| In recent years,lead halide perovskite have been reported with potential for photovoltaic,photoelectric and photo-communication due to their excellent properties,including broadband high absorption coefficient,long diffusion length and ambipolar charge-transport,etc.The power conversion efficiencies?PCEs?of lead halide perovskite solar cells has exceeding 22%in 2016 since it was first proposed in 2009,which is comparable with polycrystalline silicon.The extremely rapid developments of lead halide perovskite devices urgently require lots of theoretical supports based on fundamental research.Yet knowledge of the intrinsic property of perovskite material,especially the ultrafast photo-generated carrier and exciton dynamics,is only just emerging.In this work,exciton dynamics in CsPbBr3 perovskite quantum dots?QDs?are investigated.The ultrafast initial hot carrier cooling is analyzed;the radiative recombination is proved to be single-exciton process;the dominating mechanisms of the temperature dependent bandgap behavior are systematically discussed;the nature of the nonradiative recombination is first demonstrated to be exciton-exciton annihilation;and the ultrafast interfacial charge carrier transfer between CsPbBr3 QDs and carrier transport layers is discussed.These results are listed in detail as follows:1.High sensitivity micro confocal photoluminescence?PL?measurement system and broadband femtosecond pump-probe system are constructed,with time resolution of200ps and200fs,respectively.The PL spectrum and the transient absorption spectrum of CsPbBr3 QDs are measured by these two systems.2.Ultrafast initial intra-band carrier relaxation in CsPbBr3 QDs is analyzed.After excitation with coherent laser beam,the carrier loses coherence and then thermalizes in100fs.Subsequent carrier cooling is established in 3ps through interaction with lattice.Due to the phonon bottleneck effect and the secondary excitation by the probe beam,the carrier cooling rate is opposite to excitation density.3.Radiative recombination in CsPbBr3 dots is analyzed in detail at different excitation density and temperature.Single exciton process is identified to dominate the radiative recombination at room temperature,with a decay lifetime of 4.3ns.The temperature dependent optical bandgap behavior is found to be opposite to general semiconductor.For temperature below 220K,the optical bandgap is dominated by thermal expansion and thus shows a linear blue shift with increasing temperature.While for higher temperature up to 380K,due to the interaction between thermal expansion,electron–phonon interaction and structural phase transition around 360 K,the optical bandgap is insensitive to temperature.This luminescence monochromaticity is extremely favorable for applications in photoelectric.4.Nonradiative recombination in CsPbBr3 dots is analyzed,and the ultrafast exciton-exciton annihilation is first demonstrated.For excitation intensity lower than0.1exciton/QDs,the entire carrier recombination is dominated by single exciton process.While for Iin>0.25 exciton/QDs,carrier recombination can be divided into two time-separate process:a density-dependent initial decay?060ps?described by exciton-exciton annihilation and a density-independent long time decay?>250ps?attributed to single exciton recombination.The exciton-exciton annihilation generates a definite effect on the PL lifetime,PL Qys and PCE of perovskite solar cells.5.Ultrafast interfacial carrier transfer properties between CsPbBr3 QDs and carrier transport layers are analyzed.CsPbBr3 QDs/carrier transport layer heterojunction is prepared by spin-coating.Electron and hole interfacial transport parameters are obtained,providing important theoretical and experimental basis for the development of perovskite solar cells. |
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