EPR characterization of F4TCNQ as a dopant in Spiro-OMeTAD thin films

Abstract

Spiro-OMeTAD is a widely used hole transport material (HTM) in perovskite solar cells, contributing significantly to their high-power conversion efficiencies. In this study, Electron Paramagnetic Resonance (EPR) spectroscopy was employed to investigate F4TCNQ as a molecular dopant for spiro-OMeTAD. The doping efficiency of F4TCNQ was examined by EPR spectroscopy by varying its concentration from 0.5 to 6 mol% in two different solvents: chloroform and chlorobenzene. Spiro-OMeTAD films prepared at these dopant concentrations in chloroform were additionally characterized with UV/VIS spectroscopic and ellipsometry measurements. EPR analysis of both solutions and films revealed that F4TCNQ doping is more effective in chloroform than in chlorobenzene, indicating a strong solvent influence on the doping efficiency of spiro-OMeTAD by F4TCNQ. Furthermore, an ambient air stability study was performed on F4TCNQ-doped spiro-OMeTAD films and compared with films containing conventional additives such as tBP, LiTFSI, and FK209. The results demonstrate that F4TCNQ serves as an efficient single dopant alternative to traditional additive mixtures. Results are discussed in the context of EPR spectroscopy as a powerful tool for identifying effective dopants for HTM thin films and elucidating the role of solvent–dopant interactions in determining doping efficiency.