Tuning the Energy Level of Organic Sensitizers for High-Performance

(3) The dye-sensitized solar cell (DSC)(4) has attracted considerable ... This hurdle has precluded large-scale outdoor application and integration in...
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J. Phys. Chem. C 2009, 113, 2966–2973

Tuning the Energy Level of Organic Sensitizers for High-Performance Dye-Sensitized Solar Cells Mingfei Xu,† Sophie Wenger,‡ Hari Bala,† Dong Shi,† Renzhi Li,† Yanzhou Zhou,† Shaik M. Zakeeruddin,‡ Michael Gra¨tzel,‡ and Peng Wang†,* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China, and Laboratory for Photonics and Interfaces, Swiss Federal Institute of Technology, CH 1015, Lausanne, Switzerland ReceiVed: October 21, 2008; ReVised Manuscript ReceiVed: December 23, 2008

Cost-effective organic sensitizers will play a pivotal role in the future large-scale production and application of dye-sensitized solar cells. Here we report two new organic D-π-A dyes featuring electron-rich 3,4-ethylenedioxythiophene- and 2,2′-bis(3,4-ethylenedioxythiophene)-conjugated linkers, showing a remarkable red-shifting of photocurrent action spectra compared with their thiophene and bithiophene counterparts. On the basis of the 3-{5′-[N,N-bis(9,9-dimethylfluorene-2-yl)phenyl]-2,2′-bis(3,4-ethylenedioxythiophene)-5-yl}2-cyanoacrylic acid dye, we have set a new efficiency record of 7.6% for solvent-free dye-sensitized solar cells based on metal-free organic sensitizers. Importantly, the cell exhibits an excellent stability, keeping over 92% of its initial efficiency after 1000 h accelerated tests under full sunlight soaking at 60 °C. This achievement will considerably encourage further design and exploration of metal-free organic dyes for higher performance dye-sensitized solar cells. We have also scrutinized the physical origins of the relatively low photocurrent and photovoltage obtained with an ionic liquid electrolyte compared to a volatile acetonitrilebased electrolyte through transient and modulated photoelectrical measurements. Introduction In view of the limited availability of fossil fuels and the disastrous environmental issues concomitant with their combustion, it is imperative to develop renewable energy resources for the increasing global energy demand.1 Affluent solar energy has a widely recognized capacity to become a major sustainable energy supply.2 Incontrovertibly, the performance/price ratio will play a pivotal role in the eventual selection of various photovoltaic devices.3 The dye-sensitized solar cell (DSC)4 has attracted considerable attention as a promising technology for low-cost photovoltaic cells in the past 2 decades owing to its high efficiency of over 11%5 and its high performance under the prolonged light and thermal dual stress.6 The DSC efficiency could be further improved by the design of new sensitizers showing a good spectral overlap with the solar irradiance and by exploration of multijunction tandem device architectures.7 At the moment a major drawback of the DSC technology is the usage of volatile electrolytes to achieve a high efficiency. This hurdle has precluded large-scale outdoor application and integration into flexible cells. Among various efforts to develop solvent-free DSCs to address this issue, the use of roomtemperature ionic liquid electrolytes has proved to be the most successful strategy by far.8 However, with a low fluidity ionic liquid electrolyte the charge collection yield in DSCs decreases due to the shortened electron diffusion length.5c Enhancing the optical absorption coefficient of a stained mesoporous film to allow for the use of a thin photoactive layer can counter this effect. Apart from a * To whom correspondence should be addressed. E-mail: peng.wang@ ciac.jl.cn. † Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. ‡ Swiss Federal Institute of Technology.

Figure 1. Molecular structures of the C204 (n ) 1) and C205 (n ) 2) sensitizers.

high absorption coefficient of organic sensitizers in comparison with polypyridyl ruthenium dyes, the flexibility in tailoring organic dyes at the molecular level could inject a new momentum to boost dye-sensitized solar cells.9 In this context, an impressive device efficiency up to 7.2% has been reached by employing an indoline dye along with an ionic liquid electrolyte.10 Unfortunately, our subsequent tests showed that the stability of that cell was hampered mainly due to desorption of the indoline dye from the TiO2 nanocrystals. The conjugation of bithiophene,11 thienothiophene,12 or dithienothiophene13 with the bisfluorenylaniline fragment has been demonstrated as a viable strategy to construct organic sensitizers for making stable DSCs with ionic liquid electrolytes, showing efficiencies in the range of 5.8-7%. However, all these organic sensitizers have a narrower spectral response than state-of-the art ruthenium sensitizers. Here we design and synthesize two novel organic sensitizers coded C204 and C205, which are shown in Figure 1, featuring an electron-rich 3,4-ethylenedioxythiophene (EDOT) and a planar 2,2′-bis(3,4-ethylenedioxythiophene) (biEDOT), respectively. In contrast to their thiophene and bithiophene

10.1021/jp809319x CCC: $40.75  2009 American Chemical Society Published on Web 01/27/2009

