Charge transfer states as traps in organic solar cells

Aina Quintilla^a, Andreas Arndt^a, Uli Lemmer^a, Marina Gerhard^b, Martin Koch^b, Ian Howard^c

^aLight Technology Institute, Karlsruhe Institute of Technology, Engesserstr. 13, Karlsruhe, 76131, Germany

^bDepartment of Physics and Material Sciences Center, Philipps-Universität Marburg, Renthof 5, 35032 Marburg, Germany

^cInstitute of Microstructure Technology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Andreas Arndt, 142
Publication date: 5th February 2015

We investigate the NIR time-resolved photoluminescence of a series of P3HT:PC₆₁BM solar cells with varying blend ratios after preferential excitation of the PC₆₁BM and P3HT components respectively. Besides the rapid and diffusion-limited quenching of singlet excitons we resolve a weak emission feature in the near-infrared that our measurements confirm comes from interfacial charge-transfer (CT) states. This CT state emission becomes stronger for samples with an excess of PC₆₁BM, and also after selective excitation of the PC₆₁BM component. In this way, we show that these NIR time-resolved photoluminescence measurements provide an accurate method of observing subtle changes in the formation and dynamics of CT states at organic heterojunctions due to its high selectivity, and suggest that PC₆₁BM excitons are more likely to lead to geminately recombining CT states than are the excitons created on P3HT. We also measure the temperature dependence of the transient NIR photoluminescence and find that while the intensity of the NIR emission is temperature dependent, its lifetime is not. This interesting observation suggests that the CT states we observe are formed through a precursor state which can either form separated charges or CT states, and that the relative yield of these two pools is temperature dependent. Furthermore, it indicates that charges within these relaxed CT states are trapped at the donor-acceptor interface and cannot contribute to free-charge generation via thermal activation anymore.

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