Center for Excitonics

Events

Charge-transfer absorption and emission in polymer: fullerene solar cells

April 13, 2010 at 3pm/36-428

Koen Vandewal
Linköping University

abstract:
For an efficient conversion of photons to electrons by organic materials, the presence of a material interface between an electron donating and electron accepting material is crucial. Most successful active layers for organic solar cells today comprise a blend of conjugated polymers as electron donors and fullerenes as electron acceptors, reaching power conversion efficiencies (h) up to 8%. In order to find pathways to increase h further, fundamental properties of the electronic states at the donor/acceptor (D/A) interface and their role in determining and limiting h, are investigated in this work.

In order to probe these interfacial properties directly, highly sensitive measurements of the photocurrent and electroluminescent spectra of organic D/A photovoltaic devices are performed. For all material combinations exhibiting a decent charge carrier generation, weak absorption and emission involving a CT state with energy ECT lower than the lowest optical gap of both donor and acceptor can be observed. Efficient population and subsequent dissociation of the CT state determines the short-circuit current (Jsc).

Also the open-circuit voltage (Voc) can be related to CT properties: The principles of reciprocity and detailed balance provide a relation between the CT absorption and emission properties, and Voc.  This relation is shown to be valid for a range polymer:fullerene photovoltaic devices, at different temperatures and illumination intensities. When measured under solar illumination, we find an energetic difference between ECT and qVoc of ~0.6 eV for this type of photovoltaic devices. The origin of this difference is twofold. About 0.25 eV of this energetic loss is due to the radiative recombination through the CT state. The remaining ~0.35 eV is due to additional, non-radiative recombination mechanisms.

To conclude, upper bounds for Jsc, Voc and h are derived, under the assumption of perfect conditions for charge generation and recombination. Ideal values for CT state properties and optical gap of the main absorber are discussed.

bio:
Koen Vandewal received his master’s degree in electro-optical engineering from Gent University (Belgium) in 2004 and his Ph.D. from Hasselt University (Belgium) in 2009. Currently, he is a postdoctoral researcher at Linköping University (Sweden). Research interests include charge generation and recombination processes in organic opto-electronic devices.