Research On Crystalline Silicon Heterojunction Solar Cells

A typical crystalline silicon heterojunction solar cell is HIT?Heterojunction with Intrinsic Thin-layer?,which structure are that both sides of c-Si are intrinsic amorphous silicon thin layer and p+or n+heavily doped amorphous silicon layer,and transparent electrode ITO and collector are deposited on both side of a-Si:H to form a silicon-based heterojunction solar cell with symmetrical structure.One of the important reason for the high efficiency of the HIT is its unique and high quality intrinsic a-Si:H thin layer,which can significantly passivate interface state defects,reduce carrier recombination,improve open circuit voltage and battery performance.In addition,heterojunction materials that replace amorphous silicon doped layers with undoped compound semiconductors have also been extensively studied,typically using metal oxides such as molybdenum oxide?MoO₃?as hole collection layers.This kind of material has a wide band gap and a high work function,and can be combined with n-c-Si to form a low schottky barrier.Therefore,in recent years,it has been often reported that MoO₃ is applied to silicon-based heterojunction solar cells and achieves high conversion efficiency.In addition,some binary inorganic compound materials,due to their unique electrical and optical properties,have excellent energy band advantages when combined with crystalline silicon,and are often used in studies on heterogeneous junction materials.High efficiency solar cells need to have excellent ITO films,and excellent ITO films need to have both good conductive ability and sunlight transmission ability.However,the actual research and application found that the two always restrict each other.We prepared ITO films by magnetron sputtering method and explored the influence of process parameters on the light transmittance and resistivity,and found that substrate temperature and oxygen flow can significantly affect the photoelectric characteristics.We got the right process made the resistivity of ITO below8×10^-4?·cm,and light transmittance in the visible range over 90%.One of the biggest problems of heterojunction solar cells is the defect state of the heterogeneous interface.The high density of the defect state leads to serious minority carrier recombination,which greatly affects the electrical properties of the solar cell.Therefore,the recombination must be reduced by passivating the defect state of the interface.The most common method is to use the intrinsic a-Si:H thin layer as the passivation layer.We have studied the influence of the process parameters of plasma chemical vapor deposition?PECVD?on the passivation performance.It is found that the hydrogen dilution ratio has a great influence on the minority lifetime.When determining the proper sputtering power,working pressure and substrate temperature,the minority lifetime can reach more than 1200 seconds after annealing.The influence of different thickness of the intrinsic layer was investigated.The results show that the thickness of the intrinsic a-Si:H films above 7nm has a better passivation effect.Finally,the ITO film and the intrinsic a-Si:H film preparation technology were applied to the HIT solar cells,whose efficiency is 16.22%on the n-type texture silicon substrate.It is well known that amorphous silicon thin films have higher defect state density and narrower band gap,which can easily lead to significant light absorption in the range of ultraviolet and visible light.In addition,the preparation of n-type and p-type a-Si:H requires PECVD and dangerous gases,such as phosphane,borane and silane.The process is complex and the cost is high.However,the materials with large different work functions on both sides of crystalline silicon can effectively improve the band structure and achieve the separation of photogenic carriers.We used MoO₃with high work function and wide bandgap as the hole selection layer,and LiF with low work function as the electron selection layer to replace a-Si:H doped layer of HIT solar cell.And finally we obtained the carrier selective contact heterojunction solar cell with an efficiency of 9.5%in the laboratory.In order to further study efficient crystalline silicon heterojunction solar cells and find suitable heterojunction materials that can replace doped amorphous silicon,by means of wxAMPS software,suitable compound materials were studied to replace doped layer of HIT solar cell.It was found that ZnTe can replace the p-a-Si,and ZnO or ZnSe can replace the n-a-Si to greatly improve energy band dominance and effectively separate and collect minority carriers that can achieve an efficiency over26%.This provides a theoretical basis for the research of crystalline silicon heterojunction solar cells in the following laboratories.