title: Analysis of Triplet Exciton Loss Pathways in PTB7:PC71BM Bulk Heterojunction Solar Cells.
abstract: A strategy for increasing the conversion efficiency of organic photovoltaics has been to increase the VOC by tuning the energy levels of donor and acceptor components. However, this opens up a new loss pathway from an interfacial charge transfer state to a triplet exciton (TE) state called electron back transfer (EBT), which is detrimental to device performance. To test this hypothesis, we study triplet formation in the high performing PTB7:PC71BM blend system and determine the impact of the morphology-optimizing additive 1,8-diiodoctane (DIO). Using photoluminescence and spin-sensitive optically detected magnetic resonance (ODMR) measurements at low temperature, we find that TEs form on PC71BM via intersystem crossing from singlet excitons and on PTB7 via EBT mechanism. For DIO blends with smaller fullerene domains, an increased density of PTB7 TEs is observed. The EBT process is found to be significant only at very low temperature. At 300 K, no triplets are detected via ODMR, and electrically detected magnetic resonance on optimized solar cells indicates that TEs are only present on the fullerenes. We conclude that in PTB7:PC71BM devices, TE formation via EBT is impacted by fullerene domain size at low temperature, but at room temperature, EBT does not represent a dominant loss pathway.
authors.affiliation: Experimental Physics VI, Julius Maximilian University of Würzburg, D-97074 Würzburg, Germany.
authors.forename: Hannes
authors.initials: H
authors.lastname: Kraus
authors.affiliation: Institut für Physik, Technische Universität Chemnitz, D-09126 Chemnitz, Germany.
authors.forename: Michael C
authors.initials: MC
authors.lastname: Heiber
authors.affiliation: Experimental Physics VI, Julius Maximilian University of Würzburg, D-97074 Würzburg, Germany.
authors.forename: Stefan
authors.initials: S
authors.lastname: Väth
authors.affiliation: Experimental Physics VI, Julius Maximilian University of Würzburg, D-97074 Würzburg, Germany.
authors.forename: Julia
authors.initials: J
authors.lastname: Kern
authors.affiliation: Institut für Physik, Technische Universität Chemnitz, D-09126 Chemnitz, Germany.
authors.forename: Carsten
authors.initials: C
authors.lastname: Deibel
authors.affiliation: Experimental Physics VI, Julius Maximilian University of Würzburg, D-97074 Würzburg, Germany.
authors.forename: Andreas
authors.initials: A
authors.lastname: Sperlich
authors.affiliation: Experimental Physics VI, Julius Maximilian University of Würzburg, D-97074 Würzburg, Germany.|Bavarian Centre for Applied Energy Research (ZAE Bayern), D-97074 Würzburg, Germany.
authors.forename: Vladimir
authors.initials: V
authors.lastname: Dyakonov