Triazine dyes as photosensitizers for dye-sensitized solar cells Jian Liu a, b , Kai Wang a , Xiaopeng Zhang a , Chenghui Li a, b, * , Xiaozeng You a, b, * a State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, PR China b Academician Work Station of Changzhou Trina Solar Energy Co., Ltd., Changzhou 213022, Jiangsu Province, PR China article info Article history: Received 13 June 2012 Received in revised form 9 October 2012 Accepted 16 October 2012 Available online 23 October 2012 Keywords: Triazine Triphenylamine Dye-sensitized solar cells abstract A new series of triazine-containing donorep-acceptor organic sensitizers (TCT-(7e19)) with broad absorptions and high molar extinction coefficients have been designed and synthesized based on our previous work. All these dyes were completely characterized by 1 H NMR, ESI-MS, EA, IR, UVevis, and cyclic voltammetry. The photovoltaic performances of dye-sensitized solar cells (DSSCs) based on these dyes were investigated. The effect of conjugating length and terminating groups on the absorption properties and photovoltaic performances were discussed. An overall photon-to-electron conversion efficiency of 3.69% was achieved with the DSSC based on the dye TCT-13 (J sc ¼7.76 mA cm 2 , V oc ¼691 mV, FF¼68.8%) under AM 1.5G illumination (100 mW/cm 2 ). Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Due to the energy crisis, a variety of light-harvesting devices have been intensively investigated in recent years. 1 Among these devices, dye-sensitized solar cells (DSSCs) have received consider- able attention since Gratzel’s pioneering report in 1991. 2 Ruthenium-based dyes (such as N3, N719, and black dye) have been considered as the most efficient sensitizers, which have reached the promising solar-energy-to-electricity conversion effi- ciencies of 11% under AM 1.5 irradiation. 3 In view of the limited ruthenium resource, however, sensitizers based on non-Ru system are sorely needed. In this regard, conjugated donorep-acceptor (Dep-A) chromophores have been intensively investigated as promising candidates due to their large molar extinction co- efficient, tunable absorption wavelength, facile synthesis, and lower cost. 4 Recently, Gratzel’s group reported the porphyrin- sensitized solar cells with the highest solar-energy-to-electricity conversion efficiencies of 12.3%. 5 p-Conjugated compounds based on triazine have been studied as an attractive candidate in functional photoelectric materials owing to the intriguing structural and electronic properties of tri- azine moiety. 6 Structurally, triazine unit is a symmetric core with three active substituted places, which is promising for derivation through organic synthesis. 7 Moreover, triazine unit is a typical electron-accepting unit and would be able to improve the electron- injection and electron-transportation abilities of its conjugated derivates. 8 In our previous work, a new type of Dep-A organic sensitizers utilizing triazine as p spacer has been developed for DSSCs. 9 The cyanoacrylic acid dye TCT-1 with two electron donat- ing triphenylamine moieties exhibits better photovoltaic perfor- mance than other dyes with one donor moiety. The DSSC based on TCT-1 shows a high open-circuit photovoltage (V oc ) of 757 mV, which is comparative with some reported excellent dyes. However, its short-circuit photocurrent densities (J sc ) is quite low (3.33 mA cm 2 ) due to its narrow absorption. Based on these re- sults, we decided to tailor the chemical structure to optimize their photovoltaic properties. The molecular structures of the new sen- sitizers designed and synthesized in this work are given in Figs. 1e3. Red-shifted and broadened absorptions were realized, DSSCs with better photovoltaic performance have been obtained. The correlations between structure, absorption, and photovoltaic performance were discussed. 2. Results and discussion Increasing the conjugation length is an effective way to expand the absorption of Dep-A chromophores. 10 Therefore, we synthesized a series of p-conjugated linker between the triphenylamine and tri- azine in TCT-1 according to the synthetic protocol illustrated in Schemes 1e3. Double aryl substitutions of the cyanuric chloride with the corresponding Grignard reagents gave 2a,b. Compounds 3a,b were obtained by bromination of 2a,b using NBS. 11 The reactions of 3a,b with triethyl phosphate in the presence of Lewis acid afford the Horner reagents 4a,b. 12 The aldehydes 6a,b were readily prepared by Suzuki coupling between 4-bromotriphenylamine and the aromatic * Corresponding authors. Tel.: þ86 25 83592969; fax: þ86 25 83314502; e-mail addresses: chli@nju.edu.edu (C. Li), youxz@nju.edu.edu, youxz@nju.edu.cn (X. You). Contents lists available at SciVerse ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet 0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tet.2012.10.046 Tetrahedron 69 (2013) 190e200