Light Emission of Self‐Trapped Excitons in Inorganic Metal Halides for Optoelectronic Applications

Abstract

Self‐trapped excitons (STEs) have recently attracted tremendous interest due to their broadband emission, high photoluminescence quantum yield, and self‐absorption‐free properties, which enable a large range of optoelectronic applications such as lighting, displays, radiation detection, and special sensors. Unlike free excitons, the formation of STEs requires strong coupling between excited state excitons and the soft lattice in low electronic dimensional materials. The chemical and structural diversity of metal halides provides an ideal platform for developing efficient STE emission materials. Herein, an overview of recent progress on STE emission materials for optoelectronic applications is presented. The relationships between the fundamental emission mechanisms, chemical compositions, and device performances are systematically reviewed. On this basis, currently existing challenges and possible development opportunities in this field are presented.

Self‐Trapped Excitons in 2D SnP2S6 Crystal with Intrinsic Structural Distortion

“…In this material, photo-excited FEs are converted into STEs with different self-trapping depths through strong electron-phonon coupling. [2] The STEs may shuttle from the shallower trapped states to the deeper trapped states, resulting in longer emission wavelengths in the red-end of PL spectra (Figure S5, Supporting Information). [9] Additionally, the weak PL peak in the spectral range of 800-900 nm (1.37-1.55 eV) can be attributed to the extrinsic self-trapping process that may arise from permanent vacancies or correlations between self-trapped states, and this phenomenon is commonly found in materials with STEs emission.…”

“…Self-trapped excitons (STEs) have recently gained considerable attention due to their broadband white-light emission, long fluorescence lifetime, and large Stokes shift, which enable tremendous application prospects in various optoelectronic components such as solar cells, lightemitting diodes, and photodetectors. [1][2][3] Since Karunadasa et al first reported the high-efficiency STEs emission from hybrid perovskites in 2014, [4,5] much effort has been devoted to understand the origin and properties of this unique phenomenon. It's generally believed that the STEs are formed in materials with soft lattice and strong electron-phonon coupling.…”

Cu substitution boosts self-trapped exciton emission in zinc-based metal halides for sky-blue light-emitting diodes

“…Unlike free excitons (FEs), the formation of STEs in low electron-dimensional materials requires strong coupling between excited state excitons and soft lattices. 25 Reasonable strategies such as dimensionality reduction and doping technology, efficient luminescence in the green, yellow and blue wavelengths have been achieved in Cu-based halides, which makes STEs emission materials be of great potential for optoelectronic applications. 26–28 Xia et al prepared zero-dimensional (0D) Cu( i )-based organometallic halide (18-crown-6) 2 Na 2 (H 2 O) 3 Cu 4 I 6 with broadband green asymmetric emission at 536 nm and near-unity PLQY (91.8%).…”

Top-Emitting Microcavity Light-Emitting Diodes Based on All-Thermally Evaporated Lead-Free Copper Halide Self-Trapped-Exciton Emitters

“…23−25 Unlike the direct band emission of perovskite, CsCu 2 I 3 features a large Stokes shift and broadband emission attributed to the self-trapped excitons (STEs). 26,27 However, so far, the performance of the CsCu 2 I 3based LED device has remained inferior due to the poor charge transport and difficult charge injection of CsCu 2 I 3 , 23,24 possibly due to the distorted lattice in those STE emitters upon electrical excitation. 28 Therefore, there is a true demand to develop a device strategy that can simultaneously enhance the emission intensity and color purity of those lead-free metal halide LEDs.…”

“…Despite the great progress of metal halide perovskite light-emitting diodes (LEDs), the toxicity of heavy metal lead (Pb) seriously restricts their wide application in the field of optoelectronics . Recently, much work has focused on lead-free metal halide LEDs, which can be attributed to their ability to solve the problem of Pb toxicity. , Among them, CsCu 2 I 3 , a cesium copper iodide, has emerged as a promising alternative lead-free emitter candidate, by virtue of its low toxicity, earth abundance, robust air stability, and high photoluminescence quantum efficiency (PLQE). − Unlike the direct band emission of perovskite, CsCu 2 I 3 features a large Stokes shift and broadband emission attributed to the self-trapped excitons (STEs). , However, so far, the performance of the CsCu 2 I 3 -based LED device has remained inferior due to the poor charge transport and difficult charge injection of CsCu 2 I 3 , , possibly due to the distorted lattice in those STE emitters upon electrical excitation . Therefore, there is a true demand to develop a device strategy that can simultaneously enhance the emission intensity and color purity of those lead-free metal halide LEDs.…”