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Dielectric Properties of Polymer-Fullerene Blends for High Performance Solar Cells

Franky So, Department of Materials Science and Engineering, MSE, NCSU

It is commonly believed that the reason for lower efficiency in polymer solar cells compared to silicon solar cells is the low dielectric constant in photoactive polymers. To alleviate this problem, many researchers have attempted to find ways to increase the dielectric constant of photoactive polymers. However, it is difficult to control the polymer-fullerene blend morphology while tuning the polymer chemistry, and this strategy to enhance the solar cell performance has not been successful. Therefore, we have taken a different approach to this problem. Using several high performance polymer systems, we systematically studied how blending photoactive polymers with fullerene affects the photophysical and dielectric properties of the blends. We found several interesting results. First, most high performance polymers have a dielectric constant value smaller than 3, and there are no correlations between dielectric constant and device performance. Second, in all polymer systems we studied, we found that there is a significant increase in the value of the blend dielectric constant upon mixing a polymer with fullerene. We interpret that as an indication of the strong electronic coupling between the polymer and fullerene. Third, while the blend dielectric constant value has a weak correlation with the device performance, we found that the excited state polarizability of the blend is a strong indicator predicting the device performance. Our results indicate that while the dielectric properties of the pristine polymer might not be the critical factor, the control of the electronic coupling between the acceptor moiety and the fullerene molecule is a key factor determining the device performance.