Photoelectron spectroscopy investigation of Spiro-OMeTAD thermal stability and high-temperature behaviour for perovskite solar cells

Abstract 2,2′,7,7′-Tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) is widely used as a hole-transporting material in perovskite solar cells (PSCs); however, its intrinsic thermal stability remains insufficiently understood. Here, the thermal stability and high-temperature behaviour of pristine Spiro-OMeTAD films prepared by solution processing and vacuum deposition are systematically investigated using real-time and standard x-ray photoelectron spectroscopy, with annealing temperatures up to 500 °C under ultra-high vacuum. Spiro-OMeTAD films deposited on different substrates exhibit similar chemical environments and photoemission characteristics, demonstrating that the surface chemistry is largely insensitive to deposition method and substrate. Pristine Spiro-OMeTAD remains chemically stable up to 300 °C. Above an onset temperature of approximately 350 °C, partial decomposition of surface molecules occurs, characterised by progressive detachment of the terminating methoxy groups. In contrast, the spirobifluorene and phenylamine backbones retain structural integrity up to 500 °C, without evidence of evaporation or fragmentation, even for vacuum-deposited films. The loss of methoxy groups leads to a modification of the valence band structure, with the valence band maximum shifting towards higher binding energy and the ionisation potential increasing accordingly. These results define intrinsic thermal limits for pristine Spiro-OMeTAD and indicate that, provided other functional layers permit, processing temperatures up to 300 °C may be accessible, whereas exposure above 350 °C is expected to compromise interfacial electronic properties in PSCs.