Low-cost Crystalline Silicon Thin Film Solar Cells On The Substrate

The wide use of solar cells will be a feasible way to solve the energy and environmental protection problems. The development of solar cells is showing a tendency to improve efficiency and reduce cost. Silicon thin-film solar cells are considered the most promising cells in the future for their advantages, such as low cost, high efficiency, great stability, simple processing, and none-pollution.To achieve this aim, silicon thin film solar cells on low cost substrates are studied in this thesis. Three kinds of substrates are adopted in our work, include heavily doped mono-crystalline silicon wafer, ceramics, and silicon sheets from powder (SSP).Firstly, crystalline silicon thin films were deposited on heavily doped mono-crystalline silicon wafer by rapid thermal chemical vapor deposition (RTCVD). With the optimized deposition parameters, mono crystalline silicon films with low defects density were obtained. Based on experiments and computer simulations, optimized processing conditions for cells' fabrication were achieved. The best conversion efficiency of 15.12% (Voc=637.1mV, Jsc=30.45mA/cm², FF=0.7797, AM=1.5G ) has been reached, which is close to the best result in the world.Secondly, growth and recrystallization of silicon films on ceramic substrates were studied. Heavily doped polycrystalline silicon thin films were deposited on low cost ceramics substrates by RTCVD. Compact and uniform films with grain size in the order of some micrometers were fabricated. By means of zone melting recrystallization (ZMR) method, polycrystalline silicon thin films with large grains and relative high carrier mobility were obtained, which could act as a seeding layer. The maximum grain of these films was about one millimeter in width and some millimeters in length, and hole mobility exceeded 50cm²/Vs. Active silicon films deposited on these seeding layers showed the same morphologies. These results show that these films have great potential for photovoltaic applications.Finally, low cost SSP substrates were used in our experiments. Poly-crystalline silicon thin films were deposited on SSP substrates directly without any intermediate layer. A P⁺ layer between active layer and substrate was applied as a barrier for defects and impurities, and relative high deposition rate was achieved. Suitable processing conditions for cells' fabrication, such as phosphorous diffusion, hydrogen passivation, SiN_x anti-reflection coating deposited by plasma enhanced chemical deposition (PECVD) were also studied. Based on these works, efficiency of 8.25 % (J_sc=26.69mA/cm2, V_oc=506.8mV, FF=0.6101) and 4.5% (J_sc=20.39mA/cm², V_oc=386.9mV, FF=0.57) have been obtained on high-purity and low-purity SSP substrates respectively. These results reach the better level in the world.