Tuning the Energy-Level of Organic Sensitizers counterparts,11 these two sensitizers exhibit a remarkable redshifted spectral and photocurrent responses. Apart from the new record efficiency of 7.6% for solvent-free dye-sensitized solar cell based on metal-free organic dyes, the highefficiency cells also shows an excellent stability during a preliminary 1000 h accelerated testing. By means of intensitymodulated photocurrent and photovoltage spectroscopy as well as transient photoelectrical decay analysis, we have further detailed the origins of the relatively low photocurrent and photovoltage of a solvent-free cell compared to that employing a volatile acetonitrile-based electrolyte. 2. Experimental Section 2.1. Materials. All solvents and reagents, unless otherwise stated, were of puriss quality and used as received. 4-Bromoaniline, n-butyllithium, and tetrabutylammonium hexafluorophosphate were purchased from Aldrich. Guanidinium thiocyanate (GNCS), 2-cyanoacetic acid, and 3R,7R-dihyroxy-5β-cholic acid (cheno) were purchased from Fluka. 3,4(Ethylenedioxy)thiophene was purchased from Beili Pharmacy Co. 1-Ethyl-3-methylimidazolium tetracyanoborate (EMITCB) and 400-nm-sized TiO2 anatase particles were received as gifts from Merck and Catalysts & Chemical Ind. Co., respectively. The solvent-free synthesis of 1,3-dimethylimidazolium iodide (DMII) and 1-ethyl-3-methylimidazolium iodide (EMII) were described in our previous paper.14 N-butylbenzimidazole (NBB) was synthesized according to the literature method15 and distilled before use. 2-(Tributylstannyl)-3,4-(ethelenedioxy)thiophene,16 2,2′-bis(3,4-ethylenedioxythiophene),17 and N,N-bis(9,9-dimethylfluoren-2-yl)4-bromoaniline18 were synthesized as described in the corresponding literature. 2.2. Synthesis of 2-[N,N-Bis(9,9-dimethylfluorene-2-yl)phenyl]-3,4-(ehtylenedioxy)thiophene (1a). 2-(Tributylstannyl)3,4-(ethelenedioxy)thiophene (0.7 g), N,N-bis(9,9-dimethylfluoren-2-yl)-4-bromoaniline (0.6 g), and tetrakis(triphenylphosphine)palladium (0.12 g) were dissolved in toluene (30 mL), and the reaction was refluxed under Ar for 6 h. After cooling down to room temperature, the mixture was poured into water and extracted three times with dichloromethane. The combined organic layer was washed in turn with sodium carbonate aqueous solution and deionized water, and subsequently dried over anhydrous sodium sulfate. After rotary evaporation of the solvent under a reduced pressure, the residue was purified on a silica gel column with toluene/hexane (2:1, V/V) as eluent to give a yellow solid (0.48 g). Yield: 72%. 1H NMR (DMSOd6, 400 MHz): δ 1.37 (s, 12H), 4.24 (d, 2H), 4.30 (d, 2H), 6.57 (s, 1H), 6.99-7.02 (q, 2H), 7.18 (d, 2H), 7.26-7.28 (t, 4H), 7.30-7.33 (t, 2H), 7.50 (d, 2H), 7.59 (d, 2H), 7.72-7.75 (q, 4H). Synthesis of 5-[N,N-Bis(9,9-dimethylfluorene-2-yl)phenyl]2,2′-bis(3,4-ethylenedioxythiophene) (1b). n-Butyllithium (1.6 M in n-hexane, 6.64 mL) was added dropwise to 2,2′-bis(3,4ethylenedioxythiophene) (3 g) in dry THF (150 mL) at -78 °C under Ar. The reaction was kept at this temperature for 1 h, and tributylstannyl chloride (3.46 g) dissolved in THF (20 mL) was added. The mixture was stirred for 30 min at -78 °C and then for another 6 h at room temperature. After that the mixture was poured into water and extracted three times with dichloromethane. The combined organic layer was washed in turn with sodium bicarbonate aqueous solution and deionized water, and subsequently dried over anhydrous sodium sulfate. After rotaevaporation of the solvent under a reduced pressure, a colorless liquid of crude 5-(tributylstan-

