Solution-processable graphene linked to 3,5-dinitrobenzoyl as an electron acceptor in organic bulk heterojunction photovoltaic devices Minas M. Stylianakis a,b , George D. Spyropoulos a,c , Emmanuel Stratakis a,c,d , Emmanuel Kymakis a, * a Center of Materials Technology and Photonics & Electrical Engineering Department, Technological Educational Institute (TEI) of Crete, Heraklion, 71004 Crete, Greece b Department of Chemistry, University of Crete, Heraklion, 71003 Crete, Greece c Department of Materials Science and Technology, University of Crete, Heraklion, 71003 Crete, Greece d Institute of Electronic Structure and Laser (IESL), Foundation for Research and Technology-Hellas (FORTH), Heraklion, 71110 Crete, Greece ARTICLE INFO Article history: Received 28 March 2012 Accepted 1 August 2012 Available online 10 August 2012 ABSTRACT 3,5-Dinitrobenzoyl chloride was covalently linked to graphene oxide (GO) nanosheets pre- pared by a modified Hummers’ method, using ethylenediamine as a spacer. The linkage of the GO with the small molecule was confirmed by spectroscopic (e.g., Fourier transform infrared, Raman) and microscopic analyses. The resultant GO-ethylene-dinitro-benzoyl (GO-EDNB) consists of a controlled scale of different graphene structures and is highly dis- persable in common organic solvents. The GO-EDNB was used as the electron acceptor material in poly-(3-hexylthiophene) (P3HT) bulk heterojunction photovoltaic devices to sig- nificantly improve the performance, yielding a power conversion efficiency improvement of two orders and one order of magnitude compared with the pristine P3HT and the P3HT-GO devices respectively. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Though, the silicon (Si) solar cell technology dominates the photovoltaic world market with an over 85% share, the appli- cation of solar energy is limited mainly due to the high energy payback time of the Si cells. Organic photovoltaic cells (OPVs) are a promising alternative to Si, due to their mechanical flex- ibility, low cost and light weight. Also, their high material uti- lization through low cost printing technologies is an important plus towards commercialization. The most typical OPV structure is based on the bulk heterojunction (BHJ) con- cept, in which a polymeric electron donor and a fullerene- based electron acceptor are mixed in solution and cast into a thin film that is sandwiched between two electrodes [1]. The efficiency of the BHJ is restricted by the random network that is formed through the coating and drying of the photoac- tive solution due to phase segregation kinetics. This leads to the formation of dead ends and isolated domains that trap charge carriers and prevent them from being extracted [2]. Additionally, due to the low mobility of BHJ materials, there is rivalry between the dissociation and the recombination of the photogenerated carriers within the thin BHJ film [3]. The- oretically, the use of high aspect ratio 1D carbon allotropes, such as the carbon nanotubes should overcome the charge transport setback. But in practice, the presence of impurities and metallic nanotubes, significantly suppresses the charge mobility and favors the formation of recombination path- ways, limiting the cells performance [4,5]. Furthermore, the 0008-6223/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.carbon.2012.08.001 * Corresponding author: Tel.: +30 2810 379895; fax: +30 2810 379845. E-mail address: kymakis@staff.teicrete.gr (E. Kymakis). CARBON 50 (2012) 5554 – 5561 Available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/carbon