Franky So
University of Florida
Dept. of Materials Science and Eng.

Dielectric Effects in Photovoltaic Polymers

Location: EB1 Room 1011

Friday, October 31st 2014 - 11:00 am

Solution processable polymeric bulk heterojunction solar cells have drawn a lot of attention due to their potential for low cost roll-to-roll manufacturing. Typical polymer solar cells are made with blends consisting of a polymer donor and a fullerene acceptor. Compared with inorganic semiconductors, polymer semiconductors have significantly lower dielectric constants leading to a strong localization of the charge transfer (CT) states and an additional energy loss due to the strong exciton binding energy. It is therefore important to determine how the dielectric properties of the polymer-fullerene blends affect the energy loss in polymer solar cells. However, one of the challenges is to determine the so-called effective bandgap of a polymer-fullerene blend which is the difference of the highest occupied molecular orbital (HOMO) energy of the polymer donor and the lowest unoccupied molecular orbital (LUMO) energy of the fullerene acceptor.

In this talk, we will first describe how we used charge modulated electroabsorption spectroscopy (CMEAS) to measure the effective bandgap energy of a polymer-fullerene blend. Knowing the effective bandgap energy and the energy loss due to carrier recombination in a polymer solar cell, we can determine the additional energy loss due to dielectric effects. From our data, we found that the excess energy loss in a polymer solar cell is a strong function of the dielectric constant of the polymer blend. For polymer blends with low dielectric constants, the photo-voltage loss can be as large as 0.35 V. On the other hand, there is no voltage loss if the dielectric constant is close to 5, suggesting that these polymer solar cells behave like their inorganic counter-parts. We also found that the dielectric properties have a strong effect on delocalization of the CT states. These findings will enable chemists to design novel polymers to minimize the energy loss in a polymer solar cell.

Dr. Franky So received his Ph.D. degree in electrical engineering from the University of Southern California. He was the Manager of the OLED Program at the Motorola Corporate Research Laboratories and then the Head of Research at OSRAM Opto Semiconductors responsible for the world-wide OLED development. After 14 years in industry, he joined the University of Florida in 2005 and is currently the Rolf E. Hummel Professor of Electronic Materials and the Associate Chair of Research in the Department of Materials Science and Engineering. Dr. So is a Charter Fellow of the National Academy of Inventors, a Fellow of the Institute of Electrical and Electronic Engineers (IEEE), the Optical Society of America (OSA), and the International Society of Optics and Photonics (SPIE). He is also an IEEE Photonics Society Distinguished Lecturer. His research interest is in the area of organic light emitting diodes, organic solar cells, organic-inorganic hybrid devices, infrared sensors and radiation detectors. He is the Editor-in-Chief of Materials Science and Engineering Reports and also an Associate Editor of Organic Electronics, IEEE Journal of Display Technology, IEEE Journal of Photovoltaics, SPIE Journal of Photonics for Energy. Dr. So has edited one book and is an author of 3 book chapters. He has over 120 refereed publications, over 70 issued patents and another 20 patent applications

Dr. So received many awards and recognitions. While at Motorola, he was given the Distinguished Innovator and the Master Innovator Awards. At the University of Florida, he was given the Innovation Award, the MSE Department Faculty Excellence Award. In 2011, he was named the University of Florida Research Foundation Professor. Dr. So was also a recipient of the DOE Solid State Lighting Significant Achievement Award in 2009 and 2010.

North Carolina State University