On the combined effects of window/buffer and buffer/absorber conduction-band offsets, buffer thickness and doping on thin-film solar cell performance

In this study we perform an extensive campaign of numerical simulations of thin-film solar cell structures aimed at investigating how the conduction band offsets at buffer/window (ΔBW) and buffer/absorber (ΔAB) heterojunctions and the thickness and doping of the buffer layer combine to affect the performance parameters (Jsc, Voc, FF and η). For the two scenarios of ideal (i.e., without traps) and non-ideal (with traps) buffer/absorber interface, we vary ΔAB and ΔBW in the range -0.5 eV to 0.5 eV, and analyze, for each combination, the physical mechanisms limiting the cell performance and the way to optimize it by choosing optimal buffer doping and thickness. We show that assuming ΔAB as the main indicator of the performance potential of a cell can be misleading because ΔBW can heavily influence performance, even when ΔAB is positive (conduction band higher in the buffer than in the absorber) and near to its theoretical optimal value (0.3eV). However, we also show that ΔAB < 0 (conduction band lower in the buffer than in the absorber) is usually coupled with an efficiency loss that is even worse if ΔBW > 0. We verify our findings by simulating several examples of CIGS-based solar cells with different buffer layers (CdS, Zn1-xMgxO, In2S3, Zn(O,S)) taken from the literature; these comparisons confirm the validity of our results and suggest that the combination of ΔAB and ΔBW is the predominant factor in the design of high-efficiency solar cells. We finally propose simple quantitative guidelines for thin-film solar cell design and optimization.