J. Phys. Chem. C 2010, 114, 18283–18290
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Organic Dyes Containing Thieno[3,2-b]indole Donor for Efficient Dye-Sensitized Solar Cells Xue-Hua Zhang, Yan Cui, Ryuzi Katoh, Nagatoshi Koumura,* and Kohjiro Hara* National Institute of AdVanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ReceiVed: June 16, 2010; ReVised Manuscript ReceiVed: September 6, 2010
Three new organic dyes, comprising thieno[3,2-b]indole moiety as an electron donor, n-hexyl substituted oligothiophene units as a π-conjugated bridge, and cyanoacrylic acid group as an electron acceptor and anchoring group, have been designed and synthesized for nanocrystalline TiO2 dye-sensitized solar cells (DSSCs). Compared with our previous carbazole-based MK dyes, in the similar donor-π-acceptor structure, thieno[3,2-b]indole not only has stronger electron donating ability but also could hold the dye molecule in a more planar conformation than carbazole. The newly synthesized thieno[3,2-b]indole-based dyes exhibit their major electronic absorption in the range of 400-600 nm in the visible region in solution. Solar energy to electricity conversion efficiency (η) up to 7.8% was obtained with a DSSC using 2-cyano-3-[5′′-(4-ethyl4H-thieno[3,2-b]indol-2-yl)-3′,3′′,4-tri-n-hexyl-[2,2′,5′,2′′]terthiophen-5-yl]acrylic acid (MKZ-40) as light harvesting sensitizer under simulated AM 1.5 G irradiation (100 mW cm-2) with an aperture mask: shortcircuit photocurrent density (Jsc) ) 14.6 mA cm-2, open-circuit voltage (Voc) ) 0.70 V, and fill factor (FF) ) 0.76, suggesting that thieno[3,2-b]indole-based organic dyes are promising candidates for DSSCs. However, the DSSCs based on thieno[3,2-b]indole dyes showed shorter electron lifetime compared to that for the solar cell with a conventional carbazole dye, resulting in lower Voc. Introduction Since their discovery in 1991, dye-sensitized solar cells (DSSCs) have received extensive interest by virtue of their low cost and relatively high solar to electricity conversion efficiency (η).1 Although there are a lot of factors determining the photovoltaic performance of the DSSCs, the structural, photophysical, and photoelectrochemical properties of the sensitizers are obviously crucial ones. As a result, the pursuit of highly efficient dye stuffs has been one of the most active subjects along with the development of the DSSCs.2-21 Sensitizers employed in highly efficient DSSCs have to meet several basic requirements, the wide absorption range in visible and near-IR region, suitable energy level of the ground and excited states, fast rate constant of electron injection and slow rate constant of charge recombination, and good long-term stability. So far, Ru-complexes still represent the most efficient sensitizers.2-6 Compared with Ru-complexes, organic dyes exhibit larger molar extinction coefficients (ε), easier control of the absorption wavelength, as well as facile design, and therefore are attracting more and more research efforts. Various efficient organic dyes have been extensively investigated for the construction of DSSCs, and encouraging efficiencies have also been achieved.21 We have developed a series of carbazolebased organic dyes for efficient DSSCs in the past several years,11-14 compared with the structure of carbazole, thieno[3,2b]indole’s structure (Figure 1) is also based on the indole structure but in which a thiophene ring, instead of a second benzene ring, is fused onto the five-membered ring at the 2-3 position of the indole,22,23 whereas organic dyes based on thieno[3,2-b]indole moiety have been little studied to date. Taking account of the structure of thieno[3,2-b]indole, which features a five-mumbered electron-rich thiophene ring, the * To whom correspondence should be addressed. E-mail: n-koumura@ aist.go.jp (N.K.),
[email protected] (K.H.). Phone: +81-29-861-4638. Fax: +81-29-861-4638.
Figure 1. Structures of 4H-thieno[3,2-b]indole, and thieno[3,2b]indole-based organic dyes MKZ-39, MKZ-40, MKZ-41.
