Princeton University

School of Engineering & Applied Science

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

Franky So, North Carolina State University
E-Quad, B205
Wednesday, November 9, 2016 - 4:30pm

It is commonly believed that the reason for lower efficiencies 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.
This seminar is supported with funds from the Korhammer Lecture Series
Franky So received his PhD degree in electrical engineering from the University of Southern California in 1991. He has previously worked at Hoechst Celanese Research Division and Motorola. In 2001, he joined OSRAM Opto Semiconductors and became the Head of OLED Research. He joined the University of Florida in 2005 and became the Rolf E. Hummel Professor in Electronic Materials in the Department of Materials Science and Engineering at the University of Florida. He joined the Department of Materials Science and Engineering at the North Carolina State University where he is currently the Walter and Ida Freeman Distinguished Professor. While at Motorola, he was named the Distinguished Innovator and Master Innovator. Dr. So holds 80 issued patents and has published more than 150 journal articles. He is the Editor-in-Chief of the journal Materials Science and Engineering Reports and serves as an Associate Editor for IEEE Journal of Photovoltaics, IEEE Journal of Display Technology, SPIE Journal of Photonic Technology and Organic Electronics. So is a Distinguished Lecturer of the IEEE Photonics Society, a Charter Fellow of the National Academy of Inventors, and a Fellow of IEEE, OSA and SPIE.