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Quantum dot sensitized solar cells (QDSSCs) have attracted extensive interest as a means of fabricating highly efficient, low cost photovoltaics. QDs such as CdS, CdSe, and CdTe demonstrate size-dependent band gaps which provide wide-ranging opportunities for harvesting light energy in the visible and infrared regions. This is because these QDs have a large extinction coefficient and upon light absorption, excited electrons can be efficiently transferred to the conduction band of TiO2 5 or ZnO. In addition, due to the impact ionization effect, it is possible to utilize hot electrons in the QDs to generate multiple electron–hole pairs per photon. Thus, the power conversion efficiency of QDSSCs is expected to exceed the Shockley–Queisser limit (31%). Despite these features, the efficiencies of QDSSCs based on bare TiO2 or ZnO film are rather low. The limiting factor is often attributed to the accumulation of electrons in the semiconductor layer due to the relatively slow electron transfer, resulting in the carrier recombination at the semiconductor surface. Thus, suppressing carrier recombination at the semiconductor interface is the key to improving the performance of QDSSCs. Recently, the discovery of graphene promises a new era of electronics. Graphene has been used in solar cells due to its unique properties. Apart from the transparent electrode, graphene can also be used as the electron acceptor layer or the hole transport layer in photovolatic devices. In this communication, we first propose and demonstrate the use of a graphene-ZnO composite architecture to enhance the electron transport in QDSSCs. The device in our proposed architecture has been enhanced to almost twice the efficiency of a device without a graphene layer. A fill factor as high as 62% has been obtained, which is one of highest values based on the ZnO nanorod system up to date for QDSSCs. The fabrication procedure of the graphene-ZnO nanorod is described briefly as follows. Graphene oxide was produced by acid oxidation of natural graphite based on the modified Hummers method. GO aqueous suspension with 0.05 mg mL 1 was spin-coated on APTES-modified FTO to obtain monoto few-layer GO thin films. GO was reduced to graphene thin film on FTO/glass in hydrazine vapor at 65 1C overnight. The film thickness was controlled by repeating the spin-coating process multiple times. A ZnO seed layer was deposited on the graphene sheets by ultrasonic spray pyrolysis at 350 1C for 5 min. The substrate was then immersed into a 0.01 M zinc nitrate and 0.01 M hexamethylenetetramine solution and heated at 95 1C for 10 h. After that, the substrate was taken out, washed and dried for sensitizing with CdSe QDs. Oleic acid-capped CdSe QDs were synthesized by a one-pot growth method. The CdSe QDs were loaded on to the ZnO nanorod surface by electrophoretic deposition (ESI).w QDSSCs based on ZnO/CdSe and graphene-ZnO/CdSe photoanodes are labeled as cell A and B, respectively. Fig. 1(a) shows the absorption spectra of graphene, grapheneZnO nanorods and graphene-ZnO nanorod/CdSe QD photoanodes. While graphene shows no clear absorption features, ZnO and CdSe exhibit characteristic band edge absorption. It can be seen that the features at 465 and 575 nm appear, indicating the CdSe QDs are adsorbed onto the graphene-ZnO nanorod films. From the excitonic transition peak at 575 nm for graphene-ZnO nanorod/CdSe QD photoanode, the size of these QDs is estimated to be 3.5 nm, based on the analysis reported in the literature. a School of Electronic Science and Engineering, Southeast University, Nanjing 210096, China. E-mail: [email protected]; Fax: +86-25-83792662; Tel: +86-25-83792650 b State Key Lab of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023. E-mail: [email protected]; Tel: +86-411 84379571 c Electrical Engineering Division, Engineering Department, University of Cambridge, 9 JJ Thomson Avenue, Cambridge, CB3 0FA, UK. E-mail: [email protected] Department of Materials, Imperial College London, Exhibition Road, London, SW7 2AZ, UK Department of Materials Science and Engineering Rutgers University, Piscataway, NJ 08854, USA Department of Information Display, Kyung Hee University, Seoul 130-701, Republic of Korea w Electronic supplementary information (ESI) available: Schematic of the fabrication graphene-ZnO nanorod photoanode, FESEM images of ZnO nanorod grown on the different thicknesses of graphene layers, XRD spectra, AFM height images of graphene films with different thicknesses, TEM image of graphene, EDX analysis of ZnO/CdSe, EIS measurement, QDSSCs performance results and photoluminescence decay analysis. See DOI: 10.1039/c1cc10162e ChemComm Dynamic Article Links