Effects of graphene counter electrode and CdSe quantum dots in TiO 2 and ZnO on dye-sensitized solar cell performance A. Kathalingam 1 , Jin-Koo Rhee 1, * ,† and Sung-Hwan Han 2 1 Millimeter-wave Innovation Technology Research Center (MINT), Dongguk University, Seoul 100-715, Korea 2 Inorganic Nano-Materials Laboratory, Department of Chemistry, Hanyang University, Seoul, Korea SUMMARY Fabrication and performance study of dye-sensitized solar cells using different counter electrodes and photoanodes is reported. Spin coated, E-beam coated platinum, and graphene electrodes were used as counter electrodes. Different combinations of TiO 2 nanoparticle and ZnO nanorods (NRs) with CdSe quantum dots were prepared and used as photoanodes. The photoanodes comprising of both ZnO NRs and TiO 2 nanoparticles have shown improved performances in short-circuit current density and open-circuit voltage comparing the devices fabricated using only ZnO NR or TiO 2 nanoparticles. The inclusion of CdSe quantum dots has been found to increase the performance of dye-sensitized solar cell for all the photoanodes. In case of counter electrodes, the cells fabricated with graphene showed improved performance. Copyright © 2014 John Wiley & Sons, Ltd. KEY WORDS dye-sensitized solar cell; CdSe quantum dot; TiO 2 nanoparticle; ZnO nanorod; graphene counter electrode Correspondence *Jin-Koo Rhee, Millimeter-wave Innovation Technology Research Center (MINT), Dongguk University, Seoul, 100-715, Korea. † E-mail: jkrhee@dongguk.edu Received 4 June 2013; Revised 13 December 2013; Accepted 18 January 2014 1. INTRODUCTION As the global demand of energy is increasing, the con- sumption of fossil fuels is also increasing day by day. It will reduce the stock of fossil fuels, and also it concerns more on environmental degradation. Thus, it urges to find new alternative ways to develop efficient, cost-effective, and nonpolluting devices to harvest solar light and fulfill the future energy demands. Although the conventional silicon solar cell modules have efficiency about 20%, they are more expensive as they require ultra-pure material and high-temperature processing [1]. In this respect, dye- sensitized solar cells (DSSCs) are emerging as a viable alternative to the conventional solar cells due to their low cost and simple fabrication method [1–3]. The schematic representation of the DSSC is shown in Figure 1. A typical DSSC basically consists of four components: (i) a wide band gap (TiO 2 or ZnO) semiconductor thin film electrode (photoanode); (ii) a sensitizer (dye) adsorbed onto the surface of the semiconductor thin film; (iii) electrolyte containing a redox couple, iodide/triiodide (I  /I 3  ); and (iv) a counter electrode (CE) with a thin layer of catalytic materials such as platinum (Pt) [4,5]. When the dye- adsorbed semiconducting nanoparticle is illuminated, the dye absorbs the light and goes to excited state; subse- quently, it undergoes a charge separation and injects an electron to the conduction band of semiconductor. The released electrons are injected into the porous oxide film and conducted through the load as shown in Figure 2. Simultaneously, the oxidized dye is reduced back to its original state by a redox mediator in the electrolyte and returns to the ground state [6,7]. Although the efficiency of DSSC (11%) is lesser than silicon solar cells, it has lot of scope to increase the efficiency by suitably modifying the photoanode, dye, and CEs. For the preparation of photoanode, nanocrystalline TiO 2 is widely used because of its wide band gap, low cost, easy availability, and nontoxicity [8,9]. Many groups have attempted to improve the efficiency focusing on various aspects of photoanodes, dyes, and CEs. Zinc oxide (ZnO) has also been attracted as an alternative to TiO 2 because of its band gap and electron affinity similar to TiO 2 , higher electron mobility, the availability of low temperature syn- thesis route, and the potential for controlling the morphol- ogy through simple processing from solution. It also has an additional advantage over TiO 2 that is the ZnO can be produced in wide varieties of morphologies with a large surface area, which is an essential factor in maximizing INTERNATIONAL JOURNAL OF ENERGY RESEARCH Int. J. Energy Res. 2014; 38:674–682 Published online 4 February 2014 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.3179 Copyright © 2014 John Wiley & Sons, Ltd. 674