Berlin 2015 – scientific programme

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HL: Fachverband Halbleiterphysik

HL 36: Organic electronics and photovoltaics: OPV I (CPP with HL/TT)

HL 36.7: Talk

Tuesday, March 17, 2015, 15:45–16:00, C 130

Triplet Exciton Formation in High-Efficiency Donor-Acceptor Photovoltaic Blends — •Stefan Väth¹, Hannes Kraus¹, Andreas Baumann², Kristofer Tvingstedt¹, Andreas Sperlich¹, Vladimir Dyakonov^1,2, John Love³, and Thuc-Quyen Nguyen³ — ¹Experimental Physics VI, Julius Maximilian University of Würzburg, 97074 Würzburg — ²ZAE Bayern, 97074 Würzburg — ³University of Santa Barbara, Santa Barbara, CA 93106, USA

In donor-acceptor based bulk-heterojunction solar cells, the splitting of singlet excitons at the donor and acceptor interface is of crucial importance for charge generation. The reversed process, in which two initially free charge carriers meet at the interface to form an exciton with singlet or triplet multiplicity is rather beneficial for light emission in OLEDs but considered as one of the loss factors in OPV.

In our experiments, the occurrence of triplet excitons and CT states was probed by using spin sensitive detection of the photo- and electroluminescence. A substantial generation of molecular triplet excitons was found in high efficiency donor-acceptor OPV systems based on the low bandgap copolymer PTB7 and in the soluble small molecule p-DTS(FBTTh2)2, both blended with PC70BM as acceptor. We ascribe these findings to an electron back transfer from the CT state to the triplet state on the donor material. In summary, the fundamental understanding of the transformation processes involving the CT states, triplet excitons, as well as free electrons and holes and their dependence on nanoscale morphology and energetics of blends is essential for the optimization of OPV devices.