Charge carrier mobility,bimolecular recombination and trapping in polycarbazole copolymer:fullerene (PCDTBT:PCBM) bulk heterojunction solar cells |
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Authors: | Tracey M. Clarke Jeff Peet Andrew Nattestad Nicolas Drolet Gilles Dennler Christoph Lungenschmied Mario Leclerc Attila J. Mozer |
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Affiliation: | 1. ARC Centre of Excellence for Electromaterials Science, Intelligent Polymer Research Institute, University of Wollongong, Innovation Campus, Squires Way, North Wollongong, NSW 2500, Australia;2. Konarka Technologies, 116 John St., Suite 12, Lowell, MA 01852, USA;3. IMRA Europe, 220, Rue Albert Caquot–BP 213, 06904 Sophia-Antipolis Cedex, France;4. Canada Research Chair on Electroactive and Photoactive Polymers, Université Laval, Québec City, Québec, Canada |
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Abstract: | Organic photovoltaic devices based on the donor:acceptor blend of poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) have received considerable attention in recent years due to their high power conversion efficiencies and the ability to achieve close to 100% internal quantum efficiency. However, the highest efficiencies were all attained using active layers of less than 100 nm, which is not ideal for either maximised potential performance or commercial viability. Furthermore, more recent reports have documented significant charge carrier trapping in these devices. In this paper two charge extraction techniques (photo-CELIV and time-of-flight) have been used to investigate the mobility and recombination behaviour in a series of PCDTBT:PCBM devices. The results not only confirm significant charge carrier trapping in this system, but also reveal close to Langevin-type bimolecular recombination. The Langevin recombination causes a short charge carrier lifetime that results in a short drift length. The combination of these two characteristics (trapping and fast bimolecular recombination) has a detrimental effect on the charge extraction efficiency when active layers greater than ∼100 nm are used. This accounts for the pronounced decrease in fill factor with increasing active layer thickness that is typically observed in PCDTBT:PCBM devices. |
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Keywords: | Polymer solar cell Charge recombination Charge carrier mobility Photo-CELIV Charge extraction Polycarbazole copolymer |
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