Significant Enhancement of Open-Circuit Voltage in Indoline-Based

Article pubs.acs.org/cm

Significant Enhancement of Open-Circuit Voltage in Indoline-Based Dye-Sensitized Solar Cells via Retarding Charge Recombination Guo Li, Mao Liang,* Hui Wang, Zhe Sun, Lina Wang, Zhihui Wang, and Song Xue* Department of Applied Chemistry, Tianjin University of Technology, Tianjin, 300384, P.R. China S Supporting Information *

ABSTRACT: Indoline dyes exhibit impressive short-circuit photocurrent (JSC) but show generally low open-circuit voltage (VOC) in dye-sensitized solar cells (DSCs). To retard charge recombination in DSCs, four indoline dyes (XS41, XS42, XS43, and XS44) featuring, respectively, dipropylfluorene, hexyloxybenzene, tert-butylbenzene, and hexapropyltruxene electron donors, have been engineered. The incorporation of bulky rigid groups (i.e., dipropylfluorene and hexapropyltruxene unit) can notably retard the charge recombination at the titania/electrolyte interface. Moreover, we have developed two organic dyes (TC1 and TC2) as alternative coadsorbents to chenodeoxycholic acid (CDCA). Interestingly, it is found that regardless of the dye selection coadsorption with TC2 shows an improved VOC as well as JSC in comparison with its TC1 analogues. Dependence of photovoltage on the structure of TC1/TC2 was also investigated. The results suggest that the change in VOC is likely correlated with the molecular matching between the dyes and the coadsorbents. Combining the two contributions, high VOC in indoline-based DSCs can be realized. The results of XS41, upon coadsorption with TC2, produce a JSC of 16.1 mA cm−2, a VOC of 770 mV, and a fill factor of 0.66, corresponding to a power conversion efficiency of 8.18% under simulated AM1.5G solar light (100 mW cm−2). These findings pave a new way to achieve further efficiency enhancement of indoline dyes. KEYWORDS: indoline dyes, coadsorbent, open-circuit photovoltage, charge recombination, molecular matching

■

6.2%.22,23 An obvious advantage of the indoline dyes is their impressive JSC due to the powerful electron-donating capability of the indoline unit.14 However, they have a lower PCE than those of Ru complexes or triphenylamine dyes mainly because of relative lower VOC caused by the enhanced charge recombination on the TiO2 surface.8,11 In other words, VOC is the key parameter impeding further enhancements in PCE of indoline DSCs. If indoline dyes are endowed with an ability to suppress charge recombination, VOC will be improved, and hence further improvement of power conversion efficiency can be achieved. Therefore, new molecular design of indoline dyes is still important for more efficient DSCs. To reduce the possible charge recombination pathways occurring at the semiconductor/dye/electrolyte interface, introduction linkers15−18,22 or acceptors12 with a long alkyl group have proven to be a successful strategy. Nevertheless, works to explore indoline dyes with suitable electron donors for suppressing charge recombination are limited. In this paper, we have systematically investigated the influence of bulky electron donors on the suppression of charge recombination as well as photovoltaic performance of DSCs based on the indoline dyes (XS41−XS44,

INTRODUCTION Dye-sensitized solar cells (DSCs), as a new type of photovoltaic technology, have been considered to be a credible alternative to conventional inorganic silicon-based solar cells because of their ease of fabrication, high efficiency, and cost-effectiveness.1 To achieve high solar power conversion efficiency, great research efforts are focused on designing and synthesizing new photosensitizers: Ru-based complexes,1,2,3a,b zinc porphyrin complexes,4−7 and organic sensitizers.1,2,3a,c,8 Among them, organic sensitizers with robust availability, ease of structural tuning, and generally high molar extinction coefficients have recently received great attention. Indoline-based dyes are promising organic sensitizers for DSCs based on TiO2 films because of their good photoresponse in the visible region and high efficiency.9−24 Uchida and co-workers have reported a series of indoline dyes containing rhodanine-3-acetic acid (e.g., D205 and D149) as the acceptor and anchoring unit, showing high overall power conversion efficiencies (PCEs) of 8−9.5%.9−13 Tian, Zhu, Wang and co-workers developed a series of D−A−π−A indoline dyes with long-term stability, displaying PCEs of 3− 9.04%.14−19 Matsui and co-workers explored indoline dyes for zinc oxide dye-sensitized solar cells, giving a PCE of 4.9− 5.5%.20,21 Akhtaruzzaman, Yamamoto, and co-workers reported a series of indoline dyes incorporating a dibenzosilole or a phenylenevinylene-conjugated unit, showing PCEs up to © XXXX American Chemical Society

Received: January 17, 2013 Revised: April 2, 2013

A

dx.doi.org/10.1021/cm400196w | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

Article

Figure 1. Chemical structures of XS41−XS44.

employed as alternative coadsorbents to chenodeoxycholic acid (CDCA). Dependence of charge recombination on the structure of coadsorbents was investigated. Experimental results have shown that the addition of TC1 and TC2 (the molar ratio of XS41−XS44 (0.5 mM) to TC1/TC2 (1.5 mM) was 1:3) can notably improve the VOC of indoline dye-based DSCs, especially for TC2. Our results suggest that strategic structural modification of coadsorbent should not only focus on the horizontal direction but also on the downward direction to reduce the charge recombination. Upon coadsorption with TC2, XS41 and XS44 achieved a power conversion efficiency of 8.18 and 8.08% along with high VOC value of 770 and 775 mV, respectively.

Figure 1), by employing four bulky substitutes, i.e., dipropylfluorene, hexyloxybenzene, tert-butylbenzene, and hexapropyltruxene. Another way to control the charge recombination in DSCs is using chenodeoxycholic acid (CDCA) or deoxycholic acid (DCA) as a coadsorbent.25,26 In fact, CDCA/DCA not only inhibited the recombination reaction between electrons in the TiO2 film and the oxidized species in the redox electrolyte but also prevented close π−π aggregation between dye molecules,26−28 resulting in higher JSC than the indoline dye without coadsorbent. In spite of that, it is noteworthy that, upon coadsorption with CDCA/DCA, the improvement of the VOC is generally moderate for most indoline dyes.10,14,15 This observation indicates that CDCA/DCA may not be effective on retarding possible charge recombination.29 To further improve the performance of indoline dyes, it is necessary to explore new coadsorbents via molecular engineering. Recent studies have highlighted the importance of the exploration of new alternative coadsorbents to CDCA/DCA in DSCs.30−32 For example, Han and co-workers reported a novel coadsorbent Y1, enabled DSCs sensitized with black dye with high performance of 11.4%.30 Kim and co-workers proposed HC-acid as an alternative coadsorbent to DCA in coumarin dye (NKX2677) sensitized solar cells.31 Generally, these coadsorbents contain bulky shaped structure on the direction of paralleling to the TiO2 film. In other words, the design of a new coadsorbent mainly focused on the horizontal direction but not the downward direction. Nevertheless, we think that the effect of the molecular conjugation length of coadsorbent (i.e., the downward direction) on the performance of DSCs is also an area of concern. To the best of our knowledge, this issue has not yet been addressed. Dihexyloxy-substituted triphenylamine (DHO-TPA) employed in the organic sensitizers was known to improve the DSC performances by effective blockage of charge recombination.1−3,8 In this work, two dihexyloxy-substituted triphenylamine (DHO-TPA) dyes (TC1 and TC2, Figure 2) were

