The Physical Property Of Silicon Solar Cell Grown By Gas Source Molecule Beam Epitaxy System

Silicon solar cell is based on the photovoltaics（PV）,which has attracted attention of many researchers.The absorption coefficient is a basic parameter for a certain material,which is very important to the solar cell and photoelectric detector.PN junction induced absorption enhancement in GaAs-based and GaN-based systems was reported.GaAs and GaN are direct bandgap semiconductor materials,while silicon is an indirect bandgap material that has a wide range of applications in solar cells.Through the research on PIN-type silicon-based solar cells,it was found that the absorption coefficient of silicon in silicon-based solar cells was enhanced.We grew a series of silicon based sample,and calibrated the growth parameters of our reconstructed Gas Source Molecule Beam Epitaxy system（GSMBE VG80s）including growth rate,doping concentration et al.Four samples of PIN silicon with different intrinsic region were grew on the 2 inch boron-doped p-Si wafer with the resistivity of 1Ω.cm.The thicknesses of intrinsic layer of 4 samples were 100nm（sample A）,300nm（sample B）,1000nm（sample C）and 3000nm（sample D）,respectively.We simulated the built-in electric field intensity by the diffusion-drift model.The simulation results suggest that although the electric field intensities are different for four samples,but the built-in electric field covers the entire intrinsic layer area in each sample.Photogenerated carriers are generated inside the material,Due to the built-in electric field,only the carriers in the intrinsic region can be effectively extracted by the drift effect.A method for calculating the absorption coefficient of silicon material in solar cells by photoelectric conversion effect is derivated.The photocurrent curves of the four groups of samples are measured under a white light source.In order to analyze the contribution of photogenerated electrons to the photocurrent through the diffusion motion in the material,the photon maximum photocurrent value under the ideal condition of the surface N-doped region is calculated.When the photon energy is 3 ev,the N-type region has a light absorption rate of 87.6%.The current contribution efficiency is about 0.4%,and the contribution of photogenerated electrons to photocurrent is small through diffusion.Therefore,we can ignore the contribution of diffusion motion to photocurrent when discussing the absorption coefficient.The maximum values of the absorption coefficients for the four samples are 8.9×10⁴,1.1×10⁴,1.0×10⁴,3×10³ cm^-1 respectively.and the absorption coefficient of sample A was 30 times that of sample D.The highest absorption coefficient was improved by 1-2 orders of magnitude compared with crystalline silicon,it is close to the absorption coefficient of GaAs in the direct bandgap semiconductor material.In the statistical process,the influence of light reflection,photo-generated carrier recombination and other factors is neglected,and the measured absorption coefficient is lower than the actual value.The PIN type solar cell structure is a major factor in the improvement of the optical absorption coefficient of the silicon material,and the mechanism is currently unclear.In order to further confirm the experimental results of the absorption enhancement,the excitation power of the 730 nm laser is changed,and the absorption coefficients of the four groups of samples are measured.The experimental results are consistent with the results under white light.The absorption coefficient of the solar cell structure with unintentionally doped layer thickness of 100 nm is the highest among the four groups,and the absorption coefficient is basically unchanged with the change of excitation power.In the PIN type silicon solar cell structure.The light absorption of silicon is enhanced.In the PIN type silicon solar cell structure,the absorption coefficient of the intermediate intrinsic silicon is effectively enhanced.Through the change of the thickness of the intrinsic layer,the absorption coefficient of the silicon material will also change accordingly.The experimental results bring new challenges to the absorption mechanism of materials in this kind of structure,and provide possibilities for the preparation of new photovoltaic devices.