As the global demand for low cost renewable energy sources intensifies, interest in new routes to converting solar energy to electricity is rapidly increasing. One class of solar cells which has recently inspired significant academic and industrial excitement is the bulk heterojunction (BHJ) 'plastic' solar cell. Research by a rapidly growing community of scientists across the globe is generating a steady stream of new insights into the fundamental physics, the materials design and synthesis, the BHJ film processing and morphology, and the device science and architecture. Higher levels of performance are continuously being achieved and clear paths exist to low cost "plastic" solar cells with power conversion efficiencies greater than 10%. Future progress in the fabrication of high performance BHJ cells will depend on our ability to combine aspects of synthetic and physical chemistry, condensed matter physics, and materials science.
Research within the Center for Polymers and Organic Solids (CPOS) utilizes an interdisciplinary approach that ties together recent advances in BHJ morphology characterization, device photophysics, and thin film solution processing to identify the the limiting factors in solar cell performance by using a combination of characterization tools. New device architectures and new processing methods are implemented to increase the power conversion efficiency by controlling both the BHJ phase separation and the internal order of the phase separated components. The failure of many new semiconducting polymers to achieve high performance in BHJ solar cell devices has been blamed on 'poor morphology' without significant characterization into either the structure of the phase separated morphology or the nature of the charge carrier recombination. CPOS research utilizes a variety of techniques to understand the role of processing and thermal annealing on the performance of polymer solar cells. For example, this understanding has enabled the use of processing additives to achieve higher performance in low band gap polymers for which thermal annealing is not effective.