Rigidifying the π‑Linker to Enhance Light Absorption of Organic Dye- Sensitized Solar Cells and Influences on Charge Transfer Dynamics Zhaoyang Yao, †,§ Lin Yang, †,§ Yanchun Cai, † Cancan Yan, †,§ Min Zhang, † Ning Cai, †,§,∥ Xiandui Dong, ‡ and Peng Wang* ,† † State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China ‡ State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China § University of Chinese Academy of Sciences, Beijing 100049, China * S Supporting Information ABSTRACT: The usage of coplanar π-conjugated segments represents a feasible strategy on reducing the energy gap of organic push−pull dyes for mesoscopic titania solar cells. In this paper, we report two new dyes coded as C254 and C255 with the respective 1,4- di(thiophen-2-yl)benzene and indacenodithiophene π-linkers, in combination with the electron-releasing triphenylamine and electron-withdrawing cyanoacrylic acid units. The energy-gap reduction stemming from the rigidity of the π-linker is accompanied by a negative shift of the ground-state redox potential, which however does not affect the yield of hole injection from the oxidized state of dye molecules to a cobalt redox electrolyte. On the other side, we have identified from femtosecond transient absorption measurements a diminished rate of electron injection from the relaxed, low-energy excited state of C255 to titania, albeit a comparable rate of electron injection from the high-energy excited states of these two dyes. The bulkier C255 dye with four hexyl side chains tethered on the two sp 3 carbons of the fused indacenodithiophene unit can form a more compact self-assembling monolayer on titania, considerably attenuating the charge recombination of photoinjected electrons in titania with the cobalt electrolyte and thus enhancing the cell photovoltage and efficiency. 1. INTRODUCTION The dye-sensitized solar cell (DSC) technology is with great promise for the conversion of solar light to electricity, 1 if its power conversion efficiency (PCE) can be further improved to the level of 15% at the air mass 1.5 global (AM1.5G) conditions. Even with the state-of-the-art materials, it has already been commercialized to power some wireless key- boards, owing to its peculiar performance operated under diffusive visible light. To further boost its economical efficiency for a larger scale of applications, enormous efforts have been devoted to insightful physical analysis and new material development in the past years. Ruthenium bipyridyl and zinc porphyrin complexes have been demonstrated thus far to be the most efficient light harvesters in DSCs. 2−5 In addition, a lot of research interest has been caught on metal-free organic dyes in the past decade, due mainly to the abundance of raw materials, the flexibility of molecular design, and the bright color. 6−10 Apart from the electron-releasing and electron-withdrawing blocks in a D-π-A dye, the conjugated π-linker is also of paramount importance in modulating the energy gap and the microstructure of a self-assembling dye layer on titania. In this context, a mass of thiophene-based organic dyes full of structural variety have been prepared since the initial work by Arakawa and his co-workers 11 on integrating vinylthiophene or 2,2′-dithiophene (DT) with coumarin and cyanoacrylic acid units. 12−22 In conjunction with the hydrophilic cyanoacrylic acid electron acceptor 11 and the hydrophobic dihexyloxy- substituted triphenylamine electron donor, 23 we have evaluated the merit of improving light absorption by π-linker rigid- ification, via comparing cyclopentadithiophene (CPDT) with DT as well as cyclopenta[1,2-b:5,4-b′]dithiophene[2′,1′:4,5]- thieno[2,3-d]thiophene (CPDTTT) with 2,5-di(thiophen-2- yl)thieno[3,2-b]thiophene (DTTT). 24,25 Meanwhile, Wong et al. have exploited the coplanar indacenodithiophene (IDT) 26−28 as the central π-spacer to construct a series of D- π-A dyes with remarkable power conversion efficiencies (PCEs) up to 6.7% under AM1.5G radiation. 29 Herein we will report two new dyes shown in Figure 1a by use of either the twisting 1,4-di(thiophen-2-yl)benzene (DTB) unit 30 for C254 or IDT as the conjugated π-linker for C255. Furthermore, we will systematically analyze the impacts of structural alteration on light absorption and femtoseconds (fs) to milliseconds (ms) charge transfer dynamics, which underpin the photovoltaic performance of DSCs made with these dyes. Received: December 10, 2013 Revised: January 17, 2014 Published: January 24, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 2977 dx.doi.org/10.1021/jp412070p | J. Phys. Chem. C 2014, 118, 2977−2986