A Study On Contact Resistance Of Silicon Heterojunction Solar Cells

Silicon Heterojunction(SHJ)solar cells are of great interest in the PV market because of their high open-circuit voltage and high conversion efficiency.However,the resistive losses caused by charge carrier transport at the a-Si:H(n)/ITO and Ag/ITO interfaces of SHJ cells can lead to a decrease in fill factor(FF),which in turn degrades the performance of SHJ cells.For this reason,in this paper,the transmission line method(TLM)is used to measure the contact resistance and optimize the contact resistance at the a-Si:H(n)/ITO and Ag/ITO interfaces by several aspects,such as crystalline silicon surface weaving,silver paste viscosity,hydrogenated amorphous silicon film deposition process and ITO oxygen content,and thus improve the cell performance,with the following main findings:1)The Ag/ITO contact resistance is smallest(26.78 mΩ·cm2)at a pyramid size of 3.7 μm,when the cell performance is optimal(FF and Eff are 77.4% and22.12%,respectively),and the efficiency is improved by 1.91% and 0.53%compared to the cells with pyramid sizes of 0.6 and 1.5 μm,respectively.This is attributed to the fact that the larger the pyramid,the more adequate the contact between Ag and ITO.2)The Ag/ITO contact resistance is the lowest(16.05 mΩ·cm2)with normal viscosity silver paste,which has the best cell performance(FF and Eff are 82.3%and 23.2%,respectively);while the high viscosity silver paste is prone to agglomeration during sintering,which leads to insufficient Ag/ITO contact,which in turn increases the contact resistance and reduces the cell performance.(3)The performance of a-Si:H(i)and a-Si:H(n)films had a large effect on the a-Si:H(n)/ITO contact resistance.a-Si:H(n)contact resistance is the lowest at 2.69 mΩ·cm2 when the a-Si:H(n)deposited hot wire current is 29 A.At this time,FF is 82.63% and Eff is 24.01%.For a-Si:H(i)layer hot wire current,the lowest a-Si:H(n)/ITO contact resistance is 1.9 mΩ·cm2 when the hot wire current is 31 A.At this time,the FF is 82.16% and the Eff is 23.73%.(4)The a-Si:H(n)/ITO contact resistance increases with the increase of a-Si:H(i)film thickness,and the lowest contact resistance is 3.1 mΩ·cm2 when the a-Si:H(i)thickness is 4.1 nm.This is mainly because the larger the a-Si:H(i)film thickness,the more difficult it is for the minority carriers to tunnel through,which in turn leads to the increase of contact resistance.(5)The a-Si:H(n)/ITO contact resistance decreases and then increases with the ITO oxygen content.When the oxygen content is 4.9%,the a-Si:H(n)/ITO contact resistance is the lowest,19.17 mΩ·cm2.By optimizing the process parameters,the lowest Ag/ITO contact resistivity of16.05 mΩ·cm2,the lowest a-Si:H(n)/ITO contact resistivity of 3.1 mΩ·cm2,and the highest photoelectric conversion efficiency of 24.01% were finally obtained for the SHJ solar cell.