Dithienylthienothiadiazole-based organic dye containing two cyanoacrylic acid anchoring units for dye-sensitized solar cells G. D. Sharma,* a J. A. Mikroyannidis,* b M. S. Roy, c K. R. Justin Thomas, d R. J. Ball e and Rajnish Kurchania f Received 7th August 2012, Accepted 19th September 2012 DOI: 10.1039/c2ra21718j A new dithienylthienothiadiazole-based organic dye (D) was synthesized by a seven-step synthetic route for use as a sensitizer in dye sensitized solar cells (DSSCs). The dye contains a dithienylthienothiadiazole central unit and two cyanoacrylic acid anchoring side groups. Optical and electrochemical properties of D were evaluated. In addition, density functional theory (DFT) and time dependent density functional theory (TD-DFT) calculations were carried out. A favorable electronic excitation from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) indicated that the dye can be used as a sensitizer for DSSC applications. The photovoltaic properties of laboratory scale optimized DSSCs sensitized with D showed a power conversion efficiency (PCE) of 4.22%, which was further improved to 5.47% upon the addition of chenodeoxylic acid (CDCA) as a coadsorbant. Introduction Since the seminal work reported in 1991 by O’Regan and Gra ¨ tzel, dye sensitized solar cells (DSSCs) manufactured from mesoporous anatase TiO 2 electrodes have received much attention because they represent a promising alternative to conventional solar cells based on silicon. 1 DSSCs based on polypyridyl Ru complexes have been reported with a high point contact concentrator (PCC) of almost 12% under standard AM1.5 sunlight irradiation. 2 Although polypyridyl Ru dyes have a high power conversion efficiency (PCE) and long-term stability, the large scale application of these dyes has become a critical problem due to limited resources and the costly purification steps required for their manufacture. 3 Fortunately, metal-free dyes could provide a competitive alternative to polypyridyl Ru-based dyes due to their facile modification, high molar coefficient, relatively inexpensive preparation and straightforward compliance with environmentally friendly guide- lines. 2c,3 These attributes also make them ideal for solid state DSSCs, utilizing thinner TiO 2 layers. In particular, it is encouraging to note that a promising photon-to-current conver- sion efficiency of up to 10.3% has been achieved by Wang et al. 4 A common strategy in the design of highly efficient metal-free dyes for DSSCs is linking the electron donor and acceptor (D–A) systems through the p-conjugated bridges. Commonly referred to as D-p-A dyes, these systems have been found to possess photoinduced intramolecular charge transfer (ICT) proper- ties, 2c,3 making them suitable for DSSC applications. In recent years, the design and synthesis of D-p-A metal free organic dyes with various donor moieties such as coumarin, 5 indoline, 6 triarylamine, 7 porphyrin, 8 squaraine 9 and amine free hetero- cycles 10 has led to a number of efficient sensitizers for DSSCs. Organic dyes generally suffer due to their relatively narrow absorption spectra with very poor absorption in the longer wavelength region. However, to realize large photocurrent responses, a sensitizer must possess a broad absorption extend- ing to the near infrared (NIR) region and the absorption spectrum should overlap with the solar emission spectrum to a large extent. In addition to this, the lowest unoccupied molecular orbital (LUMO) of the dye should lie above the conduction band edge of the TiO 2 semiconductor electrode to favor electron injection in the conduction band of TiO 2 . Additionally, the occurrence of the highest occupied molecular orbital (HOMO) of the dye matching with the redox potential of the electrolyte (iodide/tri-iodide) will ensure the regeneration of the dye. Several attempts have been made to fine-tune the above properties in the organic dyes. 7–10 It was reported by Won et al. 11 that the DSSC based on terthiophene containing organic dye with two anchoring acceptor groups exhibited a higher PCE than its counterpart having one anchoring group. The higher PCE observed in the former dye was attributed to the increased amount of dye adsorbed on to the photoanode surface and the presence of a R & D Centre for Engineering and Science JEC Group of Colleges, JEC Campus, Kukas, Jaipur (Raj), 303101, India. E-mail: sharmagd_in@yahoo.com b Chemical Technology Laboratory, Department of Chemistry, University of Patras, GR, 26500, Patras, Greece. E-mail: mikroyan@chemistry.upatras.gr c Defence Laboratory, Jodhpur (Raj), 342011, India d Organic Materials Laboratory, Department of Chemistry, Indian Institute of Technology, Roorkee, 247667, Uttrakhand, India e Department of Architecture and Civil Engineering, University of Bath, Bath, BA2 7AY, UK f Department of Physics, Maulana Azad National Institute of Technology (MANIT), 462051, Bhopal (MP(, India RSC Advances Dynamic Article Links Cite this: RSC Advances, 2012, 2, 11457–11464 www.rsc.org/advances PAPER This journal is ß The Royal Society of Chemistry 2012 RSC Adv., 2012, 2, 11457–11464 | 11457 Downloaded on 02 December 2012 Published on 25 September 2012 on http://pubs.rsc.org | doi:10.1039/C2RA21718J View Article Online / Journal Homepage / Table of Contents for this issue