Fluorene-Based Sensitizers with a Phenothiazine Donor: Effect of Mode of Donor Tethering on the Performance of Dye-Sensitized Solar Cells Abhishek Baheti, † K. R. Justin Thomas,* ,† Chun-Ting Li, # Chuan-Pei Lee, # and Kuo-Chuan Ho # † Organic Materials Laboratory, Department of Chemistry, Indian Institute of Technology Roorkee, Roorkee, Uttarakhand 247 667, India # Department of Chemical Engineering, National Taiwan University, Taipei 10617, Taiwan *S Supporting Information ABSTRACT: Two types of fluorene-based organic dyes featuring T-shape/rod-shape molecular configuration with phenothiazine donor and cyanoacrylic acid acceptor have been synthesized and characterized as sensitizers for dye- sensitized solar cells. Phenothiazine is functionalized at either nitrogen (N10) or carbon (C3) to obtain T-shape and rod-like organic dyes, respectively. The effect of structural alternation on the optical, electrochemical, and the photovoltaic properties is investigated. The crystal structure determination of the dye containing phenyl linker revealed cofacial slip-stack columnar packing of the molecules. The trends in the optical properties of the dyes are interpreted using time-dependent density functional theory (TDDFT) computations. The rod-shaped dyes exhibited longer wavelength absorption and low oxidation potentials when compared to the corresponding T-shaped dyes attributable to the favorable electronic overlap between the phenothiazine unit and the rest of the molecule in the former dyes. However, the T-shaped dyes showed better photovoltaic properties due to the lowest unoccupied molecular orbital (LUMO) energy level favorable for electron injection into the conduction band of TiO 2 and appropriate orientation of the phenothiazine unit rendering effective surface blocking to suppress the recombination of electrons between the electrolyte I 3 − and TiO 2 . The electrochemical impedance spectroscopy investigations provide further support for the variations in the electron injection and transfer kinetics due to the structural modifications. KEYWORDS: phenothiazine, organic dyes, optical spectra, TDDFT computations, dye-sensitized solar cells, electrochemical impedance spectroscopy ■ INTRODUCTION Recently, dye sensitized solar cells (DSSCs) 1 have attracted much attention as an alternative to p−n junction solar cells due to their low cost, easy fabrication, fairly high power conversion efficiency (PCE), and availability of large classes of sensitizers. A lot of investigations have been performed on the components of DSSC, including dye sensitizer, redox electrolyte, and inorganic semiconductor metal oxides. 2 Among all these, sensitizers have been recognized as one of the important constituents that influences the efficiency of the DSSC. Until now, metal-containing dyes such as ruthenium-based poly- pyridyl complexes, 3 porphyrins, 4 and perovskites 5 have been demonstrated to yield high solar energy to power conversion. However, some disadvantages limited their potential for large scale applications such as scarcity of platinum group metals, relatively high cost of production, and environmental hazard- ousness associated with lead. Despite the low lab scale efficiency reports, metal-free organic sensitizers 6−8 are attractive due to the relatively low production cost and facile synthetic methodologies. Also, they have displayed superiority over the ruthenium-based dyes in molar extinction coefficients for longer wavelength intramolecular charge transfer (ICT) absorption which can be further fine-tuned by easy chemical modifications. Most of the organic dyes are designed with a simple donor-π- acceptor (D-π-A) 6−8 molecular configuration which facilitates effective photoinduced intramolecular charge transfer across the molecule. With this successful approach, a number of organic dyes featuring various arylamine donors 9 containing different conjugating bridges and cyanoacrylic acid acceptors have been synthesized for application in dye-sensitized solar cells. Although arylamine-based organic dyes have exhibited competitive efficiency (∼10%) 9 comparable to metal complexes (∼12%), 3 they still suffer from some disadvantages. The main Received: September 9, 2014 Accepted: December 31, 2014 Research Article www.acsami.org © XXXX American Chemical Society A DOI: 10.1021/am506149q ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX