P3HT:PCBM Bulk Heterojunction Solar Cells: Morphological And Electrical Characterization And Performance Optimization
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1013-Z01-02
P3HT:PCBM Bulk Heterojunction Solar Cells: Morphological And Electrical Characterization and Performance Optimization Tom Aernouts1, Peter Vanlaeke1,2, Ilse Haeldermans2,3, Jan D'Haen2,3, Paul Heremans1, Jef Poortmans1, and Jean . V. Manca2,3 1 SOLO, IMEC, Kapeldreef 75, Leuven, Belgium 2 IMO, Hasselt University, Diepenbeek, Belgium 3 IMOMEC, IMEC, Diepenbeek, Belgium
ABSTRACT The performance of organic solar cells based on the blend of regioregular poly(3hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) is strongly influenced by the morphology of the active layer on the nanoscale level. X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM) measurements demonstrated that the natural tendency of regioregular P3HT to crystallize is disturbed by the addition of PCBM. The crystallinity of the photo-active blend is typically restored by an annealing procedure resulting in improved device performance. The morphological changes upon annealing of the P3HT:PCBM blends are accompanied by electrical changes as shown in charge carrier mobility measurements. Variation of the processing solvent demonstrated however that an optimized morphology and charge transport situation can also be obtained without an additional annealing step. It is shown that in that case the as-produced active layer has already a favorable crystalline morphology. Power conversion efficiency over 4% has been achieved in this way. INTRODUCTION Many different organic solar cell structures are currently being studied worldwide, whereas the donor/acceptor bulk heterojunction solar cell is regarded as one of the most promising approaches [1]. Recently, semi-crystalline regioregular poly(3-hexylthiophene) (P3HT) has attracted a lot of attention as a donor material, as it has an absorption edge around 650 nm, combined with a high hole mobility [2]. Indeed, promising power conversion efficiencies, well over 3 and 4%, have been reported for P3HT based solar cells [3,4]. A prerequisite for reaching these high efficiencies is to optimize the relation between the layer morphology and its opto-electrical behaviour. In this work we present a combined morphological and opto-electrical study of P3HT based bulk heterojunction solar cells. It will be shown that in this material system the crystallization of P3HT plays an important role in the device performance. RESULTS AND DISCUSSION Post-production annealing has proven to be a useful way to improve the power conversion efficiency of P3HT based solar cells [5]. The current versus voltage characteristics
under illumination of a 1:2 weight ratio P3HT:PCBM blend device are shown in Fig. 1(a) before and after annealing. The optimum annealing conditions for our samples were found to be 100 ∞C for 5 minutes. The short circuit current and the fill factor are improved due to annealing, while the open circuit voltage slightly decreases. Since a re-orientation of the P3HT material occurs upon annealing of the composite, shown by measurements discussed further, a reduction of the polym
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