■

EXPERIMENTAL SECTION

Materials and Instruments. (t-Bu)3P were purchased from Puyang Huicheng Chemical Industry Limited Company, China. All other solvents and chemicals used in this work were analytical grade and used without further purification. Melting points of the samples were taken on an RY-1 melting point apparatus (Tianfen, China). 1H NMR and 13C NMR spectra were recorded on a Bruker AM-300 or AM-400 spectrometer. The reported chemical shifts were against TMS. High-resolution mass spectra were obtained with a Micromass GCT−TOF mass spectrometer. Optical and Electrochemical Measurements. The absorption spectra of dyes and sensitized films were measured by a HITACHI U3310 spectrophotometer. Fluorescence measurements were carried out with a HITACHI F-4500 fluorescence spectrophotometer. Cyclic voltammetry (CV) measurements for sensitized films were performed on a Zennium electrochemical workstation (ZAHNER, Germany), with sensitized electrodes as the working electrode, Ptwires as the counter electrode, and a Ag/AgCl electrode as the reference electrode at a scan rate of 50 mV s−1. Tetrabutylammonium perchlorate (TBAP, 0.1 M) and MeCN were used as supporting electrolyte and solvent, respectively. The results were calibrated using ferrocene as standard. CV measurements for dyes were performed according to the same procedure of sensitized films, except that Ptwires were used as the working electrode and counter electrode. Charge densities at open circuit and intensity modulated photovoltage spectroscopy (IMVS) were performed on a Zennium electrochemical workstation (ZAHNER, Germany), which includes a green light-emitting diode (LED, 532 nm) and the corresponding control system. The intensity-modulated spectra were measured at room temperature with light intensity ranging from 5 to 75 W m−2, in modulation frequency ranging from 0.1 Hz to 10 kHz, and with modulation amplitude less than 5% of the light intensity. Fabrication and Characterization of DSCs. The TiO2 paste (particle size, 20 nm) was printed on a conducting glass (Nippon Sheet Glass, Hyogo, Japan, fluorine-doped SnO2 over layer, sheet resistance of 10 Ω/sq) using a screen printing technique. The film was dried in air at 120 °C for 30 min and calcined at 500 °C for 30 min under flowing oxygen before cooling to room temperature. The heated electrodes were impregnated with a 0.05 M titanium tetrachloride solution in a water-saturated desiccator at 70 °C for 30 min and fired again to give an ca. 12 μm thick mesoscopic TiO2 film. The TiO2

Figure 2. Chemical structures of CDCA, TC1, and TC2. B



Article

yield). Mp: 232−234 °C. 1H NMR (400 MHz, CDCl3): δ 8.37 (s, 1H), 7.90 (s, 1H), 7.58−7.54 (m, 1H), 7.35 (d, J = 8.8 Hz, 2H), 7.01 (d, J = 8.8 Hz, 2H), 6.99−6.95 (m, 2H), 4.44 (t, J = 8.4 Hz, 1H), 4.01 (t, J = 8.4 Hz, 1H), 1.82−1.48 (m, 7H), 1.29−1.21 (m, 12H), 0.98− 0.96 (m, 3H). 13C NMR (100 MHz, CDCl3): δ 164.3, 151.9, 147.4, 144.3, 134.9, 133.2, 127.1, 125.9, 124.6, 123.2, 122.4, 120.6, 118.9, 108.5, 69.2, 45.6, 43.2, 42.3, 40.1, 34.7, 33.9, 31.4, 25.0. HRMS (ESI) calcd for C34H34N2O2S2 (M + H)+, 567.2140; found, 567.2147. Synthesis of (E)-2-Cyano-3-(5-(4-(5,5,10,10,15,15-hexapropyl-10,15-dihydro-5H-diindeno[1,2-a:1′,2′-c]fluoren-2-yl)1 ,2,3, 3a,4, 8b-hexa hydrocyclopen ta[b]indol-7-y l)-3propylthieno[3,2-b]thiophen-2-yl)acrylic Acid (XS44). Red yellow solid (78% yield). Mp: 242−244 °C. 1H NMR (400 MHz, DMSO-d6): δ 8.38−8.28 (m, 3H), 7.87 (s, 1H), 7.69−7.63 (m, 3H), 7.61−7.35 (m, 4H), 7.31 (s, 1H), 7.25−7.19 (m, 3H), 7.12 −7.09 (m, 1H), 5.10 (t, J = 7.6 Hz, 1H), 4.65 (t, J = 7.6 Hz, 1H), 2.94−2.83 (m, 7H), 1.99−1.92 (m, 5H), 1.47−1.39 (m, 10H), 1.13−1.09 (m, 3H), 0.55−0.46 (m, 30H). 13C NMR (100 MHz, CDCl3): δ 164.9, 147.4, 144.3, 142.4, 136.2, 127.2, 125.9, 124.6, 124.2, 122.6, 121.8, 120.9, 108.7, 69.9, 45.6, 42.1, 40.9, 34.1, 25.0. HRMS (ESI) calcd for C69H74N2O2S2 (M + H)+, 1027.5270; found, 1027.5277.

