Device optimization for organic photovoltaics with CNT networks as transparent electrode

Recently, there has been a lot of interest in flexible and high efficiency solar cells due to cost advantages of roll to roll printing. Traditionally, ITO (Indium Tin Oxide) or ZnO (Zinc Oxide) electrodes have been used as top contacts for solar cells because of their reasonable transparency and moderately low sheet resistance. However, these electrodes are not flexible and would undergo breakdown on bending of flexible substrates. Hence, several groups are working on various types of flexible electrodes which have better optical transparency as well as have high electrical conductivity. Among the various options, CNT (Carbon Nanotube) random networks have emerged as a viable alternative to ITO and ZnO, satisfying these constraints and indeed, several types of solar cells (^1–5) have been reported with CNT random networks as back contact. These experimental reports have so far not been complemented by meaningful modeling of CNT networks in solar cells for performance optimization of the solar cell device design. Here, for the first time we present comprehensive simulation results for organic excitonic solar cells with CNT networks as back contact that analyzes all elements of the solar-cell within an end-to-end theoretical framework. In our previous work, we have done extensive modeling of the CNT networks for usage as transistor channel material ^{6, 7} and here we use the previously established techniques to model CNT networks as electrodes. Our analysis shows that optimizing the CNT density is critical to achieve the best tradeoff of transparency vs. over all efficiency of the solar cell.