Fill factor enhancement with graded interface hole-selective contact for silicon heterojunction solar cells: impact of recombination losses and contact resistivity

Abstract

The development of high-efficiency silicon heterojunction (SHJ) solar cells is primarily dependent on optimizing interface recombination losses (J₀) and series resistance components (R_s), which critically influence the cell's fill factor (FF). The hole-selective p-a-Si:H/ITO contact is optimized by coupling the graded boron-doped a-Si:H and indium tin oxide (ITO) layers’ electronic properties for low J₀ and contact resistance (ρ_c). The graded p-a-Si:H layer enabled a low defective interface at the n-type c-Si wafer, which has been investigated through J₀ at various stages of device fabrication. The carrier transport modification was analyzed by evaluating the J–V characteristics of the cell obtained under light, dark, and Suns-Voc conditions. The diode quality factor (n₁ ~ 0.89 < 1), derived from the linear portion of the Suns-Voc characteristics of an optimized cell, indicates that within the voltage range of 0.60 to 0.75 V (near open-circuit voltage), the characteristics are somewhat affected by Auger recombination processes. This effect aids in achieving a higher fill factor (FF) than what is typically constrained by Shockley–Read–Hall recombination. With the optimized p-a-Si:H/ITO contact compared to the baseline cell process, the pseudo-FF enhanced from ~ 81.2% to ~ 85.1% (represents recombination and shunt losses), and the FF enhanced from ~ 75.1% to ~ 81.8% (due to reduced ρ_c of i + p/ITO from ~ 756 mΩ-cm² to ~ 406 mΩ-cm²), over all the cell efficiency has enhanced from ~ 21.0 to ~ 23.3%.