Abstract
Methylammonium lead iodide perovskite is an outstanding semiconductor for photovoltaics. One of its intriguing peculiarities is that the band gap of this perovskite increases with increasing lattice temperature. Despite the presence of various thermally accessible phonon modes in this soft material, the understanding of how precisely these phonons affect macroscopic material properties and lead to the peculiar temperature dependence of the band gap has remained elusive. Here, we report a strong coupling of a single phonon mode at the frequency of ~ 1 THz to the optical band gap by monitoring the transient band edge absorption after ultrafast resonant THz phonon excitation. Excitation of the 1 THz phonon causes a blue shift of the band gap over the temperature range of 185 ~ 300 K. Our results uncover the mode-specific coupling between one phonon and the optical properties, which contributes to the temperature dependence of the gap in the tetragonal phase.
Highlights
Methylammonium lead iodide perovskite is an outstanding semiconductor for photovoltaics
We explore the coupling between a specific phonon mode and the optical band gap in the MAPbI3 perovskite
The modulated optical band gap was measured as a function of delay time t between THz excitation and a white-light continuum pulse co-propagating with the THz pulse
Summary
Methylammonium lead iodide perovskite is an outstanding semiconductor for photovoltaics. While the large variability of the atomic coordinates within the unit cell (i.e., structural disorder) has been suggested to enhance carrier diffusion lengths[12,13,14], it results in a broad distribution of optical properties (i.e., electronic disorder), which may lead to energetic losses in a photovoltaic process[15,16,17] This soft material has thermally accessible phonon modes (thermal energy at room temperature is equivalent to ~ 6 THz)[18,19,20,21,22], which naturally displaces atoms from their equilibrium positions. Quantitative analysis of the phonon-induced shift of the optical band gap shows that this 1 THz phonon mode has a high efficiency in increasing the band gap by thermal population, consistent with the unconventional variation of the optical band gap in MAPbI3 with temperature[25, 26]
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