Princeton University

School of Engineering & Applied Science

Charge generation and recombination in bulk heterojunction organic photovoltaics

Dr. Feng Gao, Linkoping University, Sweden
E-Quad J401
Friday, December 5, 2014 - 2:00pm

The power conversion efficiency of organic photovoltaics (OPVs) has increased from around 1% to over 10% in the past two decades. In spite of the great success in device efficiency, many fundamental issues concerning basic operational mechanisms of these devices remain unknown or controversial, limiting rational design of donor and acceptor materials. Among others, charge generation and charge recombination might be two most fundamental yet most debated issues. I will talk about charge generation and charge recombination processes in polymer:fullerene, polymer:polymer, and polymer:nanocrystal blends. By combining the spectroscopic and electrical measurements, we identify that  trap-mediated recombination is responsible for the loss of photogenerated charge carriers in polymer:nanocrystal blends. We employ straightforward temperature-dependent open-circuit voltage (Voc) measurements to understanding charge generation in OPVs. At low temperatures, we observe a decrease of Voc, which is attributed to reduced charge separation. Comparison between benchmark polymer:fullerene and polymer:polymer blends highlights the critical role of charge delocalisation in charge separation and emphasizes the importance of entropy in charge generation.