Hole-Transport-Layer-Free Organic Solar Cells with Enhanced Efficiency Upon Halogenated Solvent Treatment

Typical PEDOT:PSS hole-transporting layers frequently present some issues, including mismatched energy levels, high acidity, severe hygroscopicity, etc., all of which significantly weaken device performance. Herein, an approach of halogenated solvent treatment to modulate the physical properties of indium tin oxide (ITO) substrates is employed. Four halogenated-ITO anodes (named ITO-F, ITO-Cl, ITO-Br and ITO-I) are achieved by ITO substrates immersed in hydrogen peroxide co-solvents mixed with 1,2-difluorobenzene, 1,2-dichlorobenzene, 1,2-dibromobenzene and 1,2-diiodobenzene, respectively, and upon UV illumination. The work functions of the ITO-F/Cl/Br/I anodes increase from 4.63 eV for ITO to 5.30, 5.32, 5.24, and 5.28 eV, respectively, which are close to 5.22 eV for ITO/PEDOT:PSS. Additionally, the investigated PM6:L8-BO:BTP-eC9 devices based on various anodes showed power conversion efficiencies (PCEs) of 17.81% for ITO-F, 19.48% for ITO-Cl, 17.05% for ITO-Br, 17.34% for ITO-I and 18.70% for ITO/PEDOT:PSS, respectively. In particular, a comprehensive experimental analysis is conducted to establish connections between theoretical calculations, blend morphology and physical dynamics, and further elucidate the factors that cause differences in device efficiencies. This work manifests the advantages of the proposed halogenated ITO for realizing highly efficient organic photovoltaics.