Mechanisms of Electron Transport and Recombination in ZnO Nanostructures for Dye-Sensitized Solar Cells

• Published in: Chemphyschem Vol. 15; no. 6; pp. 1088 - 1097
• Main Authors: Vega-Poot, Alberto G., Macías-Montero, Manuel, Idígoras, Jesus, Borrás, Ana, Barranco, Angel, Gonzalez-Elipe, Agustín R., Lizama-Tzec, Francisco I., Oskam, Gerko, Anta, Juan A.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 14.04.2014; WILEY‐VCH Verlag; Wiley; Wiley Subscription Services, Inc
• Subjects: Applied sciences; dye-sensitized solar cells; Efficiency; electron transport; Energy; Exact sciences and technology; nanostructures; Natural energy; photoanodes; Photovoltaic conversion; Solar cells. Photoelectrochemical cells; Solar energy; Thin films; ZnO; Dye-sensitized solar cell; Binary compound; Recombination; photoanodes; electron transport; Nanostructure; ZnO; dye-sensitized solar cells; nanostructures; Mechanism
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.201301068

ZnO is an attractive material for applications in dye‐sensitized solar cells and related devices. This material has excellent electron‐transport properties in the bulk but its electron diffusion coefficient is much smaller in mesoporous films. In this work the electron‐transport properties of two different kinds of dye‐sensitized ZnO nanostructures are investigated by small‐perturbation electrochemical techniques. For nanoparticulate ZnO photoanodes prepared via a wet‐chemistry technique, the diffusion coefficient is found to reproduce the typical behavior predicted by the multiple‐trapping and the hopping models, with an exponential increase with respect to the applied bias. In contrast, in ZnO nanostructured thin films of controlled texture and crystallinity prepared via a plasma chemical vapor deposition method, the diffusion coefficient is found to be independent of the electrochemical bias. This observation suggests a different transport mechanism not controlled by trapping and electron accumulation. In spite of the quite different transport features, the recombination kinetics, the electron‐collection efficiency and the photoconversion efficiency are very similar for both kinds of photoanodes, an observation that indicates that surface properties rather than electron transport is the main efficiency‐determining factor in solar cells based on ZnO nanostructured photoanodes. Two very different behaviors of the electron‐transport properties are found in nanostructured ZnO‐based photoanodes. Texturized samples show a voltage‐independent transport time, whereas films produced from nanocrystalline powders exhibit a voltage‐dependent signal, consistent with trap‐limited electron diffusion.