Functionalized Rosette Nanotubes as Novel Electron Donor Materials for Solution-Processed Organic Photovoltaics
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Functionalized Rosette Nanotubes as Novel Electron Donor Materials for SolutionProcessed Organic Photovoltaics Liang Shuai,1,2 Venkatakrishnan Parthasarathy,1,2 Jae-Young Cho,2 Takeshi Yamazaki,1,2 Rachel L. Beingessner,2 Hicham Fenniri3* 1
Department of Chemistry, University of Alberta, 11227 Saskatchewan Drive, Edmonton, Alberta T6G 2G2, Canada 2National Institute for Nanotechnology, 11421 Saskatchewan Drive, Edmonton, Alberta T6G 2M9, Canada 3Department of Chemical Engineering, 313 Snell Engineering Center, Northeastern University, 360 Huntington Avenue, Boston, MA 02115-5000 ABSTRACT
Two self-assembling twin guanine-cytosine (G∧C) hybrid molecules featuring porphyrin (TPPO-(G∧C)2) and oligothiophene groups (6T-(G∧C)2) were synthesized. In organic solution, these molecules self-assemble into one-dimensional rosette nanotubes (RNTs) featuring the porphyrin or oligiothiophene groups on the outer surface. Using a combination of imaging and spectroscopic techniques we established the structure of the TPPO-(G∧C)2 and 6T-(G∧C)2 RNTs and compared the HOMO and LUMO energy levels with PC61BM, a well-known electron acceptor material. These studies, in combination with solid-state photoluminescence data of PC61BM-TPPO-(G∧C)2 RNT blended thin films, indicates that these self-assembled nanomaterials have great potential as electron donor materials for solution-processed organic photovoltaics. INTRODUCTION Organic photovoltaics (OPVs) have attracted a great deal of interest due to their low cost, light-weight, flexibility and tunability [1]. Solution-processed OPVs are also suitable for largearea device fabrication using roll-to-roll technologies [2]. Intensive studies on the performance of OPV devices using the polymer electron donor (D) material P3HT and the electron acceptor (A) material PC61BM, have revealed that the microscopic network structure of the D–A bulk heterojunctions (BHJ) improves the power conversion efficiency (PCE) [3]. The well-defined nanocrystalline morphology of P3HT also significantly enhances the hole mobility and promotes charge transfer and transport. Although thermal annealing is a useful method to impose microscopic ordering on the active layer of the blended D–A thin film, it is not preferred for the mass fabrication of devices. Supramolecular self-assembly alternatively, can provide a convenient pathway to generate highly ordered nanomaterials in all dimensions [4,5]. Although this strategy has been used to improve OPV device performance, only a few examples have been reported to-date [6]. Over the last decade, we have been developing a class of organic nanotubes which are formed through the self-assembly of a G∧C hybrid motif that displays self-complementary hydrogen bonding sites (Figure 1). The twin-G∧C motif (Figure 2) features two G∧C motifs that are covalently linked through an alkyl amine spacer. In solution, single and twin-G∧C molecules self-organize into hexameric super-macrocycles, called rosettes, through the formation of 18- or 36-intermolecular hydrogen bonds, respectively [7,8].
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