Research Article Unsymmetrical Heptamethine Dyes for NIR Dye-Sensitized Solar Cells Thomas Geiger, 1 Iuliia Schoger, 1 Daniel Rentsch, 1 Anna Christina Véron, 1 Frédéric Oswald, 2 Toby Meyer, 2 and Frank Nüesch 1,3 1 Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Functional Polymers, ¨ Uberlandstrasse 129, 8600 D¨ ubendorf, Switzerland 2 Solaronix, Rue de l’Ouriette 129, 1170 Aubonne VD, Switzerland 3 Institut des Mat´ eriaux, Ecole Polytechnique F´ ed´ eral de Lausanne, EPFL Station 12, 1015 Lausanne, Switzerland Correspondence should be addressed to homas Geiger; thomas.geiger@empa.ch Received 9 April 2014; Accepted 8 May 2014; Published 4 June 2014 Academic Editor: Vincenzo Augugliaro Copyright © 2014 homas Geiger et al. his is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Seven unsymmetrical heptamethine dyes with carboxylic acid functionality were synthesized and characterized. hese near- infrared dyes exhibit outstanding photophysical properties depending on their heterocyclic moieties and molecular structure. As proof of principle, the dyes were used as photosensitizers in dye-sensitized solar cells. Using the most promising dye, an overall conversion eiciency of 1.22% and an almost colorless solar cell were achieved. 1. Introduction In order to achieve chromophores which absorb light in the far red or even in the near-infrared region (NIR), increased eforts were made by developing new molecular dye structures and synthetic strategies or by investigating basic structure property relationships. Intense absorption in the NIR is mandatory for many technologies and applications, for example, energy conversion in photovoltaic cells [1]. However, while many dyes provide absorption from the visible to the NIR [2, 3] range, only few absorb in the NIR only and are able to convert energy eiciently. Particularly for application in dye-sensitized solar cells (DSCs), only a few dye classes with appropriate molecular design are known to be viable NIR absorbers: squaraine dyes, croconine dyes, diketopyrrolopyrrole derivatives, and polymethine dyes with a single absorption maximum,  max , above 700 nm. Recently, Park et al. comprehensively reviewed the NIR sensitization in DSCs [4]. Polymethine dyes [5], in particular, the subclass of heptamethine dyes, exhibit outstanding photophysical properties, for example, absorption maximum ( max ) above 700 nm with high molar extinction coeicient () of over 100000 Lmol −1 cm −1 . Furthermore, heptamethine dyes can be obtained via straightforward and cost-efective synthesis in high yields in contrast to metal complex photosensitizers. In this context, several publications reported about synthesis and irst applications of heptamethine dyes in DSCs [6–11]. In common with all reported heptamethine dyes, the essential carboxylic acid anchor group was symmetrically positioned either at the heterocyclic moieties or the alkyl chains which are substituted to the nitrogen atoms of the heterocycles or at the cyclohexene moiety of the polymethine chain. Following the concept and molecular design of unsym- metrical squaraine dyes which were synthesized and suc- cessfully applied [12–15] in our previous works [16–18], we report on the synthesis, characterization, and application of unsymmetrical heptamethine dyes Hepta1–Hepta7 with one carboxylic acid anchor group (Figure 1). Here, the anchor group is in conjugation with one of the heterocyclic moieties (indole- or quinoline-derivative). he polymethine core is rigidiied by a cyclohexene moiety which is substituted by a chlorine atom or phenyl-group. he second terminating heterocycle (benzindole-, quinoline-, and thiazole-derivative) is chemically diferent from the irst heterocycle carrying the anchor group. In order to improve solubility in organic solvents and to hinder dye Hindawi Publishing Corporation International Journal of Photoenergy Volume 2014, Article ID 258984, 10 pages http://dx.doi.org/10.1155/2014/258984