Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV16)
Publication date: 28th March 2016
The rally in the performance of perovskite solar cells during the last three years has driven the study of perovskite materials based on various anions and organic cations in order to understand the outstanding properties of the materials and improve the efficiencies and stabilities of the corresponding cells. Methyl ammonium (MA) and formamidinium (FA) are the most frequently used cations while pure iodide or mixed with bromide or chloride are examined as anions [1].
In this work, a series of FAPbX3 perovskites (FA=Formamidinium and X= Cl, Br, I) as well as their mixed halide derivatives, have been examined by micro-Raman spectroscopy with excitation at 785 nm and at variable temperature. Off-resonance conditions secure acquisition of reliable Raman data without sample degradation or unintentional laser heating effects. The spectra at low wavenumber region 30-300 cm-1 are governed by the vibrational modes of the inorganic lead PbX6 octahedra while the modes of the FA cover the whole range from 300-3500 cm-1. The inorganic cage modes present splitting due to the lowering of symmetry and are sensitive to the phase transformations of the perovskites upon decreasing the temperature. On the other side, the frequencies of the FA modes do not split vs. temperature but present significant shifts and line narrowing. Amongst them, the most interesting vibrations are the rocking, wagging and torsion of NH2 which are observed below 1000 cm-1 and are most affected by the structural changes and the FA molecular orientation. Results are compared with those obtained at the MAPbX3 system [2] and give a complete estimate for the role of different cations and anions in the vibrational properties of the materials along with implications on their influence in the perovskites structural and electronic properties and their efficient use as light absorbers in perovskite solar cells.
[1] Hyun Suk Jung, Nam-Gyu Park, Perovskite Solar Cells: From Materials to Devices, Small 2015, 11, 10–25.
[2] R.G. Niemann, A.G. Kontos, D. Palles, E.I. Kamitsos, A. Kaltzoglou, F. Brivio, P. Falaras, P.J. Cameron, Halogen Effects on Ordering and Bonding of CH3NH3+ in CH3NH3PbX3 (X = Cl, Br, I) Hybrid Perovskites: A Vibrational Spectroscopic Study, J. Phys. Chem. C 2016, 120, 2509−2519.
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