Multiple-cation lead mixed-halide perovskites have attracted significant attention in a wide range of applications such as light-emitting devices, lasers and thin-film photovoltaics.^1-3 Solar cells based on mixed perovskites have recently demonstrated high power-conversion efficiency exceeding 22%.⁴ The wide-range tunability by intermixing of different ions, especially halides, facilitates the integration in tandem devices.⁵ However, the compositional stability could be compromised.⁶

In this contribution, we present investigations on reversible changes in the electronic structure of multiple-cation mixed-halide perovskites under illumination and applied electric fields to simulate the solar cell’s operation conditions.

We used electromodulation spectroscopy as a highly sensitive measurement tool for the determination of critical points in the band structure, e.g., the bandgap⁷. The basic principle of this technique relies on the change of the dielectric function under the influence of an applied external bias and allows for the non-invasive investigation of complete solar cells. By periodically applying an external electric field (AC bias), the dielectric function of the absorber and, in turn, the optical properties such as the reflectance R are modulated. The relative change ΔR / R is independent of experimental properties (spectral response of the detector, spectrum of the light source, etc.) and exhibits characteristic line shapes. The analysis of these leads to the precise determination of the optical resonance energy.

We found instabilities in Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃ solar cell absorbers under 1 sun illumination and applied bias leading to a decrease of the bandgap up to 70 meV.  However, interestingly these instabilities are observed to be reversible. They are attributed to segregation of the halides and are confirmed by in-situ X-ray diffraction measurements under the same conditions.

Additionally, we studied the influence of the surrounding atmosphere by changing from air to nitrogen or pure oxygen as well as applying different relative humidity. The resulting bandgap reduction strongly increases by the combined influence of high humidity and oxygen content. Thus, the effect can be minimized by an effective encapsulation. However, since the observed changes are mainly activated by a combination of illumination and electric fields, they are intrinsic to the solar cell’s operation conditions and cannot be prevented completely. Due to the fact, that multiple-cation perovskites have been considered to be stable against halide segregation up to now, a careful stability analysis for mixed perovskites seems to be necessary.⁸

In conclusion, we presented a detailed study of reversible bandgap instabilities in the mixed perovskite Cs_0.05(FA_0.83MA_0.17)_0.95Pb(I_0.83Br_0.17)₃. The observed instabilities occur under operation-relevant conditions – 1 sun illumination and applied electric fields – and cannot be avoided completely by encapsulation. This supports the necessity for further stability investigations of mixed perovskites.