Light-induced halide segregation hampers obtaining stable wide-band-gap solar cells based on mixed iodide-bromide perovskites. So far, the effect of prolonged illumination on the performance of mixed-halide perovskite solar cells has not been studied in detail. It is often assumed that halide segregation leads to a loss of open-circuit voltage. By simultaneously recording changes in photoluminescence and solar cell performance under prolonged illumination, we demonstrate that cells instead deteriorate by a loss of short-circuit current density and that the open-circuit voltage is less affected. The concurrent red shift, increased lifetime, and higher quantum yield of photoluminescence point to the formation of relatively emissive iodide-rich domains under illumination. Kinetic Monte Carlo simulations provide an atomistic insight into their formation via exchange of bromide and iodide, mediated by halide vacancies. Localization of photogenerated charge carriers in low-energy iodide-rich domains and subsequent recombination cause reduced photocurrent and red-shifted photoluminescence. The loss in photovoltaic performance is diminished by partially replacing organic cations by cesium ions. Ultrasensitive photocurrent spectroscopy shows that cesium ions result in a lower density of sub-band-gap defects and suppress defect growth under illumination. These defects are expected to play a role in the development and recovery of light-induced compositional changes.
Bibliographical noteFunding Information:
The authors acknowledge the Netherlands Organization for Scientific Research (NWO) for funding through the Joint Solar Programme III (Project 680.91.011) and the NWO Spinoza prize. This work is also part of the Advance Research Center for Chemical Building Blocks, ARC CBBC, which is co-founded and co-financed by NWO and the Netherlands Ministry of Economic Affairs (Project 2016.03.Tue). The authors further acknowledge funding from the Netherlands Ministry of Education, Culture and Science (Gravity Program 024.001.035). M.J.D. thanks the Marie Skłodowska-Curie Actions Innovative Training Network “H2020-MSCAITN-2014 INFORM—675867” for financial support. S.T. acknowledges funding by the Computational Sciences for Energy Research (CSER) tenure track program of Shell and NWO (Project No. 15CST04-2) as well as NWO START-UP from the Netherlands.
- halide segregation
- kinetic Monte Carlo simulations
- solar cells