When 3:30 PM - 5:00 PM Apr 08, 2011
Where 1670 CSE
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Russell Holmes, Department of Chemical Engineering and Materials Science, University of Minnesota

Approaches for the characterization and enhancement of exciton harvesting in organic photovoltaic cells

Organic semiconductors have received considerable attention for application in photovoltaic cells due to their compatibility with high-throughput processing techniques and lightweight, flexible substrates. Optical absorption in an organic semiconductor leads to the formation of tightly bound molecular excited states known as excitons. Consequently, the generation of photocurrent in an organic photovoltaic cell (OPV) requires that the exciton be dissociated into its component charge carriers. This is most often accomplished using a heterojunction between electron donating and accepting materials. In this scheme, photocurrent generation occurs only at the donor-acceptor (D-A) interface, and exciton diffusion to the interface is a critical step in the photoconversion process. Most organic semiconductors are characterized by exciton diffusion lengths that are considerably smaller than the optical absorption length. In an OPV, this trade-off between diffusion and absorption often necessitates the use of thin active layers to maximize exciton harvesting.


The focus of this talk will be on developing approaches that permit the accurate measurement of the exciton diffusion length, and realizing new device architectures that demonstrate enhanced exciton harvesting. In measuring the exciton diffusion length, emphasis is placed on quantifying the role of D-A excitonic energy transfer in the dissociation process. Many of the techniques currently used to estimate the exciton diffusion length incorrectly ignore these effects leading to overestimates. Here, work to overcome the short diffusion length is focused on cells that rely on the use of a continuously graded D-A film composition as a means to simultaneously optimize both exciton diffusion and charge collection. In a properly optimized graded heterojunction OPV, power conversion efficiencies >4% can be realized, exceeding the performance of conventional planar and uniformly mixed structures. 

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