Fabrication of hydrogenated microcrystalline silicon thin-film solar cells by high pressure depletion method

• Main Authors: Chi-Hua Li, 李季樺
• Other Authors: 江雨龍
• Format: Others
• Language: zh-TW
• Published: 2011
• Online Access: http://ndltd.ncl.edu.tw/handle/99727052320772555869

碩士 === 國立中興大學 === 電機工程學系所 === 99 === In this thesis, 40.68 MHz very-high-frequency plasma-enhanced chemical vapor deposition is used to fabricate μc-Si:H thin-film solar cells with changing the i-layer power density, deposition pressure and doping gas flow ratio of p-layer. The structural and electrical properties of solar cells are investigated by Raman spectrometer, x-ray diffraction, scanning electron microscope, current-voltage measurement and quantum efficiency measurement. P-type μc-Si:H thin films are fabricated by varying doping gas flow ratio of B2H6/SiH4 = 0.25%~1%. The results indicate that the ratio at 0.25% of p-layer has highest crystalline volume ratio (XC) which easy to induce the crystallization of i-layer so the incubation layer is thinner. On the contrary, the p-layer structure of amorphous phase suppresses the crystallization of i-layer, result in poor carrier conduction in p/i interface, and makes the solar cells properties deteriorate. Appropriate structure of microcrystalline P-type film can contribute to the growth and quality improvement of i-layer with microcrystalline phase. For the doping gas flow ratio of p-layer fixed at 0.5%, a series of solar cells are fabricated by changing deposition pressure of i-layer from 3 to 7 torr. The results indicate that increasing pressure decrease the crystalline volume ratio of the cells. Low-pressure deposition induces many columnar crystals with small cross section. There are many grain boundaries to make carrier recombination which reducing current response in long wavelength region. High-pressure deposition can reduce the grain boundary defects, then the electrical properties of solar cells can be improved. For the p-layer doping gas flow ratio fixed at 0.25%, a series of solar cells is fabricated by changing deposition pressure of i-layers from 5 to 8 torr. The results demonstrate that the solar cells are columnar structure for deposition pressure from 5 to 7 torr, further increasing the thickness of incubation layer the cone-shape crystalline structures are formed when pressure at 7.5 torr, and then the structure is whole amorphous phase for the 8 torr condition. Current-voltage measurement results show that the deposition pressure on the 5, 6 and 7 torr, the cells have the high current density which is typical characteristics of microcrystalline phase. The structure of 7.5-torr cell is mixed-phase of microcrystalline and amorphous phase that the current density has the middle value. The 8-torr cell is fully amorphous and has the lower current density which is the typical characteristic of amorphous phase. There have a certain percentage of microcrystalline phase for the solar cells deposited at 5 ~ 7.5 torr, therefore it has a lower open-circuit voltage of about 0.45 V. The open-circuit voltage of the amorphous phase of the 8-torr cell is increased to 0.84 V. These results illustrate that transition region from microcrystalline to amorphous occurred from 7 to 7.5 torr. The solar cell deposited at 7 torr is at boundary of transition region, and has the best performance and conversion efficiency. Good μc-Si:H solar cells must be fabricated at the boundary of transition region from microcrystalline to amorphous, which is generally controlled at high pressure condition.