| Summary: | 碩士 === 中原大學 === 電子工程研究所 === 98 === The cost of Si wafers has been the bottleneck for a widespread use of Si solar cells. The upgraded metallurgical grade silicon ( UMG-Si ) is inexpensive compared to the single crystal or solar grade polysilicon material. In this study, the UMG-Si substrates are used to fabricate solar cells with a view to reducing the production cost of Si solar cells. However, these substrate materials contain a large number of harmful internal metal impurities. These impurities tend to be the recombination centers for carriers and cause a serious degradation in the efficiency of solar cells. To solve this problem, we use an extrinsic gettering technique to remove those unwanted impurities.
The extrinsic gettering process is denoted as polysilicon film gettering (PSFG) and was conducted as follows. First, a polysilicon film was deposited using atmosphere pressure halide chemical vapor deposition ( APHCVD ) at different temperatures: 800, 900, and 1000˚C. Then this film was annealed at various temperatures ranging from 600 to 850˚C. The as-deposited polysilicon film (what is called, a sink layer) contains a large number of grain boundaries and cavities, which can be used as the dumps or places for absorbing and storing metal impurities present in UMG-Si substrate. The annealing treatment was performed to promote the interdiffusion behavior of metal impurities. The minority carrier lifetime of the UMG-Si substrate treated with PSFG was obtained from microwave photocurrent decay (μW-PCD) measurements, which was used to evaluate the effect of PSFG. Finally, the electrical properties of solar cells fabricated on the UMG-Si substrates with and without PSFG treatment were compared.
It was found that depositing a polysilicon film at at 800˚C and annealing it at 700˚C for 15 minutes would have the lifetime of UMG-Si substrate increase to 33.2 µs. Also, the higher the minority carrier lifetime of UMG-Si substrate, the higher the efficiency of solar cell fabricated thereon. Ultimately, an efficiency high to 13.66 % has been achieved for the cell fabricated on the PSFG-treated substrate, which is elevated by 1.6 % compared to that of the solar cell without PSFG treatment.
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