Metal–halide perovskite nanocrystals (NCs) are promising photonic materials for use in solar cells, light-emitting diodes, and lasers. The optoelectronic properties of these devices are determined by ...the excitons and exciton complexes confined in their NCs. In this study, we determined the relaxation dynamics of charged excitons and biexcitons in CsPbBr3 NCs using femtosecond transient-absorption (TA), time-resolved photoluminescence (PL), and single-dot second-order photon correlation spectroscopy. Decay times of ∼40 and ∼200 ps were obtained from the TA and PL decay curves for biexcitons and charged excitons, respectively, in NCs with an average edge length of 7.7 nm. The existence of charged excitons even under weak photoexcitation was confirmed by the second-order photon correlation measurements. We found that charged excitons play a dominant role in luminescence processes of CsPbBr3 NCs. Combining different spectroscopic techniques enabled us to clarify the dynamical behaviors of excitons, charged excitons, and biexcitons.
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IJS, KILJ, NUK, PNG, UL, UM
Combining the superior optical properties of their bulk counterparts with quantum confinement effects, lead halide perovskite nanocrystals are unique laser materials with low-threshold optical gain. ...In such nonlinear optical regimes, multiple excitons are generated in the nanocrystals and strongly affect the optical gain through many-body interactions. Here, we investigate the exciton–exciton interactions in CsPbI3 nanocrystals by femtosecond transient absorption spectroscopy. From the analysis of the induced absorption signal observed immediately after the pump excitation, we estimated the binding energy for the hot biexcitons that are composed of an exciton at the band edge and a hot exciton generated by the pump pulse. We found that the exciton–exciton interaction becomes stronger for hot excitons with greater excess energies and that the optical gain can be controlled by changing the excess energy of the hot excitons.
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We study the origin of photoluminescence (PL) intermittency in formamidinium lead bromide (FAPbBr3, FA = HC(NH2)2) nanocrystals and the impact of postsynthetic surface treatments on the PL ...intermittency. Single-dot spectroscopy revealed the existence of different individual nanocrystals exhibiting either a blinking (binary on–off switching) or flickering (gradual undulation) behavior of the PL intermittency. Although the PL lifetimes of blinking nanocrystals clearly correlate with the individual absorption cross sections, those of flickering nanocrystals show no correlation with the absorption cross sections. This indicates that flickering has an extrinsic origin, which is in contrast to blinking. We demonstrate that the postsynthetic surface treatment with sodium thiocyanate improves the PL quantum yields and completely suppresses the flickering, while it has no significant effect on the blinking behavior. We conclude that the blinking is caused by Auger recombination of charged excitons, and the flickering is due to a temporal drift of the exciton recombination rate induced by surface-trapped electrons.
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All inorganic CsPbBr3 perovskite quantum dots (QDs) are potential emitters for electroluminescent displays. We have developed a facile hot‐injection method to partially replace the toxic Pb2+ with ...highly stable Sn4+. Meanwhile, the absolute photoluminescence quantum yield of CsPb1−xSnxBr3 increased from 45 % to 83 % with SnIV substitution. The transient absorption (TA) exciton dynamics in undoped CsPbBr3 and CsPb0.67Sn0.33Br3 QDs at various excitation fluences were determined by femtosecond transient absorption, time‐resolved photoluminescence, and single‐dot spectroscopy, providing clear evidence for the suppression of trion generation by Sn doping. These highly luminescent CsPb0.67Sn0.33Br3 QDs emit at 517 nm. A device based on these QDs exhibited a luminance of 12 500 cd m−2, a current efficiency of 11.63 cd A−1, an external quantum efficiency of 4.13 %, a power efficiency of 6.76 lm w−1, and a low turn‐on voltage of 3.6 V, which are the best values among reported tin‐based perovskite quantum‐dot LEDs.
Suppressed trion formation: CsPb1−xSnxBr3 quantum dots (QDs) were synthesized by a hot‐injection approach. As trion formation is suppressed by the SnIV substitution, light‐emitting diodes (LEDs) based on these highly luminescent QDs performed very well, with the highest current efficiencies and external quantum efficiencies ever reported for such Sn‐based systems.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Lead halide perovskite nanocrystals (NCs) have outstanding photoluminescence (PL) properties and excellent potential for light-emitting diodes and single-photon sources. Here, we report the ...multiple-peak structures originating from excitons, trions, and biexcitons in low-temperature PL spectra of single CsPbBr3 NCs. We found fine-structure splitting in the PL peaks of bright excitons and biexcitons and also in the longitudinal–optical (LO)-phonon replicas of excitons. LO-phonon replicas of trions are clearly observed under strong photoexcitation, which do not show fine-structure splitting. From size-dependent analyses of these replicas, we clarified that both exciton–phonon and trion–phonon couplings become larger for smaller NCs and the coupling strengths of trions are larger than those of excitons in large NCs. These behaviors can be explained by the spatial distributions of the electron and hole wave functions in the NCs. Our findings provide essential information on electron–phonon couplings in perovskites and for the design of high-purity single-photon sources.
