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.
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.
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.
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.
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.
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].
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.
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 % .
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 %.