Renewable Energy Global Innovations features: Solvent-assisted microstructural evolution and enhanced performance of porous zinc oxide films for plastic dye-sensitized solar cells

Significance Statement

Recently, dye-sensitized solar cells have been attracting considerable attention. This can be highly attributed to their low fabrication cost and possibilities of several practical indoor uses such as, the replacement of transparent conducting glass for plastic substrates. Their lightness and flexibility place them as best candidates for use in mobile devices and curved surfaces. During fabrication, the metal oxide semiconductor films used are generally heated to very high temperatures so as to obtain the effective connections among semiconductor particles for better electron transport properties. A challenge is therefore presented in achieving high performance dye-sensitized solar cells since the processing of semiconductor films using plastic substrates demand application of low temperatures.

A team of researchers at Keio University in Japan, Hitomi Ohashi, Manabu Hagiwara and Shinobu Fujihara proposed a study that focused on the post chemical treatments of the low temperature-processed zinc oxide films on plastic substrates through immersion in water-ethanol solvents. They aimed at improving the cell performance; specifically, in the short circuit current density and the power conversion efficiency. Their work is now published in Journal of Power Sources.

First, the research team began by fabricating the porous zinc oxide films of two kinds of macroscopic morphologies on the indium tin oxide-coated polyethylene naphthalate substrate at low processing temperatures for use in the plastic dye-sensitized solar cells. They then immersed the fabricated films into water-ethanol solvents at a temperature of about 90 ^°C where crystal growth of the zinc oxide was anticipated. Afterward they conducted short-circuit and power conversion efficiency tests on the films.

It was observed that by immersing the films into the water-ethanol mixture, its microstructure was greatly modified. This was due to the growth of the constituting zinc oxide particles and also due to the evolution of the inter-particle connection owing to the solvents composition. The change in the microstructure could therefore be explained by the dissolution and re-precipitation of zinc oxide with the Ostwald ripening, depending on the solvent composition. They also observed that plastic dye-sensitized solar cells using the immersed zinc oxide films with the flower-like particles and the densely packed nanoparticles exhibited a power conversion efficiency of 3.9 and 4.1%, respectively.

The cell performance is largely improved especially in the short-circuit current density together with the power conversion efficiency. The immersion effect is more remarkable for the cell using the densely packed zinc oxide films. The plastic film-type fabricated dye-sensitized solar cells are hereby presented to be more economical from the fabrication and performance point of view as opposed to their transparent conducting glass-type counterparts. In fact, the plastic N719-sensitized zinc oxide cell commands a conversion efficiency as high as 4.1%.

About The Author

Hitomi Ohashi received Master of Engineering in Integrated Design Engineering from Keio University, Japan, in 2016. She was a winner of GGRN Student Poster Award at 11th International Conference on Ceramic Materials and Components for Energy and Environmental Applications (Vancouver, 2015). Now she works at one of the largest chemical companies in Japan.

About The Author

Dr. Manabu Hagiwara is a research associate in the Department of Applied Chemistry of Keio University, Japan. He received the B. Eng. and M. Eng. degrees in 2009 and 2011 from Tokyo Institute of Technology, Japan. He received the Ph.D. degree in Materials Engineering from Tokyo Institute of Technology in 2013. He has been conducting research on the development of functional oxides using chemical synthesis methods.

The main focus of his current research is on synthesizing novel lead-free ferroelectric and piezoelectric oxides, especially those with bismuth-based compositions, and on understanding the structure-property relationships in them. His research interest also includes fabrication of nano/meso-structured oxide materials for thermoelectric and electromagnetic applications.

About The Author

Dr. Shinobu Fujihara received his Ph.D. in Molecular Engineering from Kyoto University, Japan, in 1995. He has been working at Keio University since then and now is Professor. He is also a CerSJ Fellow (The Ceramic Society of Japan). He was a Visiting Scholar at RWTH Aachen University of Technology, Germany, from 2001 to 2002.

His main research fields include sol-gel processing of nanomaterials, optical design and synthesis of inorganic luminescent materials, and nanostructural control of metal oxide semiconductors for photoelectrochemical devices.

Reference

Hitomi Ohashi, Manabu Hagiwara, Shinobu Fujihara. Solvent-assisted microstructural evolution and enhanced performance of porous zinc oxide films for plastic dye-sensitized solar cells. Journal of Power Sources volume 342 (2017) pages 148-156.

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