Influence of nature of surface dipoles on observed photovoltage in dye-sensitized solar cells as probed by surface potential measurement Shyam S. Pandey a, * , Shohei Sakaguchi a , Yoshihiro Yamaguchi b , Shuji Hayase a a Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu 808-0196, Japan b Nippon Steel Chemical Co., Ltd., 46-80 Nakabaru, Sakinohama, Tobata-ku, Kitakyushu 804-8503, Japan article info Article history: Received 10 February 2009 Received in revised form 12 November 2009 Accepted 19 November 2009 Available online 26 November 2009 Keywords: Surface potential Interface DCA Dye dipole Dye aggregation Dye-sensitized solar cells abstract A contact free method of surface potential measurement using scanning Kelvin-probe microscopy (SKPM) was conducted to probe the nature of nanocrystalline TiO 2 /dye inter- face. In combination with electrical measurements an effort has been made to establish a correlation between the nature of sensitizing dye and observed surface potential with open circuit voltage (Voc) after dye-sensitized solar cell (DSSC) fabrication. Effect of cheno- deoxycholic acid (DCA) as co-adsorbent with dyes upon the enhancement of DSSC effi- ciency has been probed by SKPM, which revealed a positive shift of surface potential upon treatment of thin films of TiO 2 with DCA. The enhancement of DSSC performance upon DCA introduction has been explained by upward shift of TiO 2 conduction band edge leading to enhanced Voc along with the prevention of p-stacked aggregation resulting in enhanced short-circuit current density. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Silicon solar cells are presently playing a pivotal role in harvesting the solar energy efficiently but their high cost of production is one of the major stumbling blocks towards their reach at common mass level as a clean energy substi- tute. Dye-sensitized solar cells (DSSC) are a cheap alterna- tives to silicon solar cells for efficient conversion of solar energy into electrical energy and has attracted the atten- tion of material science community in the recent past [1,2]. Although, DSSC based on organic dyes giving a respectable photoconversion efficiency of 10–11% which is still lower than the efficiency obtained by silicon solar cells (20%) [3,4]. One of the most important and challeng- ing factor attributed to the lower efficiency of the DSSC, is the narrower optical absorption window (400–800 nm) of organic dyes, which is narrower than the optical absorp- tion window of crystalline silicon (500–1100 nm). Re- cently, Inakazu et al. have proposed the extension of optical absorption window by selective adsorption of two dyes on nanoporous titania under pressurized CO 2 atmo- sphere leading to the enhancement in photoconversion efficiency [5]. Over all photoconversion efficiency of a DSSC is basically decided by the short-circuit current density (Jsc), open circuit voltage (Voc) and fill factor (FF). There are reports about a number of organic dyes giving very high Jsc but over all poor photoconversion efficiency of the cell was mainly attributed to small observed Voc [6,7]. Voc plays an important role in controlling the photo- conversion efficiency and is basically decided by the differ- ence in the conduction band level of TiO 2 and potential of I À 3 /I À redox couple in DSSCs. In an interesting report O’Re- gan and Gratzel [8] advocated the role of electron recombi- nation in governing the Voc based on their transient photovoltage and photocurrent measurements in the DSSC. Several approaches such as introduction of nitrogen containing heterocyclic additives like pyrimidine, alkyl pyridine, etc. [9,10], molecular modification of TiO 2 surface by dipolar carboxylic acid derivatives [11,12], coating of 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.11.021 * Corresponding author. Tel.: +81 93 695 6005; fax: +81 93 695 6000. E-mail address: shyam@life.kyutech.ac.jp (S.S. Pandey). Organic Electronics 11 (2010) 419–426 Contents lists available at ScienceDirect Organic Electronics journal homepage: www.elsevier.com/locate/orgel