Improved photovoltaic performance of CdTe-based solar cells: Roles of using a hole-blocking layer and nanoscale imaging of barrier height at interfaces

Abstract

The performance of photovoltaic devices is hindered by the presence of barrier height at the interfaces as well as the presence of structural defects. CdTe solar cells, based on a CdS/CdTe heterojunction and CdCl2 vapour treatment, exhibit high efficiency. In this work, we show that the use of a hole-blocking layer has a potential to further increase the efficiency of CdTe-based cells. As a case study, we have fabricated multi-juncton CdTe-based solar cells on both pristine- and textured-silicon substrates. Here we use an n-type zinc tin oxide (ZTO) thin film as the transparent conducting oxide (TCO) layer and an n-type hole-blocking layer of MoO3 on a p-type CdTe:Cu absorber layer. In addition, we map the nanoscale barrier height at each interface, i.e. ZTO/MoO3 and MoO3/CdTe:Cu by Kelvin probe force microscopy (KPFM). We also investigate the real time photo-generated charge carrier dynamics across the heterojunctions using photo-KPFM which plays a major role in the cell efficiency. Quantitative analysis shows that barrier height between the interfaces decreases after light illumination. Based on these findings, we have fabricated solar cells on pristine- and textured-Si substrates and the maximum efficiency is found to be 8.2% for the textured-Si substrate. The present study demonstrates the fabrication of efficient hole-blocking CdTe-based solar cells and provides insights on how local barrier height affects their macroscopic performance.