zài Se wéichí guòliàng (over pressure) de qíngkuàng xiàlái tiáozhěng Cu yǔ dì sān zú yuánsù 1) shí, jīng lì yuē yǒu jǐ bǎi shènzhì shàng qiān nàmǐ de dàxiǎo, bómó chéngxiàn cūcāo de biǎomiàn; ér fù In bómó de jīng lì xiǎoyú jǐ shí nàmǐ, gù bómó biǎomiàn pínghuá guāngliàng.CIS bómó zhēng dù de guòchéng zhōng, xiān xíngchéng Cu2Se hé In2Se3 liǎng gè èr yuán xiāng (binary phase), zài jìnyībù fǎnyìng shēngchéng CuInSe2 sān yuán xiāng (ternary phase). Zài 20 shìjì 80 niándài, měiguó bōyīn gōngsī shíyàn shì suǒ zhìzuò de CIS tàiyángnéng diànchí túpò 10% de néngliàng zhuǎnhuàn xiàolǜ, dádào 12% , qí CIS bómó shì xiānhòu fēnbié zài fù Cu hé fù In de zhēngfā tiáojiàn xià zhìbèi de. Zài fù Cu de tiáojiàn xià kě dédào dà jīng lì de CJS bómó dàn què hányǒu Cu2Se èr cì xiāng, zài duō jīng bómó de shēngzhǎng guòchéng zhōng,Cu2Se wèi yètài bìng chéngxiàn zài biǎomiàn hé jīng jièmiàn (grain boundary), cǐ yǒu zhù yú jīng lì zhǎng dà; jǐn jiēzhe fù In de shēng cháng tiáojiàn jiāng kě ràng duōyú de In2Se3 èr yuán xiāng gēn fù Cu CIS mó zhōng de Cu2Se èr cì xiāng wánquán fǎnyìng ér bèi xiāochú, zuìhòu dédào de shì dà jīng lì qiě dānyī xiàng de CIS bómó, dāngrán bómó zhōng xiān xíngchéng de zǔ fèn hán shāo duō de tóng ér chéngxiàn P xíng dǎodiàn tèxìng, ér shāo hòu xíngchéng de bùfèn zé wèi In piān duō de zǔ fèn, yīn'ér jù gāo diànzǔ lǜ N xíng jiējìn běn zhēng (in­ trinsic) bàndǎotǐ de dǎodiàn tèxìng. Tóngshí qí,ARCO solar gōngsī yì fāzhǎn CIS de shài huà gōngyì, yǐ gāi gōngyì zhìzuò de CIS tàiyángnéng diànchí, qí xiàolǜ suī lüè xùn yú yǐ zhēngfā gōngyì suǒ zhìzuò de tàiyángnéng diànchí, dàn jǐn suí qí hòu. Shài huà gōngyì shì xiān fēnbié dù zhì tèdìng hòudù de Cu hé In jīnshǔ mó yǐ dáchéng tèdìng de yuánzǐ shù pèi bǐ, zài zhìyú H2Se qìtǐ huò Se zhēngqì zhī zhōng, yǐ 400 ℃yǐshàng de wēndù ràng qí fǎnyìng chéng Cu2Se. Yǐ shài huà fǎ zhìbèi de bómó, qí zǔchéng jūnyún dù lüè xùn yú zhēngfā zhě, dàn réng néng mǎnzú gāo xiàolǜ tàiyángnéng diànchí de yāoqiú, qí hǎochù zàiyú kě shǐyòng dà miànjī jiàn shè gōngyì, shènzhì dī chéngběn de mò yìn (ink printing) gōngyì, shìhé liàng chǎn de guīhuà. Lìngwài, xīnshì de shài huà fǎ cǎiyòng kuàisù tuìhuǒ (rapid thermal ling, shēngwēn sùlǜ zhìshǎo 10℃/s yǐshàng), jiāng cuì bǎn shàng Cu/ In/Se sān yuánsù yù dùcéng (precursor film) yǐ 400~500℃de wēndù zài jí duǎn shíjiān (1~5 min) nèi wánchéng shài huà fǎnyìng xiǎnrán cǐ gōngyì gèng jù chǎnliàng dà yǔ qī běn dī de yōushì. Shài huà fǎ ruò yǐ zhújiàn shēngwēn de fāngshì wéi zhī qí chéng mó guòchéng shì yī xìliè de fǎnyìng xiānhòu zài jìnxíng, yì jí xiān yǒu tóng huà nài hé gù huà shài děng duō shén èr yuán zhōngjiān xiàng de xíngchéng, zuìhòu héchéng sān yuán de CIS dānyī xiāng. Rúguǒ shǐyòng kuàisù shēngwēn jí kě tiàoguò zhōngjiān xiàng de xíngchéng. Zhíjiē héchéng CIS huàhéwù, yǐ cǐ fāngshì suǒ zhìbèi de cáiliào bìng bù huì zàochéng diànchí xiàolǜ jiǎnsǔn, zhìyú bómó xíngchéng de jīzhì zé yīn héchéng sùdù tài kuài ér nányǐ dé zhī, dàn réng kě lìyòng jiànjiē de fāngshì xúnqiú gèng hǎo de cáizhì. Mùqián yǐ CIGS wéi zhǔyào xīshōu céng de tàiyángnéng diànchí, qí zuìgāo zhuǎnhuàn xiàolǜ yǐ dá 19. 2% , Shì yóu měiguó NREL yú 2003 nián tíchū.NREL shǐyòng gǎiliáng shì de zhēngfā fǎ chēng wèi sān jiēduàn gōngyì (three-stage process), gùmíngsīyì, cǐ fǎ shì jiāng zhěnggè gōngyì fēn wéi sān gè jiēduàn lái tiáo biàn jībǎn wēndù yǔ kòngzhì bùtóng de yuánsù yuán jí qí zhēngfā wēndù. Dì yī jiēduàn shì jiāng yǐ dù shàng lǚ (Mo) jīnshǔ de yóu bōlí jiārè zhì 260℃, jǐng tóngshí tígōng yuánsù In,Ga hé Se shēngzhǎng On,Ga) xiānqū wù; dì èr jiēduàn zé guānbì yuánsù In hé Ga, gǎi tígōng yuánsù Cu hé gāng, jǐng jiārè jībǎn zhì 560°c, cǐ shí sì yuán huàhéwù Cu On, Ga) kāishǐ xíngchéng, bìng bànsuí chǎnshēng èr cì xiāng yú bómó biǎomiàn, gāi èr cì xiāng chéng yètài, yǒu zhù yú chǎnshēng dà jīng lì qiě zhìmì de zhùzhuàng jiéjīng; dì sān jiēduàn jí guānbì yuánsù Cu bìng bǎochí jībǎn wēndù zài 560℃, zài jìxù tígōng In,Ga hé Se yī xiǎoduàn shíjiān hòu yě guānbì In hé Ga, shǐ zúgòu de yuánsù In hé Ga yǔ èr cì xiāng zàicì fǎnyìng, zài biǎomiàn xíngchéng Cu2(In,Ga)4 Se7 huò Cu1(In,Ga)3 Se5 bómó, gù zuìhòu huì xíngchéng tóng shāowéi bùzú (Cu-poor) de Cu(In,Ga)4 Se2 bómó, qí zǔchéng bǐzhí wèi o. 93< Cu/ (In shí Ga)≤O. 97, Zuìhòu zài yuánsù Se qìfēn xià, zuò 5~10 min de gāowēn tuìhuǒ jìnxíng zài jiéjīng . Yǐ sān jiēduàn zhēng dù gōngyì shēngzhǎng de CIGS bómó de pōumiàn pǐn lì jiégòu yǐ cǐ fǎ zhìbèi de CIGS bómó, qí zǔchéng yuánsù zòngshēn fēnbù rú tú 1 suǒ shì 0 jièzhe jiārù Ga xíngchéng sì yuán huàhéwù, kě zēng dà zhǔ xīshōu céng de néng xì zhí, rúcǐ néng shǐ kāilù diànyā zēngjiā 20~30 mV. Zhì yú Ga de nóngdù fēnbù tīdù zé yùnyòngle a-SiGe tàiyángnéng diànchí de néng dài shèjì gàiniàn, shǐ dǎo dài xíngchéng V xíng shuāng xiélǜ, cǐ shèjì dài yǒu bèi xiàng diànchǎng yòng lái tuīdòng diànzǐ xiàng PN jié yídòng, yǒu zhù yú jiǎnshǎo zài bèimiàn jīnshǔ jiēchù jièmiàn chù fùhé (recombination) de jīhuì bìng jiāqiáng diànhè de shōují, ér guāng rùshè fāngxiàng zé dé yì yú néng xì jiàn jiǎn de xiélǜ shèjì, kuòdà guāng xīshōu bōduàn de hángài zhí wéi. Zhěngtǐ ér yán, kě zēngjiā duǎnlù diànliú. Shàngshù de cáiliào hé qìjiàn shèjì lǐniàn de chénggōng yùnyòng shì jiāng CIGS tàiyángnéng diànchí xiàolǜ tuī xiàng 15% yǐshàng de zhǔyào yuányīn. Shìshí shàng, yǐ liú qǔdài bùfèn shài chéng wèi sì yuán huàhéwù yě kěyǐ shǐ néng xì zēngjiā ér dédào xiàng CIGS yīyàng de xiàoguǒ, zhǐshì liú de zhēngqì yā bǐ shài gāo xǔduō, ruò shǐyòng zhēngfā fǎ zài kòngzhì shàng xū shǐyòng tèshū de zhēngfā yuán shèjì yǐ tèzhì fámén tiáojié liú zhēngqì de shūchū. Mùqián zài CIG shài huà liàng chǎn gōngyì zhōng, yě kěyǐ tóngshí zhì rù shài hé liú héchéng CIGSS wǔ yuán huàhéwù. Yuèdú gèng duō: Nǐ duì duōjīngguī tàiyángnéng diànchí liǎojiě duōshǎo? 展开 3,754 / 5,000 翻译结果 I-III-VI Group 2 Semiconductor Solar Cell/CuInSe2 Thin Film Process

I-III-VI Group 2 Semiconductor Solar Cell/CuInSe2 Thin Film Process

Since the 1950s, I-III-VI 2 compounds have been studied to understand their crystalline phases and material properties, and Shay and Wernick published the results of these studies in 1975. The device characteristics of PN junction solar cells fabricated with CulnSe2 and CdS were also published in 1976. After that, the electrical efficiency of CIS solar energy has been gradually improved with the continuous improvement of the process and matching materials. The current highest efficiency I-III-VI 2 thin film solar cell, its main absorber layer, its Ga/In ratio must be maintained within 0.3. . If it is higher than 0.3, the solar cell efficiency will be greatly reduced. The efficiency of solar cells made with the ternary compound CulnSe2 is slightly lower because although the material has a rather high light absorption coefficient, the energy gap of CulnSe2 is only about 1.0 eV. The addition of Ga makes the material a quaternary compound, which can increase the energy gap of CulnSe2, and also increase the open circuit voltage of the battery and reduce the current to reduce the resistance loss, thereby achieving higher battery efficiency. Before I accidentally browsed an article, the author wrote very detailed and comprehensive on the various factors affecting battery efficiency. If you are interested, please visit tycorun.com.

CulnSe2 is the most studied of all I-III-VI 2 compounds, and of course good results have been obtained. However, even if Ga is added to increase the energy gap to improve the cell efficiency, as mentioned above, when the value of x exceeds 0.3, the expected effect cannot be obtained, and when r = 0.3, the energy gap can only reach 1.15 eV. There have been some attempts to change the energy gap by means of the composition of quaternary compounds, or even pentads, in an attempt to improve cell efficiency, but this has not been the case, but has reduced the efficiency. It seems that for a long time, the successful experience of material matching and improvement obtained in CulnSe2 and Se2 cannot be directly applied to other I-III-VI 2 series compounds, especially CulnSe2, which is in addition to CulnSe2, additionally by composition. The I-III-VI 2 compounds of N-type and P-type can be obtained by formulating, as shown in Table 1, its energy gap is about 1.5 eV, as mentioned above, it is a kind of solar cell with appropriate energy gap value Material. Often these changes in material properties include interfacial and intrinsic defects.

Table 1-Energy gap and electrical properties after annealing of I-III-VI group 2 compounds
Table 1-Energy gap and electrical properties after annealing of I-III-VI group 2 compounds

The combined effect is enough to lead to considerable negative results, and it is clear that enough research efforts are still needed for these I-III-VI 2 series compounds to show their proper device performance. Therefore, the following is an introduction to the device structure and process of high-efficiency solar cells.

  1. Thin film process of CuInSe2

High-efficiency CIS solar cells use co-evaporation (co-evaporation) or solarization (selenization) reaction method to coat CIS thin films. Other methods, such as co-sputtering, have defects such as defects caused by high-energy film surface impact, and insufficient control of film composition due to In repulsion. At present, high-efficiency solar cells cannot be produced by this process; For example, the thin films plated by low-cost electrochemical deposition (electro deposition) are not suitable for the fabrication of high-efficiency solar cells due to their poor quality.