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We demonstrate evidence of localized states near the band edge of lead halide perovskite semiconductor CH3NH3PbI3 thin films. In these films, the high-temperature tetragonal and the low-temperature ...orthorhombic phases coexist at low temperatures. We determine the exact energy level of the localized states in the orthorhombic phase via photocurrent (PC) beat spectroscopy. The lack of observation of localized states in photoluminescence and absorption measurements indicates that the density of the localized states is fairly low even in solution-processed polycrystalline films. Ultrafast PC spectroscopy enables us to monitor low-density localized states in inhomogeneous polycrystalline films.
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Lead halide perovskite nanocrystals (NCs) are a class of promising light-emitting materials and have been considered as gain media in lasers. Strong exciton–exciton interactions in NCs cause an ...energy shift of the lowest optical transition and affect the optical gain threshold. Here, we clarify the dynamics of exciton–exciton interactions in highly photoexcited CsPbI3 NCs by double-pump transient absorption spectroscopy. This method provides control over the population of each excited state by varying the time interval between the two pump pulses. We find that the band-edge energy shift induced by the formation of asymmetric hot-biexcitons (comprising one ground-state exciton and one hot exciton) is smaller than that induced by hot excitons and hot biexcitons in the ensemble. We demonstrate that the generation of asymmetric hot-biexcitons reduces the optical gain threshold in the CsPbI3 NC ensemble.
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Lead halide perovskite nanocrystals (NCs) are one of the most anticipated and promising materials for light-emitting diodes and lasers because of their high photoluminescence quantum yields (PLQYs). ...However, the formation of trions (charged excitons) in the NCs reduces their PLQYs. Here, we clarify the trion formation mechanism in perovskite CsPbBr3 NCs by analyzing the excitation fluence dependence of transient absorption signals. Under weak photoexcitation, trions are formed by charge carrier trapping at surface states. In contrast, biexciton Auger recombination dominates the trion formation under strong photoexcitation. We found that the postsynthetic surface treatment suppresses the extrinsic surface-related formation of trions. The thorough understanding of the trion formation mechanisms is essential for the PLQY improvement of perovskite NCs and helps to reduce ionization of NCs in solid-state devices.
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Lead halide perovskite (APbX3) nanocrystals exhibit photoluminescence (PL) with both wide wavelength tunability and high quantum efficiency. While the Pb–X6 octahedra mainly determines the ...near-band-edge optical properties and the A-site cation affects the structural stability, the role of the A-site cation in determining the optical properties is still unclear. Here, we report the PL properties of three types of lead bromide perovskite APbBr3 nanocrystals with different cations A = HC(NH2)2 +, CH3NH3 +, and Cs+, as revealed by single-dot spectroscopy, and discuss the influence of the A-site cation on the PL spectrum. The nanocrystal size dependences of the PL energy and lifetime show no large variation with the species of the A-site cation. We find that the size of the A-site cation determines the coupling strength between electrons and longitudinal-optical phonons in the nanocrystal and thus affects the PL spectral shape, especially the low-energy tail.
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Single crystals of methylammonium lead trihalide perovskites with high optical transparency and well‐defined facets are grown via solution process. Using these high‐quality crystals, the ...photocarrier‐induced refractive‐index change is observed for the first time in organic–inorganic hybrid perovskites. The photorefractive properties are measured using a laser interferometric technique that enables to monitor the time‐resolved refractive phase shifts. It is found that these phase shifts are determined by the initial density of photogenerated carriers and they are surprisingly retained even after carrier recombination. Utilizing the extremely long‐lived phase shift, it is demonstrated that the perovskite single crystal works as a phase shifter of laser light to generate any desired polarization. Furthermore, a periodic polarization‐modulation technique is established using the time‐dependent refractive phase shift. This refractive property can be used for advanced optical applications of organic–inorganic hybrid perovskites.
Photorefractive effect is observed for the first time in organic–inorganic hybrid perovskites. The refractive‐index change is found to be determined by the initial density of photogenerated carriers and retained even after carrier recombination. Utilizing the long‐lived phase shift, it is demonstrated that the perovskite single crystal works as a phase shifter of laser light to generate any desired polarization.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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