stronger electron donating ability could be supposed than the carbazole moiety. Therefore, when the thieno[3,2-b]indole moiety is introduced into the π-conjugation system as a donor of dye-structure, a red shift of the absorption spectra and an enhancement of the light harvesting ability of the dyes could be expected. Furthermore, the planarity between thieno[3,2b]indole moiety and oligothiophene linkage would be more increased for the dye molecule based on a thieno[3,2-b]indole donor part instead of a carbazole. It is well understood that the increase of the molecular planarity can extend the π-conjugation system of the molecule, so that the enhancement of the ε could be also expected by the introduction of the thieno[3,2-b]indole structure. The structural composition of the most reported efficient organic dyes employed in DSSCs typically consists of an electron donor, an electron acceptor, a π-conjugated bridge between the donor and acceptor, and/or an anchoring group.7-21 It has been proved in previous research that, (a) the long alkysubstituted oligothiophene linkage is effective to inhibit the aggregation of the dyes by the steric hindrance, thus suppressing charge recombination and improving the Voc; (b) the long aliphatic chains can also enhance the long-term stability of the cells through preventing water-induced dye desorption from the
10.1021/jp105548u 2010 American Chemical Society Published on Web 10/01/2010
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J. Phys. Chem. C, Vol. 114, No. 42, 2010
TiO2 surface.11-14 On the basis of the above regards, we designed and synthesized a new class of organic dyes, MKZ39, MKZ-40, and MKZ-41, as shown in Figure 1, using 4-ethyl4H-thieno[3,2-b]indole moiety as an electron donor, n-hexylsubstituted oligothiophene units as a π-conjugation linkage, and cyanoacrylic acid as an electron acceptor and anchoring group, for application in DSSCs. In this article, we perform DFT and time-depended density functional theory (TD-DFT) calculations to provide a detailed characterization of the structural, electronic, and optical properties of the newly designed dyes. The synthesis, characterization, photovoltaic properties, and transient absorption properties of the dyes are reported. Moreover we compare with carbazolebased MK dyes (Figure S1 of the Supporting Information), which consist of similar π-conjugation linkage and the same acceptor/anchoring group, but using N-ethylcarbazole moiety as the electron donor.11 To the best of our knowledge, this is the first report on the photovoltaic performance of DSSCs sensitized using thieno[3,2,-b]indole-based organic dyes. Experimental Section Synthesis of the Dyes. The detail syntheses procedures and characterizations for MKZ-39, MKZ-40, and MKZ-41 are shown in the Supporting Information. Molecular Orbital Calculations. Theoretical calculations were performed on Gaussion 03 program package (revision D.01)24 using the DFT method before the synthesis of the dyes. Becke’s three-parameter hybrid functional with the LYP correlation functional (B3LYP) was employed together with 6-31G(d) basis set.25 Geometry optimizations and electronic properties of the dyes were carried out without any symmetry constraint in the gas phase and by assuming the target molecules to be isolated. We further calculated the lowest 10 singlet transitions of the dyes using the TD-DFT method, and the UV-vis spectra of these dyes were calculated using the SWizard program, revision 4.6,26 using the Gaussian model, the halfbandwidths ∆1/2,I is assumed to be 3000 cm-1. Materials and General Procedures. All starting materials and solvents for synthesis, measurements, and solar cell fabrication were purchased from Wako Chemicals, Kanto Chemicals, Tomiyama Pure Chemical Industries Ltd., Aldrich, Tokyo Chemical Industry Co., Ltd., and/or Merck and used without further purification. 1H NMR and 13C NMR spectra were recorded on a Bruker Avance 400 (400 MHz) spectrometer in CDCl3 or THF-d8, chemical shifts were reported as δ values (ppm) relative to internal tetramethylsilane (TMS). Fourier transform infrared (FTTR) spectra were measured with a PerkinElmer Spectrum One spectrophotometer with an attenuated total reflection (ATR) system equipped with a ZnSe prism. Mass spectra were measured on a JEOL MS600H mass spectrometer, elemental analyses were taken on a CE Instruments EA1110 automatic element analyzer. Absorption spectra were measured on a SHIMADZU UV3101 PC spectrophotometer, solvents used for spectroscopy experiments were spectrophotometric grade. Cyclic voltammetry measurements were carried out on a CHI610B electrochemical analyzer, dye-loaded TiO2 film, platinum, and Ag/Ag+ (0.01 M AgNO3 + 0.1 M tetrabutylammonium perchlorate in acetonitrile) were employed as working, counter, and reference electrodes, respectively. The supporting electrolyte was 0.1 M tetrabutylammonium perchlorate in acetonitrile, which was degassed with N2 for 15 min before scanning, and the scanning rate was 100 mV s-1. The potential of the reference electrode
Zhang et al. is 0.49 V versus normal hydrogen electrode (NHE), and was calibrated with ferrocene immediately after CV measurements. Fabrication of Dye-Sensitized Solar Cells. F-SnO2 (FTO)coated glass substrates were cleaned in a detergent solution by an ultrasonic bath, rinsed with water and ethanol, and then dried using N2 current. Nanocrystalline TiO2 films, 4 µm, 5 µm, 6 µm (transparent layer, consisting of only ∼20 nm nanoparticles), or 14 µm {consisting of a 9 µm transparent layer (∼20 nm nanoparticles) and a 5 µm scattering layer (60% ∼20 nm nanoparticles and 40% ∼100 nm large particles)} in thickness, were prepared using screen printing technique.27 After treated by TiCl4, the TiO2 films were immersed into the tetrahydrofuran (THF) with or without 1 mM 3a,7a-dihydroxy-5b-cholic acid (DCA), or toluene solution of the dyes (0.3 mM), for about 12 h. The dye-adsorbed TiO2 film electrode and Pt-counter electrode were assembled into a sealed sandwich solar cell with a hot-melt Surlyn film (30 µm in thickness) as a spacer between the electrodes. A drop of the electrolyte solution {0.6 M 1,2dimethyl-3-n-propylimidazolium iodide (DMPImI) + 0.1 M LiI + 0.2 M I2 + 0.5 M 4-tert-butylpyridine (TBP) in acetonitrile} was driven into the cell through the hole in the counter electrode via the suction through another drilled hole. Finally, the two holes were sealed using additional hot-melt Surlyn film covered with a thin glass slide. For the electron lifetime measurements, the transparent TiO2 film of 4 µm and the electrolyte composed of 0.6 M DMPImI, 0.1 M LiI, 0.05 M I2, and 0.5 M TBP in acetonitrile. For the stability test, the transparent TiO2 film of 6 µm and the electrolyte composed of 0.6 M DMPImI, 0.1 M LiI, 0.05 M I2, and 0.5 M TBP with 3-methoxypropionitrile as solvent were used, and the sealed DSSC device at open-circuit equipped with a
