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.
Highlights
Metal-halide perovskite (MHP) semiconductors have emerged as promising materials for future photovoltaic applications [1, 2] owing to their outstanding optoelectronic properties including high charge-carrier mobilities and long diffusion lengths [3,4,5,6,7]
To gain better insight into the importance of phonon modes and anharmonicity in MAPbI3 we chose to compare high-quality single crystals with polycrystalline thin films used in high-efficiency photovoltaic devices
Optical phonon modes of MAPbI3 associated with internal vibrations of the PbI3 framework occur at THz frequencies, we studied their properties using a combination of THz time-domain spectroscopy (THz-TDS) and optical-pump–THz-probe spectroscopy (OPTPS)
Summary
Metal-halide perovskite (MHP) semiconductors have emerged as promising materials for future photovoltaic applications [1, 2] owing to their outstanding optoelectronic properties including high charge-carrier mobilities and long diffusion lengths [3,4,5,6,7]. Given that thermal expansion is directly related to anharmonicity, the large linear thermal expansion coefficient of the prototypical MHP, CH3NH3PbI3 (MAPbI3), [10] suggests that anharmonicity is large and plays a significant role in the electronic properties of this material. An understanding of the phonon dispersion relation and electron–phonon coupling [11] in MAPbI3 is key to gaining a full understanding of its technologically significant physical properties, such as its charge-carrier mobility [6, 12]. Computational studies based on first-principles calculations have been conducted on MAPbI3 in the low-temperature regime (orthorhombic phase), allowing their theoretical phonon dispersion relations to be revealed [18, 19]. Phonons in MAPbI3 have already been extensively studied both experimentally and theoretically, allowing a comprehensive assignment of nuclear motion to observed modes. There are no substantial studies investigating how phonon modes are affected by photoexcitation or studies comprehensively comparing the phonon spectra of MAPbI3 single crystals with those of polycrystalline thin films
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