Impact of the Organic Cation on the Optoelectronic Properties of Formamidinium Lead Triiodide

Metal halide perovskites have proven to be excellent light-harvesting materials in photovoltaic devices whose efficiencies are rapidly improving. Here, we examine the temperature-dependent photon absorption, exciton binding energy, and band gap of FAPbI 3 (thin film) and find remarkably different behavior across the β–γ phase transition compared with MAPbI 3 . While MAPbI 3 has shown abrupt changes in the band gap and exciton binding energy, values for FAPbI 3 vary smoothly over a range of 100–160 K in accordance with a more gradual transition. In addition, we find that the charge-carrier mobility in FAPbI 3 exhibits a clear T –0.5 trend with temperature, in excellent agreement with theoretical predictions that assume electron–phonon interactions to be governed by the Fröhlich mechanism but in contrast to the T –1.5 dependence previously observed for MAPbI 3 . Finally, we directly observe intraexcitonic transitions in FAPbI 3 at low temperature, from which we determine a low exciton binding energy of only 5.3 meV at 10 K.