Article pubs.acs.org/JPCC
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.
■
porting materials in photovoltaic devices.8,13 The size-dependent band gap of graphene5a,14,15 and large optical absorptivity16 are particularly interesting for its application as a photosensitizing 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 cells18 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
INTRODUCTION Graphene, a flat monolayer of carbon atoms in a twodimensional (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 works5 show that quantum confinement and edge effects may introduce a band gap in narrow graphene nanoribbons (GNRs) with widths of
