Proceedings of International Conference on Perovskite Thin Film Photovoltaics and Perovskite Photonics and Optoelectronics (NIPHO23)
DOI: https://doi.org/10.29363/nanoge.nipho.2023.009
Publication date: 3rd April 2023
Perovskite photophysics in the past decade has been uprooted and revisited from the grounds several times. Initially, Metal halide perovskites were considered as excitonic semiconductors, due to their pronounced excitonic absorption and narrow-band efficient optical emission. Then ultrafast spectroscopy studies have revealed bimolecular recombination dynamics, proving that excitons are dissociated into opposite charge carriers, with beneficial effects for charge separation in solar cells. Perovskite photophysics seemed to have been rationalized: free carriers are favored over exciton by Saha equilibrium, radiative recombination is bimolecular and is the inverse process of optical absorption.
Yet several issues with such a picture started appearing, again from ultrafast spectroscopy measurements. I will review how radiometric time-resolved photoluminescence reveals that the radiative recombination rate is much lower than what expected from the absorption rate. Furthermore, the combination of transient absorption and time resolved photoluminescence in a tandem setup demonstrates that free carriers are majority even at low temperature and in 2D perovskites, when Saha equilibrium predicts instead that bound excitons should prevail.
An extensive debate on the exciton binding energy has ensued, resulting in the realization that the exciton binding energy measured in absorption, when perovskites are in their ground state, is significantly different from the binding energy after excitons have been created.
Clearly, a sound description of photophysics of halide perovskites needs additional ingredients describe the dissociation of excitons in the excited state. Phonon coherences, ultrafast electron diffraction, XAS and XRD are among the techniques that have evidenced a significant distortion of the perovskite lattice upon optical absorption, a phenomenon also known as the formation of a new quasiparticle, the polaron.
I will present a picture of perovskite photophysics that applies to both 3D and 2D materials and consists in the creation of excitons upon optical absorption, their spontaneous dissociation into opposite charge polarons and, finally optical emission by bimolecular recombination of polarons into excitons.
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