The three-source (CIS) or four-source (CIGS) co-evaporation process uses an elemental evaporation source to evaporate at a substrate temperature of 450 to 600 °C. Because the vapor pressure of the Se element is high, the ratio of Cu to Group III elements 1), the grain size is about several hundreds or even thousands of nanometers, and the film has a rough surface; while the grain size of the In-rich film is less than tens of nanometers, so the surface of the film is smooth bright. In the process of CIS thin film evaporation, two binary phases (Cu2Se and In2Se3) are formed first, and then the CuInSe2 ternary phase is formed by further reaction.

In the 1980s, the CIS solar cell produced by the Boeing laboratory in the United States broke through 10% of the energy conversion efficiency, reaching 12%, and its CIS thin film was successively prepared under Cu-rich and In-rich evaporation conditions. CJS films with large grains can be obtained under Cu-rich conditions but contain Cu2Se secondary phases. During the growth of polycrystalline films, Cu2Se is liquid and appears on the surface and grain boundaries, which helps Grain growth; followed by In-rich growth conditions, the excess In2Se3 binary phase will completely react with the Cu2Se secondary phase in the Cu-rich CIS film and be eliminated, resulting in a large-grain and single-phase CIS The film, of course, the component formed first in the film contains a little more copper and exhibits P-type conductivity, while the part formed later is a component with more In, so it has a high resistivity N-type close to intrinsic (in trinsic) ) Conductivity properties of semiconductors.

At the same time, ARCO solar company also developed the CIS solarization process. Although the efficiency of CIS solar cells produced by this process is slightly lower than that of solar cells produced by evaporation process, it is close behind. The tanning process is to first coat Cu and In metal films with specific thicknesses to achieve a specific atomic number ratio, and then place them in H2Se gas or Se vapor, and react them into Cu2Se at a temperature above 400 °C. The composition uniformity of the thin film prepared by the solarization method is slightly inferior to that of the vaporizer, but it can still meet the requirements of high-efficiency solar cells. Process, suitable for mass production planning. In addition, the new tanning method adopts rapid thermal annealing (rapid thermal ling, the heating rate is at least 10°C/s or more), and the Cu/In/Se three-element pre-plating layer (precursor film) on the extraction plate is heated at a temperature of 400~500°C. The drying reaction is completed in a very short time (1~5 min), obviously this process has the advantages of large output and low cost.

If the solarization method is gradually heated, the film formation process is a series of reactions that are carried out successively, that is, the formation of polythematic binary mesophases such as copper resistance and induration, and finally the synthesis of ternary. CIS single phase.

The formation of intermediate phases can be skipped if rapid heating is used. Directly synthesizing CIS compounds, the materials prepared in this way will not cause loss of cell efficiency. As for the mechanism of film formation, it is difficult to know because the synthesis speed is too fast, but indirect methods can still be used to find better materials.

At present, the highest conversion efficiency of solar cells with CIGS as the main absorber layer has reached 19.2%, which was proposed by NREL in the United States in 2003. NREL uses an improved evaporation method called a three-stage process. As the name suggests, this method divides the entire process into three stages to modulate the substrate temperature and control different element sources and their evaporation temperatures.

The first stage is to heat the uranium glass plated with aluminum (Mo) metal to 260°C, and to provide the elements In, Ga and Se to grow On, Ga) precursors at the same time; in the second stage, the elements In and Ga are turned off and replaced to provide Elemental Cu and vat, well heat the substrate to 560°C, at this time the quaternary compound Cu On, Ga) begins to form, accompanied by the formation of a secondary phase on the film surface, which is liquid and helps to generate large grains And dense columnar crystals; the third stage is to turn off the element Cu and keep the substrate temperature at 560 ° C, and then continue to provide In, Ga and Se for a short period of time and also turn off In and Ga, so that enough elements In and Ga are combined with the secondary phase. Reaction again to form Cu2(In,Ga)4Se7 or Cu1(In,Ga)3Se5 film on the surface, so Cu(In,Ga)4Se2 film with slightly insufficient copper (Cu-poor) will eventually be formed. Its composition The ratio is 0.93<Cu/(In+Ga)≤0.97. Finally, in the elemental Se atmosphere, do high temperature annealing for 5~10 min for recrystallization.

Cross-sectional grain structure of CIGS thin films grown by three-stage evaporation process
The depth distribution of the constituent elements of the CIGS thin film prepared by this method is shown in Figure 1. By adding Ga to form a quaternary compound, the energy gap value of the main absorber layer can be increased, so that the open circuit voltage can be increased by 20-30 mV. As for the concentration distribution gradient of Ga, the energy band design concept of a-SiGe solar cells is used to make the conduction band form a V-shaped double slope. The opportunity for recombination at the back metal contact interface enhances the collection of charges, and the light incident direction benefits from the design of the slope of the energy gap, which expands the coverage of the light absorption band. Overall, the short-circuit current can be increased.

Figure 1-Depth distribution of constituent elements of CIGS thin films
Figure 1-Depth distribution of constituent elements of CIGS thin films

The successful application of the above-mentioned material and device design concepts is the main reason for pushing the efficiency of CIGS solar cells above 15%. In fact, replacing part of the tanned with sulfur into a quaternary compound can also increase the energy gap to obtain the same effect as CIGS, but the vapor pressure of sulfur is much higher than that of tanning. If the evaporation method is used, a special evaporation source design is required for control. The output of sulfur vapour is regulated by a special valve. At present, in the mass production process of CIG tanning, it is also possible to simultaneously insert tanning and sulfur to synthesize CIGSS five-membered compounds.

Read more: How much do you know about polycrystalline silicon solar cells?

CdTe Thin Film Technology and Development Trend of CdTe Thin Film Solar Cells

CdTe Thin Film Technology and Development Trend of CdTe Thin Film Solar Cells

  1. CdTe thin film process

In order to reduce the cost of CdTe films, polycrystalline structures are often the mainstay, which can be prepared by various methods, including evaporation, close-spaced sublimation (CSS), and vapor transport deposition (vapor transport deposition). , VTD ) , chemical vapor deposition ( chemical vapor deposition , CVD ) , electrochemical deposition ( electro deposition . ED ) , screen printing ( screen printing ) , thermal plate spraying ( spray pyrolysis ) , sputtering ( sputtering ), etc. has been used [7]. Figure 7.4 organizes various CdTe coating processes and related process conditions for reference and comparison [8]. Because CdTe has ionic bonds. It is easy to form Cd and Te bonds during the film formation process, and different coating methods are used. It is not difficult to control the stoichiometric composition. Even elemental films of Cd and Te can be plated separately at room temperature and then heated up. to form compounds.

It is worth mentioning that CdTe solar cells often require an additional heat treatment procedure. Also CdCl2 is usually mixed into the air with a partial pressure of 0.04 and heat treatment at a temperature of 425 ℃ for 20 min. This promotes the growth of CdTe grains and the passivation of the crystal interface, so that the high resistance at the crystal interface disappears, and at the same time, it promotes the mutual diffusion of materials between PN heterojunctions, forming ternary compounds (such as CdS/CdTe near the interface. Produces CdTe1, S,) with a graded composition, thereby reducing the interface defect density caused by the lattice mismatch between CdTe and CdS. This procedure helps to greatly improve the efficiency of solar cells; while 0 in the air has The effect of increasing the P-type conductivity of CdTe and promoting CdS/CdTe interdiffusion. There is also a saying: Since this is a necessary follow-up process. It does not matter which method is used for the CdTe coating, as much as possible using the low-cost method.

  1. Electrical properties and device technology of CdTe

The combination of N-CdS/P-CdTe in II-VI solar cells can reach 16.5% energy conversion efficiency. The large-area CdTe solar cell module has entered the mass production stage in recent years, and its energy conversion efficiency has exceeded 10%. Different companies use different preparation methods for the P-CdTe film in the device structure, which are coating technologies such as close-range volatilization, electrochemistry, and vapor transport.

The P-type doping of CdTe is dominated by P or As, and it is usually doped under Cd overpressure, that is, Cd-rich conditions. The hole concentration of 2 × 101; cm – 3 can be obtained for the CdTe epitaxial film prepared by CVD or MOCVD, while the polycrystalline film is only 6 × 101 cm . As doped CdTe prepared by photo-assisted molecular beam epitaxy (PAMBE) has the highest hole concentration of 6. 2 × 1018 cm – 3, which is conducive to the activity of atoms on the surface under illumination , and also makes Te more easily desorbed, and the effect on the doping amount is not obvious. In the MBE process, some people have also used low-energy ion beam doping, and its hole concentration is only 2 × 1017 cm3, but the material defects caused by the ion beam impact make its electrical properties worse, and it often needs to be regrown on it. A layer of undoped CdTe to avoid reducing the short-circuit current of the solar cell.

In terms of ohmic contact, because the work function of P-CdTe is as high as 5.7 eV, no metal can form an ideal ohmic contact with it, so the metal alloy containing Cuo.12Au0.88 is rich in Te after treatment. The surface of CdTe is diffused from Cu into the lattice position where CdTe replaces Cd, which promotes the formation of high content of P-type doping on the surface of CdTe, and holes are conducted through strong electric field tunneling. In the module mass production process, the 1-contact resistance contact of the P-CdTe polycrystalline film is often fabricated by electron beam evaporation on the surface of CdTe after CdCl2 annealing treatment → Cu thin layer, and then heated to promote the diffusion of Cu into CdTe, achieve surface doping.

In summary, the fabrication of high-efficiency N-CdS/P-CdTe solar cells should be based on the following designs and steps: ① Coating the CdTe film on the transparent CdS/TCO/Glass substrate; ② The thinner the CdS light-transmitting layer, the more It can improve the spectral response of short wavelength; ③ After the CdTe coating, a heat treatment procedure is required in the atmosphere of CdCl2 and O2 to improve the grain structure and P-type conductivity of CdTe, and also promote the interaction between CdTe and CdS. The interdiffusion of CdTeS is formed to reduce the number of interface defects; ④ The CdTe layer is chemically treated to make its surface rich in Te, then plated with Cu and heated to diffuse into CdTe to increase the amount of P-type doping, and finally plated on Conductive materials such as Ni or graphite.

  1. Future development of CdTe thin film solar cells

CdTe solar cells are mass-produced by one or two companies in the United States and Germany. The energy conversion efficiency of large-area CdTe modules can currently reach 10.5% and the service life is as long as 25 years. Its unit cost has the opportunity to be $1 per watt. The following are the thin-film solar cells with the lowest cost at present. Furthermore, Cd and Te elements can be obtained from the smelting process of basic metals such as copper and iron ore. However, a negative factor in the application of this product is that the Cd contained in the material is a heavy metal element, and the environmental protection problems caused by its process and disposal have attracted much attention. Although the industry emphasizes that the pollution of Cd compounds is very different from that of elemental Cd, and that the process and waste recycling treatment procedures can meet strict requirements, a company in the United States even takes long-term samples of its employees in the production line. Examination of the Cd content in the body has not detected higher than usual results, but it is still worrying. Under the competition of other materials, although the EU’s restrictions on the Cd content of most electronic products have not yet included solar cells, the manufacturers of CdTe solar cells mass-produced in the short term have fewer competitors to produce the same products, and the products are more expensive. This is lower than solar cells made of other materials, thus making profits. Manufacturers even give a plan to pay the cost of recycling solar panels in the future, but in the long run, under the stricter environmental protection requirements in the future, it is not conducive to CdTe solar cells. development of.

Read more: What is a bulk polycrystalline silicon solar cell?

What are the characteristics of II-VI and I-III-VI compound semiconductor materials?

What are the characteristics of II-VI and I-III-VI compound semiconductor materials?

one II-VI compounds

As shown in the periodic table of elements in Fig. 1, group E elements (including Zn, CD, Hg) and group H elements (i.e. s, Se, Te, etc.) constitute group II and VI compounds with semiconductor properties, Its basic properties are shown in Fig. 1[5l. Except that CdTe and ZnSe have n-type and p-type conductive properties, other II-VI compounds can only obtain a single conductive form, such as p-type ZnTe and n-type CDs, CdSe, ZnS, etc. Therefore, there are some restrictions on the matching of materials. According to theoretical calculation, PN homojunction (Homo J function) The energy conversion efficiency of solar cell is the highest, and its energy gap is 1 About 5 EV, so CDT has naturally become the most suitable material for solar cells in h-group compounds.

periodic table of ele ments
periodic table of ele ments

II-VI compounds have stronger ionic bonds than V group. In addition, the vapor pressure of group II and VI elements is similar and their value is not low. Therefore, in the preparation of materials, it is easy to form compounds by self interactive coordination. For the growth of thin films, if the substrate temperature is set so that the same atoms (ii-ii or vi-vi) cannot form valence bonds, the elements of group H and group m only have interactive valence bonds to form group II-VI compounds; Under normal coating conditions, the temperature is not high, about 300 ℃.

Some of the crystalline structures of II-VI compounds are cubic sphalerite structures, such as ZnSe, ZnTe, CdTe, etc., while others are hexagonal structures, such as CdSe; CDs and ZnS have one or both of the above two structures according to the material preparation conditions. Different structures have different properties.

In semiconductor materials, the strength of valence bond is directly related to the energy gap. From the periodic table in Fig. 1, those with small atomic numbers of constituent elements in II-VI compounds have strong ionic bonds. For example, Zn Se is stronger than Zn te, and CD-s is also stronger than CD te; Their energy gap also reflects the strength of bidding bond. Compared with CD te, Zn te has stronger valence bond and larger energy gap.

