Dark Excitons of the Perovskites and Sensitization of Molecular Triplets

The recent emergence of metal halide perovskites as triplet sensitizers has generated new possibilities in optoelectronics, the exploitation of which requires an understanding of the characteristics of the band-edge excitons of the perovskites and mechanistic details of sensitization of long-lived molecular triplets. In this Perspective, we first examine the band-edge energetics of the perovskite nanocrystals, characterize the excitons, and introduce the dark excitons that participate in triplet sensitization. We then investigate the mechanism of sensitization of the long-lived triplets of organic molecules anchored on 3D perovskite nanocrystals or intercalated in 2D layered hybrid perovskites. At the end, we explore how the deactivation processes in sensitized triplets can be modulated to achieve photon upconversion or room-temperature phosphorescence. This critical analysis of the recent developments in these interrelated topics, identifying the key parameters and suggesting possible directions of research in this area, is likely to generate further interest, thus helping its growth.