Interfacial processes play a decisive role in determining both the performance and the long-term operational stability of perovskite solar cells (PSCs). Photoemission spectroscopy (PES) is a non-destructive and highly informative technique that provides direct access to the chemical states and electronic structure of interfaces in PSCs. In particular, synchrotron-based PES enables the investigation of buried interfaces that are otherwise difficult to probe.

In this presentation, I will demonstrate how synchrotron photoemission methods can be used to study interfacial chemical reactions, energy band alignment, and ion migration in complete perovskite device stacks. Special attention will be given to the advantages of synchrotron radiation over conventional laboratory sources, including tunable probing depth, optimized photon-energy selection, and enhanced sensitivity to subtle chemical and electronic changes at buried junctions.

In our previous work [1], we investigated device structures consisting of ITO/SnO₂/FAPbI₃/spiro-OMeTAD. We observed clear evidence of ion migration and chemical modification at the interface between the perovskite absorber and the spiro-OMeTAD hole transport layer. Lead and iodine originating from the FAPbI₃ layer were detected within the spiro-OMeTAD overlayer, indicating that deposition of the transport material induces the ion migration from perovskite layer into the spiro-OMeTAD organic matrix. The Pb 4f spectra exhibit additional components that cannot be attributed to the native perovskite environment, suggesting the formation of non-perovskite lead species. These features most likely arise from chemical interactions between perovskite derived ions and functional groups in spiro-OMeTAD. Complementary changes are observed in the N 1s region of spiro-OMeTAD, where an additional peak appears upon contact with the perovskite, indicating chemical modification of nitrogen sites at the interface. Furthermore, a systematic shift of the main N 1s peak toward lower binding energy is observed with increasing transport layer thickness. This trend indicates downward band bending in the spiro-OMeTAD near the interface with the perovskite.

Motivated by these findings, we carried out a detailed investigation of the interfacial chemistry at the FAPbI₃/spiro-OMeTAD interface. A series of samples was prepared by systematically varying the thickness of the spiro-OMeTAD layer on perovskite absorbers, both with and without surface passivation. For passivation, phenethylammonium iodide (PEAI) was employed, which forms a thin two-dimensional perovskite layer that suppresses undesirable interfacial reactions and stabilizes the surface. This study provides critical insights into the chemical and electronic structure of the FAPbI₃/spiro-OMeTAD and FAPbI₃/PEAI/spiro-OMeTAD interfaces, contributing to a deeper understanding of interfacial degradation pathways and offering guidance for the development of more stable and efficient perovskite solar cells.

References:

[1] Rahul Mahavir Varma, Bhavya Rakheja, Karen Radetzky, Evelyn Johannesson, Stefania Riva, Alberto García-Fernández, Roberto Félix, Adam Hultqvist, Soham Mukherjee, Ute B. Cappel, Tobias Törndahl, Håkan Rensmo, Energy band alignment and interfaces in FAPbI3 perovskite solar cells: a HAXPES investigation, Manuscript submitted