  1. I-ill-vi group 2 compounds

Group I VI 2 compounds can be regarded as three element compounds derived from group II VI, that is, the elements of group H are replaced by group I (Cu, Ag) and group I (al, GA, in) and combined with group VI (CS, Se, TE). Because there are two kinds of atoms arranged alternately and orderly in the lattice position occupied by cations, the unit cell seems to be formed by stacking the unit cells of two amphibolites. However, the unit length ratio of c-axis and a-axis is different due to the valence bond strength of two different cations and anions, which is not equal to 2. Such a crystalline structure is called chalcopyrite structure, as shown in Figure 2.

crystal structure of I-III-VI compounds
crystal structure of I-III-VI compounds

As can be seen from the periodic table of Fig. 1, this series of compounds include 18 compounds such as group I elements Cu and Ag, group IV elements Al, in and GA and group M elements s, Se and te. Like other compound semiconductors, their energy gaps can form quaternary or even quinary compounds through the mutual substitution of homologous elements in a certain range. The biggest difference between it and tianv or HVI compounds is that there is no composition very close to the chemical ratio. The composition stability range of single phase can reach 3% ~ 5%, and the degree of deviation from the fixed ratio composition of 1:1:2 is considerable. The above situation can be seen from the pseudo binary phase diagram of Fig. 2. The situation of deviating from the fixed specific composition will produce intrinsic point defects in the material. There are 12 kinds of ternary compounds of III-VI group, and the distribution of the number of various point defects is closely related to the chemical composition and the formation energy of defects.

Read more: how to make hydrogenated amorphous silicon solar cells?

How to make hydrogenated amorphous silicon solar cells

How to make hydrogenated amorphous silicon solar cells?

Typical single junction a-Si: H solar power grounding, (a) is in the form, and (b) is in the form of substrate. The substrate is the window of light incidence, generally glass or transparent polymer is used, and the front electrode is a layer of transparent conductive oxide (TCO) film. The commonly used materials are sn02: F, LN20,: Sn (ITO) or ZnO (AL) [28]. The quality requirements of TCO film are that the transmittance within the working spectrum range of layer I is greater than 85%, the block resistance value is less than 10 Ω / port, the contact resistance with layer P is small, the surface roughness increases the light scattering into layer I, and has sufficient chemical stability to resist the hydrogen atom etching during chemical vapor deposition. Tin oxide (fluorine) (SN 02: F) TCO films are usually prepared by atmospheric pressure CVD (a PCVD) at 500 ~ 600 ℃, and the deposited films have rough surfaces. Tin oxide (ITO) films are generally prepared by sputtering at a temperature of 200 ~ 250 ℃, and the deposited films have a smooth surface. The oxide thump is also generally produced by sputtering. The temperature ranges from room temperature to 300 ℃. The deposited film has a smooth surface and can be etched into texture by HCl. Table 6 5 shows the characteristics of these three TCO films.

P-i-n three-layer hydrogenated amorphous silicon semiconductor films are generally deposited by PECVD at a temperature of 200 ~ 250 ℃. The p layer is a window layer incident by sunlight, with a thickness of 10 ~ 30 nm. In order to improve the proportion of light incident on layer I, the effective method is to add carbon atoms into this layer to improve the energy gap value [25.26], improve the absorption efficiency of short, medium and long wavelengths, and increase the open circuit voltage of solar cells.

Adding a buffer layer [30.31] with asymptotic energy gap change at the P / I interface and adjusting the doping distribution of p layer can reduce the defects caused by the lattice stress of discontinuous energy gap, avoid the diffusion of tottering atoms from p layer to layer, reduce the back diffusion of photogenerated electrons, and distribute the internal electric field to the layer to separate photogenerated electrons and holes and reduce recombination. These effects can improve the efficiency of solar cells. The layer is the active layer of solar cell, which absorbs the incident light and generates electron hole pairs. It is separated and drifted to the front and rear electrodes by the built-in electric field, resulting in photocurrent. The thickness is 200 ~ 500 N M. the quality of the layer must meet the above device quality requirements. The general deposition rate of PECVD deposition layer is 0.1 ~ 0.5% 3 N M / s, which is an important “bottleneck” o

The N layer is the contact layer of the back electrode, with a thickness of 20 ~ 30 n M. the penetration of sunlight into the N layer is mainly medium and long wavelength. The photogenerated carriers near the I / N interface and the N layer are mainly contributed by photons with longer wavelength. In order to improve the built-in electric field strength and reduce the contact resistance with the back electrode, the N layer can be made into a microcrystalline structure. The rough interface between the N layer and the back electrode can increase the trapping of light reflected into the i layer. Adding a TCO film between the N layer and the back electrode can enhance the land capture of light [32], especially increase the absorption of 600 ~ 800 nm long wavelength light, and the short-circuit current density of solar cells can be increased. Generally, ZnO (AL) is used as the TCO film to increase reflection.

The substrate of substrate type solar cell is the back of the solar cell. Generally, stainless steel or polymer film coated with metal film is used as the back electrode in addition to the substrate. These two kinds of thin sheets have the characteristics of soft and local bending, which makes it easy to install, can be applied to different environments, and their weight is light, which is suitable for portable mobile power. Making rough Ag / ZnO film on stainless steel sheet substrate can improve the reflectivity of light and increase the short-circuit current. The deposition order of hydrogenated amorphous silicon semiconductor film is n-i-p. the N layer can use microcrystalline structure to improve the built-in electric field and reduce the contact resistance, and the N / I interface needs to add a buffer layer to alleviate the change of energy gap between I layer and N layer. The deposition of p layer is after I layer. Therefore, there is no problem of plasma etching TCO film. The TCO film of the front electrode is deposited above the p layer, and the temperature for making TCO film is limited to be no higher than the deposition temperature of n-i-p a-Si: H film. The resistance of the TCO film must be maintained at a low value to reduce the contact resistance. In order to achieve the effect of anti reflection, the thickness of the TCO film needs to be controlled at 70 ~ 80 nm. Taking ITO film as an example, the block resistance under this thickness condition will be greater than 500 / port. In order to reduce the series resistance, a metal grid must be made on TCO film to improve the filling factor and conversion efficiency of solar cells.

For a single cavity PECVD system, p-i-n three-layer a-Si: H films are deposited in this cavity in sequence. The system is simple and convenient. However, due to the cross pollution of shed and phosphorus atoms left in the cavity after deposition, the quality of solar cells is not easy to control, and the cross pollution limits the improvement of efficiency.

In order to improve the problem of cross contamination, Sanyo company of Japan has developed a continuous separation cavity system. For this purpose, the multi reaction cavity PECVD system deposits p-i-n three-layer a-Si: H films in isolated cavities. Each cavity is separated by a gate valve, and the substrate can be transported between adjacent cavities by a conveyor belt. Each cavity is deposited at the same time, and the substrate is placed and removed from the insertion cavity and extraction cavity in order to achieve the purpose of continuous production. P. The thickness of I and N layers is determined by the deposition rate and the length of the cavity. Since the thickness of layer 1 is relatively thick, the cavity length of layer I needs to be longer to maintain the continuity of production.

This system can solve the cross pollution problem of shed and phosphorus residue. Due to the design of insertion and extraction cavity, the three deposition cavities P, I and N are isolated from the atmosphere. Therefore, the cavity is not polluted by water vapor and other pollution sources in the atmosphere, so that the purity of the film can be well controlled. P. The method of depositing I and N layers by separators has been adopted by most manufacturers in the world.

The production of Si: H solar cell template adopts integrated series structure. TCO film, p-i-na-si: H film and Al film are cut by laser to form up-down series connection.

Q-switched Nd: YAG laser is usually used, and the cutting of TCO film is 1 At the laser wavelength of 06 µ m, 0.5 μ m is used for the segmentation of a Si: H film and Al film The laser wavelength is 53 µ m, and the laser cutting rate is 20 ~ 50 cm / s.

The production process of integrated structural a-Si: H solar cell template [35] has 12 procedures from broken glass cleaning to packaging, which are: ① cleaning glass; ② A thin SiO2 film of about 50 nm was plated by APCVD, and then a SnO2 film of 600 ~ lOOO nm was plated; ③ Silver glue is coated on the edge of sn02 film as anode sink line; ④ Hardening silver glue with heating furnace; ⑤ Laser cutting of sn02 film; ⑥ Clean the substrate; ⑦ P-i-n a-Si: H film was deposited by PECVD; ⑧ Then, aluminum or aluminum and oxide back reflection electrode are deposited; ⑨ By two-stage laser cutting, the back reflection electrode is cut into blocks, and then the back reflection electrode is heated by laser through the pi-na-si: H film layer and fused with the front electrode TCO film to form a series; ⑩ Electrical test and fabrication of cathode current collector Formwork performance measurement; @ Template encapsulation.

Through the above production procedures, the fabrication of large-area solar cell template can be completed. The main key affecting the production lies in the fabrication of p-i-n three-layer a-Si: H film and the integrated series connection of laser cutting. At present, there is no standardized production equipment. Manufacturers must design and construct their own production equipment to meet the requirements of production. The cost of plant construction is much higher than that of standard production equipment for single polysilicon solar cells. The quality control of p-i-na-si: H film requires certain technology and knowledge of process equipment and film process conditions.

The advantages and disadvantages of hydrogenated amorphous silicon solar cells are introduced below. Compared with silicon wafer solar cells, hydrogenated amorphous silicon thin film (a-Si: H) solar cells have the following advantages.

(1) a-Si: H thin film solar cells have better average energy yield (kW • H / kW). Figure 6 12. NEDO (new energy and Industrial Technology Development Organization) of Japan measured the one-year power generation efficiency of monocrystalline silicon and polycrystalline silicon wafer solar cells and amorphous silicon thin film solar cells of Kaneka company of Japan from August 1998 to July 1999. The results show that a-Si: H thin film solar cells are under high temperature conditions, especially in the afternoon of summer, Table 6.6 shows the field test of solar cells with different materials on the market for half a year by ECN (Netherlands Energy Research Foundation) in the Netherlands in 2000. The results show that although the conversion efficiency of amorphous silicon thin film solar cells is the lowest among these commercial solar panels, the energy yield for a long time is the highest, The reasons include better temperature coefficient under high temperature and higher photoelectric conversion performance under low illumination. Nankai University conducted a one-year field test on the same 2kwp monocrystalline silicon and a Si: H solar cell power generation system from 2004 to 2005. The results also show that the amorphous silicon solar cell module has a high total power generation. The above three independent research reports point out that amorphous silicon thin films and solar cells have excellent power generation performance.

C 2) a-Si: H thin film solar cells consume less materials. Because a-Si: H thin film solar cells have high light absorption coefficient, the required thickness of light absorption layer can be reduced by 600 times compared with silicon wafer materials.

(3) the energy PA Y-Back time of a-Si: H thin film solar cell is short. Generally, the temperature required for the production of a-Si: H solar cells is lower than 300 ℃, while the production of bulk silicon wafer solar cells requires 1000 ~ 1500 ℃. Taking the annual power generation of 30 MW as an example, the recovery time of a-Si: H is about 1 In 6 years, poly Si needs about 2.5% 2 years.

These research reports clearly point out that silicon thin-film solar cells have low temperature coefficient and can have stable power output in high temperature environment. Because silicon thin-film solar cells have high light absorption characteristics, long-term field tests show that under the condition that the sunlight energy changes continuously in different time (morning to evening) and different seasons (spring to winter), The annual average power generation efficiency is better than that of silicon wafer solar cells.

A Si: H thin film solar cells have many of the above advantages, and a-Si: H thin film solar cells can be made on low-cost glass, stainless steel or soft substrates. They can be made in a large area in production, save material consumption, and the production is not affected by wafer supply. Therefore, they have the economic advantage of reducing cost.

Hydrogenated amorphous silicon solar cells have two main disadvantages: one is that their electrical properties will deteriorate after sunlight irradiation; Second, the conversion efficiency is low.

In 1977, staebler and wronski first pointed out that the conductivity of hydrogenated amorphous silicon films will degrade under sunlight, and the conductivity can be restored after annealing. This effect is called photodegra ­ Dation effect, also known as staebler wronski effect (SWE). After more than 20 years of research, wronski pointed out in his review article in 1997 that the cause of swe effect has not been fully confirmed. It is generally accepted that the energy released when the electron hole pair recombines will break the H bond, so it will cause the light degradation effect. Hydrogen atoms play a role in compensating hanging bonds in amorphous silicon structure, but excessive hydrogen atoms will form a large number of long-chain or clustered silicon hydrogen bonds, which will loosen the structure of hydrogenated amorphous silicon film. These long-chain or clustered silicon hydrogen bonds are easy to break under sunlight, causing a large number of defects and reducing the conductivity.

At present, the efforts to improve swe are mainly to use hydrogen dilution method or plasma gas-phase reaction control to search the silicon hydrogen structure of the film from amorphous to amorphous structure, while in the device structure, it is mainly to reduce the layer thickness, improve the built-in electric field strength and reduce the carrier recombination. This method is applied to the top cell of multi junction structure. These practices can reduce the degree of light degradation to less than 20%.

