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Research on Fabrication Technology of A-Si:H/c-Si Hetero-Junction Solar Cells

Author: ZengZuo
Tutor: ZengXiangBin
School: Huazhong University of Science and Technology
Course: Microelectronics and Solid State Electronics
Keywords: hetero-junction HIT solar cell aluminum back surface field texturing silicon nitride
CLC: TM914.4
Type: Master's thesis
Year: 2011
Downloads: 31
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Abstract

Amorphous/crystalline silicon hetero-junction solar cell with intrinsic thin film (HIT solar cell) is fabricated by Plasma Enhanced Chemical Vapor Deposition (PECVD). Through new techniques such as back surface field and light trap and so on, researchers enhanced the efficiency to 23%, which is close to the highest record of crystalline silicon solar cell (24.7%). From above we know that process optimization is critical to gain high efficiency solar cell and increase its utilization percent.This paper mainly investigated the preparation of aluminum back surface field, crystalline silicon substrate texturing and silicon nitride protective film. After analyzing the experiment results we gained the optimum data. At last we fabricated HIT solar cell with wished parameters utilizing all the techniques mentioned above.The thickness of silicon wafer is thinning gradually in order to lower the cost of solar cell. When the diffusion length of minor carrier is larger than silicon wafer’s thickness, interface recombination rate will influence the solar cell efficiency considerably. Most light can’t be absorbed as the silicon wafer becomes thinner. Aluminum back surface field can decrease interface recombination and reflect the light into wafer to get secondary utilization. This paper studied the formation of aluminum back surface field at different temperature and different annealing time. Experiment results shows that low recombination rate and uniform distribution aluminum back surface field can be obtained after wafer annealed at 700℃for 150 seconds.Mirror reflection will occur in the interface if the substrate is not textured, and most of the incident light will go back to the air. With the purpose of improving utilization of sunlight, texturing process is necessary. It can increase the acceptance area for 1.73 fold. The pyramid feature gained after texturing can reflect the incident light constantly into solar cell, and the reflected light can be utilized time and again. This paper investigated surface feature and reflectivity at different etchant solution and different etching time, the experiment results show that high-class pyramid structure can be obtained after etching in solution consisted of KOH, isopropanol and de-ionized water at 83℃for 50 minutes, the average reflectivity is 10.47% and the lowest reflectivity is only 9.2%.Protection of solar cell surface is essential in order to prevent efficiency from decaying. Silicon nitride has high refractive index and can keep the solar cell from destruction of water vapor and impurity. The hydrogen plasma can passivate emission layer during deposition and lower interface recombination rate. The ratio of NH3 /SiH4 is principal factor of refractive index, this paper compare the quality of silicon nitride and its passivation effects deposited at different working mixing gas ratio. From The experiment results we know that qualified silicon nitride can be gained when the ratio of NH3 /SiH4 are 1:3 and 1:5.Utilizing all the optimal parameters gained by these experiments above, the hetero-junction with intrinsic thin film solar cell was prepared. The characteristics of HIT solar cells were measured at AM1.5. The efficiency, fill factor, open-circuit voltage and short-circuit current density of solar cell are 16%, 67%, 618mV and 38.9mA/cm2 respectively.

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