electrode was stained by immersing it into a 0.5 mM XS41−XS44 dye solution (with 5 mM CDCA or 1.5 mM TC1/TC2 added) in a mixture of DCM/EtOH (v/v, 1:1) and kept at room temperature for 36 h to complete the sensitizer uptake. To be specific, the cell performance is optimum when the molar ratios of XS41 to CDCA and XS41 to TC1/TC2 were 1:5 and 1:3, respectively (Tables S2 and S3 in the Supporting Information). To fairly evaluate the four dyes, these molar ratios of XS41 to CDCA and XS41 to TC1/TC2 were also applied to XS42−44. The sensitized electrodes were then rinsed with dry ethanol and dried by a dry air flow. Pt catalyst was deposited on the FTO glass by coating with a drop of H2PtCl6 solution (40 mM in ethanol) with the heat treatment at 395 °C for 15 min to give a photoanode. The sensitized electrode and Pt-counter electrode were assembled into a sandwich-type cell by a 25 μm thick Surlyn (DuPont) hot-melt gasket and sealed up by heating. The electrolyte is composed of 0.6 M 1,2-dimethyl-3-n-propylimidazolium iodide (DMPImI), 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine (TBP) and 0.1 M guanidinium thiocyanate (GNCS) in acetonitrile. The photocurrent−voltage (J−V) characteristics of the solar cells were carried out using a Keithley 2400 digital source meter controlled by a computer and a standard AM1.5 solar simulator (Oriel 911601000 (300 W) SOLAR SIMULATOR 2 × 2 BEAM). The light intensity was calibrated by an Oriel reference solar cell. A metal mask with an aperture area of 0.158 cm2 was covered on a testing cell during all measurements. The action spectra of monochromatic incident photon-to-current conversion efficiency (IPCE) for the solar cell were performed by using a commercial setup (QTest Station 2000 IPCE Measurement System, CROWNTECH, USA). Synthesis. The synthetic route and synthesis of intermediates of XS41−XS44 can be found in the Supporting Information. The synthesis of the XS41−XS44 dyes is described as follows. General Synthesis Procedure of Dyes. To a stirred solution of carbaldehyde (0.50 mmol) and cyanoacetic acid (1.0 mmol) in a 1:1 mixture of chloroform and acetic acid (20 mL) was added a catalytic amount of piperidine (50 μL). The reaction mixture was refluxed for 12 h, and then the solvent was removed in vacuo. The resulting solid was dissolved in CH2Cl2 and sequentially washed with brine and dried with anhydrous Na2SO4. The solvent was evaporated, and the remaining crude product was purified by column chromatography (CH2Cl2:methanol = 20:1 as eluent) to give the product. Synthesis of (E)-2-Cyano-3-(5-(4-(9,9-dipropyl-9H-fluoren-2yl)-1,2,3,3a,4,8b-hexahydrocyclopenta[b]indol-7-yl)-3propylthieno[3,2-b]thiophen-2-yl)acrylic Acid (XS41). Red solid (72% yield). Mp: 230−232 °C. 1H NMR (400 MHz, DMSO-d6): δ 8.30−8.26 (m, 2H), 7.80 (s, 1H), 7.76−7.55 (m, 3H), 7.48−7.45 (m, 1H), 7.40 (d, J = 7.6 Hz, 1H), 7.29−7.27 (m, 2H), 7.25 (d, J = 7.6 Hz, 1H), 6.72 (s, 1H), 5.03 (t, J = 8.4 Hz, 1H), 3.86 (t, J = 8.4 Hz, 1H), 1.88−1.80 (m, 2H), 1.58−1.51 (m, 4H), 1.23−1.19 (m, 6H), 1.14− 1.13 (m, 6H), 0.91−0.86 (m, 3H), 0.68−0.65 (m, 6H). 13C NMR (100 MHz, DMSO-d6): δ 165.4, 154.3, 146.6, 146.1, 145.7, 143.2, 140.7, 137.5, 136.8, 135.3, 134.3, 133.2, 133.0, 132.8, 132.5, 128.8, 126.6, 126.2, 121.7, 121.2, 119.7, 116.4, 113.2, 110.2, 69.7, 57.2, 46.3, 44.3, 36.6, 34.7, 23.4, 18.6, 15.7, 14.8. HRMS (ESI) calcd for C43H42N2O2S2 (M + H)+, 683.2766; found, 683.2771. Synthesis of (E)-2-Cyano-3-(5-(4-(4-(hexyloxy)phenyl)1,2, 3,3a, 4,8b-hexahydrocy c lopenta [b]indol-7-yl) -3propylthieno[3,2-b]thiophen-2-yl)acrylic Acid (XS42). Purple solid (83% yield). Mp: 236−238 °C. 1H NMR (400 MHz, CDCl3): δ 8.03 (d, J = 8.8 Hz, 2H), 7.52 (s, 1H), 7.31−7.28 (m, 2H), 6.91 (d, J = 8.8 Hz, 2H), 6.79 (s, 1H), 6.32 (s, 1H), 4.72 (t, J = 6.8 Hz, 1H), 3.99 (t, J = 6.8 Hz, 1H), 3.96 (t, J = 6.4 Hz, 2H), 1.92−1.66 (m, 4H), 1.71− 1.69 (m, 6H), 1.38−1.10 (m, 8H), 0.96−0.92 (m, 6H). 13C NMR (100 MHz, CDCl3): δ 162.5, 159.1, 152.2, 147.3, 146.2, 145.3, 144.2, 138.7, 138.0, 133.6, 132.8, 129.7, 127.6, 126.2, 125.6, 124.8, 124.1, 121.4, 121.2, 118.6, 116.3, 110.2, 69.5, 45.2, 35.2, 35.0, 31.5, 27.8, 26.3, 23.2, 22.5, 20.9, 17.6. HRMS (ESI) calcd for C36H38N2O3S2 (M + H)+, 611.2402; found, 611.2407. Synthesis of (E)-3-(5-(4-(4-tert-Butoxyphenyl)-1,2,3,3a,4,8bhexahydrocyclopenta[b]indol-7-yl)-3-propylthieno[3,2-b]thiophen-2-yl)-2-cyanoacrylic Acid (XS43). Purple solid (70%

■

RESULTS AND DISCUSSION UV−vis Absorption Properties. The UV−vis absorption spectra of the four dyes and sensitized films are shown in Figure 3 and Figure 4, respectively. Table 1 summarizes the

Figure 3. Absorption spectra of XS41−XS44 in DCM.

photophysical properties of all the as-synthesized dyes. The maximum absorption peaks (λmax) for XS41, XS42, XS43, and XS44 are located at 492, 495, 465, and 500 nm, respectively. By comparison, the introduction of additional donors with larger π conjugation into the indoline unit promotes red-shifting in absorption spectra and enhances maximum molar absorption coefficients (ε).19 For example, the λmax of XS41, XS42, and XS44 is red-shifted by about 27, 30, and 35 nm related to that of XS43, respectively. The use of the fluorene and truxene as the electron donor confers XS41 and XS44 with higher molar absorption coefficients of 3.3 × 104 and 4.5 × 104 M−1 cm−1, respectively. To understand how the additional donors as well as coadsorbents worked in our system, we compare the absorption spectra of XS41−XS44 sensitized films (3 μm) with those of (XS41−XS44) + CDCA and (XS41−XS44) + TC1. Small bathochromic shifts are noticed in (XS41−XS44) + CDCA sensitized films relative to the corresponding spectrum in XS41−XS44 (9, 7, 11, and 3 nm for XS41, XS42, XS43, and XS44, respectively), indicating that the possible π-stacked aggregates are not serious. Recently, Liu et al. proposed that the most effective way to prevent π-aggregation was the C



Article

Table 2. Electrochemical Data for Dyes dye

λint (nm)a

E0−0b/eV

ED/D+/V

ED*/D+c/V

XS41 XS42 XS43 XS44

579 567 558 586

2.14 2.18 2.22 2.11

0.83 0.87 0.95 0.82

−1.31 −1.31 −1.27 −1.29

a

The intersect of the normalized absorption and the emission spectra (Figure S1 in Supporting Information). bE0−0 = 1240/λint. cED*/D+ were estimated from ED*/D+ = ED/D+ − E0−0.

