Preserving Bond Ionicity under Illumination to Achieve Photostable Halide Perovskites

Abstract

Lead halide perovskites (LHPs) with the formula ABX3 are very efficient materials for optoelectronic devices: solar cells, light-emitting diodes, lasers, photodetectors, γ-detectors, and field effect transistors. But they are very fragile and sensitive to humidity, temperature, and operating conditions such as voltage bias and light. The reason is their weakly ionic structure with twice lower atomic ionicities than those in oxides. Photostability is difficult to achieve in LHP, due to the nature of electronic states that originate mainly from the halide anions in the valence band and B cations in the conduction band; hence, the charge transfer lowers the ionicity at the B–X bond. To date, the A cations used have been optically inactive and only served as one-electron donors for the charge balanced BX3 frame. We propose a mechanism of photoexcitation acting between two different A cations, namely SLi3 and SH3, while the inorganic frame remains unchanged under illumination preventing B–X bond breakage and halide migration. Demand for a specific crystal localization of the optical excitations is a new factor for achieving photostability, in addition to a control of the ionic radii, defects, morphology, and surface and interface stabilization.