Proceedings of Online International Conference on Hybrid and Organic Photovoltaics (OnlineHOPV20)
Publication date: 22nd May 2020
Two – dimensional (2D) halide perovskites represent a promising class of materials for light emitting and optoelectronic applications, owing to their narrow photoluminescence (PL) from tightly bound excitons. However, while devices based on multidimensional lower-dimensionality perovskites have achieved high PL efficiencies, purely 2D perovskites show low emissivity at room temperature. It is not yet well understood whether this response stems from crystal defectivity, or if it is a consequence of the complex exciton photophysic of these materials, which shows a rich excitonic landscape composed of multiple states with distinct lattice coupling, relaxation dynamics and oscillator strengths.
We report on the fine structure of excitonic emission in the two dimensional lead free perovskite PEA2SnI4: following the work on the presence of dark excitons in lead-halide perovskites. We characterize the material optical constants between 5 and 300 K, identifying the spectral structures therein. We find an increase in PLQY going from 5 K to 200 K, which we investigate by means of transient absorption and time-resolved PL spectroscopy: we observe a monotonous increase of excitonic lifetime with temperature as well as complex dynamics in the exciton and free carrier continuum bleach decays. These observations give evidence of the presence of a dark excitonic state acting as an excited population reservoir, with an activation energy of 8-10 meV: we build a quantitative model for exciton dynamics accounting for transfer to and from the dark state, and discuss how interplay between the thermally activated back-transfer and exciton – phonon scattering affects the PLQY of 2D perovskites.
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