Events

Overcoming the Exciton Diffusion Bottleneck in Organic Photovoltaic Cells

May 20, 2009 at 3pm/36-428

Russell J.Holmes
Department of Chemical Engineering and Materials Science, University of Minnesota

abstract:
Organic materials are attractive for application in photovoltaic cells due to their compatibility with lightweight, flexible substrates, and high throughput processing techniques. Optical absorption in these materials leads to the creation of a bound electron-hole pair known as an exciton. The exciton is mobile, and diffuses to a heterojunction where electron-hole separation and photocurrent generation may take place. In most organic materials, the exciton diffusion length is much shorter than the optical absorption length. This “exciton bottleneck” limits the active layer thickness and reduces cell absorption efficiency. Routes around the bottleneck have centered on the use of mixed donor-acceptor film morphologies to increase the area of the dissociating interface. While promising, these architectures are difficult to optimize, and can introduce resistance for the collection of photogenerated carriers.

This talk will examine two alternate techniques to overcome the exciton bottleneck. First, the use of energy transfer to a long-lived phosphorescent sensitizer will be described as a means to enhance the exciton diffusion length in fluorescent, electron donating materials. A second approach involving the use of donor-acceptor films with engineered film composition and morphology will be discussed as a means to simultaneously maximize the exciton diffusion and charge collection efficiencies.

bio:
Russell Holmes is an Assistant Professor in the Department of Chemical Engineering and Materials Science at the University of Minnesota. He completed his M.A. and Ph.D. in Electrical Engineering at Princeton University, and holds a B.Sc. (Honours) in Physics from the University of Manitoba, Canada. Prof. Holmes’ research is focused on the fundamental optoelectronic properties of organic and hybrid materials, and their application in organic light-emitting devices, photovoltaic c