Synthesis and photovoltaic properties of organic sensitizers containing electron- deficient and electron-rich fused thiophene for dye-sensitized solar cells Xiaobing Cheng, Mao Liang * , Siyuan Sun, Yongbo Shi, Zijian Ma, Zhe Sun, Song Xue * Department of Applied Chemistry, Tianjin University of Technology, Tianjin 300384, PR China article info Article history: Received 4 March 2012 Received in revised form 15 April 2012 Accepted 28 April 2012 Available online 3 May 2012 Keywords: Dye-sensitized solar cell Organic sensitizers Electron-deficient fused thiophene abstract Diverse fused thiophenes with electron-rich and electron-deficient blocks have been synthesized and employed as the p-conjugated spacers of organic dyes for the dye-sensitized solar cells (DSSCs). The effects of these fused thiophenes were investigated by their absorption spectra, electrochemical and photovoltaic properties. For a typical device a maximum power conversion efficiency of 6.11% was ob- tained under simulated AM 1.5 irradiation (100 mW cm 2 ): a short-circuit current (J SC ) of 14.47 mA cm 2 , an open-circuit voltage (V OC ) of 670 mV, and a fill factor (FF) of 0.63. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction There has been considerable interest for developing cheap and easily accessible renewable energy sources due to increasing en- ergy demands and concerns about global warming. Compared to traditional silicon-based solar cells, dye-sensitized solar cells (DSSCs) are viewed as promising candidate for renewable clean energy sources in virtue of their low manufacturing cost and im- pressive photovoltaic performance. 1 To achieve high solar power conversion efficiency, great research efforts are focused on de- signing and synthesizing new photosensitizers: Ru-based com- plexes 2 and organic sensitizers. 3 The former holds the record of validated efficiency of over 11%. 2b Organic sensitizers with robust availability, ease of structural tunability, and generally high molar extinction coefficients, have emerged as a competitive alternative to the Ru-based counterpart. 4e24 So far, the metal-free organic sensitizers have gained promising solar energy-to-electricity con- version efficiencies (h) comparable to Ru-based complexes. 25e28 Aside from electron donor (D) and acceptor (A) in a typical D-p-A system, the conjugated bridging segment (p) is widely recognized as its significance for performance control of DSSCs. Introduction of fused ring building blocks 20,26,29e33 involved of rigid thieno[3,2-b] thiophene (TT), dithieno[3,2-b:2 0 ,3 0 -d]thiophene (DTT), cyclo- pentadithiophene, dihexyl-substituted dithienosilole, and benzo [1,2-b:4,5-b 0 ]dithiophene in the p-spacer is proved to be effective strategies for enhancing the light-harvesting capacity, leading to impressive performances under standard global air mass 1.5 (AM 1.5G) illumination. The majority of research on fused ring building blocks has focused on electron-rich fused thiophene; however, electron-deficient fused thiophene has not yet to be explored for organic dyes. It is known that electron-deficient materials were successfully in application of bulk heterojunction organic photo- voltaic cells (OPVs). 34e36 It can be expected that developing of new type fused ring building blocks may lead to synthesis of interesting photosenzitizers with tailor-made photophysical, electrochemical, and other properties to fulfill those requirements for high perfor- mance of DSSCs. Herein, we report the design, syntheses, and photovoltaic properties of a series of new triphenylamine dyes (XS35e38) in- corporating electron-rich and electron-deficient fused thiophene units. For obtaining high photovoltaic performance, binary spacers of a combination of EDOT and different fused thiophenes were introduced. In the case of XS36 and XS38, electron-rich fused thiophene units, DTT, and 3-methylthieno[3,2-b]thiophene (MTT) are introduced, respectively. Meanwhile, electron-deficient fused thiophene units, dithieno[3,2-b:2 0 ,3 0 -d]thiophene-4,4- dioxide (DTTO) and 3-methylthieno[3,2-b]thiophene-4,4-dioxide (MTTO) are introduced into the structure of XS35 and XS37 , re- spectively. These designed dyes allow us to better understand the structureeproperty relationship of metal-free organic chromo- phores. Molecular structures of these two dyes are shown in Fig. 1 . 2. Results and discussion The synthetic routes of four organic dyes XS35e38 are shown in Scheme 1 . DTT 37 and MTT 38 units were prepared according to * Corresponding authors. Tel.: þ86 22 60214250; fax: þ86 22 60214252; e-mail addresses: liangmao717@126.com (M. Liang), xuesong@ustc.edu.cn (S. Xue). 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. doi:10.1016/j.tet.2012.04.113 Tetrahedron 68 (2012) 5375e5385