Performance and Stability of Dye Solar Cells on Stainless Steel

OF DOCTORAL DISSERTATION HELSINKI UNIVERSITY OF TECHNOLOGY P.O. BOX 1000, FI-02015 TKK http://www.tkk.fi Author Kati Miettunen Name of the dissertation Performance and Stability of Dye Solar Cells on Stainless Steel Manuscript submitted 05.06.2009 Manuscript revised 09.09.2009 Date of the defence 23.10.2009 Monograph Article dissertation (summary + original articles) Faculty Faculty of Information and Natural Sciences Department Department of Applied Physics Field of research New Energy Technologies Opponent(s) Prof. Juan Bisquert Supervisor Prof. Peter Lund Instructor Prof. Peter Lund, D.Sc. Janne Halme Abstract This study is on nanostructured dye solar cells which are easy to manufacture and use low cost materials. Dye solar cells have conventionally been deposited on conductive glass sheets. To reduce the costs and enable roll-to-roll mass production, the glass substrates should be replaced with flexible metal and plastic substrates. The change of the substrates has a profound effect on the cell, e.g. on electrochemical properties, optics, temperature treatments, and lifetime which all change when using alternative substrates.This study is on nanostructured dye solar cells which are easy to manufacture and use low cost materials. Dye solar cells have conventionally been deposited on conductive glass sheets. To reduce the costs and enable roll-to-roll mass production, the glass substrates should be replaced with flexible metal and plastic substrates. The change of the substrates has a profound effect on the cell, e.g. on electrochemical properties, optics, temperature treatments, and lifetime which all change when using alternative substrates. In this work, the focus is on using stainless steel and ITO-PET substrates in dye solar cells. The electrochemical characteristics of the substrates were analyzed and the need for additional coatings to improve the performance or the stability was evaluated. The optimization of the other cell components in different metal-plastic cell configurations was also examined. An important part of the work was to decouple the effects of the different cell components on the cell performance. For this purpose, a method to study individual electrode performance based on electrochemical impedance measurements of a complete cell is presented. Issues related to the up-scaling of the cell, namely current collection and spatial performance distribution, are also covered in this work. A novel, segmented cells configuration is presented to study the spatial performance distribution. It was discovered that there are large variations in the cell performance leading to significant efficiency losses. The spatial distribution was linked with the usual electrolyte filling technique which resulted in an uneven distribution of a common electrolyte component, 4-tert-butylpyridine. Finally, the lifetime of the stainless steel based cells was examined. Interestingly, the cells with stainless steel photoelectrode substrates aged much faster than those with stainless steel counter electrode substrates. To examine the aging mechanisms, a segmented cell design was also developed specially for the degradation studies. With the segmented cells, it could be confirmed that the degradation of the stainless steel photoelectrode cells was not due to changes in the electrolyte. The aging of the stainless steel counter electrode cells was, on the contrary, linked with the corrosion of the stainless steel substrate by the electrolyte.