Cyanovinyl substituted benzimidazole based (DepeA) organic dyes for fabrication of dye sensitized solar cells Subbaiah Manoharan, Sambandam Anandan * Nanomaterials and Solar Energy Conversion Lab, Department of Chemistry, National Institute of Technology, Tiruchirappalli 620 015, India article info Article history: Received 13 December 2013 Received in revised form 3 February 2014 Accepted 11 February 2014 Available online 22 February 2014 Keywords: Benzimidazole Cyanovinyl Organic dye Efficiency p-bridge Acceptor abstract Benzimidazole based metal free organic dyes (DepeA) with and without cyanovinyl unit have been designed and synthesized via one step processes and tested as sensitizers in the dye sensitized solar cells (DSSC). In such donor-p-bridge-acceptor dyes, N,N-diethylaniline and triphenylamine were act as donors and benzimidazole derivatives were act as p-bridge as well as acceptor. The relationship between mo- lecular structure and optoelectronic properties of the synthesized dyes as well as their DSSC perfor- mances were studied by UVevis and fluorescence spectroscopy, cyclic voltammetry (CV) and Density Functional Theory (DFT) calculation. Introduction of cyanovinyl unit in between the donor and benz- imidazole leads to the significant improvement in the optoelectronic properties and DSSC performance. In general, the cyanovinyl unit containing dyes based DSSCs show more than two times higher efficiency than the dyes without cyanovinyl unit based DSSCs. Among the fabricated DSSC, the dye contain tri- phenylamine donor and benzimidazole with cyanovinyl unit based DSSC shows maximum power con- version efficiency of 3.47% under AM1.5 illumination (85 mW/cm 2 ). Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction The current situation in the energetic and environmental crisis has awakened the interest in the conserved utility of renewable energy sources. In this regard, dye-sensitized solar cells (DSSCs) certainly appear as one of the most promising devices for con- verting the solar energy and have attracted significant attention as low-cost alternatives to conventional semiconductor photovoltaic devices [1]. Among the DSSC components the photoanode play like a heart of the device in which sensitizer molecules were chem- isorbed on the TiO 2 surface [2,3]. Recently, metal-free organic dyes possess the common structure of donor-p-bridge-acceptor (Depe A) have much attention as alternative sensitizers in DSSC compared to their metal organic counterparts due to their high flexibility in structure modification, environmental concerns and low cost [4e 6]. In the last decade, variety of organic dyes with DepeA con- struction have been investigated for DSSCs, including dyes based on triphenylamine, [7,8] indoline, [9,10] phenothiazine, [11,12] carba- zole [13,14], phenoxazine [15,16] and achieved efficiency more than 8%. To improve the efficiency and stability of the DSSC more efforts have been spent through the modification and optimization of dye structure by many research communities [6]. For that variety of donors has been utilized, but predominantly such dyes consist of thiophene or ethenyl derivatives as p-bridges [4]. Even though those p-bridges are very effective, it has some synthetic drawbacks such as multi steps reaction, utilization of expensive catalysts and reagents, finally it needs very rigorous inert atmosphere [17,18]. In this concern, we choose benzimidazole derivatives which are pro- duced from inexpensive and easily available starting materials, simple synthesis methodologies, good electron transporting ability and high thermal stability [19,20]. Introduction of cyanovinyl unit in benzimidazole is very attractive since the presence of cyano group will increase the electron withdrawing nature of the benz- imidazole which decreases the lowest unoccupied molecular orbital (LUMO) energy level of the dye molecules. It indicates that the cyanovinyl group helps to increase the intramolecular charge transfer and reduces the optical band gap of the dye molecules [21,22]. Herein we present four benzimidazole based organic dyes namely 2-(4-(diethylamino)phenyl)-benzimidazole-5-carboxylic acid (EB), 2-(4-(diphenylamino)phenyl)-benzimidazole-5-carboxy lic acid (PB), 2-(1-cyano-2-(4-(diethylamino)phenyl)vinyl)-benz- imidazole-5-carboxylic acid (ECB) and 2-(1-cyano-2-(4-(dipheny- lamino)phenyl)vinyl)-benzimidazole-5-carboxylic acid (PCB) as a sensitizer towards fabrication of metal free DSSCs (Scheme 1). Triphenylamine and N,N-diethylaniline were used as donors to * Corresponding author. Tel.: þ91 431 2503639; fax: þ91 431 2500133. E-mail addresses: sanand99@yahoo.com, sanand@nitt.edu (S. Anandan). Contents lists available at ScienceDirect Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig http://dx.doi.org/10.1016/j.dyepig.2014.02.010 0143-7208/Ó 2014 Elsevier Ltd. All rights reserved. Dyes and Pigments 105 (2014) 223e231