Investigation Of High Efficiency Dopant-free Interdigitated Back Contact Heterojunction Solar Cells

Single junction crystalline silicon(c-Si)solar cells featuring interdigitated back contact heterojunction(IBC-SHJ)structure have the advantages of both interdigitated back contact(IBC)and heterojunction(HIT)structures,which can obtain high short-circuit current density and high open-circuit voltage,and the photoelectric conversion efficiency(PCE)has reached 26.6%.However,integrating the interdigital p-type and n-type amorphous silicon(a-Si:H)layers on the rear surface of c-Si substrate requires complex fabrication processes,and the photoelectric losses inherent to the doped a-Si:H layers themselves restrain further promotion on efficiency.These problems have prompted people to seek alternative new functional materials and simple deposition processes.The dopant-free heterojunction materials,such as molybdenum oxide(MoO_x)and lithium fluoride(LiF),due to their own work functions,can effectively extract holes or electrons in contact with c-Si.It has a simple manufacturing process and a wider range of material selection,which has become a research focus in recent years.This thesis focuses on the research of high-efficiency dopant-free IBC-SHJ solar cells.The main research contents and results are as follows:(1)To evaluate the passivation performance of the c-Si/a-Si:H interface,it was found that as the thickness of the a-Si:H film increases,the passivation performance increases,and the surface recombination velocities decreases gradually.Then the change of contact resistivity between MoO_x or LiF films and c-Si substrate with different thickness a-Si:H films was analyzed.It was found that as the thickness of a-Si:H films increased,their contact resistivity increased significantly,and the contact resistivity of the hole transport layer increases faster.Finally,theoretical simulations show that the IBC-SHJ solar cells performance is positively correlated to the improved passivation quality and reduced rear contact resistivity.And the IBC-SHJ solar cells with about 4 nm-thick a-Si:H interlayer have a simulated efficiency close to 24%due to the lower surface recombination velocities and acceptable rear contact resistivity.After analyzing the power loss of IBC-SHJ solar cells with 0 nm or 4 nm a-Si:H films,it was found that by improving the passivation quality of the silicon wafer surface,reducing the contact resistivity of the hole transport layer,or using high-quality silicon wafer can effectively improve the efficiency of cells.(2)The rear-side structure of dopant-free IBC-SHJ solar cells was demonstrated by combining evaporated carrier-selective materials and two-steps hard masks alignments.We fabricated dopant-free IBC-SHJ solar cells using a-Si:H as rear surface interfacial passivation layer,MoO_x and LiF as the back-sided hole-and electron-selective layers,respectively.This preparation method greatly simplifies the process flow.Adding a-Si:H film on the rear-side of silicon wafer can effectively improve the passivation quality of the cells,which makes the short circuit current density and open circuit voltage of the cells improved significantly,but at the same time the contact resistivity of the cells will also increase significantly,which will reduce the fill factor(FF)of the cells.In the end,the dopant-free IBC-SHJ solar cells with 4 nm a-Si:H layer can achieve high conversion efficiency.In addition,the position of the evaporated metal target and isolation in between the busbar and the c-Si substrate are highly relevant to leakage and recombination and have great impact on the device performance.The conversion efficiency of dopant-free IBC-SHJ solar cells above 20%were finally obtained.