J. Phys. Chem. C, Vol. 113, No. 7, 2009 2967 nyl)-2,2′-bis(3,4-ethylenedioxythiophene) was obtained and used for the next step without any purification. The Stille coupling was performed according to the above procedure for 1a. Purification on a silica gel column with dichloromethane/hexane (1:1, V/V) as eluent gave a yellow solid. Yield: 68%. 1H NMR (DMSO-d6, 400 MHz): δ 1.37 (s, 12H), 4.20-4.35 (m, 8H), 6.58 (s, 1H), 7.00-7.03 (q, 2H), 7.11 (d, 2H), 7.26-7.28 (t, 4H), 7.30-7.34 (t, 2H), 7.50 (d, 2H), 7.60 (d, 2H), 7.73-7.76 (q, 4H). Synthesis of 3-{5-[N,N-Bis(9,9-dimethylfluorene-2-yl)phenyl]-3,4-(ethylenedioxy)thiophene-2-yl}-2-cyanoacrylic Acid (3a, C204). Compound 1a (0.45 g), DMF (0.44 mL), and phosphorus oxychloride (0.082 mL) were dissolved in 1,2dichloroethane (30 mL), and the mixture was stirred for 4 h at room temperature under Ar. After that, 20 mL of water was added into the flask, followed by neutralizing with sodium acetate. The mixture was extracted three times with dichloromethane, and the combined organic phase was dried over anhydrous sodium sulfate. After rotary evaporation of the solvent under a reduced pressure, crude 2a as an orange solid (0.45 g) was obtained and used for the next step without any purification. Compound 2a (0.45 g), 2-cyanoacetic acid (0.071 g), and piperidine (0.03 mL) were added to acetonitrile (20 mL), and the mixture was refluxed for 14 h. Then the solvent was removed and 20 mL of water was added. The mixture was acidified with 20% aqueous HCl and extracted three times with chloroform. The organic phase was dried over anhydrous sodium sulfate. After rotary evaporation of the solvent, the residue was purified on a silica gel column with chloroform as eluent to give the crude product, which was then dissolved in chloroform and washed with HCl aqueous solution. The removal of solvent under reduced pressure gave a purple solid (0.32 g). Yield: 65%. 1H NMR (DMSO-d6, 400 MHz): δ 1.38 (s, 12H), 4.41 (d, 4H), 7.06 (d, 2H), 7.13 (d, 2H), 7.25-7.34 (m, 6H), 7.51 (d, 2H), 7.66 (d, 2H), 7.74-7.78 (t, 4H), 8.07 (s, 1H). 13C NMR (DMSOd6, 100 MHz): δ 26.6, 64.63, 65.61, 117.1, 119.1, 119.6, 121.1, 121.9, 122.6, 123.7, 124.2, 126.8, 127.1, 127.8, 132.4, 134.6, 137.5, 13.80, 145.8, 148.1, 154.9, 164.0, 171.9. ESIMS m/z calcd for (C46H36N2O4S): 712.85. Found: 712.80. Synthesis of 3-{5′-[N,N-Bis(9,9-dimethylfluorene-2-yl)phenyl]-2,2′-bis(3,4-ethylenedioxythiophene)-5-yl}-2-cyanoacrylic Acid (2d, C205). Performing a similar procedure as described above with 1b as starting material gave 3b (C205) as a purple solid in 68% yield. 1H NMR (DMSO-d6, 400 MHz): δ 1.38 (s, 12H), 4.38-4.54 (m, 8H), 7.04 (d, 2H), 7.12 (d, 2H), 7.25-7.34 (m, 6H), 7.51 (d, 2H), 7.66 (d, 2H), 7.74-7.78 (t, 4H), 8.17 (s, 1H). 13C NMR (DMSO-d6, 100 MHz): δ 26.6, 64.9, 65.0, 65.4, 65.7, 117.4, 118.5, 118.9, 119.5, 121.1, 122.6, 123.0, 123.1, 125.8, 126.7, 126.8, 127.0, 134.0, 136.1, 137.5, 138.1, 139.7, 140.9, 146.3, 146.5, 148.2, 153.1, 154.1, 164.3. ESI-MS m/z calcd for (C52H40N2O6S2): 853.01. Found: 852.50. 2.3. Computation. All calculations were performed in the Gaussian 03W program package. Without any symmetrical constraints, the geometrical structures of the C204 and C205 sensitizers were optimized by employing the density functional theory (DFT) method combined with Becke’s three-parameter hybrid functional19 and Lee-Yang-Parr’s gradient-corrected correlation functional.20 In particular, a 6-31G(d) basis set was applied for all atoms.21,22 Incorporating the optimized model in the ZINDO/S method, we calculated the lowest 40 singlet-singlet electronic transitions. Subsequently, the ZINDO/S results were entered into the SWizard program (http://www.sg-chem.net/

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swizard/) to compute the absorption profile as a sum of the Gaussian band using the following equation:23

ε(ω) ) 2.174 × 10

9

∑ I

(

(ω - ωI)2 fI exp -2.773 ∆1⁄2,I ∆1⁄2,I2

)

(1)