The stable efficiency of single junction a-Si: H solar cells is recorded. The data show that the stable power generation efficiency is less than 10%. The power generation efficiency is 2 / 3 lower than that of monocrystalline polysilicon solar cells. The application of large power supply is limited due to the large area required.

The method to improve the power generation efficiency can be realized by improving the film quality and increasing the application of solar spectrum. The method to improve the film quality is mainly to gradually change the deposited film from amorphous structure to microcrystalline silicon (µ c-Si) structure by hydrogen dilution method. The increase of crystalline composition in the film changes the structure, electrical and optical properties, and the optical energy gap can gradually change from L. 7 ev to 1 1 E V adjustment.

At present, many efforts are focused on changing the structure of a-Si: H films, especially transforming the amorphous structure of a-Si: H films into hydrogenated polystructured Si (PM Si: H) [43 ~ 45], hydrogenated protocrystalline Si (PC Si: H) [46.47] and hydrogenated microcrystalline Si (µ C Si: h) [48 ~ 51] films with high order and different crystalline phases through plasma process control. Due to the increase of the order of the film structure, these films have better resistance to light degradation. Generally, the basic method of making the above three films is hydrogen dilution, which controls different dilution ratio and plasma reaction to obtain different crystallization phases. PMSI • H thin film is a kind of stick phase thin film with sparse distribution of silicon nanocrystals with the size of 2 ~ 4 nm and doped with amorphous silicon network (the crystallization ratio is about 2%), and its composition is not affected by the film thickness. PC Si: H film is a kind of film whose structure gradually changes from the incubation phase containing crystal nucleus to the mixed phase of amorphous and microcrystalline [(a + µ C) Si: H phase] with the increase of thickness, and finally reaches the microcrystalline phase (µ c-Si: H phase). Its structure changes with the increase of thin film thickness. µ c-Si: H film is a kind of film that directly changes from very thin incubation phase to microcrystalline phase (MC Si: H phase). PM Si: H thin films generally form macromolecular groups in the gas phase reaction of plasma close to generating dust at high gas pressure, and the reaction groups are deposited into the thin films to form silicon nanocrystals. In order to make the macromolecular reactant reach the substrate smoothly, the temperature gradient between the plasma and the substrate must be controlled. PC Si: H films are generally controlled by adjusting the dilution ratio of hydrogen, and the thickness of incubation layer corresponding to the transformation from amorphous phase to amorphous and microcrystalline mixed phase is used as the basis for process selection. The fabrication of µ c-si:h thin films is carried out at a very high hydrogen dilution ratio. The high hydrogen atom concentration in the plasma is conducive to the formation of µ C: H thin films. The most important control factor of the above three film manufacturing methods with different crystalline phases is how to control the gas-phase reaction in the plasma to form different reaction groups, especially the control of the size of silicon clusters and the content of hydrogen atoms.

Generally, the crystal structure of single junction solar cells with high stability and efficiency is the transition zone from amorphous silicon to microcrystalline silicon. The grains with crystal size of 20 ~ 30 nm are surrounded by amorphous structure. Maintaining appropriate hydrogen content is the key point of making good electrical solar cells. The efficiency of solar cells made of microcrystalline silicon is 10.52% [1.5]

Effectively improving the conversion efficiency can be achieved by multi junction solar cells or tandem cells.

The i layer of the upper battery is a Si: H film, and its energy gap value is about 1 8 E V, used to capture blue light photons. The i layer of medium electric ground is doped by 10%,..:, The energy gap of a-Si Ge: H film with 15% wrong atoms is about 1 6 e v, used to capture green photons; The i layer of the lower battery is a-Si Ge: H thin film doped with 40% and 50% wrong atoms, and its energy gap is about 1 4 E V to capture red photons. Due to the expansion of the application range of the solar spectrum, the stability efficiency is effectively improved. The experimental data show that the area of O. 25 C and FF are 2.5 respectively 30 V 、7. 56 MA / cm2 and 0 70. The stable conversion efficiency can reach about 13%. San yo’s use of a-Si: H / a-Si Ge: H double junction solar cells can also effectively improve the conversion efficiency. The experimental data show that in the area of lcm2, FF is 1.5% respectively 54 V 、10. 8 MA / cm2 and O. 73, the conversion efficiency is 11 7 % 。

In recent years, the international research on multi junction solar cells mainly focuses on the research and development of a-Si: H / µ c-Si: H double junction solar cells. Compared with the three junction structure, the manufacturing process of double junction solar cells is less, the required equipment is cheaper, the process parameters are easy to control, and the current matching is easy to achieve. A-Si: H / µ c-Si: H is an all silicon process without adding additional wrong atoms, which can save the cost of wrong Wan and avoid the problem of wrong pollution. The energy gap of µ C — Si: H thin film can be reduced to about 1 1 eV, so it can effectively absorb long wavelength light and replace the function of a-sige: H film. However, the thickness of P.C Si: H film which can effectively absorb red light photons needs to be about 2 p.m When PECVD l~3 A / S deposition rate of 56 MHz is used to make this film, the production rate will be too slow. Therefore, the current focus is on the production of c-Si: H films by VHF-PECVD at 30 ~ 130 MHz. Due to the high ion density in UHF plasma, the deposition rate can be improved, the ion energy is low and the damage to the films is low, so µ c-Si: H films with good quality can be deposited quickly. Internationally, the research and development achievements of Kaneka and MHI in Japan are the most representative. Kaneka company adds TCO interlayer to a-Si: H / TCO interlayer / µ c-Si: H double junction structure, and the FF is 1.5% respectively under the area of 1 cm2 41 V 、14. 4 mA / cm2 and 72 8. The conversion efficiency is 14 7% 0 in the study of double junction structure, another noteworthy is the a-Si: H / a-sige: H structure of Sanyo company as mentioned above, which is characterized by using a Si: H and a SiGe: Hi layers with a thickness of about 100 nm respectively, reducing the film thickness, saving consumption and improving the production rate, and maintaining the use of the original 13 56 m Hz PECVD system, relative to V H F ­ PECVD system makes it easier to quantify production in a large area.

What is a thin film polycrystalline silicon solar cell?

What is a thin film polycrystalline silicon solar cell?

The types of high-efficiency thin-film polycrystalline silicon solar cells are classified by structure, and are roughly divided into natural surface texture and enhanced absorption with back reflector (NSTAR) solar cells and P-i-N tandem solar cells The following two categories will be introduced and discussed in detail.

1. Surface texture/back reflection enhanced absorption (N STAR) solar cell
The N STAR battery will be mainly published by the Japanese solar cell factory Kanaka Corporation. The company has many years of rich experience and excellent technology in this battery structure. The back reflection layer of the first generation NSTAR battery is not textured, and then the back reflection layer of the second generation battery is textured, and then to the third generation battery The cell incorporates light trapping technology to increase its efficiency from 10.7 % to 14.7 %, significantly improving the conversion efficiency.

1) NSTAR solar cell structure
The main structure of the NSTAR cell is glass/back reflector/NiP polysilicon/indium tin oxide (glass/back reflector/NiP poly-Si/ITO), in which the active i layer is a low temperature plasma chemical vapor Deposition method (plasma-enhanced chemical vapor deposition, PECVD).

Figure 1 shows the structure of the first-generation NSTAR thin-film polycrystalline silicon solar cell. One of the characteristics of the cell is that the surface presents a natural textured structure. The uppermost surface structure has a leaf-like shape. The cell with a thickness of 4µm has a roughness of 0.12µm. It is found by XRD measurement that the thin film polysilicon has a columnar structure and a preferred orientation of (110); its crystalline volume percentage is nearly 90% determined by ellipsometry analysis. Figure 2 ( a ) shows the structure of the first generation NSTAR, which is mainly characterized by natural surface texture and a flat back reflection layer; Figure 2 ( b ) shows the structure of the second generation NSTAR, the back reflection layer After texture treatment, it can improve its light absorption efficiency.

What is a thin film polycrystalline silicon solar cell?
Figure 1 Structure of the first generation NSTAR thin-film polycrystalline silicon solar cell
What is a thin film polycrystalline silicon solar cell?
Figure 2 NSTAR structured thin-film polycrystalline silicon solar cells: (a) first generation (flat back mirror); (b) second generation (textured back mirror)

The third-generation NSTAR structure is to add an interlayer to increase the light trapping effect. As shown in Figure 3, its structure is an amorphous silicon/microcrystalline silicon (a-Si/μc-Si) PiN stack with an interlayer. The battery has an intermediate layer between the upper a-Si and the lower μc-Si.

What is a thin film polycrystalline silicon solar cell?
Figure 3 The third generation NSTAR structure

2) Manufacturing steps of NSTAR solar cells
The experimental steps for the first-generation NSTAR structure (Figure 1) are described below. Typical NSTAR cell structure is ITO(800nm)/P-μe-Si:H(20nm)/i-poly-Si(4.7μm)/N–poly-Si(300nm)/P”-poly-Si(300nm) )/glass, the production steps are as follows.

The PECVD conditions for making P–poly-Si are RF power density=40mW/cm², H2/SiH4=40, B2H6/SiH4=10-6, pressure is 1 Torr, temperature is 200°C, and brick concentration is 1016cm-3. The PECVD conditions of N+-poly-Si are RF power density=200mW/cm², .H2/SiH4=20, PH3/SiH4=10-2 and pressure 1Torri, then a back reflection layer is formed on the glass substrate, and then PECVD is used to deposit The N-type Si thin film is deposited on the back reflection layer. Next, the i-poly-Si film is also deposited on the N-type Si film by PECVD, and then the P-type Si film is deposited to form a P-i-N junction. Indium tin oxide (ITO) is deposited on top of the solar cell as a transparent conductive electrode. The Ag grid electrode is made at the top. The maximum temperature for all manufacturing processes is 550°C.
The third-generation NSTAR structure is shown in Figure 3.

3) Efficiency of NSTAR solar cells
Japan’s Kaneka company has developed a stable 8% amorphous silicon single-junction large-area solar cell module through advanced process equipment, the size of which is 910mm × 55mm. Since the fall of 1999, the company has been capable of mass production of about 20MW of solar energy per year. Battery. When the company developed its next-generation thin-film silicon solar cells, the company focused on thin-film polysilicon and amorphous silicon tandem solar cells. In 1996, Meier of the University of Neuchatel invented a-Si/mc-Si stack cells with 7% microcrystalline silicon (c-Si) cells and an initial efficiency of 13%. In 1997, Kaneka used PECVD to manufacture low-temperature thin-film polycrystalline silicon solar cells on glass substrates with a cell thickness of 2.0 μm and a conversion efficiency of 10%. The company’s current focus is on improving the efficiency and mass production of a-Si/poly-Si stacked modules. Figure 4 shows the development timeline of Kaneka’s silicon thin-film-based solar cells and modules. It can be seen from the figure that the company is working on hybrid (HYBRID) solar cells (i.e. a-Si/poly-Si stack cells). In terms of mixed use), it has reached a state of stable production for many years.

What is a thin film polycrystalline silicon solar cell?
Figure 4 The development process of Kaneka’s thin-film polysilicon and hybrid solar cells

Figure 5 shows the photovoltage characteristics of a 2.0 μm thick NSTAR cell [with Japan Quality Assurance (JQA) as the measurement standard], its intrinsic efficiency is 10.7%±.5%, and its aperture efficiency is 10.1% ±.5%, open circuit voltage (Voc) is 0.539V±0.005V, short circuit current, current density (Jsc) (essential) is 25.8±0.5mA/cm², short circuit current density (Jsc) (pore size) is 24.35±0.5mA /cm², the difference between the intrinsic efficiency and the aperture efficiency is that the aperture has a silver electrode on the ITO.

What is a thin film polycrystalline silicon solar cell?
Figure 5 Illumination I-V characteristics of 2.0μm thick NSTAR cells

Figure 6 shows an a-Si/interlayer/poly-Si hybrid cell with an area of ​​1 cm², which can achieve an initial efficiency of 14.7% under optimized deposition conditions.

What is a thin film polycrystalline silicon solar cell?
Figure 6 Illumination I-V characteristics of hybrid solar cells with interlayer thin films

As shown in Figure 7, the large-area 910mm×455mm hybrid solar cell module mass-produced by Kaneka in 2004, its initial efficiency can reach 13.5% [Voc=137V, Isc=0.536A (Jsc=14.0mA/cm²), FF =0.706].

What is a thin film polycrystalline silicon solar cell?
Figure 7 Illumination I-V characteristics of a 910mmX455mm hybrid solar cell module under AM1.5 conditions

2. P-i-N tandem solar cells
1) Introduction of P-i-N tandem solar cells
Recently, hydrogenated amorphous silicon (a-Si:H) single-junction solar cells have achieved efficiencies of up to 13% through continuous optimization of materials, interface fabrication, and device geometry. But in any case, because the band gap of a-Si devices is 1.7~1.8 eV, the average efficiency of a-Si can only reach 14%~15% according to theoretical calculations, and in practical applications, a-Si Si has obvious photo-induced degradation, and it has not been completely solved. To address the a-Si efficiency barrier, a stacked structure can be used in combination with narrow-gap materials to make the most of the solar radiation spectrum.

