The effort to develop earth‐abundant kesterite solar cells has led to an approximate doubling of the power conversion efficiency over the past five years to 12.6%, primarily due to increases in short‐circuit current and fill factor; open‐circuit voltage has resisted similar change, limiting further efficiency improvement. In the present investigation, Auger nanoprobe spectroscopy, X‐ray/ultraviolet photoelectron spectroscopy, and device characterization are used to provide a comprehensive understanding of the role of grain boundaries and interfaces in limiting performance in kesterite‐based devices. High photovoltaic performance is found to correlate with grain boundaries that are Cu‐depleted and enriched with SnOx. The formation of this bulk‐like oxide at grain boundaries with type I band offset provides a unique effective passivation that limits electron‐hole recombination. Building on these new insights, photovoltaic device simulations are performed that show optimized electrostatic designs can compensate for bulk defects, allowing efficiencies closer to the theoretical limit.

Dual-source evaporation approach is applied to deposit AgBi2I7, AgBiI4 and Ag2BiI5 films; a planar junction AgBiI4-solar cell is demonstrated.

The kesterite material Cu 2 ZnSn(S,Se) 4 (CZTSSe) is an attractive earth-abundant semiconductor for photovoltaics. However, the power conversion efficiency is limited by a large density of I-II antisite defects, which cause severe band tailing and open-circuit voltage loss. Ag 2 ZnSnSe 4 (AZTSe) is a promising alternative to CZTSSe with a substantially lower I-II antisite defect density and smaller band tailing. AZTSe is weakly n-type, and this study reports for the first time on how the carrier density is impacted by stoichiometry. This study presents the first-ever photovoltaic device based on AZTSe, which exhibits an efficiency of 5.2%, which is the highest value reported for an n-type thin-film absorber. Due to the weakly n-type nature of the absorber, a new architecture is employed (SnO:F/AZTSe/MoO 3 /ITO) to replace conventional contacts and buffer materials. Using this platform, it is shown that the band tailing parameter in AZTSe more closely resembles that of CIGSe than CZTSSe, underscoring the strong promise of this absorber. In demonstrating the ability to collect photo-generated carriers from AZTSe, this study paves the way for novel thin-film heterojunction This article is protected by copyright. All rights reserved. architectures where light absorption in the n-type device layer can supplement absorption in the ptype layer as opposed to producing a net optical loss.