Tailoring Opto-electrical properties of ultra-thin indium tin oxide films via filament doping: Application as a transparent cathode for indoor organic photovoltaics

Either side of organic photovoltaics incorporates a transparent conducting electrode for light entry, which becomes particularly crucial under indoor light conditions with low light intensity. However, since electrical resistivity and optical transparency with respect to film thickness are mutually contradictory, sufficiently thick (> 150 nm) electrodes are inevitably required to ensure the appropriate electrical conductivity, at the sacrifice of transmittance. This paper introduces an electric field induced filament doping method to realize ultra-thin indium tin oxide with high conductivity. The proposed method allows for injecting metal dopants (i.e., Ni) into ultra-thin indium tin oxide under an electric field, enabling substantial resistance reduction while retaining high transmittance and low surface roughness. Optimum light absorption and effective carrier transport via filament doping provides improved performance for indoor organic photovoltaics. As a proof-of-principle demonstration, we fabricate poly(3-hexylthiophene):indene-C60 bisadduct based inverted organic photovoltaics with 10 nm Ni-doped indium tin oxide as a transparent cathode, leading to a power conversion efficiency of 14.6 +/- 1.8% under the 1000-lux light-emitting diode. This efficiency is 40% higher than that achieved from the device using commercially available 150 rim indium tin oxide.