On the origin of 1.7 eV PL peak in mixed I/Br organolead perovskites

MAPbI3-xBrx (MA=CH3NH3) perovskites have been studied mainly because adjusting the component I/Br ratio allows the bandgap to be tuned to cover almost the whole visible spectrum, [1,2] so that an intermediate band gap semiconductor can be obtained and used as the high-band-gap material in tandem/spectrally split cells. Besides, Br inclusion results in a significant improvement of the short term stability of nanostructured solar cells for x=0.6 [1]. However, it was reported that these mixedhalide perovskites show photoluminescence (PL) spectra that are dominated by a peak whose position (~1.7 eV) is independent of the composition in the range 0.6<x<3 [2, 3]. This peak was ascribed to recombination centers coming from the simultaneous presence of iodide and bromide in the perovskite lattice [2] or photo-induced halide migration that can result in iodide-rich domains having x~0.6 [3]. On the other hand, aging the mixed perovskite films was found to greatly reduce the amplitude of the 1.7 eV peak [4], even if the influence of light soaking in aged samples was not analyzed. At present ion migration is a hypothesis supported by a variety of experimental results, but still lacking fully conclusive experimental evidence. Here we report about an investigation of the influence of composition, aging, and photoexcitation on the properties of MAPbI3-xBrx perovskites, aiming to point out the relations between optical properties and crystallographic textures. The obtained results will be discussed considering the thermodynamics of alloy formation [5] and the dependence on x of the Urbach energy [2]. . References [1] J.H. Noh, S.H. Im, J.H. Heo, T.N. Mandal, S.I. Seok, Nano Lett. (2013), 13, 1764- 1769 [2] P. Fedeli, F. Gazza, D. Calestani, P. Ferro, T. Besagni, A. Zappettini, G. Calestani, E. Marchi, P. Ceroni, R. Mosca, J. Phys. Chem. C (2015), 119, 21304-21313 [3] E.T. Hoke,D.J. Slotcavage, E.R. Dohner, A. R. Bowring, H.I. Karunadasa, and M.D. McGehee, Chem. Sci. (2015), 6, 613-617 [4] A. Sadhanala, F. Deschler, T.H. Th omas, S.E. Dutton, K.C. Goedel, F.C. Hanusch, M.L. Lai, U. Steiner, T. Bein, P. Docampo, D. Cahen, R.H. Friend, J. Phys. Chem.Lett. (2014), 5, 2501-2505 [5] F. Brivio, C. Caetano, A. Walsh, J. Phys. Chem. Lett. (2016), 7, 1083-1087