Amorphous Silicon Thin Films Deposited By PECVD And Their Influence On The Performance Of HIT Solar Cells

HIT solar cells with high efficiency, low cost and quick preparation process caused wide attention all over the world. Sanyo Co. announced they have achieved a novel HIT solar cell with a high efficiency (over 23 %).The intrinsic layer, window layer and transparent electrode strongly influenced HIT solar cells performance. In this paper, we wanted to increase the light absorption and improve the carrier separation and transport by researching structural and photoelectric characteristics of these three layers, and optimizing the preparation conditions, and prepared highly efficiency single-sided HIT solar cell in the laboratory level. The window layer and the intrinsic layer were deposited by PECVD; the transparent electrode was deposited by magnetron sputtering; and n type crystalline silicon was used as substrate.First, through the study of the intrinsic layer (Intrinsic hydrogenated amorphous silicon thin film), we have come to the conclusion that H2 flow influenced the structural characteristics of a-Si:H films, when H2 flow was small, films were amorphous, and photoconductivity of 4.22×x10-5 (Ω·cm)-1, dark conductivity of 7.95×10-11 (Ω·cm)-1. Deposition power and pressure mainly affected the deposition rate. The Eg mainly related to local density of states and the smallest Eg of 1.7 eV. The optimized preparation conditions were:deposition power of 50W, deposition pressure of 200 Pa, the hydrogen flow of 160 seem. Through the study of the intrinsic layer thickness, we obtained the intrinsic layer major effected short-circuit current, the optimum intrinsic layer thickness was 5 nm.Next, we studied the window layer (p-type amorphous silicon thin film). After doping trimethyl boron, dark conductivity of the film increased 4,5 orders of magnitude, and the thin film was amorphous. When the doping concentration was 1.5%, Eg was 2.2 eV. The optimized preparation conditions were:deposition power of 50 W, deposition pressure of 200 Pa, doping concentration of 1.5%, the hydrogen flow rate of 75sccm. After studying of window layer doping density and thickness influence on HIT solar cells performance, we obtained the window layer main effected open circuit voltage and short-circuit current, the optimum doping concentration was 1.5%, optimum thickness was 30nm.Then through the study of transparent electrode (ITO), a high quality ITO film with highly transmittance above 90%, lower resistivity of 0.003Ω-cm was obtained as transparent of HIT solar cells. The optimized preparation conditions were:sputtering power of 350 W, substrate temperature of 200℃, Ar flow of 40 sccm.Finally, after stratification optimization, HIT solar cells performance was improved, especially Isc was significantly improved. In this paper an optimized single-sided HIT solar cell structure of Ag/ITO/p-a:Si/i-a:Si/n-c:Si/Al provided the maximum efficiency of 8.63%, Vocof 430 mV, Isc of 40.45 mA/cm2, FF of 49.6%.