Photoelectron Spectroscopy of Co-evaporated and Spin-Coated LiTFSI-Doped Spiro-OMeTAD Reveals the Interface Energetics Inside a MAPI-based Perovskite Solar Cell

The electronic doping mechanism of the Spiro-OMeTAD (2,2′,7,7′-Tetrakis­[N,N-di­(4-methoxyphenyl)­amino]-9,9′-spirobifluorene) with LiTFSI (lithium bis­(trifluoromethylsulfonyl)­imide) and tBP (4-tert-Butylpyridine) has not been fully understood. Spiro-OMeTAD is a highly studied hole transport layer in solar cells and, especially, perovskite solar cells (PSCs). Its p-doping not only improves the hole transport characteristics and thus the photocurrent but also increases the band bending at the interface with the perovskite absorber and thus the cell photovoltage. A better understanding of the p-doping of Spiro-OMeTAD and its interface energetics with MAPI would contribute to optimization strategies for a better overall efficiency of the PSC. Until now, doping mechanisms were studied separately for liquid and thermal evaporation processing. It was impossible to pinpoint the similarities and differences between the two approaches. Using an integrated ultra-high vacuum cluster tool (DAISY-SOL), we prepared LiTFSI-doped Spiro-OMeTAD samples by spin-coating under an inert atmosphere and by thermal co-evaporation in a vacuum. Without contact with the atmosphere, the thin films were then characterized by photoelectron spectroscopy to deduce their electronic properties. It has been evidenced that with thermal co-evaporation, LiTFSI alone dopes the Spiro-OMeTAD layer, while for spin-coating, the additive tBP is required to prevent LiTFSI precipitation. Both processes give a more p-doped Spiro-OMeTAD layer by shifting its Fermi level toward the highest occupied molecular orbital (HOMO) at best by 0.65 eV. This results in a final energy difference between the Fermi level and the HOMO onset of 0.4 eV for the most p-doped sample. The co-evaporation process of LiTFSI and Spiro-OMeTAD was then used to perform interface experiments of doped Spiro-OMeTAD on MAPI and of Au on doped Spiro-OMeTAD to study a simplified solar cell structure SnO2 | MAPI | Spiro-OMeTAD | Au. Results confirm that the band alignment is suitable for electron blocking and hole extraction. Under light, a surface photovoltage of at least 0.65 eV is measured at the MAPI | Spiro-OMeTAD interface, making it the functional key interface. These interface experiments provide a not only detailed but also quantitative picture of the band alignment both in the dark and in operation under illumination under open circuit conditions.