Page - 164 - in Photovoltaic Materials and Electronic Devices

recombinationof theelectronswiththeoxidizeddyemolecules, reducingefficiency and oxidized redox species. In order to enhance dye adsorption, the thickness of TiO2 should be increased. However, this recombination problem is aggravated in TiO2 nanocrystals by reason of a depletion layer on the TiO2 nanocrystallite surface, and its severity increases as the photoelectrode film thickness increases [5]. In response to this problem, the paper proposes a ZnO-based DSSC technology as a replacement for TiO2 in solar cells. Zinc oxide has received a great deal of attention as a photoanode in dye-sensitized solar cells (DSSCs) due to its large exciton-binding energy (60 meV) and large band gap (3.37 eV) [6]. Furthermore, its electronmobilityishigherthanthatofTiO2 bytwo-to-threeordersofmagnitude[7]. Therefore, ZnO is anticipated to demonstrate faster electron transport as well as decreased recombination damage compared to TiO2. Nevertheless, studies have reported that the entire efficiency of TiO2 DSSCs is higher than that of ZnO DSSCs. The efficiency of TiO2 thin-passivation shell layers is higher than the highest reported efficiency of ZnO DSSCs [8], in which the principal problem is the dye adsorption process in ZnO DSSCs. Because of the high carboxylic acid binding groupsinthedyes, thedissolutionofZnOandprecipitationofdye-Zn2+ complexes occurs. This phenomenon results in a poor overall electron injection efficiency of thedye[9]. Several approaches exist for enhancing the efficiency of ZnO DSSCs. One methodistointroduceasurfacepassivationlayertoamesoporousZnOframework; nevertheless, this may aggravate the dye adsorption problems. Alternatively, conventional particulate structures can be changed by replacing the internal surface area and morphology of the photoanode. Nevertheless, the surface area and diffusion length are incompatible. Augmenting the photoanode thickness empowered a higher number of dye molecules to be fixed; this, however, increases the possibility of electron recombination because of the extended distance through which electrons diffuse to the transparent conductive oxide (TCO) collector. This trapping process results in augmented scattering and slows down the electron transport which increases the recombination of the electrons with the oxidized redox species or the oxidized dye molecules, hence reducing efficiency. One probable strategy for ameliorating electron transport in DSSCs is to supersede the nanoparticle photoelectrode with a single-crystalline nanorod (or nanosheet, nanobelt, nanotip) photoelectrode. Electrons can be led through a direct electron path within a nanorod rather than by multiple-scattering transport between nanoparticles. Inresearch, theelectrontransport is tenstohundredsoftimesslower in nanoparticle DSSCs than in nanorod-based DSSCs [10–12]. Therefore, many works have been performed on the synthesis of TiO, and ZnO nanostructures for applications inDSSCs[13–15]. 164