Optical Properties of Nanostructured Electrodes

Abstract

A versatile and powerful new lithographic fabrication method has been used to fabricate a number of nano-architectured ordered 2-D indium tin oxide (ITO) and silver (Ag) electrodes. By careful tuning of the dimensions of the nanofeatures in the electrodes, the surface area can be enhanced as desired, in-turn changing resistivity and free carrier concentrations accordingly. Absorption spectra of the samples show the existence of a new optical bandgap, in addition to the bulk bandgap, that is smaller. Nanostructured electrodes show enhanced transparency compared to their planar counterparts and demonstrate typical surface plasmon characteristics. The resonance frequency can be tuned as well by changing the dimensions of the nanofeatures in the electrodes. INTRODUCTIONMetal and metal oxide electrodes play a significant role in many state-of-the-art technologies including photonics [1], membranes [2], biological supports [3], sensing [4], electrochromics [5], and in various green technologies, such as, photocatalytics [6], Li-ion batteries [7] and photovoltaics [8]. There is strong on-going effort in these areas to create one-and twodimensional electrode structures that provide tunability in key electrode properties. Tailored nanostructured interfaces enable modification and tunability of the optical transparency, band gap, carrier concentration and mobility, and low tortuosity for improved electrode -hole transport characteristics. Organic photovoltaics (OPVs) suffer from low hole mobility and Hsu et al [9] have showed that insertion of nanostructured ITO in the active layer of organic solar cells may act to balance the carrier transport. Nanostructured ITO will increase the effective mobility of holes by creating a shorter path for the holes to reach the anode. Tailored active layer electrode interfaces can also enable optimization of exciton generation and dissociation within the space -charge uncertainty limits along with selective charge transport, possibly improving charge collection from both electron and hole rich regions. Enhanced surface area in nanostructured ITO also enables easy access to analytes in electrochemical sensors and other applications. Further advantages can include improved light harvesting efficiency in OPVs using these structures, along with enhanced mechanical stability and shorter radial diffusion distances [10]. The existence of void volume in the pillar arrays reduces the effective refractive index of the electrode and hence can be useful for antireflection coatings [11,12] These structures are fabricated using a modified imprinting process [13], resulting in a thermoplastic nanostructured scaffold that can be subsequently coated with a wide variety of metal and metal oxides using traditional deposition techniques. Such a process allows facile integration with existing TCO and device manufacturing methodologies, lower operating and start-up materials cost and providing the potential for high throughput manufacturing, including roll-to-roll production. The dimensions and distribution of...