DOI: 10.1002/adfm.200700438 Elaboration of P3HT/CNT/PCBM Composites for Organic Photovoltaic Cells** By Solenn Berson, Rémi de Bettignies , Séverine Bailly , Stéphane Guillerez, and Bruno Jousselme* 1. Introduction Bulk-heterojunction photovoltaic devices based on blends of conjugated polymers such as poly(3-hexylthiophene) (P3HT) and the fullerene derivative 1-(3-methoxycarbonyl) propyl-1- phenyl [6,6]C 61 (PCBM) have achieved efficiencies of almost 4–5 % under AM1.5 illumination. [1] This technology compared with silicium or Grätzel devices offers many advantages. Indeed, solution-based fabrication techniques used for this kind of device lend themselves to very low cost, high-throughput manufacturing techniques, such as simply coating or printing (roll-to-roll techniques). Ultra thin, flexible, and low-weight devices are additional attractive fea- tures that may be integrated into large-area devices that should permit the development of products for new markets. The power conversion efficiency of these bulk-heterojunc- tion polymer photovoltaic cells is directly correlated to the open-circuit voltage (V oc ), the short-circuit current (I SC ), and the fill factor (FF) delivered by these devices under illumina- tion. In order to enhance the photovoltaic performances, the increase of these crucial factors is necessary. It has recently been demonstrated that the V oc of bulk-heterojunction devices are correlated to the HOMO level of the donor. [2] In order to increase the V oc , there is an important need to identify and synthesize new polymers with a high oxidation potential. On the other hand, the current density is directly correlated to the mobility of the charges in the semi-conducting material, which avoids charge recombination at the interface between the two materials and allows good charge collection at the elec- trodes. However, in bulk-heterojunction systems, the intimate blending of the electron and hole transporting materials leads to a detrimental charge carrier recombination in these devices. This obstacle was overcome by thermal treatment to obtain a nanostructured donor–acceptor morphology that increases the current density. [3] Similarly, the formation of nanostructured materials could be achieved by the elaboration of fibrillar structures of P3HT in solution followed by spin-coating deposi- tion. [4] This also leads to an increase of the mobility and to bet- ter cell performances. With an aim to improve the performance of the photovoltaic cells, carbon nanotubes (CNTs) have recently been incorpo- rated into the devices. CNTs present remarkable electronic, mechanical, and chemical properties that open a way to future applications in material molecular electronics, energy storage, sensors, new composites, etc. Adv. Funct. Mater. 2007, 17, 3363–3370 © 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 3363 – [*] Dr. B. Jousselme, S. Berson CEA-Grenoble DRFMC/SPrAM/LEMOH Structures et Propriétés d’Architectures Moléculaires, UMR5819 17 rue des martyrs 38054 Grenoble, cedex 09 (France) E-mail: bruno.jousselme@cea.fr Dr. B. Jousselme CEA-Grenoble LETI/DIHS/LIMN Laboratoire d’Intégration Mémoires et Nanodispositifs 17 rue des martyrs 38054 Grenoble cedex 09 (France) R. de Bettignies, S. Bailly, S. Guillerez CEA-Grenoble DRT/LITEN/DTS/LCS, INES-RDI Laboratoire des Composants Solaires 50 avenue du Lac Léman BP 332 73377 Le Bourget du Lac (France) [**] We would like to thank the referees for useful comments that were in- corporated in the manuscript. We are indebted to the French Ministry of Research for financial support, through the Micro and Nanotech- nology Program for the post-doctoral grant ‘RTB: Post CMOS molé- culaire 200 mm’ assigned to B.J. We also thank Arkema for generous gifts of MWNT and P. Rannou for his assistance in size exclusion chromatography characterizations. Supporting Information is avail- able online at Wiley InterScience or from the author. This Full Paper focuses on the preparation of single-walled or multi-walled carbon nanotube solutions with regioregular poly(3-hexylthiophene) (P3HT) and a fullerene derivative 1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C 61 (PCBM) using a high dissolution and concentration method to exactly control the ratio of carbon nanotubes (CNTs) to the P3HT/PCBM mix- ture and disperse the CNTs homogeneously throughout the matrix. The CNT/P3HT/PCBM composites are deposed using a spin-coating technique and characterized by absorption and fluorescence spectroscopy and by atomic force microscopy to un- derline the structure and the charge transfer between the CNTs and P3HT. The performance of photovoltaic devices obtained using these composites as a photoactive layer mainly show an increase of the short circuit current and a slight decrease of the open circuit voltage which generally leads to an improvement of the solar cell performances to an optimum CNT percentage. The best results are obtained with a P3HT/PCBM (1 : 1) mixture with 0.1 wt % multi-walled carbon nanotubes with an open cir- cuit voltage (V oc ) of 0.57 V, a current density at the short-circuit (I sc ) of 9.3 mA cm –2 and a fill factor of 38.4 %, which leads to a power conversion efficiency of 2.0 % (irradiance of 100 mW cm –2 spectroscopically distributed following AM1.5). FULL PAPER