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Charge-transfer absorption and emission in organic donor/ acceptor solar cells

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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. In order to probe interfacial properties directly, highly sensitive measurements of absorption, photocurrent, photoluminescence and electroluminescence spectra of organic D/A photovoltaic devices are performed. For 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. We find that the internal quantum yields of photocurrent generation are similar for both pure phase excitations and direct excitation in the low energy part of the CT absorption band, indicating that properties of this relaxed CT state will determine the free charge carrier generation rate and short-circuit current (Jsc).

Also the open-circuit voltage (Voc) can be related to CT state properties: The principles of reciprocity and detailed balance provide a relation between the CT absorption and emission actions, 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.

In order to optimize Voc, material combinations with ECT close to the optical gap of the main light absorbing material (Eg) are desirable. A range of D/ A combinations with different Eg-ECT values are investigated and possible detrimental effects on photocurrent generation are discussed.

This talk is part of the Optoelectronics Group series.

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