| Summary: | 碩士 === 國立中央大學 === 照明與顯示科技研究所 === 103 === In this study, the intrinsic hydrogenated amorphous silicon (a-Si:H) thin films in heterojunction with intrinsic thin layer (HIT) solar cell was prepared by Plasma Enhanced Chemical Vapor Deposition (PECVD). PECVD has several advantages, such as stable deposition rate, good coverage uniformity, low temperature growth and easy integration with other processes. The passivation quality of a-Si:H thin films was investigated by tuning the process parameters such as power, dilution ratio, and substrate temperature. Firstly, the thin film properties were measured and analyzed by Spectroscopic Ellipsometer. And the surface passivation quality of a-Si:H was determined by photo-conductance lifetime tester. Finally, a-Si:H films were applied to amorphous silicon / crystalline silicon heterojunction solar cells and improved the open-circuit voltage of solar cells.
The results show that the 50nm a-Si:H films can obtained high effective lifetime and good passivation quality for all process parameters with as-deposited condition, but after annealing treatment the passivation quality will decay because the Si-H bonds become disorder thus result in the reduction of effective lifetime. On the other hand, for the ultra-thin (5nm) a-Si:H films, after annealing treatment the effective lifetime increased. The high crystalline volume fraction will generated defects in the films or interface, after annealing process can improved the defects ratio, increase the Si-H bond at the substrate interface and compensate the surface dangling bonds, reduce recombination center and thus enhance the lifetime, that also prove the best passivation quality can be obtained with suitable amorphous to microcrystalline transition region. For ultra-thin (5nm) a-Si:H films, we obtained the high passivation quality under the condition of H2/SiH4=4, power of 20W, substrate temperature of 140℃, and after annealing process under the temperature 300 ˚C for 120 sec. The effective lifetime of a-Si:H film increased to 1.2 msec, the implied Voc increased to 0.694 V and cause the surface recombination velocity (SRV) decreased to 10 cm/s. In addition, the lifetime can reach 4.7 msec, implied Voc 0.725 V, the SRV drops to 2.98 cm / s on the 4-inch FZ-n silicon substrate. Moreover, the characteristics of HIT solar cell on CZ-n silicon substrate were shown as follow: Voc = 0.66 V, Jsc = 36.71 mA/cm2, F.F. = 71.75 %, efficiency = 17.26 % in the area of 1 cm2.
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