Here comes the sun: A conversion efficiency as high as 5.4 % has been achieved on dye‐sensitized ZnO solar cells with photoelectrode films consisting of polydisperse aggregates, compared to 2.4 % for the films with only nanosized crystallites. The aggregation of nanocrystallites with a broad size distribution is effective in enhancing the light‐harvesting efficiency by inducing light scattering within the photoelectrode films.

The interest in dye-sensitized solar cells has increased due to reduced energy sources and higher energy production costs. For the most part, titania (TiO 2 ) has been the material of choice for dye-sensitized solar cells and so far have shown to exhibit the highest overall light conversion efficiency ∼ 11 %.[1] However, zinc oxide (ZnO) has recently been explored as an alternative material in dye-sensitized solar cells with great potential.[2] The main reasons for this increase in research surrounding ZnO material include: 1) ZnO having a bandgap similar to that for TiO 2 at 3.2 eV, [3] and 2) ZnO having a much higher electron mobility ∼ 115-155 cm 2 V -1 s -1 [4] than that for anatase titania (TiO 2 ), which is reported to be ∼ 10 -5 cm 2 V -1 s -1. [5] In addition, ZnO has a few advantages as the semiconductor electrode when compared to TiO 2 , including 1) simpler tailoring of the nanostructure as compared to TiO 2 , and 2) easier modification of the surface structure. These advantages [6] are thought to provide a promising means for improving the solar cell performance of the working electrode in dye-sensitized solar cells. It was reported [7] that the surface structure, the particle size and shape, and the porosity are all important factors for optimizing the solar cell performance of dye-sensitized solar cells. With ZnO, these factors can easily be tailored through the modification of solution growth and wet-chemical methods to fabricate various nanostructures. In addition, the surface structure and crystallinity of ZnO for dye-sensitized solar cells can be easily modified through the use of aqueous solution methods to increase the surface area.[8] For example, aligned ZnO nanowires can be prepared by electrochemical deposition, VLS or nucleation growth, or thermal evaporation; whereas, the growth of TiO 2 nanowires are less likely to occur and much more difficult to obtain using such solution growth methods.So far, the highest overall light conversion efficiency obtained for ZnO nanoparticle film has been ∼ 5 % [9] by utilizing additional compression methods for better particle packing. With typical nanoparticle film processing techniques, the highest overall light conversion efficiency obtained for ZnO has been ∼ 1.5 %.[10]

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ZnO films consisting of either polydisperse or monodisperse aggregates of nanocrystallites were fabricated and studied as dye‐sensitized solar‐cell electrodes. The results revealed that the overall energy‐conversion efficiency of the cells could be significantly affected by either the average size or the size distribution of the ZnO aggregates. The highest overall energy‐conversion efficiency of ∼4.4% was achieved with the film formed by polydisperse ZnO aggregates with a broad size distribution from 120 to 360 nm in diameter. Light scattering by the submicrometer‐sized ZnO aggregates was employed to explain the improved solar‐cell performance through extending the distance travelled by light so as to increase the light‐harvesting efficiency of photoelectrode film. The broad distribution of aggregate size provides the ZnO films with both better packing and an enhanced ability to scatter the incident light, and thus promotes the solar‐cell performance.

In this paper, we report the significant effects of dye loading conditions on the overall light conversion efficiency of zinc oxide (ZnO) film electrodes in dye-sensitized solar cells. A comparison of the ZnO film electrodes was also made with TiO 2 film electrodes prepared with similar dye loading conditions. It was found that using a higher and lower dye concentration requires a shorter and longer immersion time, respectively, for optimal sensitization of ZnO to obtain maximum efficiencies. A similar trend was found for the TiO 2 film electrode as well; however, smaller differences in the overall light conversion efficiencies were observed with varying dye concentration and immersion time. It was found that the chemical stability was an issue for the ZnO film electrodes but was not pertinent for the TiO 2 film electrodes. The film quality and structure of the ZnO film differed after prolonged immersion in high dye concentration, where the formation of N3 dye and Zn 2+ aggregates and/or the deterioration of the ZnO colloidal spheres and nanoparticles on the surface may have occurred. On the other hand, the film quality and structure of the TiO 2 film was not appreciably affected by prolonged immersion in high dye concentration, where the nanoparticle structure was not affected.