Figure 5. Energy diagram of the dyes.

−0.5 V vs NHE,35 providing enough driving force for electron injection.36 Dependence of Intermolecular Electron Hopping on the Structure of Coadsorbents. To understand the molecular interaction between the dyes and coadsorbents, cyclic voltammetry was carried out in a typical three-electrode electrochemical cell with TiO2 films stained with dyes as the working electrode.16,32,37,38 As shown in Figure 6a, the current intensity of XS41 sensitized film (1.08 mA) is 2.7-fold higher than that of XS41 + CDCA (0.4 mA), sharply contrasting a slight decrease of the XS41 adsorbed amount (Table S1 in the Supporting Information). By comparing the absorbance change of a dye solution before and after dye uptaking with a TiO2 film (12 μm), the surface coverage (Γ) of XS41 and XS41 + CDCA anchored on TiO2 film was determined to be 2.55 × 10−7 and 2.27 × 10−7, respectively. A reported mechanism of charge transfer in the working reported electrode is concerned with an intermolecular electron hopping across the TiO2 nanoparticle surface after the oxidation of dye molecules on the film.37,38 Wang et al. proposed that organic dyes with strong π−π stacking allowed facile intermolecular electron hopping during oxidation in the CV experiment.16,39 These results suggest that CDCA is effective in preventing possible intermolecular electron losses between two neighboring dye molecules, consistent with a previous report.40 In the cases of XS42−44, similar results were observed (Figures 6b−d). In sharp contrast to CDCA, TC1 and TC2 exhibit a different CV behavior. As shown in Figure 6 and Figure S3 (Supporting Information), the current intensities of (XS41−XS44) + TC1/ TC2, TC1, and TC2 are much higher than their (XS41−XS44) + CDCA analogues. From the CV data, it is clear that TC1 and TC2 are less effective in suppressing the intermolecular electron hopping on the films. Interestingly, this result is different from an earlier report. Kim and co-workers proposed that HC-acid is more effective than DCA in preventing the electron loss mentioned above.31 The current intensity of HCacid sensitized film (about 0.07 mA) is much lower than that of NKX2677 + DCA (about 0.15 mA). This discrepancy indicates

Figure 4. Absorption spectra of sensitized electrodes (3 μm) based on XS41−XS44.

Table 1. Photophysical Data for Dyes dye XS41 XS42 XS43 XS44

λmaxa/nm (ε/M−1 cm−1) 492 495 465 500

(33 000) (24 000) (21 000) (45 000)

λmaxb/nm

λmaxc/nm

red-shiftd/nm

435 416 416 445

444 423 427 448

9 7 11 3

a

The absorption spectra in DCM. The absorption peaks of the XS41− XS44. b(XS41−XS44) + CDCA. cSensitized TiO2 films (3 μm). dThe red-shifted degree in absorption spectra of (XS41−XS44) + CDCA with respect to those of XS41−XS44.

introduction of long alkyl groups into planar π-linker segment.19 We therefore attribute such small variation of absorption spectra to the presence of 3-propylthieno[3,2b]thiophene impeding interaction between molecules in these complexes. It is also intriguing to note that the λmax of XS44 + CDCA is red-shifted by only 3 nm compared to the λmax of XS44, a result that is attributed to the bulky nonplanar structure of hexapropyltruxene in the donor part of the indoline unit. In contrast, it is difficult to determine the antiaggregation ability of TC1 since the λmax of TC1 (417 nm) is smaller than that of XS41 (435 nm). Electrochemical Properties. Cyclic voltammetry was conducted in acetonitrile solution to verify whether the ground and excited oxidation potentials of the XS41−XS44 dyes are suitable for fabrication of TiO2-based DSCs with iodidecontaining redox electrolytes (Table 2). As shown in Figure 5, all the HOMO levels of the four dyes are more positive than the Nernst potential of the I−/I3− redox couple (0.4 V vs NHE33), ensuring regeneration of the oxidized dyes by I− after electron injection.34 On the other hand, the LUMO levels of the four dyes (−1.27 to −1.31 V vs NHE) are much more negative than the conduction band of TiO2 at approximately D



Article

Figure 7. J−V curves of studied DSC devices under AM 1.5G simulated solar light (100 mW cm −2).

Table 3. Photovoltaic Parameters Studied DSCsa dye XS41 XS41 XS41 XS41 XS42 XS42 XS42 XS42 XS43 XS43 XS43 XS43 XS44 XS44 XS44 XS44 TC1 TC2

Figure 6. CV lines of sensitized electrodes based on XS41−XS44.

that the chemical structure of coadsorbent has a strong impact on the intermolecular electron hopping. Both TC1/TC2 and HC-acid have a bulky shaped structure (containing hexyloxybenzene and fluorene unit, respectively) in their donor section. The main difference between TC1/TC2 and HCacid lies in the fluorene unit which makes the HC-acid more rigid than TC1/TC2. The ability of preventing intermolecular electron hopping is therefore suggested to be determined by the bulkiness of rigidity groups. The validity of this deduction could be further examined by the fact that the current intensity of XS44 sensitized film (0.28 mA) is much lower than those of XS41 (1.08 mA). Note that the total amounts of XS41 and XS44 on TiO2 film are determined to be 2.55 × 10−7 and 2.32 × 10−7 mol/cm2, respectively (Table S1 in Supporting Information). Photovoltaic Performance of DSCs. The current density−voltage (J−V) characteristics of DSCs based on XS41, XS42, XS43, and XS44 were evaluated under 100 mW cm−2 simulated AM1.5G solar light, and the curves are shown in Figure 7. XS41 and XS44 sensitized cells gave JSC of 15.0 and 14.8 mA cm−2, VOC of 670 and 725 mV, and fill factors (FF) of 0.67 and 0.66, corresponding to PCEs of 6.73 and 7.08%, respectively. Under identical conditions, XS42 and XS43 provided a moderate PCE of 5.61% and 5.89%, respectively. The performance data are summarized in Table 3. DSCs fabricated using XS41/XS44 showed a distinct enhancement in JSC (0.9−1.7 mA) compared to DSC devices prepared with XS42/XS43. This could be attributed to the broader absorption, higher molar extinction coefficient, and retardation of recombination (see the following section), causing a broader IPCE action area (Figure 8a).

+ CDCA + TC1 + TC2 + CDCA + TC1 + TC2 + CDCA + TC1 + TC2 + CDCA + TC1 + TC2

JSC/mA cm−2

VOC/mV

FF

PCE (%)

15.0 15.3 14.9 16.1 13.3 13.6 13.5 14.8 13.9 14.2 14.1 15.0 14.8 15.2 15.0 15.8 8.5 11.3

670 688 725 770 621 641 680 723 633 660 690 743 725 690 744 775 770 697

0.67 0.66 0.67 0.66 0.68 0.67 0.67 0.66 0.67 0.68 0.67 0.66 0.66 0.67 0.66 0.66 0.65 0.66

6.73 6.95 7.23 8.18 5.61 5.84 6.15 7.06 5.89 6.37 6.52 7.36 7.08 7.03 7.36 8.08 4.25 5.19

a

Measurements were performed under AM 1.5 irradiation on the devices.

