Self-Trapped Excitons in All-Inorganic Halide Perovskites: Fundamentals, Status, and Potential Applications

Abstract

Photoluminescence is a radiative recombination process of electron–hole pairs. Self-trapped excitons (STEs), occurring in a material with soft lattice and strong electron–phonon coupling, emit photons with broad spectrum and large Stokes shift. Recently, series halide perovskites with efficient STE emission have been reported and showed promise for solid-state lighting. In this Perspective, we present an overview of various photoluminescence phenomena with the emphasis on the mechanism and characteristics of emission derived from STEs. This is followed by the introduction of STE emission in hybrid halide perovskites. We then introduce all-inorganic STE emitters and focus in particular on the mechanism of STEs in double-perovskite Cs2AgInCl6 and strategies for efficiency improvement. Finally, we summarize the current photoluminescence and electroluminescence applications of STE emitters as well as the potential in luminescent solar concentrators and provide an overview of future research opportunities.

Optical Functional Units in Zero‐Dimensional Metal Halides as a Paradigm of Tunable Photoluminescence and Multicomponent Chromophores

“…Moreover, Figure 3b,d show the decay curves of (C 9 NH 20 ) 9 [Pb 3 Br 11 ](MBr 4 ) 2 and (C 9 NH 20 ) 9 [Pb 3 Cl 11 ](MCl 4 ) 2 , respectively, and the nanosecond lifetime is consistent with other Pb‐based hybrid halides 9. The above optical parameters demonstrate that the emission of (C 9 NH 20 ) 9 [Pb 3 X 11 ](MX 4 ) 2 is derived from the contribution of [Pb 3 X 11 ] 5− part, and the typical STEs mechanism can be well‐explained for the broadband emission (full width at half maximum (FWHM) ≈75 nm for Br‐based samples and ≈61 nm for Cl‐based samples) and large Stokes shift (≈200 nm for Br samples and ≈186 nm for Cl samples) 10. It must be pointed out that there is a special compound among the following materials, (C 9 NH 20 ) 9 [Pb 3 Br 11 ](MnBr 4 ) 2 , which has an extra emission band under 450 nm excitation and thus forms multiple‐emission.…”

Rb2CuX3 (X = Cl, Br): 1D All‐Inorganic Copper Halides with Ultrabright Blue Emission and Up‐Conversion Photoluminescence

“…This fact suggests the intrinsic nature of Rb 2 CuX 3 blue emission, and the absence of saturation at high excitation power excludes the presence of permanent defects emissions . Therefore, we attribute this intense blue‐emission to STEs often observed in metal halide all‐inorganic systems 7a,14,19,21. Based on our single exponential fitting of the time‐resolved PL data for Rb 2 CuCl 3 single crystals (Figure S13, Supporting Information), a decay lifetime of 12.21 µs was extracted.…”

Self‐Trapped Exciton to Dopant Energy Transfer in Rare Earth Doped Lead‐Free Double Perovskite

“…Moreover, as expected, the excitation strength at 365 nm was larger and broader than that at 450 nm in the PLE spectrum (Figure b), indicating the rare earth excitation via energy transfer channel was more efficient than direct excitation. That is not surprising because the STE luminescence belongs to intrinsic luminescence and consequently enjoys a large absorption strength and a broad excitation region . Accordingly, this energy transfer channel can overcome the weak absorption cross‐section of f–f transition …”

“…Admittedly, the possibility of energy transfer from free exciton (FE) to Ho 3+ cannot be entirely dismissed. However, on account of the transient transformation of FE to STE accomplished within several hundred femtoseconds, finishing in several hundred femtoseconds as well is the prerequisite for an effective energy transfer process from FE to Ho 3+ …”