Publications associated with Dark matter

Impact of the organic cation on the optoelectronic properties of formamidinium lead triiodide

Journal of Physical Chemistry Letters American Chemical Society 9 (2018) 4502-4511

CL Davies, J Borchert, CQ Xia, RL Milot, H Kraus, MB Johnston, L Herz

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 FAPbI3 (thin film) and find remarkably different behavior across the β–γ phase transition compared with MAPbI3. While MAPbI3 has shown abrupt changes in the band gap and exciton binding energy, values for FAPbI3 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 FAPbI3 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 MAPbI3. Finally, we directly observe intraexcitonic transitions in FAPbI3 at low temperature, from which we determine a low exciton binding energy of only 5.3 meV at 10 K.

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