Figure 8. IPCE action spectra of studied DSCs.

The J−V curves indicate VOC to be in the order of XS42 (621 mV) < XS43 (633 mV) < XS41 (670 mV) < XS44 (725 mV). Interestingly, the VOC of XS42 is lower than that of XS43, E



Article

coadsorbent should not only focus on the horizontal direction but also on the downward direction. Dependence of Photovoltage on the Conduction Band Movement and Charge Recombination. As commonly understood, the VOC of a DSC intrinsically represents the difference between Fermi level of TiO2 (EF,n) and Fermi level of a redox electrolyte (EF,redox), i.e., VOC = EF,redox − EF,n.48 The EF,n of TiO2 can be expressed as eq 1

though the former possesses longer alkyl chains. In a study of molecular design of organic dye toward retardation of charge recombination at the semiconductor/dye/electrolyte interface, Nishida et al. proposed that having 3D structure to increase the distance between dyes and acceptors (such as I3−) is important to increase the electron lifetime.41 Compared to the hexyloxybenzene, tert-butylbenzene with 3D structure could be more efficient for retarding the interaction between electrons in TiO2 and electron acceptors in the electrolyte. Not surprisingly, XS41 and XS44 provide higher VOC due to their nonplanarly structure. In particular, among the four dyes the XS44 shows the maximum VOC of 725 mV, proving the strong ability of the hexalkyltruxene group to retard charge recombination (see next section).42−47 As expected, the addition of CDCA has a positive impact on improving the VOC values of XS41−43 based DSCs. However, no significant increase in VOC was observed (e.g., 18 mV for XS41, 20 mV for XS42, and 27 mV for XS43). Furthermore, with the increase of the CDCA concentration, the VOC value slightly increased, but JSC decreased significantly with increasing CDCA concentrations (e.g., XS41 + CDCA, Table S2 in Supporting Information). These results suggest that the usage of the CDCA coadsorbent may not be effective on retarding the charge recombination in DSCs. It is interesting to note that the presence of CDCA in XS44 dye solutions leads to a 35 mV decrease of VOC from 725 to 690 mV, a result that is attributed to increased charge recombination in the device (see next section). To further reduce the charge recombination in XS41−XS44 sensitized cells, TC1 was introduced as a coadsorbent. As shown in Table 3, it is found that regardless of the dye selection the addition of TC1 to these dyes resulted in significant improvement in VOC in comparison with its CDCA analogues. Evidently, this improvement in VOC largely contributes to the higher power conversion efficiencies of cells fabricated using (XS41−XS44) + TC1. According to the discussion in the electrochemical properties section, the inherent antimolecular stacking ability of TC1 is weaker than CDCA. That means the efficiency enhancement of (XS41−XS44) + TC1 with respect to XS41−XS44 largely depends on the suppressed charge recombination rather than preventing of intermolecular electron loss. Therefore, strategic structural modification of coadsorbent which focused on the horizontal direction for suppressing charge recombination is successful. Nevertheless, there is much room left for further enhancement by reducing the possible charge recombination to improve the VOC. In parallel with the studies on TC1 as coadsorbent, current− voltage measurements based on (XS41−XS44) + TC2 were made to evaluate the impact of the molecular conjugation length of coadsorbent on the performance of DSCs. Surprisingly, the superior performance of the TC2 as coadsorbent relative to TC1 in terms of VOC is particularly noteworthy. The VOC values for XS41 + TC2, XS42 + TC2, and XS43 + TC2 are 770, 723, and 743 mV, respectively. Compared with XS41−43, the introduction of TC2 as coadsorbent successfully realized a significant increase in VOC over 100 mV, while that of TC1 lies in the range 55−59 mV. In the case of XS44, the advantage in voltage of XS44 + TC2 (775 mV) over XS44 + TC1 (744 mV) is also maintained, although the enhancement (31 mV) is moderate. This important observation indicates that tuning the molecular conjugation length of coadsorbent can notably impact the VOC of cells. We therefore suggest that strategic structural modification of

⎛n ⎞ E F, n = ECB + kBT ln⎜ c ⎟ ⎝ Nc ⎠

(1)

where kB is the Boltzmann constant, T the temperature (293 K in this work), nc the free electron density, and Nc the density of accessible states in the conduction band. Considering that EF,redox would not change strongly in DSCs with a fixed redox electrolyte, VOC is intimately correlated to the ECB and nc. To clarify the VOC performance of dyes, the relative conduction band positions and electron lifetime of DSCs based on XS41− XS44 were investigated. In this work, charge densities at open circuit, measured by the charge extraction technique,49 were used to compare the conduction band positions in different DSCs. The electron lifetimes at open circuit (τoc), measured by controlled intensity modulated photovoltage spectroscopy (IMVS), can be obtained by fitting either the real or imaginary part of the IMVS response (eqs 2 and 3).50,51 X1 re(ΔVoc) = 1 + (ωτocre)2 (2) im(ΔVoc) = −

X2ωτocim 1 + (ωτocim)2

(3)

where im(ΔVOC) is the imaginary modulation of the photopotential (ΔVOC); re(ΔVOC) is the real modulation of the re im photopotential (ΔVOC); X1, X2, τoc , and τoc are the fit parameters; and ω is the circular frequency of the light modulation. To correctly evaluate the recombination rate of different devices at the same equivalent value of the position of the conduction band, the electron lifetimes of all the samples in the representation at Vecb (Vecb = VF − ΔEc/q)52,53 were recorded. Figure 9 shows the relation between VOC and extracted charge density (Q) at open circuit. The curves for the DSCs based on the same dye (i.e., dye, dye + CDCA, dye + TC1, dye + TC2) are roughly parallel to each other. Among the different dyes, the Q−VOC relation varied within 31 mV (Table 4). At a fixed Q, a higher VOC value indicates a negative shift of CB, while a decreased VOC means a positive shift of CB.54,55 As shown in Table 4, for the XS41, XS42, and XS43, the CDCA causes the TiO2 bands to move upward by about 14, 18, and 9.5 mV, respectively. It is clear that the CB shift significantly contributes to the VOC change in the devices based on (XS41−XS43) + CDCA. In contrast, the XS44 + CDCA cell showed a positive shift of 8 mV in the Q−VOC relation compared to the XS44 case, which contributes to the VOC attenuation of 35 mV. At the same Vecb, the electron lifetime of the XS44 + CDCA cell is shorter than that of XS44 (Figure 10d), indicating a more pronounced electron recombination in the XS44 + CDCA cell. These results are in agreement with the assumption that CDCA/DCA may not be effective on retarding of interfacial charge recombination in DSCs. F



Article

Figure 9. Charge density at open circuit as a function of VOC for studied DSCs.

