Flexible, Scalable, Transparent Conductors for Solar Photovoltaics: Laser Processed Silver Nanonetworks
Metal nanowire networks, or nanonetworks have emerged as promising alternatives for transparent conducting oxides used in photovoltaics. Currently, indium-tin-oxide (ITO) is the industry standard for transparent conductors, although ITO is inherently brittle and cannot be used in flexible settings. Furthermore, high quality ITO is created in an expensive, high vacuum/voltage, environmentally unsound manner. Alternatively, silver nanonetwork electrodes can be synthesized in normal laboratory conditions, and have favorable mechanical, optical, and electrical properties. Nanonetwork electrodes tend to suffer from large surface resistance due to narrow contact area at the nanowire-nanowire junctions in the network. To overcome this, we have devised a laser processing technique which takes advantage of a plasmonic resonance in silver nanowires to weld junctions and increase the contact area creating a superior transparent electrode. The laser processing of silver nanonetworks results in >75% reduction in macroscopic sheet resistance of the electrode while preserving the optical transparency, key metrics when evaluating a transparent conductor. This laser process transfers optical energy to a plasmonic resonance of the nanowires, allowing the localization of energy to the junctions while preserving the long range connectivity of the network. We have investigated the plasmonic effects of this laser processing through finite-difference time-domain simulations, and have verified these findings with microscopy of silver nanonetworks exposed to varying amounts of laser radiation. Finally, we have validated our process by laser processing silver nanonetworks on otherwise complete hybrid organic photovoltaic devices, which show >45% enhancement in fill factor compared to control devices.