1
Metal Nanoparticle Decorated ZnO Nanostructure Based DyeāSensitized Solar Cells
Gregory Thien Soon How1, Kandasamy Jothivenkatachalam2, Alagarsamy Pandikumar3, and Nay Ming Huang4
1 Department of Physics, University of Malaya, Malaysia
2 Department of Chemistry, Anna UniversityāBIT Campus, Tiruchirappalliā620024, Tamilnadu, India
3 Functional Materials Division, CSIRāCentral Electrochemical Research Institute, Karaikudiā630006, India
4 Faculty of Engineering, University Xiamen Malaysia, Malaysia
1.1 Introduction
Solar energy has always been an ideal renewable energy source that is clean, abundant, inexpensive, and widely distributed regionally in the world [1ā3]. Understanding this, the emergence of dyeāsensitized solar cells (DSSCs) for converting solar energy to electricity has been very promising due to the ease of the manufacturing process, the low fabrication cost, the fact that it is nonpolluting, and the relatively high efficiency [1, 4ā6]. It is known that a typical DSSC consists of various subsections, includinng a nanocrystalline semiconductor oxide photoanode, dye sensitizer, redox couple electrolyte, and counterelectrode [3, 4]. The main idea behind the operating principle of DSSCs is based on the optical excitation of a dye that results in the injection of an electron into the conduction band of a wide band gap semiconductor oxide. The oxidized dye molecule is regenerated afterwards when it is reduced to its ground state by gaining one electron from a redox couple that is found in the electrolyte around the sensitized semiconductor oxide nanostructured film [3ā5]. Since the first outstanding research work on DSSC was demonstrated by OāRegan and Gratzel in 1991 [5], each of its components has been extensively investigated and optimized, with the aim to maximize the power conversion efficiency (PCE) of DSSCs [4, 7, 8]. Recently, a PCE of 12.3% has been achieved by using the cosensitization of two dyes and a Co(II/III) tris(bipyridyl)ābased redox electrolyte [9]. Hence, study to find a suitable and high performance DSSC output has greatly increased over the years.
Amongst all the materials studied for use in DSSCs, nanocrystalline TiO2 has been most commonly employed as the metal oxide semiconductor material in high efficiency DSSCs [4ā6]. Several methods were used for the preparation of the TiO2 nanoparticles in DSSCs, such as solāgel [10, 11], gasāphase pyrolysis [12], or the commonly used hydrothermal synthesis method [13, 14]. However, hydrothermal methods are not ideal because both synthesis and purification processes take a prolonged time to achieve wellāformed and highly crystalline TiO2 particles [12]. To minimize the costs of metal oxide semiconductor materials for DSSCs, simple preparation methods are essential to control the formation of crystal structure, crystallization, and particle size [15]. Besides TiO2, there are reports of other alternative metal oxides, such as SnO2, Nb2O5, and ZnO, being used as porous semiconductor materials for DSSC photoelectrodes [16ā20].
ZnO is an another attractive and alternative photoanode to replace TiO2 as an electron conductor owing to its higher bulk electron mobility and easily tunable morphology, which allows the rational design and development of hierarchical ZnO nanostructures able to simultaneously optimize charge carrier path and dye loading [19, 20]. Hence, ZnO is considered an excellent backbone to produce highāefficiency DSSCs. The ZnO characteristic of higher electron mobility (ā¼205ā1000 cm2 Vā1 sā1) than TiO2 (ā¼0.1 ā 4 cm2 Vā1 sā1), enables the rapid diffusion transport of photoinjected electrons when it is employed as a photoanode material in DSSCs. In addition, ZnO is a suitable material for the fabrication of mesoporous photoanodes in DSSCs; it has a band gap of 3.2 eV and a conduction band edge position of ā4.3 eV, both of which are similar to TiO2 [15ā17]. Moreover, ZnO can be easily prepared into tunable nanostructures, such as nanoparticles, nanowires, nanotubes, nanorods, nanosheets, and tetrapods, providing numerous alternatives for op...