where ε is the molar extinction coefficient given in the unit of M-1 cm-1, ω is the energy of all allowed transitions expressed in cm-1, fI is the oscillator strength, and ∆1/2 is the halfbandwidths and is assumed to be 3000 cm-1. 2.4. UV-vis, Emission, ATR-FTIR, and Voltammetric Measurements. Electronic absorption spectra were measured on a UNICO WFZ UV-2802PC/PCS spectrometer. Emission spectra were recorded with a PerkinElmer LS55 luminescence spectrometer. The emitted light was detected with a Hamamatsu R928 red-sensitive photomultiplier. The ATR-FTIR spectra were measured using a BRUKER Vertex 70 FTIR spectrometer. A CHI660C electrochemical workstation was used for square-wave voltammetric measurements in combination with a mini threeelectrode electrochemical cell equipped with a 5 µm radius Pt ultramicroelectrode as working electrode. A Pt wire and a silver wire were used as counter and quasi-reference electrodes, respectively. The redox potential of ferrocene internal reference is taken as 0.64 V vs NHE. 2.5. Device Fabrication. A screen-printed double-layer film of interconnected TiO2 particles was used as mesoporous negative electrode. A 7 µm thick film of 20-nm-sized TiO2 particles was first printed on the fluorine-doped SnO2 conducting glass electrode and further coated by a 5 µm thick second layer of 400-nm-sized light-scattering anatase particles. The detailed preparation procedures of TiO2 nanocrystals, pastes for screenprinting, and double-layer nanostructured TiO2 film have been reported in our previous paper.24 The TiO2 electrode was stained by immersing it into a dye solution containing 300 µM C204 (or C205) and 2 mM (or 10 mM) cheno in a mixture of acetonitrile and tert-butyl alcohol (volume ratio 1/1) for 5 h. After washing with acetonitrile and drying by air flow, the sensitized titania electrode was assembled with a thermally platinized conducting glass counter electrode. The electrodes were separated by a 25 µm thick Surlyn hot-melt gasket and sealed up by heating. The internal space was filled with a liquid electrolyte using a vacuum back-filling system. The electrolyteinjecting hole on the counter electrode glass substrate, made with a sand-blasting drill, was sealed with a Surlyn sheet and a thin glass cover by heating. Two electrolytes were used for device evaluation. Electrolyte I is 1.0 M DMII, 50 mM LiI, 30 mM I2, 0.5 M tert-butylpyridine, and 0.1 M GNCS in the mixed solvent of acetonitrile and valeronitrile (V/V, 85/15);5c electrolyte II is DMII/EMII/EMITCB/I2/NBB/GNCS (molar ratio 12/12/ 16/1.67/3.33/0.67), where the iodide and triiodide concentrations are 3.182 and 0.238 M, respectively.8 Devices A and B were made by employing the C204 sensitizer with electrolytes I and II, respectively. Devices C and D were fabricated using the C205 sensitizer with electrolytes I and II, respectively. 2.6. Photovoltaic Characterization. A 450 W xenon light source (Oriel) was used to give an irradiance of 100 mW cm-2 (the equivalent of one sun at AM 1.5G) at the surface of a testing cell. The spectral output of the lamp was matched in the region of 350-750 nm with the aid of a Schott K113 Tempax sunlight filter (Pra¨zisions Glas & Optik GmbH, Germany) so as to reduce the mismatch between the simulated and true solar spectra. Various incident light intensities were regulated with neutral wire-mesh attenuators. The current-voltage characteristics of the cell under these conditions were obtained by applying external potential bias to the cell and measuring the generated

photocurrent with a Keithley model 2400 digital source meter (Keithley). This process was fully automated using Wavemetrics software (http://www.wavemetrics.com/). A similar data acquisition system was used to control the incident photon-to-current conversion efficiency (IPCE) measurement. Under full computer control, the light from a 300 W xenon lamp (ILC Technology) was focused through a Gemini-180 double monochromator (Jobin Yvon Ltd.) onto the photovoltaic cell under test. The monochromator was incremented through the visible spectrum to generate the IPCE (λ) as defined by IPCE (λ) ) 12400(Jsc/ λφ), where λ is the wavelength, Jsc is the short-circuit photocurrent density (mA cm-2), and φ is the incident radiative flux (mW cm-2). The photovoltaic performance was measured by using a metal mask with an aperture area of 0.159 cm2. Solar cells covered with a 50 µm thick of polyester film (Preservation Equipment Ltd., UK) as a 400 nm UV cutoff filter were irradiated at open circuit under a Suntest CPS plus lamp (ATLAS GmbH, 100 mW cm-2) in ambient air at 60 °C. The photovoltaic measurements were carried out at room temperature after allowing the cells to cool down and equilibrate during 2 h. 2.7. Transient Photoelectrical Measurements. In the transient photoelectrical decay experiment, steady-state light was supplied with a homemade white-light-emitting diode (LED) array by tuning the driving voltage, and a green LED array controlled with a fast solid-state switch was used to generate a perturbing pulse with a width of 200 ms. The pulsed green light and steady-state white light were both incident on the photoanode side of a testing cell. The green pulse was carefully controlled by the driving potential of diodes to keep the modulated photovoltage below 5 mV. We used green light as a probe to generate a photovoltage perturbation near the opencircuit photovoltage (Voc) of the cell under the white light and measured the voltage decay process thereafter. Normally, the transient signals follow a monoexponential decay; thus, the recombination rate constant, kr, can be extracted from the slope of the semilogarithmic plot. The capacitance (Cµ) of the TiO2/ electrolyte interface at the Voc are calculated by Cµ ) ∆Q/∆V, where ∆V is the peak of the photovoltage transient and ∆Q is the number of electron injected during the red light flash. The latter was obtained by integrating a short-circuit photocurrent transient generated from an identical green-light pulse. This method may underestimate the actual injected electrons by the fraction that is lost due to recombination during the electron collection. The error is thought to be less than 30% in the worst case. More critically, it does not affect the shape of the calculated capacitance versus potential but only the magnitude. 2.8. IMVS and IMPS Measurements. The intensitymodulated photovoltage spectroscopy (IMVS) and intensitymodulated photocurrent spectroscopy (IMPS) were carried out on a ZAHNER CIMPS system. Stationary as well as sinusoidal modulated light was supplied with a green LED with a maximum wavelength at 546 nm. The LED was controlled by a potentiostatic feedback loop. The selected alternating current (ac) amplitude is in the range of 5-15% of the stationary direct current (dc) value. The transfer function for the IMPS and IMVS were determined by correlating the system response with the actually stimulation signal. The potential applied to a testing cell was controlled by a potentiostat. IMPS was carried out at short-circuit while IMVS was at open-circuit. The measured short-circuit photocurrent efficiency (Φext(ω)) of IMPS and the real and imaginary parts of modulated photovoltage ∆Voc of IMVS were fitted using the Levenberg-Marquardt algorithm.