Table 4. Relative CB Positions of Studied DSCs dye XS41 XS41 XS41 XS41 XS43 XS43 XS43 XS43 TC1 TC2

+ CDCA + TC1 + TC2 + CDCA + TC1 + TC2

ΔEc/mV 0 −14 −13 −31 0 −9.5 +5 −5 0 −14

dye XS42 XS42 XS42 XS42 XS44 XS44 XS44 XS44

+ CDCA + TC1 + TC2 + CDCA + TC1 + TC2

Figure 10. Electron lifetime as a function of Vecb for studied DSCs.

a

ΔEc/mV

that the significant enhancement of VOC values observed for (XS41−XS43) + TC1/TC2 is largely because of the increased electron lifetimes (i.e., retarded electron recombination rate) rather than the different positions of the conduction band edge of TiO2. To further explore the different roles played by TC1 and TC2 in affecting the VOC performance, we compare the Q−VOC plots of (XS41−XS44) + TC1 and (XS41−XS44) + TC2. For all four dyes, the insertion of TC2 moves the CB negatively in comparison to those of TC1. For example, as compared to the XS41 + TC1 cell, the XS41 + TC2 cell showed an upward CB shift by 18 mV. A similar effect was observed for DSCs based on TC1 and TC2 alone (Figure S2 in Supporting Information). Recently, Hagfeldt and co-workers proposed that the larger dyes should induce a more negative conduction band edge shift than smaller dyes having a smaller dipole,56 giving an explanation of the different CBs yielded by DSCs based on the TC1 and TC2. As a result, the negative shift of CB is an important factor for the significant improvement of VOC from (XS41−XS44) + TC1 to (XS41−XS44) + TC2. Besides the contribution from the CB shift, the superior performance of the TC2 as a coadsorbent relative to TC1 in terms of VOC can also be attributed to the repression of charge recombination, which is related to electron lifetime as shown in Figure 10. It is found that regardless of the dye selection coadsorption with TC2 confers an enhanced electron lifetime in comparison with its TC1 analogues. Therefore, the observed enhancement of VOC for (XS41−XS44) + TC2 can be attributed to the collective effect of retarded charge recombination and negative shift of CB.

0 −18 −15 −24 0 +8 +10 −8

a ΔEc values were determined by the Q−VOC plots. Relative CB positions of DSCs based on a single dye were set as zero. Negative value means an upward CB shift, while a positive value indicates a downward CB shift.

Compared with XS41−XS43, the introduction of TC1 and TC2 as coadsorbents successfully increases VOC to around 57 and 100 mV, respectively. According to the Q−VOC plots, the insertion of TC1 and TC2 in XS41−XS43 moves the CB within 15 and 31 mV, respectively. Evidently, the small CB shift observed could not explain the large VOC difference in comparison with the XS41−XS43 analogues. Figure 10a, b, c, and d shows the electron lifetime as a function of Vecb for the DSCs based on XS41, XS42, XS43, and XS44, respectively. At the same Vecb, the electron lifetime of the (XS41−XS43) + TC1/TC2 cells are much longer than those of (XS41−XS43), indicating that charge recombination between electrons in the TiO2 film and electron acceptors in the electrolyte is significantly retarded by coadsorption with TC1/TC2. Note G



Article

Dependence of Charge Recombination on the Structure of Coadsorbents. As illustrated by Figure 10, the electron lifetime increases in the order of (XS41−XS44) + CDCA < (XS41−XS44) + TC1 < (XS41−XS44) + TC2, which is in good accordance with that for VOC, suggesting the same order of decreased charge recombination rate. It is well-known that effective surface blocking is essential for long electron lifetime.42−47,56−59 The superiority of TC1/TC2 relative to CDCA regarding reduction of charge recombination can be understood from the structure of the molecules. The chemical structure of TC1/TC2 has a more bulky structure than CDCA, especially on the horizontal direction as observed in Figure 11. Figure 12. Simple sketch map of the adsorbed XS41 + TC1 and XS41 + TC2 on the TiO2 surface.

interfacial back electron transfer from the conduction band of the TiO2 film to the I3− ions. This enables attainment of higher photovoltage. On the other hand, XS41 + TC1 forms less regularly packed dye layers due to a mismatch between molecules, resulting in voids on the TiO2 surface and thus shorter TiO2 electron lifetimes.

■

CONCLUSIONS

In summary, four units (i.e., dipropylfluorene, hexyloxybenzene, tert-butylbenzene, and hexapropyltruxene) with different rigidity were introduced into the indoline donor section for repression of charge recombination. VOC values of the four dyes are in the order of XS42 < XS43 < XS41 < XS44, indicating that the introduction of an additional donor, especially with a bulky rigid group, can notably reduce the charge recombination at the titania/electrolyte interface. To further improve the VOC of XS41−XS44, TC1 and TC2 as the alternative coadsorbents to CDCA were explored. After coadsorption with TC1/TC2, the XS41−XS44 cells showed notably higher VOC that could be mainly attributed to the suppressed charge recombination. Interestingly, it was found that the surface blocking of the dye layer largely depended on the degree of molecular matching between XS41−XS44 and TC1/TC2. (XS41−XS44) + TC2 located well above the TiO2 surface retarded the rate of interfacial back electron transfer from the conduction band of the TiO2 film to the I3− ions, and this enabled attainment of higher photovoltage. We therefore propose that strategic structural modification of coadsorbent should focus not only on the horizontal direction but also on the downward direction to retard the charge recombination. In other words, molecular matching between the dyes and coadsorbents is a major factor we should take into account. In addition, the TC2 dye causes a downward displacement of CB compared to the TC1 dye. The collective effect of the CB shift and retarded charge recombination rate can account for the observed V OC enhancement from (XS41−XS44) + TC1 to (XS41−XS44) + TC2. Importantly, we demonstrated that high VOC can be realized without sacrificing JSC by employing indoline dyes featuring a bulky rigid donor combined with a matched coadsorbent. Our work provides a platform for further exploration of high effective indoline dyes toward slow charge recombination.

Figure 11. Molecular structures of CDCA, TC1, TC2, and XS41 derived from density functional theory (DFT) calculations (b3lyp).

