Influence of the bromide content on the properties of CH3NH3PbI(3-x)Brx perovskite films

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]. We report about the properties of MAPbI3-xBrx films having different compositions x obtained by mixtures of MAPbI3 and MAPbBr3 solutions that allow synthetic processes suitable for solar cell production being fast and reproducible. Tuning the Br content and measuring the bandgap energy by the Tauc analysis of diffuse reflectance measurements allow us to achieve an EG(x) relation that does not depend on film properties other than composition, contrary to the one previously reported in the literature [1]. Besides, in those films we evaluate the dependence on x of the disorder through the Urbach energy E0, finding a close relation between E0 and grain size, which points out that defect states responsible for the sub-bandgap absorption are localized at the grain boundaries in these materials. Photoluminescence (PL) spectra are dominated by a peak whose position (~1.7 eV) is independent of the composition in the range 0.6