Recent progress in defect engineering for kesterite solar cells

Kesterite Cu2ZnSn(S, Se)4 (CZTSSe) thin film solar cells have been regarded as one of the most promising thin film photovoltaic technologies, offering a low-cost and environmentally friendly solar energy option. Although remarkable advances have been achieved in kesterite solar cells, the performance gap relative to mature thin film photovoltaic technologies such as CIGSe and CdTe remains large. Significant open-circuit voltage (VOC) deficit has been recognized as the main limiting factor to performance improvement, with undesirable intrinsic defects being a key culprit contributing to the low VOC. To realize the promise inherent in kesterite CZTS to become an earth-abundant alternative to existing thin film photovoltaic technologies with comparable performance, significant research effort has been invested to tackle the challenging defect issues. In this review, recent progress and achievements relevant to engineering improvements to the defect properties of the semiconductor have been examined and summarized. Promising strategies include: (i) manipulating the synthesis process to obtain a desirable reaction pathway and chemical environment; (ii) introducing cation substitution to increase the ionic size difference and supress the related band tailing deep-level defects; (iii) applying post deposition treatment (PDT) with alkaline elements to passivate the detrimental defects. These advances obtained from work on kesterite solar cells may lead to future high performance from this material and may be further extended to other earth-abundant chalcogenide photovoltaic technologies.