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Figure 3. Schematic energy levels and isodensity plots of HOMO and LUMO of the C204 and C205 sensitizers.

Figure 2. (A) Electronic absorption and emission spectra of the C204 and C205 sensitizers dissolved in chloroform. (B) Normalized electronic absorption spectra of the C204 and C205 sensitizers anchored on transparent mesoporous titania films. The inset is transmission spectra in the long wavelength range.

3. Results and Discussion The electronic absorption and emission spectra of the C204 and C205 sensitizers dissolved in chloroform are shown in Figure 2A. While the trivial ultraviolet absorptions for DSCs peak at 358 and 371 nm, the molar extinction coefficients of their low-energy bands at 525 and 544 nm, mainly stemming from the intramolecular charge-transfer transitions, are 33.5 × 103 and 38.5 × 103 M-1 cm-1, respectively. Obviously, with the increase of the conjugation length, the charge-tranfer transition absorption is not only red-shifted but also enhanced. The emission of the C204 and C205 sensitizers in chloroform is centered at 688 and 697 nm, respectively. The excitation transition energy (E0-0) was roughly estimated to be 2.08 and 2.00 eV by taking the crossing-point of the absorption and emission spectra. We explored the origins of these transitions by calculating the electronic states (Figures S1 and S2 in the Supporting Information) of the C204 and C205 dyes with the ZINDO/S method in the Gaussian 03W program suite. The low-energy absorptions stem from the πfπ* transitions from HOMO to LUMO and HOMO-1 to LUMO while the high-energy absorptions are mainly due to the electron transitions from HOMO to LUMO+2 and HOMO-1 to LUMO+2 (Tables S1 and S2 in the Supporting Information). The isodensity surface plots of HOMO and LUMO are presented in Figure 3. Obviously, the HOMO of the C204 and C205 sensitizers is populated over the substituted triarylamine and EDOT moieties, while the LUMO is delocalized through the fragments of EDOT and cyanoacrylic acid groups with sizable contribution from the latter. This spatially directed separation of HOMO and LUMO is an ideal condition for dye-sensitized solar cells, which not only facilitates the ultrafast interfacial electron injection from

the excited dye to the TiO2 conduction band but also slows down the recombination of the injected electron with the oxidized sensitizers due to their remoteness. In addition, the hole localization on the triarylamine unit facilitates the electron donor to approach, promoting the fast dye regeneration. Moreover, both HOMO and LUMO exhibit overlapping extension to the EDOT or biEDOT fragment, enhancing the electronic coupling parallel to the electronic transition dipole moment between the two states, which in turn increases the oscillator strength between these two electronic states. Since the charge-transfer absorption bands are very sensitive to the solvent polarity, we also measured the UV-vis spectra of these two sensitizers in ethanol. As depicted in the Supporting Information (Figure S3), these two bands are maximized at 484 and 508 nm, which are 48 and 52 nm red-shifted with respect to their thiophene and bithiophene counterparts in ethanol, respectively.11 In view of the fact that the absorption maxium of the bithiophene-based sensitizer is only 16 nm red-shifted with respect to thiophene,11 the spectral enhancement achieved here through the introduction of EDOT or biEDOT as linker into D-π-A dyes is considerable. Apparently, the oxygen atoms of the EDOT unit decreases the aromatic character of thiophene unit and extends the conjugation length, and this effect will be detailed in our further publication. Furthermore, our DFT calculations show that EDOT has a smaller torsion angle with respect to its adjacent phenyl group in the triarylamine donor than thiophene, which should play a critical role for the enhanced absoprtion of C204 in contrast to its thiophene counterpart. The planar geometry of biEDOT itself together with a small torsion angle with respect to the neighboring phenyl explains the spectral enhancement of C205 compared to its bithiophene counterpart. We also note that these spectral shiftings are not counteracted by decrease of molar extinction coefficients. This new finding highlights the importance of using an electron-rich and planar conjugation unit for the further design of organic sensitizers. The absorption spectra (Figure 2B) of the C204 and C205 dyes anchored on transparent mesoporous titania film shows maxima at 463 and 499 nm, respectively. The optical HOMO-LUMO gaps of the C204 and C205 dyes anchored on the titania film derived from the inset of Figure 2B are 1.94 and 1.78 eV, respectively. The redox potentials of the C204 and C205 sensitizers were accurately measured by employing the ultramicroelectrode technique in combination with square-wave voltammetry. As presented in Figure 4, the LUMO of both dyes at -0.87 V vs NHE is more negative than the conduction band edge (-0.5 V vs NHE) of titania, providing sufficient thermodynamic driving

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Figure 4. Square-wave voltammograms of a Pt ultramicroelectrode in DMF solution containing the C204 or C205 sensitizer. Supporting electrolyte was 0.1 M tetrabutylammonium hexafluorophosphate.

