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Lecture Vladimir Dyakonov


13 January 2012 FWN-Building 5118.-152, Nijenborgh 4, 9747 AG, Groningen
Speaker: Prof. Dr. Vladimir Dyakonov
Affiliation: Julius-Maximilians University of Würzburg and Bavarian Center for Applied Energy Research e.V. (ZAE Bayern), Germany
Title: Probing triplet states in OPV materials
Date: Fri Jan 13, 2012
Start: 11.00
Location: FWN-Building 5118.-152
Host: Maria A. Loi
Telephone: +31 50 363 4119


Triplet excitons and organic photovoltaics sound controversial for the first moment. Moreover, probing them is generally a challenging task. The formation of triplets is usually assumed from the transient photoluminescence or absorption experiments.  We developed and applied the technique of photoluminescence detected spin resonance (ODMR) which is intrinsically multiplicity sensitive thus allowing us a direct identification of triplets and other spin carrying states, e.g. charge transfer states inside the materials. In the intermixed conjugated polymers and fullerenes, such as P3HT:PC60BM  – one of the main working horses of the OPV technology so far- nearly all the generated excitons are split into free charge carriers and no triplets can bee seen. We surprisingly found that in highly performing OPV bulk-heterojunctions such as P3HT:PC80BEH (PCE=4%) and poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl][3- fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7):PC70BM blends (PCE=7%) and several other low-band gap copolymer blends triplet excitons are formed. We propose a scenario, in which the triplet exctions formation is due to electron back transfer (EBT) from the charge transfer state to the polymer triplet state. We discuss the physical picture in which pushing up the LUMO level of acceptors may end up in an unwanted reduction of the overall performance of the bulk-heterojunction solar cells.

Last modified:22 October 2012 2.31 p.m.