What is a thin film polycrystalline silicon solar cell?
Figure 8 a-Si single heterojunction solar cell structure

The advantages of a tandem solar cell integrating a Si and poly Si are as follows:
(1) Combining small energy gap poly-Si with high energy gap a-Si.
(2) The mature hydrogen passivation poly-Si thin film growth technology can be applied.
(3) There is no Steabler-Wronski effect at the underlying poly-Si junction.
(4) Low cost.
The efficiency of a-Si/poly-Si quadruple tandem solar cells can be as high as 20%.

2) Upper layer a-Si unijunction cell
The structure of a-Si/poly-Si four terminal tandem solar cell is an upper a-Si cell and a lower poly-Si cell. 5.23 shows an a-Si single heterojunction solar cell with the structure Glass/T CO/ P µc-SiC/ P a-SiC/a-SiC/ ia-Si/N µc-Si/ ITO/ Ag, in which the textured Glass/TCO structure has an optical confinement effect.

In this solar electric tree planting, the gas source is used as the plasma excitation gas, and the ECR (electron cyclotron resonance) plasma-enhanced CVD method is used for deposition at a low temperature of 180°C and a microwave power of 200 W fl£ A P 11.c-SiC electrode layer with an energy gap of 2.7 eV and a high dark conductivity of 0.1 S/cm was fabricated; then PaSiC/a-SiC/ia was formed by means of RF PECVD -Si/N PC-Si heterojunction structure; then ITO with a thickness of about 80 nm was fabricated by electron beam evaporation; finally, a silver backside electrode was used to provide high photon reflectivity. The device processes are all carried out at an average temperature of 200°C (except for the C P µc-SiC electrode layer). Because the Pµc-SiC layer is grown by ECR PECVD, the TCO layer is bombarded by dense hydrogen plasma in ECR plasma. So there are serious flaws. To eliminate this disadvantage, the TCO layer is overlaid on a plasma-resistive ZnO layer.

Figure 9 shows the optimized light output characteristics of a-Si single heterojunction solar cell, its efficiency is 12.3%, Voc=0.916 V, j SC = 19. am A/cm² and FF = 70.6 % .

What is a thin film polycrystalline silicon solar cell?
Figure 9 Light output characteristics of a-SiC single heterojunction solar cells

3) Lower poly Si battery
The underlying cell structure is ITO/P u c-SiC/P a -SiC/N poly-Si/N u c-Si/Al. Among them, the poly-Si substrate is a cast-wafer with a thickness of 250-300 μm and a resistivity of 0.5-5 Ω/cm. The fabrication steps of this cell are as follows: first, an N μc-Si layer is deposited on the backside of the acid-etched poly-Si wafer substrate by conventional methods to provide BSF effect between N-type poly-Si and Al electrodes and good Ohmic contact; a p-type a-SiC buffer layer is deposited on a clean poly-Si surface. The fabrication temperature is about 100 °C, and the microwave power is 200 W; then, at a higher temperature of 250 °C and 320 A P-type μc-SiC layer was deposited with a microwave power of W; finally, an ITO film with a thickness of 800 Å was deposited on the substrate by electron beam evaporation as the anti-reflection layer and front electrode.

4) a-Si and poly Si four-terminal stack cells
Figure 10 shows the structure of a-Si and poly-Si four-terminal stacked cells; Figure 11 shows the light output characteristics of a-Si and poly-Si four-terminal stacked cells. This four-terminal stack cell uses a-Si as its upper cell, and its intrinsic layer (i-layer) thickness is 100 nm; another P µc-SiC/N poly-Si heterojunction device is used as the lower cell. Among them, the upper cell efficiency is 7.25% ( Voc= 0.917 V , Jsc = 10.4m A/cm² , FF = 76.0% ), while the lower cell efficiency is 13.75% (Voc = 0.575 V , Jsc = 30.2 mA /cm² , FF = 79. 2 % ), so the total conversion efficiency of the entire stacked cell is as high as 21. 0 %.

What is a thin film polycrystalline silicon solar cell?
Figure 10 Structure of a-Si and poly-Si four-terminal stacked cells
What is a thin film polycrystalline silicon solar cell?
Figure 11 Light output characteristics of a-Si(a) and poly-Si(b) four-terminal stacked cells
What is a bulk polycrystalline silicon solar cell?

What is a bulk polycrystalline silicon solar cell?

Polycrystalline silicon solar cells can be divided into two types: bulk polycrystalline silicon (bulk multicrystalline) and thin-film polycrystalline silicon (thin-film polycrystalline). This section first introduces bulk polycrystalline silicon solar cells.

Monocrystalline silicon solar cells have disadvantages such as high cost and small wafer size. Polycrystalline silicon solar cells are another choice for solar cells due to their advantages of reducing cell cost and increasing the use area. However, polysilicon defects and potential barriers at the grain boundaries cause solar cell short-circuit current and conversion efficiency to decrease in order to reduce these negative effects. In fact, some methods need to be developed to tune these grain boundaries and to use ITO (indium-tin-oxide) films [32] as the top electrode, which have high conductivity and high visible light transmittance.

Generally speaking, the fabrication steps of bulk polycrystalline silicon solar cells are as follows: substrate cleaning (such as organic cleaning), surface polishing (particle removal), preferential grain etching, doping of PN interface (such as POCI, doping Miscellaneous), back metallization treatment (such as Al back electrode), ITO front electrode treatment, etc. The main production process and conditions are roughly as follows: use a 10 cm × 10 cm polysilicon substrate. The thickness is about 350um, the resistivity is in the range of 1~50/cm, the life of the minority current is more than 5,us, and the grain size varies from 5μm to 50um. The average size is 16.9um; the polysilicon substrate uses various etching methods. Preferential grain etching is carried out, then phosphorous (phosphorous) doping is used to form the interface, and X-type emitter interface is formed by diffusion. The ITO film is prepared by magnetron sputtering. Figure 1 shows the structure of a bulk polycrystalline silicon solar cell.

What is a bulk polycrystalline silicon solar cell?
Figure 1 Structure of a bulk polycrystalline silicon solar cell

1. Surface texture of bulk polycrystalline silicon solar cells
For solar cells, silicon surface texturing is a very important key technology. Especially polycrystalline silicon solar cells. Its main purpose is to reduce surface reflection and increase cell efficiency, and surface texture can reduce light reflection from 35% to 50% to 20% to 25%. The traditional monocrystalline silicon solar cell etching method cannot be directly applied to the polycrystalline silicon surface because it has grains with different crystal orientations. Therefore, it is important to study the surface texture of polysilicon. There are different methods such as dry phase wet etching for the surface texture process of polysilicon crystal clusters, and the etched effects are also different.

Generally speaking, the wet method of silicon texture is to use HF-H NO3-based solvent, or use alkaline water-based solvent machine containing inorganic or organic salt (salt). A hemispherical structure is formed on the silicon surface by means of alkaline solvent etching. The anisotropic texture (anisotropic texturing) etched on the silicon surface by means of an acidic solvent is usually a pyramid or a tilted pyramid. For example, Park et al. use the spray method. The etching solution is a combination of HF-HNOx (1:20) based solvent, sulfuric acid (H2SO4), NaNO2 and other additives. Another method of silicon texturing is dry etching, such as reactive ion etching (RIE) or electron discharge etching.

1) Wet etching method
The wet etching method introduced here is a negative potential (NPD) method, in which the dissolution of silicon occurs only when the potential is lower than -10 V, while the surface texture of the silicon and the current time and potential are very important. Clear relationship, for single crystal silicon, increasing KOH concentration and negative potential can reduce etch time and roughness.

Figure 2 shows the current-time relationship curve of the NP D method in the electrolyte with a low alkaline concentration of 24% (mass fraction) KOH, and the potential range is -30~10 V. The greater the negative potential, the current will increase: 0.75 A at -10 VF, 2 A at -20 V, and 3 A at -30 V. In addition, Figure 2 shows that the current recordings at a 10 V and a 20 V were stable, but there was a significant reduction in cathodic current, probably due to the elimination of defect areas. The etch rate of polysilicon increases with the increase of the negative NPD potential, which is about 15.5 µ.m/h at 10 V, and can reach 60 µ.m/h at 30 V. However, at low potential and low etching rate, the defect area cannot be completely removed, resulting in a stable etching state within 600 s.

What is a bulk polycrystalline silicon solar cell?
Figure 2 Current-time relationship curve of NPD method at low alkaline concentration of 24 % (mass fraction) KOH

Figure 3 shows the SEM micrograph of polysilicon textured at -30V for 600 s by NP D method at a low alkaline concentration of 24% (mass fraction) KOH. Figure 3(a) shows the two main crystallographic orientations (100) and (110) of the polysilicon substrate resulting in two textured surface metallographic phases; steps.

What is a bulk polycrystalline silicon solar cell?
Figure 3 Below 24 % (mass fraction) KOH concentration. SEM microstructure of polysilicon textured at -30 V for 600 s

Figure 4 shows the current-time curves of NPD at different alkaline concentrations (20%~50% KOH). The figure shows that 20% KOH can get the best cathode current of about 3.75 A at a 30 V; when the electrolyte concentration is slowly reduced to 24% KOH, the current is almost constant. However, when the electrolyte concentrations were 32%, 38% and 50% KOH, the maximum current values ​​were 3.4~3.6 A, and the final currents dropped significantly to 1.5 A, 1.3 A and 1.5 A. 1 A, so increasing the KOH concentration will cause the current to decay rapidly.

What is a bulk polycrystalline silicon solar cell?
Figure 4 Current-time curves of NPD under different alkaline concentrations (20%~50% mass fraction KOH)

Figure 5 shows the effect of alkaline electrolyte concentration on polysilicon surface; the conditions are – 30 V, 120 so Figures 5 Ca), C b) surface roughness under the condition of 24 % KOH It is obvious that only some areas have jagged damage Figures 5(c) and (d) are the anisotropic textured surfaces obtained at a concentration of 32 % KOH, and the jagged damage area is completely removed; Figures 5 (e) and (f) are 38 % and 50 %, respectively. % KOH. Its textured surface is not a general metallographic phase, but shows grain boundaries.

What is a bulk polycrystalline silicon solar cell?
Figure 5 Metallographic structures of polysilicon textured surfaces with KOH electrolytic concentrations of (a) and (b) 24%, (c) and (d) 32%, (e) 38% and (f) 50%, respectively

Figure 6 shows the light reflection spectrum of the polysilicon substrate after etching at -30 V and 120 s under the conditions of KOH electrolysis concentration of 24%, 32% and 38% by NPD method. The top curve in the figure is the polished polysilicon. The graph shows that the reflectance has a maximum value at a wavelength of 0.4µm and a minimum value at a wavelength of 0.72µm. The minimum reflectance of the obtained polysilicon textured surface at 32 % electrolyte concentration is 25.7 % (at 0.72 μm wavelength), while at 24 % and 38 % KOH concentration, the minimum reflectance is 28.3 % and 28.3 %, respectively. 34.7%. The above results show that the best surface metallography and the smallest reflectance can be obtained when the electrolyte concentration is 32 %. In addition, the main advantages of NPD texturing are the use of non-toxic electrolytes and a fast texturing process.

What is a bulk polycrystalline silicon solar cell?
Figure 6. Light reflection spectrum of polysilicon substrates after etching at -30 V and 120 s with KOH electrolysis concentration of 24%, 32% and 38% by NPD method

2) Dry etching method
Wet etching can etch a uniform surface on monocrystalline silicon, but on polycrystalline silicon wafers, due to the changeable crystal orientation, the uneven surface is caused, so that the polycrystalline silicon conversion efficiency cannot achieve the best effect. Inomata et al mentioned M. Takayama et al used NaOH solution to etch the surface in the fabrication process of 15 cm × 15 cm large-scale solar cells, and the maximum conversion efficiency obtained was 16. 4 % (Is c = 7. 96 A , VOC = 0. 611 V , FF = 0.759 ) , while Y. lnomata can obtain 17.09 % Usc = 8.136 A , V cx: = 0.621V , A = o.7615) conversion efficiency. The reason is that wet etching cannot effectively reduce surface reflection, because different crystal orientations of polysilicon surface have different etching rates, and when photolithography technology is used, wet etching is not suitable for mass production. Therefore, it is more suitable to use the RIE method to form a low-reflection polysilicon surface as a texture technology for large-area high-efficiency polysilicon solar cells.

The gas, gas flow, reaction pressure and RF power introduced in the RIE fabrication process will affect the results of polysilicon etching. The following is a brief introduction to the RIE method proposed by Y. Inomata, the result of which is to effectively form a uniform pyramid-like structure on the polysilicon surface, and this method can easily control the surface by controlling the flow rate of chlorine gas (Cl2). aspect ratio. The results shown in Figure 7 are a comparison of RIE dry etched and previously NaOH wet etched surfaces, with a clear reduction in reflectivity. The research team found that the maximum short-circuit current and the maximum open-circuit voltage can be obtained under the condition of a chlorine flow rate of 4.5 sccm.

