Publication date: 14th January 2021
Lead halide perovskites uniquely offer long carrier diffusion lengths and high resilience against electronic defects, which are highly surprising for a solution-processed semiconductor. The microscopic mechanisms behind such favorable properties have not been fully understood to date. Main hypotheses revolved around dynamic structural responses and formation of large polarons in order to explain the slower carrier recombination and defect tolerances. Nevertheless, these dynamic structural aspects have not been monitored before mainly due to lack of available techniques that can track atomic-scale motions after photoexcitation. In this work, we perform optical pump / femtosecond diffuse x-ray scattering probe measurements using an x-ray electron free laser, the so-called LCLS at SLAC/Stanford [1]. The experiments clearly show that localized expansive strain fields build up following photoexcitation. It takes ~20 ps for the local distortions, on average, to reach about 5 nm in diameter. These lattice distortions then relax over the time window of the carrier recombination. Our finding undoubtedly demonstrates that the charge carriers in the perovskites strongly distort the atomic structure transiently while the distortion size extending over many unit cells, hence forming large polarons. In this experiment, we resolve the spatio-temporal evolution of the polarons for the first time enabled by phonon-momentum-resolved measurements in our experiment based on time-resolved diffuse x-ray scattering.
[1] B. Guzelturk et al. Nature Materials (2020) DOI: 10.1038/s41563-020-00865-5
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