1. Consideration of semiconductor materials
Generally speaking, the chips used in solar cells can be divided into P-type and N-type, but because the diffusion length of excess minority carriers (electrons) in P-type is longer than the diffusion of excess minority carriers (holes) in N-type The length is too long. In order to obtain a larger photocurrent, a P-type chip is generally selected as the substrate.
2. Considerations of spectral response
Figure 1 is the solar radiation spectrum of AMO and AMI. In solar radiation, the most energy is in the short wavelength region close to UV. in solar cells. Since only some photons with energy greater (Eg) than the energy gap of the solar cell material can be absorbed by the material, other photons with energy less than the energy gap of the material cannot be absorbed and penetrated; in addition, if a material with a small energy gap is selected. Although most of the spectrum can be absorbed, the intrinsic concentration ni of the material is large, the dark current of the cell will increase and the efficiency of the solar cell will decrease. Generally speaking, the material energy gap of solar cells is between 1~2 eV, and the corresponding light wavelength is 0.6~1.3µm, so the material energy gap is between silicon (Si), gallium arsenide (GaAs), Indium phosphide (lnP) is a very good solar cell material.
3. Considerations for Shallow Interfaces
Based on the above considerations, we chose the material as monocrystalline silicon (Si) to consider the design of the shallow interface. When the light wavelength is less than the short wavelength of 400nm, the light absorption coefficient of silicon will become very large (>105cm1), so most of the incident photons will be absorbed near the surface depth (0.1~0.3µm) to generate electron-hole pairs. For the light absorption coefficient of a material, the larger the light absorption coefficient, the stronger the absorption of photons near very shallow surfaces. To improve the conversion efficiency of the cell, the key is to reduce the interface depth of the silicon solar cell to less than 0.3µm, so as to enhance the absorption of short wavelengths and improve the efficiency.
4. Considerations for Anti-Reflection Layers
For single crystal silicon, the light reflectivity on the silicon surface under normal illumination is 30%~35%. In order to reduce the surface reflectivity, layers of materials with different refractive indices can be used to stack up anti-reflection layers. As shown in Figure 2, the interface reflection coefficient R between air and medium can be calculated according to the following formula
In the formula, n0 is the refractive index of air; n1 is the refractive index of the anti-reflection layer; ns is the refractive index of the semiconductor. Since n0=1 (air), when n1=(ns) 1/2, the reflection coefficient R=0, the refractive index of single crystal silicon at the incident light wavelength of 400~1100nm is 6.0~3.5, so in order to reduce the surface reflectivity, The refractive index of the antireflection layer is preferably between (3.5) 1/2 = 1.87 and (6) 1/2 = 2.5. Commonly used anti-reflection layer materials are SiO2 (n=1.5), Si3N4 (n=2.0), Ta2O5 (n=2.25), TiO2 (n=2.3) and so on.
5. Considerations for Surface Passivation
The surface passivation (surface passivation) treatment is generally to grow a layer of oxide layer to reduce the surface carrier recombination speed at the Si-SiO2 interface.
The total carrier recombination rate per unit area of the semiconductor surface is
where nth is the electron thermal speed; σn is the electron-capture cross-section; σp is the hole capture cross-section; Nst is the surface recombination rate (#/cm3); ps is the hole concentration on the surface; ns is the electron concentration on the surface; E is the energy level of the defect center. Under low injection, ns≈ND>>ps, ns﹥﹥ni, exp(Et-EFi/kT), so the above formula can be rewritten as
Here Sp=vstσpNst, Sp is the surface recombination velocity of holes. The surface recombination velocity has a great influence on the reverse saturation current. The faster the surface recombination speed, the greater the reverse saturation current, resulting in a decrease in the photocurrent. , which affects the efficiency of solar cells.
6. Considerations for textured structures
Anisotropic chemical etching on the single crystal silicon surface in the (100) lattice direction will form a tiny inverted pyramid structure in the (111) lattice direction. As shown in Figure 3, this textured structure can cause multiple reflections of incident light, which can increase the travel path of light in addition to reducing the reflection of incident light. For long-wavelength incident light, since the light absorption coefficient α of single crystal silicon material is very low, the incident light will be reflected at the bottom and then refracted by the surface textured structure to increase the light absorption.