force for electron injection from the excited dyes to titania. In addition, their ground-state oxidation potentials (1.00 and 0.89 V vs NHE) are both higher than the redox potential (0.4 V vs NHE) of the iodide/triiodide couple. This could lead to fast dye regeneration, avoiding the geminate charge recombination between oxidized dye molecules and photoinjected electrons in the nanocrystalline titania film. It is noted that adding one more EDOT unit does not have an influence on the LUMO level as well as ground-state reduction potentials but lifts up the HOMO level significantly, narrowing the HOMO and LUMO gaps. This pattern proved by the ZINDO/S calculation (Figure 3) differs from that observed for the thiophene and bithiophene counterparts.11 We believe that this should be ascribed to the strong electron-donating character of the employed EDOT unit. It is noted that the measured electrochemical LUMO/HOMO gaps are 1.78 and 1.66 eV for C204 and C205, which are smaller than the above-mentioned optical gaps. This difference is a common feature of organic optoelectronic materials and may be related to the solvent effect. However, we have noted that the measured electrochemical HOMO/LUMO gaps are in good agreement with the energy gaps (1.77 eV for C204 and 1.65 eV for C205) derived from the low-energy onset of photocurrent action spectra presented in Figure 5A. The ATR-FTIR spectrum (Figure S5 in the Supporting Information) of the C205 dye anchored on a TiO2 film clearly shows the bands at 1603 and 1381 cm-1 for the asymmetric and symmetric stretching modes of the carboxylate groups, indicating that the carboxylic acid group is deprotonated and engaged in the adsorption of the dye on the surface of TiO2.25 From the ATR-FTIR data, we infer that the dye is anchored on the surface though the carboxylate groups via a bidentate chelation or a bridging of surface titanium ions rather than ester type linkage.25 The NtC signal remains at 2209 cm-1, indicating that cyano groups may not take part in the dye-adsorption process. The sharp aromatic ring modes are at 1576, 1447, 1363, and 1269 cm-1 while the C-N stretching modes of triarylamine are at 1322 cm-1. The band at 1145 cm-1 is assigned to the stretching vibration mode of the ethylenedioxy group. The peaks at 2857 and 2923 cm-1 are due to the symmetric and asymmetric -CH2- stretch vibrations of the EDOT unit. The corresponding CH3 peak is observed at 3002 cm-1, while the C-H stretching mode of aromatic rings is at 3053 cm-1. The C204 dye anchored on the transparent titania film shows a similar ATR-FTIR spectroscopy. Plots of incident photon-to-collected electron conversion efficiencies (IPCE) versus wavelength for devices A and C employing the C204 and C205 sensitizers in combination with an acetonitrile-based electrolyte are shown in Figure 5A. The photocurrent action spectra of both sensitizers exhibit a high

Figure 5. (A) Photocurrent action spectra of devices A and C with the corresponding C204 and C205 dyes. (B) J-V characteristics measured under an irradiance of 100 mW cm-2 AM 1.5G sunlight and in the dark. (a) Device A in the dark, (b) device A under light, (c) device C in the dark, and (d) device C under light. The electrolyte composition is as follows: 1.0 M DMII, 50 mM LiI, 30 mM I2, 0.5 M tert-butylpyridine, and 0.1 M GNCS in the mixed solvent of acetonitrile and valeronitrile (V/V, 85/15).

plateau where the incident photon to current conversion efficiency reaches ∼90%. Considering the light absorption and scattering loss by the conducting glass, the maximum efficiency for absorbed photon-to-collected electron conversion efficiency (APCE) is almost unity over a broad spectral range, suggesting a very high charge collection yield. The red-shifted photocurrent action spectrum of device C with the C205 dye compared to that of device A with C204 is consistent with the absorption measurements on stained mesoporous titania films shown in Figure 2B. Note that without cheno coadsorption, cells exhibit lower IPCEs, although more dyes have been grafted to the titania film, which may be caused by a low charge generation yield owing to a rapid deactivation of excitons in the case of dye aggregation. This will be clarified in our further pump-probe measurements. As presented in Figure 5B, the short-circuit photocurrent density (Jsc), open-circuit photovoltage (Voc), and fill factor (FF) of device A with the C204 sensitizer under an irradiance of AM 1.5G full sunlight are 13.82 mA cm-2, 762 mV, and 0.751, respectively, yielding an overall conversion efficiency (η) of 7.92%. In contrast, the photovoltaic parameters (Jsc, Voc, FF, and η) of device C with the C205 sensitizer are 15.68 mA cm-2, 746 mV, 0.711, and 8.32%, respectively. Note that by employing a similar titania film and electrolyte, the cell made with the counterpart dye (JK-2) with a bithiophene unit shows a power conversion efficiency of 7%.26 We attribute the improved efficiency with the C205 dye to its better spectral match with the solar irradiance by introducing the biEDOT unit. In order to lower the cost of photovoltaic power production, a substantial improvement in device efficiency of DSCs is still necessary. Nevertheless, stable, low-cost, flexible, and lightweight solar cells based on a plastic matrix are attractive, even