What is a bulk polycrystalline silicon solar cell?
Figure 7 Comparison of surface reflection spectra of textured polysilicon formed by chlorine RIE with previous NaOH wet etch

2. Battery manufacturing and characteristics
Figure 8 shows the structure of a high-efficiency bulk polysilicon cell. The substrate of the battery is P-type polysilicon 15 cm×15 cm provided by the company, the thickness of the substrate is 270 µm, the surface texture is formed by passing 4.5 chlorine gas through RIE, and the front surface emitter is formed by diffusing PO Cl3 doping source. B SF is formed by screen printing and firing method of aluminum bonding, and Si N film is deposited by PE CVD as bulk passivation and anti-reflection layer, under N2/H 2 600℃ Bottom annealing, the top contact electrode (iron/silver) is patterned by vapor deposition and lift-off method, and then copper plating layer is used as the uppermost metal. Table 1 shows the efficiency performance of this solar cell, under two different emitter sheet resistances (52.0./□ and 89.0./□), it is clear that the fabrication using RIE yields a higher wettability than the previously mentioned NaOH Etch better short-circuit current and open-circuit voltage, and its highest efficiency can reach 17.09%, as shown in Figure 9.

What is a bulk polycrystalline silicon solar cell?
Figure 8 High-efficiency bulk polysilicon cell structure
What is a bulk polycrystalline silicon solar cell?
Figure 9 I-V curves of bulk polycrystalline silicon solar cells with the highest efficiency of 17.09%
What are the structural considerations for polycrystalline silicon solar cells?

What are the structural considerations for polycrystalline silicon solar cells?

Generally speaking, solar cell structure design mainly considers two directions: one is to improve efficiency; the other is to reduce manufacturing cost. In order to improve efficiency, factors such as material, light absorption, junction depth, anti-reflection layer, surface passivation, texture, thin film stacking, etc. need to be considered. Through various considerations and analysis, the optimized photovoltaic (photovoltaic, PV) characteristics can be obtained. . The following mainly discusses the light storage, thin film stacking, hydrogen passivation and impurity adsorption of thin film polycrystalline silicon solar cells.

1. Light trapping technology
Light trapping technology is one of the main methods to increase the efficiency of solar cells. Because thin-film solar cells are only a few microns thick, they cannot absorb enough incident light and thus cannot obtain sufficient photocurrent. The trapped light can prolong the optical path of the incident light, so that the incident light can increase the multiple reflections of the light in the solar cell and the degree of absorption of the light by the active layer.

Light trapping technology often uses the following four methods:
(1) Surface texture reduces frontal reflection.
(2) Use a flat high reflectivity material as the bottom reflective layer.
(3) Internal light trapping.
(4) Add anti-reflection layer (AR coating).

Usually, the light-receiving surface of a solar cell is a flat mirror surface. If light is formed through surface texture, there will be multiple reflections on the active layer to reduce the reflection of light on the surface, so that the travel distance of the light is lengthened and the absorption of light by the battery is increased.

Figure 1(a) shows the bottom layer with a flat and high reflectivity reflective layer, and Figure 1(b) shows the bottom layer as a reflective layer through texture to increase its reflective optical path. In addition, the reflective layer can also be used as the back electrode of the solar cell. If the thickness of the reflective layer is less than 4µm, the pyramid structure cannot be formed, and the light trapping effect cannot be achieved. Therefore, the thickness of the reflective layer has an inseparable relationship with the trapping of light.

What are the structural considerations for polycrystalline silicon solar cells?
Figure 1(a) Bottom flat high reflectivity reflective layer; (b) Bottom textured reflective layer

Another light trapping technology is called internal light trapping. This structure is to sandwich a transparent layer in a stacked solar cell, so that the incident light can be reused in the upper cell (amorphous silicon) to reduce the light attenuation of amorphous silicon. effect, thereby improving the efficiency of solar cells. Figure 2 shows the technical structure of an interlayer with internal light trapping. Figure 3 is a conceptual diagram showing the internal light trapping.

What are the structural considerations for polycrystalline silicon solar cells?
Figure 2 The technical structure of the interlayer (linterlayer) with internal light trapping
What are the structural considerations for polycrystalline silicon solar cells?
Figure 3 shows a conceptual diagram of interior light trapping

Adding anti-reflection film is another option for light trapping technology. For example, Si has a refractive index of 3.50 to 6.00 in the wavelength range of 400 to 1100 nm, so the reflection loss is 54% in the short wavelength range and 34% in the long wavelength range. In order to reduce the reflection loss, an anti-reflection coating (anti-reflection coating) is made of transparent materials with different refractive indices. The relationship between the optimal refractive index n and thickness d of the anti-reflection film and the wavelength λ of the incident light is:
λ=4nd·n²=nsino
In the formula, nsi is the refractive index of Si; no is the refractive index of the environment. In the air environment, no=1, so the optimal refractive index of the anti-reflection film is n=(nsi) 1/2. Table 1 shows the refractive index of common antireflection coating materials.

What are the structural considerations for polycrystalline silicon solar cells?
Table 1 Refractive index of common anti-reflection coating materials

2. Stack structure
Although the efficiency of solar cells made of thin-film polysilicon can reach 10%, the device efficiency is still lower than that of polysilicon solar cells made of bulk materials, so a breakthrough must be made in the structure of solar cells. Therefore, a two-layer or three-layer (hybrid) solar cell structure is used to achieve the desired efficiency. In order to make all the fabrications can be made at low temperature, the fabrication process of multilayer films from Player, i-layer to N layer are all made by CVD, forming a PiN stacked multilayer film structure. Reduce costs and simplify production steps. Because the solar cells of the multilayer thin film structure can absorb different incident light, and can use the existing materials and process methods to achieve better device characteristics, the use of stacked multi-layer solar cells has the following advantages:

(1) It can absorb a wider spectrum and use incident light as efficiently as possible.
( 2 ) A higher open circuit voltage (Voc) can be obtained.
(3) It can suppress the photo-degradation of solar cells.

In addition, the bottom layer of the stacked structure can use polysilicon (poly-Si) to absorb infrared wavelengths, and the upper layer can use amorphous silicon (Ca-Si) to reduce the surface recombination rate of polysilicon, so the current leaking from the rough surface of polysilicon will Reduced due to amorphous silicon layer.

Using the solar cell with this stacked structure can certainly improve the efficiency of the solar cell. If a transparent interlayer can be added to the structure, and applied and improved to achieve the internal light trapping effect, the efficiency of the solar cell can be further improved. effectiveness. Therefore, thin-film solar cells with stacked structure will be the focus of future development.

3. Hydrogen passivation of polysilicon
In order to reduce the intrinsic cost of solar cells, most low-cost polysilicon or thin-film polysilicon use substrates with lower intrinsic cost, resulting in poor crystalline quality and high impurity and defect content, which will seriously affect the diffusion length of charge carriers. Thus, the efficiency of the battery is greatly limited. An effective method to change this poor electrical quality of polysilicon is hydrogen passivation. This method uses hydrogen to remove impurities and defects in silicon and passivate grain boundaries. There are several methods for introducing hydrogen into silicon. The most common methods are diffusing hydrogen in plasma or depositing layers such as silicon nitride. Since these deposited layers are usually formed by plasma-enhanced chemical vapor deposition (PECVD), they may cause plasma-induced damage; recently, the use of microwave remote plasma oxygen passivation has been developed. (microwave-induced remote plasma hydrogen passivation, RPHP) method to avoid this problem. Another technique is the implantation of hydrogen ions, which involves implanting high concentrations of hydrogen near the surface of the silicon.

Commonly used polysilicon hydrogen passivation methods are as follows:
(1) SiN:H by PECVD, hydrogen diffusion from the silicon nitride deposition layer.
(2) Low-energy hydrogen ion implantation (HID) is performed with Kaufman ion swimming.
(3) Remote plasma hydrogen passivation (CRPHP).

In the high temperature fabrication step, using Si H4 as the supply gas for the PECVD deposition of silicon nitride, hydrogen can be diffused into the silicon layer at the same time, and the formed silicon amide layer (SiN layer) can be used as the front anti-reflection layer. . The main disadvantage of this method is that it requires high temperatures in excess of 600°C. The low-energy hydrogen ion implantation method can control the high concentration of hydrogen in the silicon towel, however, the backside of the silicon chip will cause defects due to ion bombardment. As for the remote plasma hydrogen passivation method, microwaves are used to generate low-temperature plasma remotely, and the low-temperature plasma is diffused to the test piece, so there is no ion bombardment phenomenon. The equipment structure is shown in Figure 4.

What are the structural considerations for polycrystalline silicon solar cells?
Figure 4 Remote plasma hydrogen passivation method and equipment

Remote plasma hydrogen passivation method equipment The remote plasma hydrogen passivation method is a plasma-enhanced annealing technology. The preferred production process parameters are to pass 40 sccm of hydrogen (H2) and 50 sccm of argon at 400 °C. (Ar) and 10 sccm of oxygen (O2) for 1 h at a pressure of 1 mbar and a microwave power of 200 W. As shown in Figure 5, the R P HP method has the best improvement effect on the effective diffusion length Leff of the minority carriers in the silicon crystal. Especially on grain boundaries.

What are the structural considerations for polycrystalline silicon solar cells?
Figure 5 Improving the effective diffusion length of minority carriers in solar cells by RPHP

4. Impurity adsorption of polysilicon
To improve the efficiency of polycrystalline silicon solar cells, effective impurity adsorption is widely used in solar cell processes. For polysilicon materials, phosphorus (P) adsorption and aluminum adsorption are the two most commonly used and effective warm-adsorption techniques. It can greatly improve the electrical properties of polysilicon. Phosphorus adsorption can use POCl3 diffusion to diffuse phosphorus atoms into polysilicon, while aluminum adsorption can be performed by electron beam evaporation of a 0.5 µm aluminum film on the silicon surface, followed by a process of high temperature annealing for several hours.

5. Polysilicon Film Deposition Technology
One of the main purposes of using thin-film polysilicon in solar cells is to improve efficiency and reduce costs. At the same time, the main process used in the solar cell can follow the existing mature technology of semiconductor technology, so it is highly compatible with general semiconductor technology in equipment and mass production. The two main thin film growth techniques required for polycrystalline silicon solar cells are described below:

(1) Vapor phase growth method.
( 2 ) Solid phase crystallization method.

Generally speaking, the low temperature polysilicon thin film process is mainly two kinds of vapor phase growth method and solid phase crystallization method. For the cost-effectiveness and performance improvement of solar cells, the following three items are very important:
(1) Manufactured at low temperature so that low-cost substrates can be used.
(2) Develop large-area technology, such as the application of amorphous silicon (a-Si: H) production technology.
(3) Reduce film thickness and develop optical enhancement structures. Vapor deposition is the most widely used technique for making polysilicon thin films, usually using plasma-enhanced or catalytic (hot-wire CVD) methods.

1). Vapor phase growth method
The vapor phase growth method is a more expensive and complex method for depositing silicon because of the large amount of precursor and diluent gases used in the vapor phase growth process. The main reason for using the vapor phase growth method is that a high-quality silicon thin film can be obtained.

The process methods of the vapor phase growth method are roughly as follows:
(1) Atmospheric pressure CVD (atmospheric pressure CVD, APCVD).
(2) Low pressure CVD (low pressure CVD, LPCVD).
(3) Rapid thermal CVD (rapid thermal CVD, RTCVD).
(4) plasma enhanced CVD (plasma enhanced CVD, PECVD).
( 5 ) Electron cyclotron resonance CVD (electron cyclotron resonance CVD, ECRCVD) o
(6) Hot wire CVD (hot-wire CVD, HWCVD).

2). Solid phase crystallization
It will be introduced that Japan Sanyo Company uses solid phase crystallization (SPC) to grow polycrystalline silicon thin films and use them on solar cells to improve conversion efficiency and reduce costs. The experimental results show that the solar cell has high light sensitivity in the long wavelength region, and there is no light-induced degradation phenomenon after exposure to light.

The SPC method uses a PECVD a-Si film as a pre-step for forming a polysilicon film. Figure 6 is a schematic diagram of the SPC method, which is mainly composed of two steps. The first step is to deposit a phosphorus-doped a-Si film on the substrate by PECVD; the second step is to recrystallize the phosphorus-doped a-Si film into an N-type polysilicon film using a low-temperature annealing method at about 600 °C.

This SPC method has the following three main advantages:
(1) The production process is very simple.
(2) The production process temperature is low.
(3) Suitable for making large-area solar cells.

How much do you know about polycrystalline silicon solar cells?

How much do you know about polycrystalline silicon solar cells?

At present, more than 95% of commercial solar cells are made of silicon. The advantages of silicon are mainly due to its abundant raw materials, mature process technology and no toxicity. The cost of silicon wafers accounts for 40% to 60% of the entire process, so material cost is an important issue. Both single-crystalline silicon (sc-Si) and multi-crystalline silicon (mc-Si) wafers are widely used, especially polycrystalline silicon wafers have great application potential due to their low cost advantages. Gradually increasing trend yang. The conversion efficiency of commercial polycrystalline silicon solar cells is generally 12% to 15%, and can be as high as 17% with more sophisticated solar cell designs. The potential of polysilicon is very high, and its efficiency has been increased to about 20% in the laboratory recently, which greatly increases its commercial viability.

How much do you know about polycrystalline silicon solar cells?
Commercial solar cells

The performance of polycrystalline silicon solar cells is mainly limited by the minority carrier recombination rate. During the crystallization process, the material will produce different defect structures, which determine and limit the efficiency of the cell. Generally speaking, dislocations and intra-grain defects, such as internal impurities, atomic clusters or precipitates, are the main reasons for the recombination of carriers; for relatively large grains on the centimeter scale, The grain boundaries become irrelevant.

