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

Multiple-cation mixed-halide (Cs,FA,MA)Pb(I,Br)3 perovskites containing cesium, formamidinium (FA), and methylammonium (MA) possess excellent properties for a wide range of optoelectronic applications such as thin-film photovoltaics or lasers. We investigate the role of excitons and the exciton binding energy EB, relevant for the effectiveness of charge separation in solar cells, as well as the temperature-dependent bandgap energy Eg which is used as an indicator for crystal phase transitions. Generalized Elliott fits of absorption spectra offer the possibility to determine both EB and Eg. However, since excitonic effects are non-negligible 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-containing mixed perovskite compounds are below the ones of 24–32 meV and 36–41 meV for pure methylammonium lead iodide (MAPbI3) and bromide (MAPbBr3), respectively, and, thus, facilitate charge-carrier separation in photovoltaic applications. Furthermore, temperature-dependent (T = 5–300 K) studies of Eg in (Cs,FA,MA)Pb(I,Br)3 indicate a suppressed crystal phase transition by the absence of any phase-transition related signatures such as the well-known jump of about 100 meV in MAPbI3. 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 of the phase transition in Cs0.05(FA0.83MA0.17)0.95Pb(I0.83Br0.17)3 by temperature-dependent X-ray diffraction.