Tuning the Energy-Level of Organic Sensitizers

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Figure 6. J-V characteristics of devices B and D with the corresponding C204 and C205 dyes measured under an irradiance of AM 1.5G sunlight of 100 mW cm-2 and in the dark. (a) Device B in the dark, (b) device B under light, (c) device D in the dark, and (d) device D under light. The solvent-free electrolyte composition is DMII/EMII/ EMITCB/I2/NBB/GNCS (molar ratio 12/12/16/1.67/3.33/0.67).

if their solar conversion efficiencies are moderate, i.e. in the 5-10% range. However, for these devices the use of organic solvents is nondesirable, as they would permeate across polymeric cell walls. Thus, we employed a solvent-free ionic liquid to evaluate the preliminary efficiency of these two organic sensitizers. As presented in Figure 6, under the standard AM 1.5G full sunlight, the photovoltaic parameters (Jsc, Voc, FF, and η) of device B using the C204 sensitizer are 13.58 mA cm-2, 708 mV, 0.760, and 7.31%, respectively. Device D with the C205 sensitizer exhibits a Jsc of 14.85 mA cm-2, a Voc of 696 mV, and a FF of 0.736, reaching a power conversion efficiency of 7.61%. At various lower light intensities, the efficiencies are even higher, up to 8.5%. This is the first time that such high efficiencies have been achieved for solvent-free dye-sensitized solar cells based on metal-free organic sensitizers. Note that with a similar ionic liquid electrolyte, only 6% efficiency under the AM 1.5G full sunlight was obtained for the JK2 dye (C205 counterpart) with a bithiophene unit.27 We exposed solvent-free devices B and D, with a UV absorbing antireflection film adhered on the photoanode matrix, to the 1000 h accelerated testing at 60 °C, in a solar simulator with a light intensity of 100 mW cm-2. UV photons provoke a direct band-to-band excitation of titania nanocrystals, generating holes in the valence band, which in turn may attack some organic materials in our cells. However, to some extent this electron-transfer reaction can be suppressed by the capture of holes with iodide in the electrolytes. Note that the contribution of total UV photon flux to the standard AM 1.5G solar spectrum is very small. We remark that the use of an antireflection film not only removes the UV absorption but reduces the visiblelight reflection, resulting in overall enhanced efficiencies for DSCs. As presented in Figure S6 in the Supporting Information as well as in Figure 7, devices B and D both exhibit excellent stabilities, maintaining over 92% of their initial efficiencies after the 1000 h accelerated aging. While we did observe 63 and 47 mV drops in open-circuit photovoltage for devices B and D, respectively, it is worth noting that there is almost no decrease in fill factor and photocurrent during the whole tests, proving the stability of the C204 and C205 dyes. Different from the reference cells with indoline dyes, where the electrolyte becomes red after short-term aging, we do not see any desorption during long-term device operation for cells with the C204 and C205 dyes, ensuring a stabilization of their photocurrents. The small change in photovoltage may reflect the augmentation of surface states in the mesoporous titania film. This problem could be solved by using some special additives to counter this effect,

Figure 7. Detailed photovoltaic parameters of device D with the C205 dye measured under an irradiance of 100 mW cm-2 AM 1.5G sunlight during successive full sunlight soaking at 60 °C. The solvent-free electrolyte composition is DMII/EMII/EMITCB/I2/NBB/GNCS (molar ratio 12/12/16/1.67/3.33/0.67).

Figure 8. Plots of chemical capacitance of devices C and D with the C205 dye versus Voc. Electrolyte in device C: 1.0 M DMII, 50 mM LiI, 30 mM I2, 0.5 M tert-butylpyridine, and 0.1 M GNCS in the mixed solvent of acetonitrile and valeronitrile (V/V, 85/15). Eleectrolyte in device D: DMII/EMII/EMITCB/I2/NBB/GNCS (molar ratio 12/12/16/ 1.67/3.33/0.67).

building a self-healing titania/electrolyte interface for the longterm device operation. Keeping in mind the preferential importance of employing solvent-free ionic liquid electrolytes to fabricate high efficiency DSCs rather than using volatile electrolytes, we are very interested in scrutinizing the origins of the relatively low Voc and Jsc of device D in contrast to device C, shedding light on further efficiency enhancements. In order to understand the interaction of different electrolytes with titania partially uncovered by organic molecules, we estimated the chemical capacitance (Cµ) at the titania/electrolyte interface by measuring the transient response of Voc and Jsc to a swift small-amplitude light flash.28 As shown in Figure 8, it appears that there are less electron-trapping states below the conduction band edge for the cell with a volatile electrolyte (device C), resulting mostly from the presence of coordinatively unsaturated titanium species at the surface of the titania nanocrystals. These have been passivated most likely by the GNCS and NBB additives. However, for the solvent-free ionic liquid electrolyte (device D), the passivating action of GNCS and NBB seems to be less effective, probably due to the extraordinary high ion concentrations. The less electron trapping states below the titania conduction band edge in device C and a higher electron density will raise the electron quasi-Fermi level in the titania film for a given illumination intensity. Also, the calculated electrolyte

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Figure 9. Plots of charge recombination rate constant versus (A) opencircuit photovoltage for devices C and D with the C205 dye. Electrolyte in device C: 1.0 M DMII, 50 mM LiI, 30 mM I2, 0.5 M tertbutylpyridine, and 0.1 M GNCS in the mixed solvent of acetonitrile and valeronitrile (V/V, 85/15). Eleectrolyte in device D: DMII/EMII/ EMITCB/I2/NBB/GNCS (molar ratio 12/12/16/1.67/3.33/0.67).

