cfaed Seminar Series

cfAED Seminar Series

Prof. Dieter Neher , University of Potsdam, Institute of Physics and Astronomy; Germany

Quantifying Free Charge Generation and Extraction in Organic and Hybrid Solar Cells

22.09.2014 (Monday) , 16:00 - 17:30
TU Dresden, room BEY98 ( ) , George-Bähr-Str. 1 , 01069 Dresden

A guest talk by Prof. Dieter Neher, University of Potsdam, Institute of Physics and Astronomy, Karl-Liebknecht-Str. 24-25, D-14476 Potsdam, Germany

The photon-to-charge conversion in organic and hybrid solar cells comprises several elementary steps, including the formation of interfacial charge-transfer states (CTS) via exciton dissociation at the DA heterojunction, the split-up of these states into free charge carriers, and free charge extraction to the electrodes. Understanding of the efficiency and dynamics of these processes is essential for a comprehensive understanding of these devices.

By applying state of the art charge extraction methods we are able to quantify the efficiency of free charge formation and extraction under application-relevant illumination conditions. We find that the charge generation involves thermalized CT states, meaning that the mutual Coulombic binding of the electron-hole pair forming the CTS must be compensated by forces originating from the specific energetics at the mesoscale in efficient devices. We find that for the very same donor-acceptor combination, a phase-separated structure with rather pure domains results in a less field-dependent generation, pointing to the importance of blend morphology for efficient separation of the CTS into free charge.

For these optimized systems, charge extraction becomes a major objective. A new method is employed to quantify the effective mobility of charge extraction under application-relevant conditions. It is shown that the performance of some polymer:fullerene blends is ultimately limited by an insufficient hole mobility, which places the understanding und optimisation of this particular property into the focus of ongoing research work.

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