Ultrafast photo-induced phonon hardening due to Pauli blocking in MAPbI₃ single-crystal and polycrystalline perovskites

Abstract Metal-halide perovskite semiconductors have attracted intense interest over the past decade, particularly for applications in photovoltaics. Low-energy optical phonons combined with significant crystal anharmonicity play an important role in charge-carrier cooling and scattering in these materials, strongly affecting their optoelectronic properties. We have observed optical phonons associated with Pb–I stretching in both MAPbI₃ single crystals and polycrystalline thin films as a function of temperature by measuring their terahertz conductivity spectra with and without photoexcitation. An anomalous bond hardening was observed under above-bandgap illumination for both single-crystal and polycrystalline MAPbI₃. First-principles calculations reproduced this photo-induced bond hardening and identified a related lattice contraction (photostriction), with the mechanism revealed as Pauli blocking. For single-crystal MAPbI₃, phonon lifetimes were significantly longer and phonon frequencies shifted less with temperature, compared with polycrystalline MAPbI₃. We attribute these differences to increased crystalline disorder, associated with grain boundaries and strain in the polycrystalline MAPbI₃. Thus we provide fundamental insight into the photoexcitation and electron–phonon coupling in MAPbI₃.