Defects in halide perovskite semiconductors: impact on photo-physics and solar cell performance

Imperfections such as heterogeneity at different length scales, static versus dynamic disorders, defects in the bulk, surface imperfections, grain boundaries, and interface imperfections of solution-processed hybrid metal-halide perovskite semiconductors are known to be detrimental to the solar cell performance. These imperfections influence voltage losses and charge transport by the formation of undesirable non-radiative channels. Photo-generated charge carriers recombine via these non-radiative channels and hamper the performance of perovskite solar cells (PSCs). Scientists are aiming to decode the nature of these defects by a better understanding of their origins and by developing novel engineering techniques for the passivation of defect states. In this review article, we explain the different kinds of imperfection and discuss their impact on charge carrier transport in PSCs through optical studies. Furthermore, we summarize the efforts made in the community to passivate these defect states by various kinds of additive engineering such as solvent additives, small-organic-molecule additives, potassium-ion additives, graded 3D-2D perovskite materials, etc. Finally, this review provides an insight into defect dynamics and passivation strategies that allows us to understand the nature of defects and helps in the development of future trends in passivation methods.