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