Zinc oxide based dye-sensitized solar cells: A review

Zinc oxide (ZnO) is the closest alternative to TiO2 as the semiconductor material in a dye-sensitized solar cell (DSSC). This is to be attributed to the facts that both TiO2 and ZnO have same electron affinities and almost the same band gap energies, and ZnO has much higher electron diffusivity than TiO2, a high electron mobility, a large excitation binding energy, is available at low-cost, and stable against photo-corrosion. The article provides a broad survey of ZnO based DSSCs, and highlights the potential of utilizing a ZnO photoanode in the place of a TiO2 photoanode in a DSSC. The merits of a ZnO photoanode, over against those of a TiO2 photoanode, are discussed in detail, associated main problems are mentioned, and their solutions are suggested. Parameters to improve the performance of a DSSC are revealed and solutions to optimize them are suggested. Discussions are made on ZnO based flexible, quasi-solid state, and solid state DSSCs. The advantages and disadvantages of ZnO as semiconductor material in DSSCs are weighed. All architectures reported till date are cited, and the techniques used to achieve such hierarchical structures are mentioned. A thorough discussion is made on the dyes used for ZnO based DSSCs. Organic dyes and metal-free dyes are found to be most suitable for such DSSCs. Optimum particle size of ZnO, its stability, its suitable facet for the application in a DSSC, and the best redox couple for a ZnO based DSSC are discussed with evidences. Great emphasis is given on ZnO films that are doped with various materials. The review also discusses miscellaneous works on ZnO based DSSCs. A hitherto never discussed concept of usage of MOFs in a ZnO based DSSC concludes the review.