Effect of the Incorporation of a Low-Band-Gap Small Molecule in a Conjugated Vinylene Copolymer: PCBM Blend for Organic Photovoltaic Devices P. Suresh, † P. Balraju, † G. D. Sharma,* ,† John A. Mikroyannidis,* ,‡ and Minas M. Stylianakis ‡ Molecular Electronic and Optoelectronic Device Laboratory, Physics Department, JNV University, Jodhpur 342005, India, and Chemical Technology Laboratory, Department of Chemistry, University of Patras, GR-26500 Patras, Greece ABSTRACT The effect of the incorporation of a low-band-gap small-molecule BTD-TNP on the photovoltaic properties of vinylene copolymer P:PCBM bulk heterojunction solar cells has been investigated. The introduction of this small molecule increases both the short-circuit photocurrent and the overall power conversion efficiency of the photovoltaic device. The incident photon-to-current efficiency (IPCE) of the device based on P:PCBM:BTD-TNP shows two distinct bands, which correspond to the absorption bands of P:PCBM and BTD-TNP. Furthermore, it was found that the IPCE of the device has also been enhanced even at the wavelengths corresponding to the absorption band of P:PCBM, when the thermally annealed blend was used in the device. This indicates that the excitons that are generated in copolymer P are dissociated into charge carriers more effectively in the presence of the BTD-TNP small molecule at the copolymer P:PCBM interface by energy transfer from P to the small molecule. Therefore, we conclude that the BTD- TNP small molecule acts as light-harvesting photosensitizer and also provides a path for the generated exciton in copolymer P toward the P:PCBM interface for efficient charge separation. The overall power conversion efficiency for the P:PCBM:BTD-TNP photovoltaic device is about 1.27%, which has been further enhanced up to 2.6%, when a thermally annealed blend layer is used. KEYWORDS: low-band-gap small molecule • vinylene copolymer • photovoltaic device • bulk heterojunctions INTRODUCTION S olar cells based on organic semiconducting materials are of tremendous interest (1-10) because of their attractive properties such as flexibility, ease of fabri- cation, and low cost of materials. The most common ap- proach for the fabrication of an efficient conjugated polymer photovoltaic device is to prepare a blend of a conjugated polymer (donor) and a fullerene derivative, such as [6,6]phen- yl-C 61 -butyric acid methyl ester (PCBM) (acceptor) to produce a bulk heterojunction. At present, the power conversion efficiency of photovoltaic devices employing bulk hetero- junction active layers is in the range of 5-5.5%, and the highest efficiency of 6.5% has been reported for tandem solar cells (11-18). Photovoltaic devices with optimized performance have been developed using a poly(3-hexylth- iophene) (P3HT):PCBM blend and show an ideal incident photon-to-current efficiency (IPCE) in only the mid-wave- length region of visible light. However, the barrier that must be overcome to produce efficient organic photovoltaic de- vices is to extend the light absorption by the photoactive layer to the longer-wavelength and near-infrared regions, where the maximum photon flux lies, without a reduction in the open-circuit voltage. The overall power conversion efficiency of the organic device depends upon the following factors: the light-harvesting efficiency of the materials in the visible and infrared regions, exciton diffusion to the donor- acceptor interface, photoinduced charge separation, and mobility of the charge carriers produced by photoinduced charge separation (19, 20). In recent years, significant progress has been made in the synthesis and processing of low-band-gap polymers (21-28), which are capable of absorbing a broad range of solar photons. However, only a few of them have achieved power conversion efficiencies comparable with those of devices fabricated from P3HT (29, 30). A possible route to overcome the low photovoltaic performance observed in a polymer solar cell might be to incorporate a low-band-gap small- molecule donor in the blend with energy levels intermediate to those of polymer and fullerene derivatives (31, 32). Small molecules generally have high mobility and are easily puri- fied and more prone to long-range order than conjugated polymers (33-35). During selection of the small molecule, it is crucial that the energy levels of the small molecule lie between those of the conjugated polymer and PCBM, in order to avoid the charge trapping in the small molecule. For this purpose, we have used the BTD-TNP small molecule, whose energy levels lie between the copolymer P and PCBM. This molecule * Corresponding authors. Tel: 91-0291-2720857 (G.D.S.), +30 2610 997115 (J.A.M.). Fax: 91-0291-2720856 (G.D.S.), +30 2610 997118 (J.A.M.). E-mail: sharmagd_in@yahoo.com (G.D.S.), mikroyan@chemistry.upatras.gr (J.A.M.). Received for review April 8, 2009 and accepted June 29, 2009 † JNV University. ‡ University of Patras. DOI: 10.1021/am900244y © 2009 American Chemical Society LETTER 1370 VOL. 1 • NO. 7 • 1370–1374 • 2009 www.acsami.org Published on Web 07/06/2009