Direct Observation of the Exciton Self-Trapping Process in CsCu₂I₃ Thin Films

Abstract

Low-dimensional copper halides, such as CsCu 2 I 3 , have emerged as promising LED materials featuring strongly Stokes-shifted photoluminescence with high quantum yield. Previous calculations suggest an exciton self-trapping mechanism; however, direct experimental evidence for this process is still lacking. Here, we present femtosecond UV−vis transient absorption experiments of CsCu 2 I 3 thin films. The films were analyzed by SEM, XRD, and 133 Cs/ 63 Cu NMR for crystallinity and defects. Unique spectral dynamics is observed. The band gap absorption exhibits a characteristic double-peak structure arising from the 130 meV spin− orbit splitting of the copper d electrons. Emission at the direct band gap disappears because of the formation of the lowest-energy self-trapped exciton state. We determined the time constant of 12 ps for the trapping process of thermally relaxed free excitons, with an energy barrier of at least 60 meV. The data are successfully modeled by global kinetic analysis, providing also accurate time constants for charge carrier cooling processes.

Enhanced Responsivity of CsCu2I3 Based UV Detector with CuI Buffer‐Layer Grown by Vacuum Thermal Evaporation

“…There is no obvious CuI absorption peak in other samples and the results are also consistent with the XRD results. The corresponding direct optical bandgaps ( E g ) of Cs‐Cu‐I thin film have been calculated using the Tauc equation, as shown in Figure 5b,d,f, which is consistent with the previous works, [ 22,30,31 ] as shown in Table S1 in the Supporting Information.…”

Pressure‐assisted cooling to grow ultra‐stable Cs3Cu2l5 and CsCu2l3 single crystals for solid‐state lighting and visible light communication

“…Therefore, the luminescent mechanism is expected to derive from the effect of STEs. Besides, the PL spectra of both single crystals grown with pressure‐assisted exhibit large Stokes shifts, broad PL FWHM (Figure 3A,D) and long PL decay time (Figure 2B,D), which may be attributed to the effects of STEs 36–39 . Furthermore, the temperature‐dependent PL characterization was performed to confirm the existence of STEs in both copper halide single crystals.…”

“…Besides, the PL spectra of both single crystals grown with pressureassisted exhibit large Stokes shifts, broad PL FWHM (Figure 3A which may be attributed to the effects of STEs. [36][37][38][39] Furthermore, the temperature-dependent PL characterization was performed to confirm the existence of STEs in both copper halide single crystals. Negligible change of PL peaks was found with the increase of measuring temperature from 273 to 460 K, as shown in Figure 3B,E.…”

Oriented-Structured CsCu2I3 Film by Close-Space Sublimation and Nanoscale Seed Screening for High-Resolution X-ray Imaging

“…For the energy range of the fluoroscopy and radiography application (30–50 keV), CsCu 2 I 3 exceeds all of these scintillators due to its Cs K edge (35.98 keV) and I K edge (33.17 keV) absorption. Previous research has well-studied the kinetic formation mechanism of self-trapped excitons (STEs) of CsCu 2 I 3 by transient absorption spectrum. , The relaxation process in CsCu 2 I 3 thin films sequentially goes along hot free exciton (FE), cooled FE, hot STE, cooled STE, and ground state. Here we additionally recorded the photoluminescence decay spectrum of CsCu 2 I 3 single crystals, where the photoluminescence decay gives the information on radiative recombination rate.…”