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:
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.