Optimization of interdigitated back contact silicon heterojunction solar cells: tailoring hetero-interface band structures while maintaining surface passivation

Interdigitated back contact silicon heterojunction (IBC-SHJ) solar cells have the potential for high open circuit voltage (V-OC) due to the surface passivation and heterojunction contacts, and high short circuit current density (J(SC)) due to all back contact design. Intrinsic amorphous silicon (a-Si: H) buffer layer at the rear surface improve the surface passivation hence V-OC and J(SC), but degrade fill factor (FF) from an S'' shape J-V curve. Two-dimensional (2D) simulation using Sentaurus device'' demonstrates that the low FF is related to the valence band offset (energy barrier) at the hetero-interface. Three approaches to the buffer layer are suggested to improve the FF: (1) reduced thickness, (2) increased conductivity, and/or (3) reduced band gap. Experimental IBC-SHJ solar cells with reduced buffer thickness (<5 nm) and increased conductivity with low boron doping significantly improves FF, consistent with simulation. However, this has only marginal effect on efficiency since J(SC) and V-OC also decrease due to poor surface passivation. A narrow band gap a-Si:H buffer layer improves cell efficiency to 13.5% with unoptimized passivation quality. These results demonstrate that tailoring the hetero-interface band structure is critical for achieving high FF. Simulations predicts that efficiences >23% are possible on planar devices with optimized pitch dimensions and achievable surface passivation, and 26% with light trapping. This work provides criterion to design IBC-SHJ solar cell structures and optimize cell performance. Copyright (C) 2010 John Wiley & Sons, Ltd.