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

Abstract

Interdigitated back contact silicon heterojunction (IBC‐SHJ) solar cells have the potential for high open circuit voltage (VOC) due to the surface passivation and heterojunction contacts, and high short circuit current density (JSC) due to all back contact design. Intrinsic amorphous silicon (a‐Si:H) buffer layer at the rear surface improve the surface passivation hence VOC and JSC, 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 JSC and VOC 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 © 2010 John Wiley & Sons, Ltd.

Optimization of interdigitated back contact silicon heterojunction solar cells by two-dimensional numerical simulation

“…The FF increases from 55% to >78% when E g of buffer layer reduces from 1.72 eV to 1.65 eV. All the above improvement in FF can be explained by the modification in band alignment and band offsets [10]. Using the present device dimensions, with a front SRV of 10 cm/s (a value consistent with our lifetime measurements of ~2 milliseconds), an efficiency of 22.8% can be reached (Fig.…”

Back-Contacted Silicon Heterojunction Solar Cells: Optical-Loss Analysis and Mitigation

“…This can result in J sc losses. This effect is known as electrical shading and has been extensively investigated both for homojunction and heterojunction back-contacted devices [32]- [34]. Eventually, back-contacted SHJ devices require bulk materials with sufficiently long carrier diffusion lengths to perform at their best [35], [36].…”

Amorphous silicon oxide window layers for high-efficiency silicon heterojunction solar cells

“…8(e)). We therefore conclude that the drop in FF we observed in cells with thin a-SiO x :H passivation layers must indeed be a result of impeded hole collection due to a higher VB offset 51,56 which leads to an accumulation of holes. 57 The effect depends on the working point of the device and becomes more significant when moving from short-circuit to open-circuit conditions.…”