This project will to systematically study, identify, and potentially overcome the various physical phenomena in both materials and in devices that lead to degradation of organic solar cells when subjected to light. Organic photovoltaic (OPV) devices are an increasingly important photovoltaic (PV) energy conversion technology. Recent advances in efficiency to ~12% range in both single and tandem junction OPV solar cells are very encouraging for eventual commercial deployment. However, the devices are known to degrade rapidly when exposed to light, losing 20-30% of the initial efficiency within ~100 hours of illumination, even when encapsulated or kept in inert atmospheres. A major market for this technology is building-integrated products, since in principle, the OPV devices can be laminated onto existing window frames. For such commercial deployment, it is essential that the degradation be reduced significantly, to <10% range over the lifetime of the product, which is typically ~20 years for building products. Similarly, another major market segment, providing power for rural populations in developing countries, also requires relatively long life, even though they do not require the same power conversion efficiency as grid-connected central power in the U.S. This project will systematically investigate the changes in fundamental physical parameters such as optical absorption, hole mobility, deep state densities in both the absorber materials and at the hetero-junction interface when OPV materials and devices are subjected to illumination.
The PIs will study the evolution of defects using both electrical measurements such as capacitance-frequency at different temperatures, and structural measurements such as spin resonance. PIs will study the kinetics of defect evolution over time under varying intensities of light so as to establish kinetic laws that govern defect evolution. Then, the PIs will systematically explore the thermal annealing of these defects over time, thereby finding out activation energies for annealing. The PIs will correlate these kinetics and annealing energies to the structure, morphology and composition of the organic materials, and the specific technology used for fabricating the devices. A number of different materials such as P3HT and PCDTBT will be studied and the relationship between the various kinetic parameters to the nature of the bonding in the materials will be established. We will use these results to design and fabricate better polymers which are likely to be more stable while maintaining power conversion efficiency.
The broader impact consists of the industrial impact of the work, and in educating both graduate and undergraduate students in the field of OPV devices and materials in particular, and solar energy conversion devices in general. Significant attention will be paid to transfer the research results into education by including new lab sections in existing courses. Both women and under-represented minority group students are expected to play a significant role in the research. The results of the research will be broadly disseminated to scientists and engineers through publications, and by offering webinars through IEEE. Dissemination to the general public will be done by giving talks both in the U.S. and overseas.
