Temperature-dependent studies of excitors binding energy and phase-transition suppress on in (Cs,FA,MA)Pb(I,Br)3 perovskites

Multiple-cation mixed-halide (Cs,FA,MA)Pb(I,Br)(3) perovskites containing cesium, formarnidinium (FA), and methylammonium (MA) possess excellent properties for a wide range of optoelectronic applications such as thin-film pliotovoltaics or lasers. We investigate the role of excitons and the excitor) binding energy E-B, relevant for the effectiveness of charge separation in solar cells, as well as the temperature dependent bandgap energy E-g which is used as an indicator for crystal phase transitions. Generalized Elliott fits of absorption spectra offer the possibility to determine both E-B and E-g. However, since excitonic effects are non-negligibk even at room temperature, a careful and detailed analysis of the spectra is crucial for a correct interpretation. Therefore, an additional evaluation based on a so-called f-sum rule is applied to achieve an improved reliability of the results at higher temperatures. The obtained EB values of 20-24 meV for Cs-contailaing mixed perovskite compounds are below the ones of 24-32 meV and 36-41 meV for pure methylammonium lead iodide (MAPbI(3)) and bromide (MAPbBr(3)), respectively, and, thus, facilitate charge-carner separation in photovoltaic applications. Furthermore, temperature-dependerit (T = 5-300 K) studies of E-g in (Cs,FA,MA)Pb(I,BrP)(3) indicate a suppressed crystal phase transition by the absence of any phase-transition related signatures such as the well-Imown jump of about 100 meV in MAPbI(3). We verify these results using temperature-dependent electroreflectance spectroscopy, which is a very reliable technique for the direct and non-destructive determination of optical resonances of the absorber layer in complete solar cells. Additionally, we confirm the suppression i of the phase transition in Cs-0.05(FA(0.83)MA(0.17))(0.95)Pb(I0.83Br0.17)(3) by ternperature-deperiderit X-ray diffraction. (C) 2019 Author(s).