Cesium lead halide perovskite materials have recently attracted attention in view of their optical and electronic properties which make them excellent candidates for potential applications in lasers, light emitting diodes and photodetectors. In this work, we provide the experimental and theoretical evidence for sequential photon absorption/re-emission in CsPbBr3 perovskite microwires. Using two-photon excitation, we recorded PL lifetimes and emission spectra as a function of the lateral distance between PL excitation and collection positions along the microwire, with separations exceeding 100 µm. As the propagation length increases, the PL spectrum develops a new emission peak that is red-shifted by 20 nm from the main emission and is accompanied by the appearance of the well-resolved rise times in the PL kinetics. We undertake quantitative modeling that accounts for bimolecular recombination and photon recycling within the microwire waveguide, and find that it is sufficient to account for the observed decay modifications. The model relies on a high radiative efficiency in CsPbBr3 perovskite microwires to explain the photon recycling observed. Such findings provide crucial information about the potential impact of photon recycling and waveguide trapping on optoelectronic properties of cesium lead halide perovskite materials
To further optimize the solar cell devices based on hybrid perovskites materials, understanding the major contributions of organic cations to ion migrauion and surface degradation is urgently needed. In this presentation, I show the tremendous impact of the structure of organic cations on halogen migration, vacancies, and interstitials, carrier lifetimes as well as surface degradation of perovskites using a combination of experimental and theoretical investigations. We found that Br- vacancies and interstitials have much lower formation energies and much higher density in MAPbBr3 compared to FAPbBr3 counterpart. The results also demonstrated clearly that the transition energy barrier for Br migration through vacancies within the bulk phase is much lower in MAPbBr3 than in FAPbBr3. Finally, we found that the rotation barrier of the organic cation is much higher in the case of FAPbBr3 than for MAPbBr3, which points to a much stronger H-bonding with Br- in the former case. Our results imply that incorporating organic cations with stronger H-bonding capacity, appropriate structure and more restricted motion inside the inorganic framework, is beneficial for suppressing ion migration and thus improving the performance of hybrid perovskite-based optoelectronic devices.
Photoinduced charge transfer is a key step in the mechanism of charge generation in organic solar cells. Charge transfer typically occurs from a photoexcited conjugated polymer donor to an electron acceptor. In an effort to better understand the primary events in solar cells, we have investigated photoinduced charge transfer in model donor-acceptor systems consisting of pi-conjugated oligomer donors that are covalently linked to diimide electron acceptors. These studies utilized oligo(thiophene), oligo(phenylene ethynylene) and oligo(fluorene) pi-conjugated systems with lengths varying from 4 to 12 repeat units linked to naphthalene diimide electron acceptors. Excitation with 100 femtosecond pulses at wavelengths correspoinding to the conjugated oligomer absorption band(s) leads to rapid photoinduced charge transfer to produce a charge separated state, (oligomer+)-(NDI-), which subsequently decays on timescales ranging from 100 ps to 5 ns. The dynamics of the forward and reverse electron transfer reactions depend strongly on the structure and length of the pi-conjugated oligomers, with the fastest rates occurring for oligo(thiophene)s, and considerably slower rates for oligo(phenylene ethynylene)s. The talk will discuss the structure-property relationships and energetic correlations that control the dynamics of charge separation and recombination.
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