Donor and Acceptor Behavior in a Polyfluorene for Photovoltaics Henry M. P. Wong, ² Peng Wang,* ,², | Agnese Abrusci, ² Mattias Svensson, ‡ Mats R. Andersson, ‡ and Neil C. Greenham* ,² CaVendish Laboratory, J. J. Thomson AVenue, CB3 0HE Cambridge, United Kingdom, and Materials and Surface Chemistry, Chalmers UniVersity of Technology, S-412 96 Go ¨teborg, Sweden ReceiVed: December 12, 2006; In Final Form: January 29, 2007 We investigate photovoltaic devices based on a red-absorbing conjugated polymer poly(2,7-(9,9-dioctyl- fluorene)-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) (APFO-3). We show that the polymer acts as an electron donor when blended with ZnO nanoparticles, giving a short-circuit quantum efficiency of 28%. When blended with poly(3-hexylthiophene) (P3HT), however, the APFO-3 acts as an electron acceptor, giving a short-circuit quantum efficiency of 12%. We study this charge-transfer process by comparing photoinduced absorption spectra of the hybrid blends with the absorption spectra of chemically doped APFO-3, which allows us to distinguish features due to positive and negative polarons. We also present dark current measurements of single-carrier devices which demonstrate that APFO-3 has similar mobilities for electrons and holes, consistent with ambipolar behavior in photovoltaic devices. Introduction The operation of efficient polymer photovoltaic devices requires that photogenerated excitons are dissociated into electrons and holes at the heterojunction between two polymers, followed by efficient transport of the electrons and holes to the electrodes on their respective polymers. There are many examples of electron-donating, hole-transporting polymers such as poly(3-hexylthiophene) (P3HT) 1-4 and poly(2-methoxy-5- (3′ ,7′ -dimethyloctyloxy)-p-phenylene vinylene) (OC 1 C 10 -PPV). 5-9 Electron-accepting, electron-transporting polymers are less common, with well-known examples including poly(9,9′- dioctylfluorene-co-benzothiadiazole) (F8BT) and the cyano- substituted poly(p-phenylenevinylene) derivative CN-PPV. 10 Recent measurements of ambipolar transport in polymer field- effect transistors demonstrate that many polymers previously thought to support only hole transport in fact have comparable carrier mobilities for electrons and holes, in the absence of strong trapping effects. 10-13 It is therefore apparent that the main challenge for photovoltaic devices is to find pairs of materials with the correct offset in electron affinity and ionization potential to dissociate excitons at the heterojunction between them. Typically, an offset of 0.4 eV in both electron affinity and ionization potential is required. Here, we investigate the photovoltaic properties of the red polyfluorene copolymer poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5- (4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)) (APFO-3). Its chemi- cal structure and energy levels are presented in Figure 1a, d. The lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) levels for APFO-3 are 3.5 and 5.8 eV, respectively. 14 The relatively high electron affinity and modest ionization potential (a consequence of the low band gap) suggest that this polymer may be able to act as an electron acceptor or electron donor, depending on the material with which it is combined. Indeed, it has previously been shown that APFO-3 is a good electron donor in devices using fullerenes 15 or CdSe nanocrystals 16 as the electron acceptor, and a similar polymer to APFO-3 has been shown to act as electron acceptor in blends with P3HT. 17 In this paper, we demonstrate electron-donating behavior of APFO-3 using a ZnO nanocrystals as a new electron acceptor, 4,9 and we show that APFO-3 can act as an electron acceptor in combination with P3HT (Figure 1b). We confirm charge transfer by photoluminescence (PL) quenching measurements, and we spectroscopically investigate the properties of positive and negative polarons formed on APFO-3 after charge transfer, in comparison with absorption spectra of chemically doped APFO-3. Dark current measure- ments in single-carrier diodes are used to estimate the electron and hole mobilities of APFO-3, and the combination of efficient charge transfer, charge transport, and charge extraction is demonstrated by quantum efficiency measurements on solar cells using APFO-3 as a donor or acceptor material. Experimental Methods APFO-3 was synthesized according to a procedure reported previously. 15 The molecular weights were M n ) 4900 and M w ) 11800 relative to polystyrene standards. P3HT with molecular weight of M n ) 26000 was supplied by Merck. ZnO nano- crystals were synthesized using a published procedure. 18 ZnO nanocrystals used in this paper had a spherical shape with a diameter of approximately 5 nm, determined by transmission electron microscopy (Figure 1c). No additional surfactants or ligands are needed to disperse the nanocrystals. The nanocrystals as synthesized could form a stable solution in chlorobenzene with a concentration of 40 mg/mL. For the APFO-3/ZnO system, a 40 mg/mL solution of ZnO in chlorobenzene was mixed with a 10 mg/mL APFO-3 solution in chloroform to make up a ratio of 1:2 (APFO-3/ZnO) by weight. The resulting blend solution required 6.25% of additional methanol to avoid precipitation of ZnO nanocrystals. For the APFO-3/P3HT system, we used * Corresponding authors. E-mail: peng.wang@ciac.jl.cn; ncg11@ cam.ac.uk. ² Cavendish Laboratory. ‡ Chalmers University of Technology. | Present address: State Key Laboratory for Polymer Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, P.R. China. 5244 J. Phys. Chem. C 2007, 111, 5244-5249 10.1021/jp068536f CCC: $37.00 © 2007 American Chemical Society Published on Web 03/14/2007