Figure 10. Plots of (A) effective electron lifetime and (B) diffusion coefficient versus light intensity for devices C and D with the C205 dye. Electrolyte in device C: 1.0 M DMII, 50 mM LiI, 30 mM I2, 0.5 M tert-butylpyridine, and 0.1 M GNCS in the mixed solvent of acetonitrile and valeronitrile (V/V, 85/15). Eleectrolyte in device D: DMII/EMII/EMITCB/I2/NBB/GNCS (molar ratio 12/12/16/1.67/3.33/ 0.67).

equilibrium potential of device C is 17 mV positive-shifted compared to that of device D. These together explain the higher Voc of device C than device D. The capture of conduction band electrons by triiodide at the titania/electrolyte interface is known to depend on the electron occupancy of trapping states. By adjusting the output of the white-light-emitting diodes to produce various steady bias light, we generated different electron quasi-Fermi levels of titania films and thus varied Voc and measured the pseudofirst-order recombination rate constant (kr) from the transient decay of Voc caused by the green light flash. Figure 9 presents the semilogarithmic plots of kr versus Voc for devices C and D. Obviously, along with the increase of Voc, the recombination becomes faster. The shortening of electron lifetime along with increasing steady bias light intensity (or Voc) is intrinsically related to the augmentation of occupied trapping states, resulting in a fast detrapping of electrons to the conduction band for recombination with triiodide in the electrolytes. At a given Voc, device D with a solvent-free ionic liquid electrolyte shows a feature of much higher charge recombination rate than device C. This difference is probably caused by the high triiodide concentration of ∼0.24 M in device D compared to 0.03 M in device B. We also resort to the intensity-modulated photovoltage spectroscopy (IMVS)29 to compare the interfacial charge recombination for devices C and D. IMVS measures the periodic photovoltage response of a testing cell to a small sinusoidal perturbation of light superimposed on a large steady background level, providing information on electron lifetime under opencircuit conditions. Figure 10A presents the plot of electron lifetime versus incident light intensity. Indeed, under a given light intensity there is low electron occupancy in trapping states of device D in comparison with device C. On the basis of transient and periodic measurements, we conclude that the observed high charge recombination rate in our ionic liquid cells is mainly related to the high concentration of triiodide. Intensity-modulated photocurrent spectroscopy (IMPS)30 employs the same light perturbation as IMVS but measures the periodic photocurrent response, detailing the dynamics of charge transport and back reaction under short-circuit conditions. The electron transport in the mesoporous titania films may be described by the previously proposed multiple trapping-detrapping (MTD) model.31 Along with the increase of light intensity, more deep traps in the mesoporous titania film may be filled with photoinjected electrons. The detrapping of electrons from shallow traps is much faster, resulting in a highly effective electron diffusion coefficient (Dn), as shown in Figure 10B. The

electron diffusion coefficients in these two devices do not differ significantly, implying effective charge screening for electron transport32 in our ionic liquid device due to a very high concentration of cations, even though there is a huge difference in the viscosities of the employed two electrolytes. 4. Conclusions In summary, two new metal-free organic sensitizers possessing high molar extinction coefficients have been synthesized and demonstrated as efficient sensitizers for dye-sensitized solar cells. By introducing electron-rich EDOT and biEDOT units, the light absorptions as well as photocurrent spectral responses of these two dyes have been remarkably red-shifted compared to their thiophene and bithiophene counterparts. On the basis of the C205 sensitizer, we have set the new efficiency record of 7.6% measured under the irradiance of AM 1.5G full sunlight for solvent-free dye-sensitized solar cells with metal-free organic sensitizers. These cells also exhibit excellent stabilities during long-term accelerated tests under light-soaking and thermal dual stress. With the aid of transient and periodic measurements, we have shown that, compared to the cell with an acetonitrile-based electrolyte, a dye-sensitized solar cell with an ionic liquid electrolyte has a feature of low electron diffusion coefficients and short electron life times in the mesoporous titania film. We have also highlighted the intrinsic physics of a low open-circuit photovoltage for solar cells with ionic liquid electrolytes. Further efforts are underway to address this issue by smart engineering of the titania/dye/electrolyte interface and designing new sensitizers that allow the use of positive equilibrium potential ionic liquid electrolytes without an adverse effect on dye regeneration. Acknowledgment. The National Key Scientific Program (No. 2007CB936700), the National Science Foundation of China (Nos. 50773078 and 60877028), the “CAS 100-Talent Program”, and the “CAS Knowledge Innovation Program” have supported this work. S.W., S.M.Z., and M.G. thank the US Air Force project (FA 8655-08-C-4003) for financial support. Supporting Information Available: Synthetic route, additional calculation results, ATR-FTIR, and device stability data. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Szuromi, P.; Jasny, B.; Clery, D.; Austin, J.; Hanson, B. Science 2007, 315, 781. (b) Clery, D. Science 2007, 315, 782.

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