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Bournonite (CuPbSbS3) and enargite (Cu3AsS4) have recently been used as absorber layers in thin-film photovoltaic devices due to their ideal bandgap and ferroelectric properties. An understanding of the ferroelectric domain structure in these materials is required so that the benefits of the internal depolarizing electric fields can be fully exploited. Here, the atomic structure and electronic properties of domain walls (DWs) are elucidated through a combined aberration-corrected scanning transmission electron microscopy and density functional theory study. ∼90° and 180° DWs are observed in bournonite. As the 180° DW is charge neutral, it cannot contribute to the anomalous photovoltaic effect that leads to high open circuit voltages. The ∼90° DW shows a slight offset across the boundary, but the contributions of this to the anomalous photovoltaic effect are negligible. The DWs are also electrically passive, i.e., they do not result in significant recombination and do not block charge carrier transport. A high density of stacking faults (SF) was, however, observed in enargite. The SFs have a large number of defect states within the bandgap, which would lower the device efficiency through Shockley–Read–Hall recombination. [ABSTRACT FROM AUTHOR]

Polycrystalline materials are ubiquitous in technology, and grain boundaries have long been known to affect materials properties and performance. First principles materials modeling and electron microscopy methods are powerful and highly complementary for investigating the atomic scale structure and properties of grain boundaries. In this review, we provide an introduction to key concepts and approaches for investigating grain boundaries using these methods. We also provide a number of case studies providing examples of their application to understand the impact of grain boundaries for a range of energy materials. Most of the materials presented are of interest for photovoltaic and photoelectrochemical applications and so we include a more in depth discussion of how modeling and electron microscopy can be employed to understand the impact of grain boundaries on the behavior of photoexcited electrons and holes (including carrier transport and recombination). However, we also include discussion of materials relevant to rechargeable batteries as another important class of materials for energy applications. We conclude the review with a discussion of outstanding challenges in the field and the exciting prospects for progress in the coming years. [ABSTRACT FROM AUTHOR]