Nanostructured indium-tin-oxide electrodes for organic photovoltaics

Abstract

The use of organic, semiconducting polymers provides an attractive option to advance commercial solar photovoltaic technologies. These devices have the potential to be spray-coated on flexible, light-weight substrates and produced cheaply in high volumes; however, performance remains limited in practice. Interpenetrating, transparent nanostructures with near metallic conductivity have been suggested to provide an expedited track for improved charge extraction while also serving as a 3-D structural support for the photo-absorbing polymer region—extending device power conversion efficiency and lifetime. For tin-doped, indium-oxide nanostructures there is trade-off between high conductivity, transparency, and the desired highly-branched 3-D ‘nanotree’ morphology; when fabricating highly conductive nanotree films, wires with high conductivity are achievable through deposition at elevated rates, however increasing the deposition rate results in a reduction of the number of branches. Further, reliable characterization and optimization of the electrical properties of the nanotree arrays is a challenge due to the laterally disconnected architecture, and requires the adaptation of a typically planar device architecture to accommodate the 3-D structures. In this work, the fabrication of a four-terminal device architecture paired with ohmic contacts removes the contribution from the measurement leads and minimizes the contribution for the lead-nanostructure interface, allowing for the exploration of the electronic interfaces internal to the nanostructure arrays. These electrical test structures are then utilized to explore post-processing anneals to optimize the NW conductivity, while maintaining the highly-branched structures suitable for integration within OPV devices. It was found that the oxygen vacancy distribution within individual structures can be affected, measurable through analysis of the shifts in the observable electrical properties of the four-terminal characterization devices. These observations extend the technological feasibility of these nanostructures and present areas of potential improvement in future applications.