F. Ruf, M. F. Aygüler, N. Giesbrecht, B. Rendenbach, A. Magin, P. Docampo, H. Kalt, and M. Hetterich

APL Materials 7, 031113 (2019)

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 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-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 E_B 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 E_g 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 Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃ by temperature-dependent X-ray diffraction.