Cunming Liu1 Kaibo Zheng2 David Gosztola1 Sophie Canton3 Xiaoyi Zhang1

1, Argonne National Laboratory, Lemont, Illinois, United States
2, Lund University, Lund, , Sweden
3, Deutsches Elektronen Synchrotron, Hamburg, , Germany

Lead free perovskites, especially Bi-based perovskites, are excellent candidates for replacing Pb-based perovskite absorbers in the large scale commercialization of perovskite solar cells due to their higher stability but much less toxicity. However, before their practical application, it is of such importance to capture the fate (relaxation, trapping, recombination and etc.) of their photoexcited charger carriers for helping optimize the photovoltaic performance. By employing the ultrafast laser initialized time-resolved X-ray absorption near edge structure (TR-XANES) spectroscopy, which directly interrogates the orbitals of atoms forming the materials, we have characterized the ~ 4 nm nanocrystals of a representative lead free Bi-based perovskite. Through examining the Bi L3 edge, we have found that most of photoexcited electrons are freely wandering (delocalized) in the conduction band. By contrast, the results probed at Br K edge uncover that the photogenerated holes are strongly localized at Br 4p orbitals in the valence band, forming strong exciton-polarons alive for ┬Ás. Our time-resolved element-specific study of lead-free perovskites for the first time will advance the understanding of the photophysical behavior of photogenerated charge carriers in solar cell materials.

This research used resources of the Advanced Photon Source and the Center for Nanoscale Materials, U.S. Department of Energy (DOE) Office of Science User Facilities operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.