Insights on the working principles of flexible and efficient ITO-free organic solar cells based on solution processed Ag nanowire electrodes

We report a detailed study on the operating principles of silver nanowire (Ag NW) networks as a transparent conducting electrode. Current limitations to achieve high conversion efficiencies are overcome with the combination of Ag NW and specifically designed ZnO nanoparticles (NP). The new transparent electrode presents improved optoelectronic properties with respect to the most widely used ITO-indium tin oxide. We demonstrate that there are non-conducting void spaces left between neighboring nanowires onto the holding substrate as a consequence of processing the Ag NW film from a solvent dispersion. Moreover, the fabrication of efficient ITO-free organic solar cells (OSC) based on Ag NW relies on filling the resulting void spaces with a highly selective and conductive supporting thin layer. This slight modification of the Ag NW film enables the great majority of photogenerated charge carriers - that would otherwise be recombined at voids - to be efficiently collected by the nanowires. In this way, we report - to our knowledge - one of the highest ITO-free OSC (3.8%) also fabricated onto flexible substrates (3.2%) with commercially available Poly (3-Hexylthiophene) (P3HT) and Phenyl-C61-butyric acid methyl ester (PCBM). This breakthrough could once and for all bring the potential of flexible low-cost organic photovoltaics into reality. (C) 2012 Elsevier B.V. All rights reserved.