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Monomolecular and Bimolecular Recombination of Electron–Hole Pairs at the Interface of a Bilayer Organic Solar Cell |
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| Authors: | Tobias Hahn Steffen Tscheuschner Frank‐Julian Kahle Markus Reichenberger Stavros Athanasopoulos Christina Saller Guillermo C. Bazan Thuc‐Quyen Nguyen Peter Strohriegl Heinz Bässler Anna Köhler |
| Affiliation: | 1. Experimental Physics II, University of Bayreuth, Bayreuth, Germany;2. Departamento de Física, Universidad Carlos III de Madrid, Leganés, Madrid, Spain;3. Macromolecular Chemistry I, University of Bayreuth, Bayreuth, Germany;4. Department of Chemistry and Biochemistry, Center for Polymers and Organic Solids, University of California, Santa Barbara, CA, USA;5. Bayreuth Institute of Macromolecular Science (BIMF), University of Bayreuth, Bayreuth, Germany |
| Abstract: | While it has been argued that field‐dependent geminate pair recombination (GR) is important, this process is often disregarded when analyzing the recombination kinetics in bulk heterojunction organic solar cells (OSCs). To differentiate between the contributions of GR and nongeminate recombination (NGR) the authors study bilayer OSCs using either a PCDTBT‐type polymer layer with a thickness from 14 to 66 nm or a 60 nm thick p‐DTS(FBTTh2)2 layer as donor material and C60 as acceptor. The authors measure JV‐characteristics as a function of intensity and charge‐extraction‐by‐linearly‐increasing‐voltage‐type hole mobilities. The experiments have been complemented by Monte Carlo simulations. The authors find that fill factor (FF) decreases with increasing donor layer thickness (Lp) even at the lowest light intensities where geminate recombination dominates. The authors interpret this in terms of thickness dependent back diffusion of holes toward their siblings at the donor–acceptor interface that are already beyond the Langevin capture sphere rather than to charge accumulation at the donor–acceptor interface. This effect is absent in the p‐DTS(FBTTh2)2 diode in which the hole mobility is by two orders of magnitude higher. At higher light intensities, NGR occurs as evidenced by the evolution of s‐shape of the JV‐curves and the concomitant additional decrease of the FF with increasing layer thickness. |
| Keywords: | charge carrier extraction fill factor organic semiconductors planar heterojunctions |