The widths of TC1, TC2, and CDCA were estimated to be 18.6, 14.8, and 7.1 Å, respectively. TC1 and TC2 with considerably large “protected” area can readily block the acceptor species in the electrolyte approaching the TiO2 surface, thereby suppressing charge recombination. Interestingly, the 31−53 mV higher VOC caused by the coadsorbent alteration from TC1 to TC2 is sharply contrasted with a 73 mV attenuation for the TC2 (VOC = 697 mV) cell with respect to that of TC1 (VOC = 770 mV). It is known that the halogen bonding between iodine and some electron-rich segments of dye molecules could cause a larger charge recombination rate at the titania/electrolyte interface.60−62 A TC2 containing thiophene unit (sulfur atom) prefers the formation of dye−iodine complexes in comparison with TC1, leading to a lower VOC for TC2 sensitized cells. However, this interaction between the TC2 and electrolyte reduced when TC2 is mixed with XS41−XS44. It is thus suspected that the surface blocking of dyes on the TiO2 surface depends on the molecular matching between dyes and coadsorbents. Taking XS41 for an example, the lengths of TC1, TC2, and XS41 were estimated to be 11.6, 13.2−15.5, and 14.6 Å (Figure 11), respectively. Clearly, molecular matching between XS41 and TC2 is better than that between XS41 and TC1. As presented in Figure 12, different degrees of molecular matching may result in a distinctive surface blocking. XS41 + TC2 located well above the TiO2 surface retards the rate of H



■

Article

(22) Akhtaruzzaman, M.; Seya, Y.; Asao, N.; Islam, A. l.; Kwon, E.; Shafei, A. E.; Han, L. Y.; Yamamoto, Y. J. Mater. Chem. 2012, 22, 10771. (23) Akhtaruzzaman, M.; Islam, A.; Yang, F.; Asao, N.; Kwon, E.; Singh, S. P.; Han, L. Y.; Yamamoto, Y. Chem. Commun. 2011, 47, 12400. (24) Funabiki, K.; Mase, H.; Saito, Y.; Otsuka, A.; Hibino, A.; Tanaka, N.; Miura, H.; Himori, Y.; Yoshida, T.; Kubota, Y.; Matsui, M. Org. Lett. 2012, 14, 1246. (25) Kay, A.; Grätzel, M. J. Phys. Chem. 1993, 97, 6272. (26) Hara, K.; Dan-oh, Y.; Kasada, C.; Ohga, Y.; Shinpo, A.; Suga, S.; Sayama, K.; Arakawa, H. Langmuir 2004, 20, 4205. (27) Ren, X.; Feng, Q.; Zhou, G.; Huang, C.-H.; Wang, Z.-S. J. Phys. Chem. C 2010, 114, 7190. (28) Neale, N. R.; Kopidakis, N.; van de Lagemaat, J.; Grätzel, M.; Frank, A. J. J. Phys. Chem. B 2005, 109, 23183. (29) Shi, Y.; Liang, M.; Wang, L.; Han, H.; You, L.; Sun, Z.; Xue, S. ACS Appl. Mater. Interfaces 2013, 5, 144. (30) Han, L.; Islam, A.; Chen, H.; Malapaka, C.; Chiranjeevi, B.; Zhang, S.; Yang, X.; Yanagida, M. Energy Environ. Sci. 2012, 5, 6057. (31) Song, H. M.; Seo, K. D.; Kang, M. S.; Choi, I. T.; Kim, S. K.; Eom, Y. K.; Ryu, J. H.; Ju, M. J.; Kim, H. K. J. Mater. Chem. 2012, 22, 3786. (32) Kang, M. S.; Kang, S. H.; Kim, S. G.; Choi, I. T.; Ryu, J. H.; Ju, M. J.; Cho, D.; Lee, J. Y.; Kim, H. K. Chem. Commun. 2012, 48, 9349. (33) Hagfeldt, A.; Grätzel, M. Chem. Rev. 1995, 95, 49. (34) Wenger, S.; Bouit, P. A.; Chen, Q.; Teuscher, J.; Di Censo, D.; Humphry-Baker, R.; Moser, J. E.; Delgado, J. L.; Martin, N.; Zakeeruddin, S. M.; Grätzel, M. J. Am. Chem. Soc. 2010, 132, 5164. (35) Grätzel, M. J. Photochem. Photobiol. C 2003, 4, 145. (36) Hara, K.; Sato, T.; Katoh, R.; Furube, A.; Ohga, Y.; Shinpo, A.; Suga, S.; Sayama, K.; Sugihara, H.; Arakawa, H. J. Phys. Chem. B 2003, 107, 597. (37) Heimer, T. A.; D’Arcangelis, S. T.; Farzad, F.; Stipkala, J. M.; Meyer, G. J. Inorg. Chem. 1996, 35, 5319. (38) Bonhote, P.; Cogniat, E.; Tingry, S.; Barbe, C.; Vlachopoulos, N.; Lenzmann, F.; Comte, P.; Grätzel, M. J. Phys. Chem. B 1998, 102, 1498. (39) Ren, X.; Feng, Q.; Zhou, G.; Huang, C.−H.; Wang, Z.−S. J. Phys. Chem. C 2010, 114, 7190. (40) Wang, Z.-S.; Cui, Y.; Dan-oh, Y.; Kasada, C.; Shinpo, A.; Hara, K. J. Phys. Chem. C 2007, 111, 7224. (41) Nishida, J. I.; Masuko, T.; Cui, Y.; Hara, K.; Shibuya, H.; Ihara, M.; Hosoyama, T.; Goto, R.; Mori, S.; Yamashita, Y. J. Phys. Chem. C 2010, 114, 17920. (42) Liang, M.; Lu, M.; Wang, Q.; Chen, W.; Han, H.; Sun, Z.; Xue, S. J. Power Sources 2011, 196, 1657. (43) Lu, M.; Liang, M.; Han, H.; Sun, Z.; Xue, S. J. Phys. Chem. C 2011, 115, 274. (44) Zong, X. P.; Liang, M.; Fan, C. R.; Tang, K.; Li, G.; Sun, Z.; Xue, S. J. Phys. Chem. C 2012, 116, 11241. (45) Zong, X. P.; Liang, M.; Chen, T.; Jia, J. N.; Wang, L. N.; Sun, Z.; Xue, S. Chem. Commun. 2012, 48, 6645. (46) Ning, Z. J.; Zhang, Q.; Pei, H. C.; Luan, J. F.; Lu, C. G.; Cui, Y. P.; Tian, H. J. Phys. Chem. C 2009, 113, 10307. (47) Lin, S. H.; Hsu, Y. C.; Lin, J. T.; Lin, C. K.; Yang, J. S. J. Org. Chem. 2010, 75, 7877. (48) Usami, A.; Seki, S.; Mita, Y.; Kobayashi, H.; Miyashiro, H.; Terada, N. Sol. Energy Mater. Sol. Cells 2009, 93, 840. (49) Duffy, N. W.; Peter, L. M.; Rajapakse, R. M. G.; Wijayantha, K. G. U. J. Phys. Chem. B 2000, 104, 8916. (50) Schlichthörl, G.; Huang, S. Y.; Sprague, J.; Frank, A. J. J. Phys. Chem. B 1997, 101, 8141. (51) Schlichtho1rl, G.; Park, N. G.; Frank, A. J. J. Phys. Chem. B 1999, 103, 782. (52) Raga, S. R.; Barea, E. M.; Fabregat-Santiago, F. J. Phys. Chem. Lett. 2012, 3, 1629. (53) Fabregat-Santiago, F.; Garcia-Belmonte, G.; Mora-Seró, I.; Bisquert, J. Phys. Chem. Chem. Phys. 2011, 13, 9083.