How much do you know about polycrystalline silicon solar cells?
polycrystalline silicon solar cell

Most of the cost of solar cells comes from the cost of substrates and manufacturing. Early solar cells are dominated by monocrystalline silicon. Because it can provide good conversion efficiency and use mature semiconductor manufacturing technology, it is generally used under non-cost considerations. In non-electric applications or artificial satellites or scientific experiments that require small area and high power generation, such as automobiles, etc. If it is to be commercialized and popularized, the cost of the product is an urgent problem that we need to solve, so the solar cell technology of polysilicon and amorphous silicon came into being.

How much do you know about polycrystalline silicon solar cells?
polycrystalline silicon solar cells

There are two types of polycrystalline silicon solar cells: bulk polycrystalline silicon (bulk silicon) and thin-film polycrystalline silicon (thin-film pol e silicon). Because thin-film polycrystalline silicon has the advantages of reducing the dependence on wafers and reducing cost, so polycrystalline silicon thin-film solar cells are used. is an important trend yang. Because the thickness of the light absorbing layer of the solar cell is 2~3 times the thickness that the sunlight can absorb, and most of the electron-hole pairs act at the interface, it is only necessary to ensure that the grain size of the polysilicon film is larger than that of the film. thick, so that there are more minority carriers for effective power generation at the interface than short-lived carriers that flow into the grain boundaries, thereby suppressing the influence of grain boundaries, and then using an inexpensive substrate to make tandem silicon thin films (tandem thin films) -film) structure to form thin-film solar cells, and large-area solar cell modules can be fabricated.

What are the applications of monocrystalline silicon solar cells?

What are the applications of monocrystalline silicon solar cells?

Since the energy crisis occurred in the 1970s, people have begun to apply solar cells to general livelihood purposes. At present, in countries such as the United States, Japan and Israel, solar energy installations have been widely used and are moving towards the goal of commercialization. Among these countries, the United States established the world’s largest solar power plant in California in 1983, which can generate up to 16 MW of electricity. South Africa, Botswana, Namibia and other countries in southern Africa have also set up programs to encourage the installation of low-cost solar cell power systems in remote rural areas. The most active country in the implementation of solar power generation is Japan. In 1994, Japan implemented the subsidy and incentive method to promote the “mains parallel solar photovoltaic power system” of 3000 W per household. In the first year, the government subsidizes 49% of the funds, and the subsidy thereafter decreases year by year. “Mains parallel type solar photovoltaic power system” is when the sunshine is sufficient, the solar cell provides electricity to the load of the home, and if there is excess electricity, it will be stored separately. When the power generation is insufficient or does not generate electricity, the required power is provided by the power company. By 1996, 2,600 households in Japan had installed solar power generation systems, with a total installed capacity of 8 MW. A year later, there were 9,400 installations, with a total installed capacity of 32 MW. In recent years, due to the rising awareness of environmental protection and the government’s subsidy system, it is estimated that the demand for household solar cells in Japan will also increase rapidly. In terms of industry, the total output of solar cells in Japan in 1999 was 86 MW, and in 2000 it had increased to 120 MW, ranking first in the world for two consecutive years. Recently, many Japanese solar cell manufacturers, such as Sharp Corporation and Mitsubishi Heavy Industries, have expanded their production plants. In the United States, the “Million Roofs Solar Power” plan proposed by former President Clinton is planned to be completed before 2010. The construction of 1 million solar power generation systems has been completed. In addition to Japan and the United States, Detong also started in 1990. Begin to implement the home-based plan, each household’s installed capacity of solar power is 15 kW, and the government will subsidize 70% of the funds. By 1995. 2,250 households have installed solar energy systems, with a total installed capacity of 5.6 MW. In addition, the Dutch government expects that the total installed capacity of solar energy systems can reach 1,450 MW in 2020. Other countries, such as Switzerland, Norway and Australia, have also implemented plans to install thousands of solar cells each year. in Taiwan, China. At present, the main manufacturers of solar cells include Guanghua, Motech and Shilin Electric. Guanghua Development Technology Co., Ltd. has been mainly producing amorphous silicon solar cells since 1988. Mainly used in super-eliminating electronic products, such as sub-tables, calculators, etc.

In 1999, Moody Corporation began to set up a factory in the same district of Tainan Science and Industry. It mainly produces solar cells of polycrystalline silicon and monocrystalline silicon. Shihlin Electric has also sent a research and development team to the United States for training to learn the manufacturing and packaging technology of solar panels used by satellites. At the same time, after the successful launch of China Satellite 1 in 1999, it further invested in the research and development of solar cells for people’s livelihood. In addition, the Institute of Materials of Industrial Technology Research Institute has also successfully developed the manufacturing and packaging technology of solar cells, and transferred the technology to Moody Corporation and Shihlin Electric Company to promote the solar power generation business. In recent years, Taiwanese manufacturers have gradually become interested in investing in the solar cell business. The main reason is that in addition to the shortage of supply in the international market, another factor is that Taiwan has vigorously promoted solar cell power generation since 1999, and started to promote various incentives. As a result, the number of companies investing in this business has also increased significantly. At present, there are still some difficulties in the implementation of solar power generation in Taiwan. The main reason is that it is obviously much more convenient to apply for utility power compared to the application procedures for general utility power and solar power generation, and the installation of solar power generation must first be invested in a funds. Based on economic considerations, it is indeed difficult for the general public to accept. even so. Look at it from another angle. Taiwan has favorable conditions such as abundant sunshine, sound development of semiconductor and power electronics industries, and strong official promotion, plus possible tourism, crisis, and popularization of environmental awareness. The solar power generation business indeed has a very large space for development in Taiwan. I believe that as long as the manufacturing cost can be greatly reduced. It can occupy a place in the field of solar power generation in the world.

In addition, the development and use of solar energy. The inability to generate electricity at night is a major disadvantage of solar cells. But for this shortcoming, scientists use two ways to overcome. The first way is to convert sunlight energy during the day into other forms of energy for storage, such as batteries, flywheel devices, pumped storage power plants, etc., and release the stored energy at night. Another way is the “satellite solar power station” (SPSS) that the United States and Japan are working on Places, such as near the equator, launch satellites with solar cells or thermal power generation systems, and use artificial satellites to absorb solar energy in space to generate electricity. Due to the avoidance of factors such as day and night, temperature difference and climate, artificial satellites can continuously and stably receive solar energy, convert it into electrical energy, and then transmit it back to the earth in the form of microwaves. After being received by the earth’s microwave receiving station, it is converted back into electrical energy. delivered to various places. Currently. Because scientists continue to study. Coupled with the advancement of semiconductor industry technology, the efficiency of solar cells is gradually increasing, and the unit cost of power generation systems is also decreasing year by year. Therefore, as the efficiency of solar cells increases and the cost decreases, the use of solar cells will become more and more common.

The solar cell technology has been transferred from the space technology application in the 1950s to the general commercial use of people’s livelihood. With the reduction of cost and environmental protection considerations, the use of monocrystalline silicon solar cells has become more and more common. The main applications are as follows.

(1) Household power generation system 5 from 20W to tens of thousands of watts. Depends on demand.

Located in California, USA, it was built in 1983 and completed in 1986. It is a 6 MW PV power plant; there are more than 20 relatively small-scale PV systems. It has also been successively adopted by many power companies in the past 10 years, as an experimental auxiliary device or installed on residential roofs to provide household electricity. A 6-year PV experiment program of the New England Electric Power Company (NEES) selected some residential buildings to install 2.2 kW C 10 pieces of 220 W) PV photovoltaic panels. The result is an average saving of about 50% of summer electricity bills. And users respond well. The Sacramento Electric Power Company in California) installed and tried two 1000 kW PV systems from 1984 to 1986 in accordance with the requirements of local residents to pay attention to ambient air quality; since 1993, a large number of medium-scale PV systems have been installed, and the total installed capacity has now over 3.7 MW .

The application of solar photovoltaic power generation system is quite extensive, with different application occasions. The system architecture is also different. For example, systems used in remote areas without electricity are stand-alone systems. In areas where there is electricity, the utility power parallel system can be used. When the power generated by the solar power generation system is greater than the load power, the excess power can be sent back.

What are the applications of monocrystalline silicon solar cells?
solar building

The independent solar power generation system mainly includes solar cell modules, charge controllers, batteries, converters and lighting loads. The solar cell first converts light energy into direct current, then charges the battery through the charge controller, and finally converts the direct current into alternating current through the converter to supply the lighting load. The solar power independent system has three possible operation modes: ① When the output power of the solar cell is greater than the load power, the excess power will be stored in the storage battery; otherwise. When the output power of the solar cell is less than the load power, the insufficient power will be provided by the battery. ② Add ATS between the converter and the load. There are two power sources for the load, one is the solar photovoltaic power generation system. One is the power system. When the power of the photovoltaic power generation system is sufficient to supply the load, the photovoltaic power generation system supplies the load power; when the power of the photovoltaic photovoltaic power generation system is insufficient, the ATS switches to the power system instead of the power system. This ensures that the load has an adequate source of power. ③ Integrate the charge controller with the converter.

What are the applications of monocrystalline silicon solar cells?
Schematic diagram of solar independent power generation system

(2) Traffic: electric vehicles, charging systems, road lighting systems and traffic signals.

What are the applications of monocrystalline silicon solar cells?
solar transportation

(3) High-power solar power generation system.

(4) Agriculture: Power systems such as irrigation and pumping

(5) Telecommunications and communications: wireless power, wireless communications.

(6) Backup power: disaster recovery.

(7) Power supply for low-power commodities.

(8) Outdoor positioning monitoring system: electronic bus stop signs, billboards, etc.

What are the applications of monocrystalline silicon solar cells?
Solar Outdoor Billboard System

The traditional energy sources that people mainly rely on today are limited. It is estimated that the remaining oil reserves are 1,033.8 billion barrels, which can be used for 43 years; the natural gas reserves are 146M, which can be used for 62 years; the coal reserves are 9,841.2 billion tons, which can be used for 230 years; the uranium reserves are 395 million. 10,000 tons can be used for 64 years. In addition, in recent years, as the issue of global warming has been paid attention to by countries all over the world, major countries in the world have actively developed clean renewable energy such as solar photovoltaic energy to replace fuel power generation in recent years, in order to alleviate the pollution problems caused by traditional power generation methods. Therefore, increasing the development and use of solar photovoltaic energy is an important direction for human life and survival. The solar photovoltaic industry is one of the most important energy technologies in the 21st century. Many companies have vigorously developed and promoted them. Many companies have also expanded their production capacity. In the past five years, the global solar cell production has grown at an average annual growth rate of more than 30%. , showing its unlimited development potential in the field of renewable clean energy. Taiwan has a complete semiconductor industry base and excellent conditions for the development of solar cells. Due to the promotion of policies and the vigorous development of the world market. At present, the solar cell industry has gradually emerged. Integrate related capital, technology, equipment and other manufacturers and research units, and cooperate with each other to develop competitive product technology. in order to enhance international competitiveness. To build a sustainable energy business.

What are the common types of high-efficiency monocrystalline silicon solar cells?

What are the common types of high-efficiency monocrystalline silicon solar cells?

At present, the commercialized monocrystalline silicon solar cells have an efficiency of 15%~20%, and the service life of the module is about 20 years; by 2010, it is hoped that the efficiency will be increased to 25%, and the chip thickness will be reduced to 50µm, which will reduce the cost to half of the current one. The module service life is expected to exceed 30 years; by 2030, the efficiency is expected to increase to more than 30%. Due to the current market expansion and high product competitiveness, some large companies are actively investing in the development of new technologies. The main development directions are: ① the improvement of the quality and thickness of silicon chips; ② the improvement of battery efficiency and cost reduction. As for how to improve the conversion efficiency of solar cells (greater than 25%), it has always been the direction of the industry and academia.

