Study On The Mechanism Of Efficiency Improvement Of Amorphous Silicon/crystalline Silicon Heterojunction Solar Cells In Light-thermal Treatment

Existing studies have shown that light-thermal treatment can improve the energy conversion efficiency of amorphous/crystalline silicon heterojunction（HJT）solar cells.The present works aim to understand the mechanism（s）involved,and then achieve the efficiency control of HJT cells.The research content is divided into two parts:theory and experiment.The theory part simulated the motion of hydrogen atoms in HJT cells through molecular dynamics.In the experimental part,the role of doped a-Si:H layer in the light-thermal process was studied by preparing hydrogenated amorphous silicon/crystalline silicon（a-Si:H/c-Si）structures and HJT cells.Main contents and conclusions of the research are summarized as follows:1.Based on previous reports,light-induced charging of hydrogen atoms in the cells was assumed.Simulations of the movement of hydrogen in HJT cell structures,with various levels of the applied electric field intensity（0-1.5 V/?）,and at different temperatures（160-200°C）,had been carried out by molecular dynamics.The results show that electric field induced build-up of hydrogen exists at a-Si:H（i）/c-Si interface,which reaches a maximum under an electric field intensity of 0.3～0.5 V/?,at 180°C or lower.Such a level of intensity is found well matching that of the electric field provided by the doping layers of the HJT cells,and the temperatures agree well with the optimal temperature of the light-thermal processes.As hydrogen at the a-Si:H（i）/c-Si interface has been identified as the key for passivation,it is postulated that the built-up of hydrogen at the interface results leads to the benefit of performance enhancement of HJT cells in light-thermal processes.It is further found by the simulations that,assuming hydrogen atoms becoming charge-free in darkness,the build-up of hydrogen will disappear in dark-thermal processes,and the reported loss of the benefit in subsequent annealing can thus well be explained.2.The simulation results of hydrogen motion were experimentally verified by treating different types of a-Si:H/c-Si samples and HJT cells with different doping concentrations.The results show that the light-thermal treatment can increase the microstructure factor R^*of doped a-Si:H/c-Si samples,and samples with n-type and p-type a-Si:H layers is increased by 0.3 and 0.2,respectively.It can be explained by the increase of Si-H₂bond in the interface,and supports the hypothesis that the hydrogen atoms are charged.After light-thermal treatment,the??_??（the increment of open-circuit voltage）of HJT cells versus the concentration of p layer is similar to the accumulation law of interface hydrogen in the simulation,which verifies the driving role of electric field in hydrogen drift.In addition,during the process of preparation,the minority carrier life of the cells that completed CVD deposition after 24 h in the dark can be improved significantly,pointing to the possibility that hydrogen can be charged by trapping hot carriers.