2008 GCEP Report Project title : Advanced Materials and Devices for Low Cost and High Performance Organic Photovoltaic Cells

Exciton harvesting is of fundamental importance for the efficient operation of organic photovoltaic devices. The quantum efficiencies of many organic and hybrid organicinorganic devices are still limited by low exciton harvesting efficiencies. This problem is most apparent in planar heterostructures that suffer from a direct tradeoff between light absorption and exciton harvesting. One approach to overcome small diffusion lengths is the use of triplet excitons. We have an ongoing project investigating pentacene/C60 solar cells to determine if triplets are the dominant exciton species following photoexcitation. Simulations of exciton harvesting suggest that excitons in pentacene are triplets. These triplets are likely formed by an exciton fission route which further has implications for beating the Shockley-Queisser limit and experiments are ongoing to verify this. In addition to using triplets to harvest excitons over long distances, we have developed a new scheme using long range resonant energy transfer to harvest singlet excitons over 25 nm away from the donor-acceptor interface in organic solar cells using resonant energy transfer. These results represent dramatic improvements over our previous findings and show that this scheme holds promise for the future design of highly efficient organic photovoltaics. We present theory and experiment demonstrating a scheme to harvest singlet excitons over 25 nm away from a donor-acceptor interface using resonant energy transfer. Improvement in materials choice could yield effective diffusion lengths as large as 40 nm using long-range transfer, while minimizing the energy loss to less than 0.1 eV making this a promising approach for developing highly efficient organic photovoltaics. Finally, we have successfully synthesized several new low-band gap polymers. Several of these polymers have bandgap of 1.7 eV or lower. Evaluation of the performance of these new materials is underway.