Monocrystalline silicon solar cells are currently more than 20% efficient and have been commercialized, as shown in Figure 1. Figure 1(a) is the solar cell Sun Power A-300 developed by Sun Power Company, which is characterized by designing the electrode parts on the same side and the back side>, so that the front side of the cell does not have any shielding area, and its highest efficiency has reached 21.5 %; Figure 1(b) is a solar cell developed by BP Solar, which uses laser to embed the front electrode into the cell to increase the carrier collection effect and improve the efficiency, and the highest efficiency can reach 20.5%; Figure 1 ( c) HIT high-efficiency solar cells developed for Sanyo Company, the highest efficiency can reach 20.1%. In addition, there are several other high-efficiency solar cell structures, including emitter passivated and rear locally diff used (PERL) solar cells, grating solar cells, point-contact solar cells , obliquely evaporated contact solar cells, metal insulating layer semiconductors, solar cells, screen printing (screen printing) solar cells, etc., the monocrystalline silicon solar cells with different structures will be introduced and discussed below.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 1 Commercialized monocrystalline silicon solar cell structure

1. Emitter Passivated Back Local Diffusion Solar Cell
In recent decades. The high-efficiency monocrystalline silicon solar cell is most famous for the emitter-passivated backside local diffusion (emitter and rearlocally diffused, PERL) device developed by the University of New South Wales, Australia, and its efficiency is as high as 24.7%, as shown in Figure 2 Show. The structure is surface textured with inverted pyramids, and is also coated with double anti-reflection layers of MgF2 ( n = 1. 38 ) and ZnS ( n = 2. 4 ) to increase the Light absorption to increase photocurrent generation: Passivation of silicon surface with thermal oxide layer. To avoid photocarrier recombination at the boundary; the design of local diffusion on the back forms a back surface field (BSF), which can bounce minority carriers, and due to the design of local diffusion, the majority of carriers are avoided on the boundary. The composite mountain increases the collection of majority carriers; BBr3 and PBr3 liquid sources are used for doping at the metal contact position to reduce the contact resistance.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 2 PERL solar cell structure

2. Buried Contact Solar Cells
In the past 15 years, the efficiency of solar cells has been improved a lot. The most striking structure is the buried-contact solar cell (BCSC), which was developed by the University of New South Wales in Australia, and was developed by BP Solar in the United States. commercialized, and its structure is shown in Figure 3. The BCSC solar cell structure combines the advantages of the early PE SC structure and the recent PERL structure. Part of the cell is etched and surface textured, and then the cell surface is diffused and passivated to achieve the best resistance through oxide layers and nitrides. Reflection and surface passivation, and then use YAG-Laser to carve grooves (grooving) on ​​the surface of the battery. The depth of the groove should not exceed 60µm, otherwise it will affect the open circuit voltage of the battery. In addition, in order to increase its mass production speed, The laser grooving process can also be changed to mechanical grooving. Although the use of the mechanical process may result in poor cell uniformity, the subsequent etching can be used to smooth the groove; The secondary diffusion process and the deposition of the back aluminum electrode, and then use the electroplating technology to deposit three metal alloys of inlay, copper and silver on the groove and the back of the battery.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 3 Buried Contact Solar Cell (BCSC) Structure

The solar cell efficiency of the BCSC structure is higher than that of the general commercial screen-printed solar cells, which not only improves the current and voltage output, but also improves the series resistance effect. Since it is easier to absorb the incident light of the blue-ray technology, the current output is improved; in addition, the carrier combination rate of the electrode is reduced, so the voltage output is improved. The fill factor is also increased due to the improved open circuit voltage and lower series resistance. Therefore, the overall efficiency of the battery has been improved a lot, reaching 19.9%.

In addition, the general buried structure can be slightly changed into a double-sided contact (double-sided contact, DSBC) solar cell structure, as shown in Figure 4. It uses lower temperature and lithography process, spin coating liquid diffusion source to reduce the cost of solar cells, which has been proved by experiments. Its conversion efficiency is 17%, which is not as efficient as the general buried structure. The reason is that the liquid diffusion source tends to drive in the trenches. If the manufacturing process is improved, it is expected to reach 20%.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 4 Double-sided buried contact solar cell structure

3. Grating solar cells
The grating structure solar cell is one of the solar cells designed in recent years. Its main concept is to use various etching techniques to make the cell surface structure into a grating shape to increase the utilization of the incident light source. A research team used the reactive ion etching (RIE) process to etch the cell surface into gratings with depths ranging from 10 to 30 µm, as shown in Figure 5, and found that the grating structure can better absorb incident light (visible light band). As shown in Figure 6, it is found that the effect of using a 2-dimensional grating structure is better than that of a 1-dimensional structure. If passivation treatment is added, the recombination rate of electron-hole pairs can be reduced, and the short-circuit current density and internal quantum efficiency are also improved. Can be improved a lot.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 5 Grating structure
What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 6 Comparison of light absorption by 1D and 2D grating structures

Later, other research teams used ZnO as the main material of the solar cell grating structure, used photolithography to define the pattern, and etched the grating depth of about several hundreds of nanometers, as shown in Figure 7 using SEM scanning. The fill factor of the battery can reach 68%, and it is found that it has a better response to the red and blue wavelengths, that is, the grating structure increases the utilization of the incident red and blue wavelengths.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 7 Grating structure using ZnO thin film

4. Thin intrinsic layer heterojunction HIT solar cells
The thin intrinsic layer heterojunction HIT (heterojunction with intrinsic thin layer) solar cell was developed by Japan Sanyo Company and has been commercialized. It uses an N-type silicon chip, which is different from the general battery using a P-type. The thickness of the overall HIT cell does not exceed 200µm, and a thin amorphous layer (i/P, i/N layer) is deposited on the upper and lower layers of the N-type silicon chip, and the front and back sides of the cell are both transparent conductive oxide layers (TCO) , which also acts as an anti-reflection layer, as shown in Figure 8. The fabrication of this cell emphasizes that no high temperature diffusion is required to form the PIN interface, and the fabrication temperature is lower than 200 °C, so it is easier to use thinner silicon chips and reduce costs.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 8 Structure of Sanyo’s HIT solar cell

Sanyo started mass production of HIT solar cells in 1999, and in April 2003 published a commercial record of 21.5% high-efficiency HIT solar cells, which achieved the efficiency improvement goal through process improvement, mainly by improving the silicon thin film. quality, making it more efficient. Later, Sanyo company developed a new conductive adhesive with higher conductivity, which can obtain higher filling factor and short-circuit current value, and its conversion efficiency also reached 21.5% (Voc = O. 712 V, lsc = 3. 837 A , FF = 78. 7 % ), and the battery area is 100. 3 cm². In addition, San yo also mentioned that the conversion efficiency of general solar cells will decrease with the increase of temperature, but the efficiency of HIT solar cells has the slowest decrease rate, which is 0.25%/℃, as shown in Figure 9, showing that the Battery performance is good.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 9 Conversion efficiency versus temperature of HIT solar cells

5. Backside Contact Solar Cells
In order to reduce the cost of the solar cell process, Sun Power Corporation of the United States has developed a high-efficiency, low-cost back-contact solar cell, as shown in Figure 10. The FZ chip with a resistivity of 2.0Ω/cm and a thickness of 200 µ.m is mainly used to texture the front side of the battery, passivate the front and back sides of the battery with an oxide layer, and reduce the surface reflection with a double-layer anti-reflection layer. The N+/P contact is formed on the backside by diffusing from the side. The aluminum electrode is designed to be finger-shaped and all distributed on the backside to increase the utilization of incident light. The overall cell area is 22 cm², and the highest cell efficiency reaches 23%.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 10 The structure of the back contact solar cell developed by SunPower

In addition, the thickness of the backside contact cell affects the conversion efficiency of the cell. If the thickness of the battery is reduced, the electron-hole pairs will be reduced, because a large number of photons cannot be absorbed; ② The collection efficiency of minority carriers can be increased, and the reduction of the thickness will shorten the moving distance of the carriers; ③ The dark current can be reduced; ④ Reduces the effect of edge current carrying on recombination. The resistivity of the battery is also closely related to the battery efficiency. In addition to affecting the carrier mobility, it also affects the bulk recombination current (bulk recombination current), as well as the parallel resistance and the edge carrier recombination rate. In order to reduce costs, the following chips such as high-quality CZ chips can also be used. These two chips also have the effect of carrier lifetime greater than 1ms, and the battery conversion efficiency can reach more than 19%.

6. Point contact solar cell
In 1986, the Stanford University research team developed a point contact solar cell, as shown in Figure 11. The maximum conversion efficiency of the cell can reach 28.3% under the condition of collecting light of 050 suns.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 11 Point contact solar cell structure

The main feature of this structure is to reduce the emitter area on the back of the battery, which is similar to the back contact structure developed by Sun Power. Both positive and negative electrodes are designed on the back. It can be used more effectively; in addition, since the back electrode is composed of several layers of metal, the series resistance of the structure is quite low, so that the output power loss will not be too much, about several percentages. However, the area of ​​the solar cell that was originally designed with this structure is quite small, about 1.21 cm², which makes modularization very difficult.

The main process of point contact solar cells is as follows 2. Using < 100 > FZ grade N-type chip, the thickness is 130 233µm, the resistivity is 100 200Ω/cm, and the diffusion of P-type and N-type is about 1000 Ω/cm. The SiO2 oxide layer is deposited by TCA process in the environment of ℃, the thickness is about 1000Å, and the sheet resistance value is 5~6Ω/port, which also reduces the surface bonding rate. At the same time, the oxide layer can also be used as an anti-reflection layer. The electrode materials of N-type and N-type are all aluminum, and the resistivity of the electrode is (1~2)×10-6Ω/cm.

7. OECO solar cells
Oblique plating contact (obliquely evaporated contact, OECO) solar cell is developed by German research institute ISF H (Institute Fur Solar) in recent years, its main feature is the use of special aluminum metal film oblique plating (obliquely evaporated) equipment. Different from the general vertical method, the evaporation process adopts the inclined method, so that the electrode can be plated on the side of the trench without any mask and alignment, and the width of the metal electrode can be easily adjusted. As shown in Figure 12, the front electrode of the battery is placed on the side of the parallel groove. Therefore, the shielding area of ​​the metal electrode is very small. This cell uses an MIS structure, so the thickness of the thin oxide layer must be carefully controlled. At present, the conversion efficiency of such solar cells is 18%~21%. Figure 13 shows the main structure of OECO solar cells.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 12 Evaporation system of OECO solar cells
What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 13 Main structure of OECO solar cell

8. Metal insulating layer semiconductor solar cells
As early as the 1970s, MIS structured solar cells attracted everyone’s attention. Whether in theory or in practical preparation, MIS structured solar cells can make up for the shortcomings of Schottky barrier solar cells. MIS solar cells are also known as MIS solar cells. for low open circuit voltage solar cells. The MIS insulating layer is quite thin, and in addition to controlling the huge dark current, it can also control the type of majority or minority carriers. In the 1990s, the MIS-IL (metal-insulator-semiconductor inversion-layer) solar cell was developed by the German IS FH organization, as shown in Figure 14. The conversion efficiency is 15.7% on a 2 cm × 2 cm FZ chip; on a 10 cm × 10 cm CZ chip. Its conversion efficiency is 15.3%. In order to achieve higher conversion efficiency, the battery conversion efficiency can reach 18.5% by improving the following three parameters:
(1) Reduce the loss of surrounding carrier recombination.
(2) Use the grid electrode on the front of the battery to improve the electrode impedance.
(3) Reduce the carrier recombination loss on the back of the battery.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 14 Solar cell structure of MIS-IL

The following are the main fabrication steps of solar cells with MIS-IL structure:
(1) Chemical etching (texturing the surface of the battery, such as a pyramid).
(2) The backside of the battery is deposited with an aluminum electrode by an evaporation method.
(3) A tunnel oxide layer is grown at 500°C.
(4) Use the metal mask to define the pattern and deposit the aluminum electrode on the front side of the battery.
(5) The excess aluminum electrodes are removed by etching.
(6) Immersion in calcium to increase the positive charge density on the silicon surface.
(7) Deposit Si Nx on the entire front side of the cell using PE CVD.

In 1997, the ISFH institute developed a more efficient MIS-N+P solar cell, as shown in Figure 15, mainly by changing the following manufacturing processes:
(1) Place the MIS electrode on the N+ diffused emitter.
(2) The electrodes on the front and back of the battery are made of aluminum.
(3) Double-layer SiNx is deposited by PECVD, which is used as a double-layer anti-reflection layer (DLAR) as a passivation layer. Compared with the MIS-IL structure solar cell, the open circuit voltage and fill factor of the MIS-N+P structure are not improved. The open circuit voltage is increased from 595mV to 656mV, the fill factor is also increased from 74.4% to 80.6%, and the overall battery efficiency is increased from 18.5% to 20.9%. Generally speaking, the production process and structure design of MIS structure solar cells are not difficult, the cost is not high, and the efficiency has reached more than 20%. If it can be improved and researched and developed, it is expected to become the mainstream of monocrystalline silicon solar cells.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 15 Structure of MIS-N+P solar cell

9. Screen-printed solar cells
Since the advent of screen printing technology, it has been used in quite a few occasions. In addition to the printing of circuit boards, it is also used in the electrode manufacturing process of solar cells. The process is quite fast, simple and low-cost. At present, many manufacturers of solar cells, in order to increase the speed of mass production, mostly use screen printing technology to print the electrode part on the emitter (30~55Ω/□) instead of the shallow emitter (shallow emitter, 90~ 100Ω/□) to avoid high electrode impedance, the structure of the screen-printed solar cell is shown in Figure 16.

What are the common types of high-efficiency monocrystalline silicon solar cells?
Figure 16 Structure of screen-printed solar cell

Heavy doping of the emitter will reduce the short-wavelength response, and will make the emitter saturation current higher, which will make the solar cell less efficient. Therefore, in order to increase the efficiency of solar cells, a high sheet resistance value of the emitter can be used to provide an effective emitter surface passivation. Usually screen printing is a part of the solar cell manufacturing process, which belongs to the latter part of the solar cell manufacturing process. Aluminum-containing glue is often used. The aluminum glue is printed on the back of the battery by screen printing, and is placed in a 200 ℃ environment first, and then removed. After removing excess water, the silver-containing jelly is printed on the anti-reflection layer of the battery, and then the battery is put into the furnace tube (CIR or RTP furnace tube) for sintering, thus completing the production of screen-printed solar cells. Using screen-printed solar cells, the efficiency can exceed 18%.