Interface engineering of p-n heterojunction for kesterite photovoltaics: A progress review

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) is considered one of the most promising thin-film photovoltaic (PV) technologies due to its bandgap tunability (1.0 ~ 1.5 eV) and high absorption coefficient (>104 cm−1). However, the highest power conversion efficiency (PCE) of CZTSSe has so far only reached up to 12.6%, much lower than the theoretical limit defined by the Shockley–Queisser (SQ) theory. The large open-circuit voltage (Voc) deficit and inferior fill factor (FF) are prevalent in kesterite PV and hamper the improvement in efficiency. In this review, unfavourable energy band alignment at the CZTSSe/buffer junction, as well as defective interface are identified as two obstacles at the p-n heterojunction. These issues contribute to the interface induced recombination, thus significantly reducing efficiency. Subsequently, we review recent advances in strategies to improve the efficiency by altering the characteristics of the interface, covering alternative buffer layers, heterojunction treatments and passivation layers. Finally, future research directions of heterojunction engineering are proposed as schemes towards the ideal interface in kesterite solar cells.