Illumination-induced phase segregation and degradation of perovskites revealed by hard x-ray photoelectron spectroscopy
Roberto Félix1, Carolin Rehermann2, Evelyn Handick1, Claudia Hartmann1, Regan G. Wilks1,3, Eva Unger2,4, Marcus Bär1,3,5,6
1Department Interface Design, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany
/2Young Investigator Group Hybrid Materials Formation and Scaling, Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany
/3Energy Materials In-Situ Laboratory Berlin (EMIL), Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, Berlin, Germany
/4Chemical Physics and NanoLund, Lund University, Lund, Sweden
/5Helmholtz-Institute Erlangen-Nürnberg for Renewable Energy (HI ERN), Erlangen, Germany
/6Department of Chemistry and Pharmacy, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany

Solar cells based on hybrid organic-inorganic perovskites (HOIP) – particularly those using APbX3 [A = CH3NH3+ “MA”, HC(NH2)2+, Cs+ and X = I-, Cl-, Br-] absorbers – have reached power conversion efficiencies surpassing 23%, record performances on par with (or above) conventional photovoltaic technologies. HOIPs are of particular interest as a top cell for low-cost multi-junction solar cell devices to boost the performance of existing Si or thin-film solar cells. The long-term stability of perovskite-based solar devices is a major challenge for their technological exploitation and commercialization; currently, a significant deterioration in efficiency within first hours of operation is observed. Such performance losses are often linked to a combination of extrinsic instability (e.g., degradation by exposure to moisture, heat, light, oxygen, etc.) and intrinsic instability (e.g., composition defects, ion migration, etc.) of HOIPs. Furthermore, for their application in multi-junction top cells, the band gap of HOIPs have to be increased, which can be accomplished by using a higher Br fraction in mixed halide (bromide-iodide) perovskites. However, the open circuit voltage (VOC) of respective solar devices saturates at a [Br]/([I]+[Br]) fraction of ca. 0.25, corresponding to an absorber band gap of ca. 1.75 eV. This phenomenon seems to be related to an illumination-induced phase segregation, the so-called “Hoke Effect”. An understanding of its underlying mechanism, critical to improving the stability and the band gap tunability of HOIPs, remains incomplete.
To study this effect, we investigate the properties of MAPb(Br0.75,I0.25)3 and CsPb(Br0.75,I0.25)3 absorbers using hard x-ray photoelectron spectroscopy under various illumination conditions. The absorbers’ chemical and electronic structures were monitored in ultra-high vacuum and under solar, as well as, x-ray irradiation. Although changes are detected on both absorbers, MAPb(Br0.75,I0.25)3 exhibits a more pronounced tendency to segregate into “iso-ionic” phases. The role of the perovskites’ A cation in the revealed underlying mechanisms is discussed.