A fullerene–single wall carbon nanotube complex for polymer bulk heterojunction photovoltaic cells Cheng Li, a Yuhong Chen, a Yubing Wang, a Zafar Iqbal, a Manish Chhowalla b and Somenath Mitra* a Received 19th December 2006, Accepted 20th February 2007 First published as an Advance Article on the web 12th March 2007 DOI: 10.1039/b618518e A novel immobilized fullerene–single wall carbon nanotube (C 60 –SWCNT) complex was synthesized via a microwave induced functionalization approach. It has been used as a component of the photoactive layer in a bulk heterojunction photovoltaic cell. As compared to a control device with only C 60 , the addition of SWCNTs resulted in an improvement of both the short circuit current density J SC and the fill factor (FF). This device takes advantage of the electron accepting feature of C 60 and the high electron transport capability of SWCNTs. The results indicate that C 60 decorated SWCNTs are promising additives for performance enhancement of polymer photovoltaic cells. Introduction Organic photovoltaics (OPVs) are a promising low cost alternative to silicon solar cells, thus a great deal of effort is being devoted, in both academic and industrial laboratories, to increase the power conversion efficiency and scale-up of the production processes. An attractive feature of the OPVs based on conjugated polymers is that they can be fabricated by a coating process (e.g., spin coating or inkjet printing) to cover large areas, and may be formed on flexible plastic substrates. This was made possible by the discovery of photoinduced electron transfer from the excited state of a conjugated polymer (as the donor) onto fullerene C 60 (as the acceptor). 1 Photovoltaic cells based on polymer/C 60 planar heterojunc- tions was first demonstrated in 1993. 2 Blending a conjugated polymer and C 60 (or its functionalized derivatives) resulted in higher charge separation and collection efficiencies due to the formation of bulk donor–acceptor (D–A) heterojunctions. 3,4 Much effort has gone into finding the best combination of D–A pairs, and the optimum fabrication process. Energy conversion efficiency of OPVs has been approaching 5% under one sun irradiation using conjugated polymer poly(3-hexyl- thiophene) (P3HT) as the electron donor and fullerene derivative (6,6)-phenyl-C 61 -butyric acid methyl ester (PCBM) as the electron acceptor. 5 To achieve high performance, usually 50 wt% or more PCBM is required in the blend to create large numbers of exciton dissociation sites and an extensive percolation network for electron transport. PCBM is effective in bulk heterojunc- tion solar cells because of its high solubility in organic solvents, such as toluene, and better electron mobility as compared to C 60 . On the other hand, C 60 is a stronger electron acceptor than PCBM, 6 and is more efficient in charge separation. In addition, PCBM is intrinsically more expensive than C 60 because it involves the derivatization of C 60 by complicated synthesis routes. This increases the overall cost of the photovoltaic devices. Carbon nanotubes (CNTs), especially single wall carbon nanotubes (SWCNTs) are excellent at electron transport. Applications of CNTs in OPVs have been of much interest. SWCNTs have been employed as electrodes, 7,8 and blended with conjugated polymers to form bulk heterojunctions in the active layers. 9,10 Kymakis et al. first reported a photovoltaic device based on the blend of SWCNTs and a conjugated polymer poly(3-octylthiophene) (P3OT). 9 Adding SWCNTs to the P3OT matrix improved the photocurrent by more than two orders of magnitude. The authors argued that the improve- ment was due to charge separation at polymer–nanotube junctions and efficient electron transport through nanotubes. The relatively low power conversion efficiency (0.04%) of the device under 100 mW cm 22 white illumination, however, suggested incomplete exciton dissociation at low nanotube concentration (,1.0 wt%). Doping a higher percentage of SWCNTs into the polymer matrix may cause short circuits since the lengths of carbon nanotubes are comparable to the total thickness of photovoltaic films. In a recent work, Pradhan et al. physically blended functionalized multi-walled carbon nanotubes (MWNTs) into a P3HT polymer to provide extra dissociation sites and assist in hole-transport in a P3HT- MWNT/C 60 double-layered device. 10 The power efficiency under 100 mW cm 22 white illumination was rather low (,0.01%). This may have been caused by poor exciton diffusion toward the donor–acceptor interface in the bilayer structure as well as inefficient electron transport across the C 60 layer. The major advantage of SWCNTs lies in their superior electron transport properties. On the other hand, the spherical C 60 molecules with a larger surface to volume ratio are more efficient in separating photogenerated carriers than nanotubes when distributed within the polymer matrix. However, it is not easy to disperse SWCNTs in a photoactive matrix. In previous studies, 9–11 SWCNTs were first purified and then blended with a polymer matrix. Such composites have been found to be metastable and uniform distribution in a polymer matrix is an a Department of Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102, USA. E-mail: Somenath.Mitra@njit.edu b Department of Materials Science and Engineering, Rutgers University, Piscataway, New Jersey 08854, USA PAPER www.rsc.org/materials | Journal of Materials Chemistry 2406 | J. Mater. Chem., 2007, 17, 2406–2411 This journal is ß The Royal Society of Chemistry 2007