ZnO/Graphene Quantum Dot Solid-State Solar Cell Mrinal Dutta, †,‡ Sanjit Sarkar, † Tushar Ghosh, and Durga Basak* Department of Solid State Physics, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032, India * S Supporting Information ABSTRACT: Graphene quantum dots (GQDs) synthesized by a direct chemical method have been used in combination with ZnO nanowires (NWs) to demonstrate their potential as a solar harvesting material in photovoltaic cells exhibiting an open circuit voltage of 0.8 V. The excited state interaction between the photoexcited GQDs and the ZnO NWs has been verified from the charge-transfer process by both emission spectroscopy and photovoltaic measurements. This work has implications for less expensive and efficient next generation solid-state solar cells. ■ INTRODUCTION Graphene, a flat monolayer of carbon atoms in a two- dimensional (2D) honeycomb lattice, has become the pin-up among all of the carbon materials since its discovery by Novoselov and his group in 2004. 1 It is now considered a wonder kit among all of the promising building blocks for future nanodevices because of the superior electronic, thermal, and mechanical properties as well as chemical stability. 1a,2 Currently available micrometer-sized graphene sheets (GSs) produced by reduction of exfoliated graphene oxide (GO), micromechanical cleavage, solvothermal synthesis, and other physical and chemical routes generally are highly conducting, 3 which makes them suitable for flexible conductors in electronic circuits. GSs are also used in solar cells as electrodes for efficient charge transfer. 4 However, because these GSs do not have an energy band gap, their direct application in optoelectronics is limited. Theoretical and experimental works 5 show that quantum confinement and edge effects may introduce a band gap in narrow graphene nanoribbons (GNRs) with widths of <∼10 nm and smooth edges. On the other hand, quantum confinement and edge effects are found to be more pronounced in graphene quantum dots (GQDs) from which many new fascinating phenomena are expected. 6 The synthesis of GQDs has been done successfully by different methods. Pan and co-workers 7 presented a hydrothermal method for cutting preoxidized GSs into GQDs (approximately 10 nm in size), while Shen et al. 6b have prepared GQDs by hydrazine hydrate reduction of GO by a surface-passivation technique with polyethylene glycol (PEG). An electrochemical route has been shown to be successful for the synthesis of GQDs by Li and co- workers. 8 GQDs have also been synthesized from small molecule precursors such as 3-iodo-4-bromoaniline and other substituted benzene derivatives by solution chemistry in a bottom up approach. 9 However, depending on the synthesis, the physical properties of GQDs widely vary; thus, their applications in opto-electronic devices have become a great challenge. The major area in which GQDs have shown promise is luminescent materials for bioimaging, 10 single electron transistors, 11 light-harvesting assemblies, 12 and electron-trans- porting materials in photovoltaic devices. 8,13 The size-depend- ent band gap of graphene 5a,14,15 and large optical absorptivity 16 are particularly interesting for its application as a photosensitiz- ing material in photovoltaic devices. Theoretical results show that the broad range of the whole solar spectrum can be covered only by tuning the band gap of graphene achieved by varying their sizes. 5a,14 Thus, exploring GQDs to work as a light-harvesting material becomes very important and timely in the present scenario of energy crisis, especially when solid sensitized solar cell devices are found to be more reliable and sustained for a long time with respect to the dye-based solar cells. Despite the recent progress in quantum dot-sensitized photovoltaic devices, 17 the use of mostly higher cost, toxic, and hazardous acceptor materials (CdSe and PbTe) becomes a serious impediment for large-scale applications. On the other hand, ZnO has been widely used in organic as well as hybrid solar cells 18 due to its salient characteristics such as low cost, easy synthesis of 1D nanostructures, nontoxicity, high stability, and good optoelectronic properties. 19,20 The applications of ZnO in solar cells also include its use as electrode buffer layers or transparent electrodes. 21 Graphene-based materials have shown promising applications for energy materials as well as energy storage. 22 The GQDs have a great potential to be used as a sensitizer for the solar cells as demonstrated by Yan et al. 12 for the first time. Therefore, because they are nontoxic, biocompatible, and cheaper, these QDs might be the right choice as a replacement for benign sensitizing materials for ZnO in solar cells. In this paper, we thus report a combination GQDs with ZnO NWs that can be efficiently harnessed in solar cells. Most notably, use of this combination in the solar cell is unique and novel. The emission spectroscopy shows that a charge-transfer process takes place at the interface between GQDs and NWs. This charge separation at the interface has further been confirmed from the photovoltaic performance of the cells made up by ZnO NWs-GQD-based composite. The Received: March 29, 2012 Revised: August 23, 2012 Published: September 4, 2012 Article pubs.acs.org/JPCC © 2012 American Chemical Society 20127 dx.doi.org/10.1021/jp302992k | J. Phys. Chem. C 2012, 116, 20127-20131