Role of Localized States in Photoluminescence Dynamics of High Optical Gain CsPbBr₃ Nanocrystals

Abstract

the visible to near-IR wavelengths of the electromagnetic spectrum. [9,10] Though organic-inorganic metal halides based perovskites can produce high photoluminescence (PL) yield, their instability makes it difficult for practical use. On the other side, all inorganic perovskite, e.g., cesium lead halides (CsPbX 3 ), based colloidal quantum dots attract much attention due to its very high PL yield (PLQY ≈ 90%), [11] narrow emission peak (enhanced color purity), low trap density, and reduced blinking [12] with high stability. Recent studies show ultralow threshold density (≈4.5 µJ cm −2 ) for amplified spontaneous emission (ASE) in CsPbBr 3 nanocrystals (NCs) film. [13,14] In past few years the successful employment of CsPbBr 3 quantum dots as light-emitting diodes, [9] single photon emitter, [15] advanced lasing material, [13] and photodetectors [16] make it necessary to have an in-depth study of different photophysical processes in this material. Some recent studies have been devoted to investigating excitonic recombination processes in this material at ultrafast timescale. [17] To date, most of these studies have been done at room temperature (RT). However, to understand the photoexcited carrier dynamics, temperature-dependent time-resolved PL (TRPL) study is an essential tool. Since PL lifetime can be significantly affected by the presence of surface traps, temperature-dependent TRPL gives us an insight of the trap-related kinetics in the NC. [18] Some recent studies [19,20] of the fine structure analysis of the excitonic and PL spectra of CsPbX 3 (X = Cl, Br, I) NCs show the presence of trion formation at low T. These trions are red-shifted by few meV from free excitons peak and decay radiatively with much faster rate than the excitons which results in faster PL decay at low T. [19,21] It has also been argued that the lowest triplet excitonic states are bright (emission red-shifted by few meV) in this material which is the cause of the higher photon emission rate. [22] Here, we have studied temperature-dependent TRPL decay of CsPbBr 3 NCs along with the steady-state absorption and PL to get insight into the influence of surface trap states on the emission kinetics in highly luminescent NCs. From timeresolved emission spectroscopy, we have found that the PL decaying faster at low temperature is in contrast to many other colloidal NCs such as PbS and CdS, [23,24] where PL lifetime gets extended by lowering the temperature. However, similar lengthening of PL lifetimes at elevated temperatures has also Temperature-dependent time-resolved photoluminescence (TRPL) spectroscopy of high optical gain CsPbBr 3 nanocrystals (NCs) films along with temperaturedependent steady-state absorption and photoluminescence (PL) spectra is studied. A red shift in the optical band edge and the corresponding PL peak is observed by lowering the temperature from room temperature, analogous to hybrid perovskite semiconductors. However, the faster PL decay of CsPbBr 3 NCs film at a low temperature as compared to room temperature is in con...

Recent Progress in Metal Halide Perovskite Micro‐ and Nanolasers

“…For instance, Dey et al reported trap/defect‐assisted (from the localized states) ASE (from the localized states) in CsPbBr 3 NCs with a value of optical gain coefficient (51 cm −1 ) and a loss coefficient (15 cm −1 ). The NCs possess an ASE threshold of 5.8 µJ cm −2 pumped sub‐nanosecond (840 ps) excitation pulse width, this threshold value is comparable to many of the polyfluorene‐based waveguide …”

“…To relate the output light variation with excitation length to gain, the excited region of length ( L ) with SE occurs uniformly in the excited state. A straightforward calculation shows that if the net gain experienced by the light is g tot = g′ ‐α (cm −1 ), then the detected light intensity is given bywhere AP 0 indicates the SE proportional to excitation intensity, g tot is the optical gain of the waveguide, L is the excitation beam length, g ′ is the gain due to stimulated emission, and α is the optical loss . As pumping length is varied, the observation of an exponential increase in the emitted light intensity could be considered as a direct evidence of the existence of gain.…”

“…A measured gain threshold of 16 µJ cm −2 , and an optical gain coefficient of 3200 ± 830 cm −1 are observed at 1.58 eV. (a,b) Reproduced with permission . Copyright 2018, Wiley‐VCH.…”

Ultrastable CsPbBr3 Perovskite Quantum Dot and Their Enhanced Amplified Spontaneous Emission by Surface Ligand Modification

“…Regardless of the rapid progress in perovskite ASE or lasing, the intrinsic instability of CsPbBr 3 upon attacks by ambient moisture is still a serious challenge and hindrance for their practical applications owing to the surface defect, surface long ligands and ionic migration behaviors . For example, their surface long ligands and low stability could result in nonradiative recombination of excitons and photoluminescence emission degradation .…”

Spontaneous Formation of Nanocrystals in Amorphous Matrix: Alternative Pathway to Bright Emission in Quasi‐2D Perovskites

“…In P4 and P4L samples the emission intensity then starts decreasing at ≈175 K, as shown in Figure S14 of the Supporting Information. The decrease of the emission intensity with increasing temperature due to temperature activated nonradiative process is commonly observed in many materials, including perovskite nanocrystals, although nanocrystals may exhibit slower thermal quenching compared to perovskite thin films and single crystals . However, in the P4L samples with TOPO after the change of the peak shape at ≈75 K (disappearance of weak shoulder corresponding to defect emission), the emission intensity stays constant, as shown in Figure f.…”