ASSOCIATED CONTENT

S Supporting Information *

Syntheses and characterizations of new compounds, Figures S1−S7, and Tables S1−S3 (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Fax: +86 22 60214252. Tel.: +86 22 60214250. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We are grateful to the National Natural Science Foundation of China (21003096, 21072152, 21103123) for financial support.

■

REFERENCES

(1) (a) Yella, A.; Lee, H. W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.; Nazeeruddin, M. K.; Diau, E. W. G.; Yeh, C. Y.; Zakeeruddin, S. M.; Grätzel, M. Science 2011, 334, 629. (b) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.; Pettersson, H. Chem. Rev. 2010, 110, 6595. (2) Yen, Y.-S.; Chou, H.-H.; Chen, Y.-C.; Hsu, C.-Y.; Lin, J. T. J. Mater. Chem. 2012, 22, 8734. (3) (a) Mishra, A.; Fischer, M. K. R.; Bäuerle, P. Angew. Chem., Int. Ed. 2009, 48, 2474. (b) Mishra, A.; Pootrakulchote, N.; Wang, M.; Moon, S.-J.; Zakeeruddin, S. M.; Grätzel, M.; Bäuerle, P. Adv. Funct. Mater. 2011, 21, 963. (c) Haid, S.; Marszalek, M.; Mishra, A.; Wielopolski, M.; Teuscher, J.; Moser, J.-E.; Humphry-Baker, R.; Zakeeruddin, S. M.; Grätzel, M.; Bäuerle, P. Adv. Funct. Mater. 2012, 22, 1291. (4) Li, L.-L.; Diau, E. W.-G. Chem. Soc. Rev. 2013, 42, 291. (5) He, H. S.; Gurung, A.; Si, L. P.; Sykes, A. G. Chem. Commun. 2012, 48, 7619. (6) He, H. S.; Gurung, A.; Si, L. P. Chem. Commun. 2012, 48, 5910. (7) Shrestha, M.; Si, L. P.; Chang, C.-W.; He, H. S.; Sykes, A.; Lin, C.-Y.; Diau, E. W.-G. J. Phys. Chem. C 2012, 116, 10451. (8) Liang, M.; Chen, J. Chem. Soc. Rev. 2013, 42, 3453. (9) Horiuchi, T.; Miura, H.; Uchida, S. Chem. Commun. 2003, 3036. (10) Horiuchi, T.; Miura, H.; Sumioka, K.; Uchida, S. J. Am. Chem. Soc. 2004, 126, 12218. (11) Ito, S.; Zakeeruddin, S. M.; Humphry−Baker, R.; Liska, P.; Charvet, R.; Comte, P.; Nazeeruddin, M. K.; Péchy, P.; Takata, M.; Miura, H.; Uchida, S.; Grätzel, M. Adv. Mater. 2006, 18, 1202. (12) Ito, S.; Miura, H.; Uchida, S.; Takata, M.; Sumioka, K.; Liska, P.; Comte, P.; Pechy, P.; Grätzel, M. Chem. Commun. 2008, 5194. (13) Kuang, D.; Uchida, S.; Humphry-Baker, R.; Zakeeruddin, S. M.; Grätzel, M. Angew. Chem., Int. Ed. 2008, 47, 1923. (14) Zhu, W. H.; Wu, Y.; Wang, S.; Li, W.; Li, X.; Chen, J.; Wang, Z. S.; Tian, H. Adv. Funct. Mater. 2011, 21, 756. (15) Wu, Y. Z.; Marszalek, M.; Zakeeruddin, S. M.; Zhang, Q.; Tian, H.; Grätzel, M.; Zhu, W. H. Energy Environ. Sci. 2012, 5, 8261. (16) Cui, Y.; Wu, Y.; Lu, X.; Zhang, X.; Zhou, G.; Miapeh, F. B.; Zhu, W. H.; Wang, Z. S. Chem. Mater. 2011, 23, 4394. (17) Qu, S. Y.; Qin, C. J.; Islam, A.; Wu, Y. Z.; Zhu, W. H.; Hua, J. L.; Tian, H.; Han, L. Y. Chem. Commun. 2012, 48, 6972. (18) Li, W. Q.; Wu, Y. Z.; Li, X.; Xie, Y. S.; Zhu, W. H. Energy Environ. Sci. 2011, 4, 1830. (19) Liu, B.; Liu, Q. B.; You, D.; Li, X. Y.; Naruta, Y.; Zhu, W. H. J. Mater. Chem. 2012, 22, 13348. (20) Higashijima, S.; Inoue, Y.; Miura, H.; Kubota, Y.; Funabiki, K.; Yoshida, T.; Matsui, M. RSC Adv. 2012, 2, 2721. (21) Matsui, M.; Fujita, T.; Kubota, Y.; Funabiki, K.; Jin, J.; Yoshida, T.; Miura, H. Dyes Pigm. 2010, 86, 143−148. I



Article

(54) Kopidakis, N.; Neale, N. R.; Frank, A. J. J. Phys. Chem. B 2006, 110, 12485. (55) Lu, X.; Feng, Q.; Lan, T.; Zhou, G.; Wang, Z.-S. Chem. Mater. 2012, 24, 3493. (56) Marinado, T.; Nonomura, K.; Nissfolk, J.; Karlsson, M. K.; Hagberg, D. P.; Sun, L.; Mori, S.; Hagfeldt, A. Langmuir 2010, 26, 2592. (57) Miyashita, M.; Sunahara, K.; Nishikawa, T.; Uemura, Y.; Koumura, N.; Hara, K.; Mori, A.; Abe, T.; Suzuki, E.; Mori, S. J. J. Am. Chem. Soc. 2008, 130, 17874. (58) Hao, X.; Liang, M.; Cheng, X.; Pian, X.; Sun, Z.; Xue, S. Org. Lett. 2011, 13, 5424. (59) O’Regan, B. C.; López-Duarte, I.; Martínez-Díaz, M. V.; Forneli, A.; Albero, J.; Morandeira, A.; Palomares, E.; Torres, T.; Durrant, J. R. J. Am. Chem. Soc. 2008, 130, 2906. (60) Privalov, T.; Boschloo, G.; Hagfeldt, A.; Svensson, P. H.; Kloo, L. J. Phys. Chem. C 2009, 113, 783. (61) Planells, M.; Pellejá, L.; Clifford, J. N.; Pastore, M.; De Angelis, F.; López, N.; Marder, S. R.; Palomares, E. Energy Environ. Sci. 2011, 4, 1820. (62) Tuikka, M.; Hirva, P.; Rissanen, K.; Korppi-Tommola, J.; Haukka, M. Chem. Commun. 2011, 47, 4499.

J


Significant Enhancement of Open-Circuit Voltage in Indoline-Based

Recommend Documents