Excited-State and Charge Carrier Dynamics in a High-Photovoltage

Article pubs.acs.org/journal/apchd5

Excited-State and Charge Carrier Dynamics in a High-Photovoltage and Thermostable Dye-Sensitized Solar Cell Shu Chen,†,§ Lin Yang,†,§ Jing Zhang,‡ Yi Yuan,‡ Xiandui Dong,† and Peng Wang*,‡ †

Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China Department of Chemistry, Zhejiang University, Hangzhou 310028, China § University of Chinese Academy of Sciences, Beijing 100049, China ‡

S Supporting Information *

ABSTRACT: Apart from the harvesting of more infrared solar photons for a higher photocurrent, improving the photovoltage and thermal stability of a dye-sensitized solar cell are the other two key challenging issues for its performance enhancement. Herein we report a metal-free donor−acceptor dye (SC-4) characteristic of a triphenylaminedihexylbithiophene electron donor and a benzothiadiazolebenzoic acid electron acceptor. This organic dye can be utilized for the fabrication of sensitized titania solar cells exhibiting excellent photovoltages of 1005 and 825 mV, respectively, when a volatile tris(1,10-phenanthroline)cobalt-based electrolyte and a nonvolatile iodine-based ionic liquid composite electrolyte are applied, respectively. With respect to the control dye C239 using the traditional electron acceptor cyanoacrylic acid, dye SC-4 displays not only an enhanced photovoltage owing to slower interfacial charge recombination but also an improved stability of photocurrent and efficiency even under a long-term thermal aging at 85 °C, because of negligible desorption of dye molecules from the surface of titania. The photovoltage drop of dye-sensitized solar cells under the thermal stress is identified for the first time as the intrinsic instability of the interface between titania and electrolyte, which needs to be judiciously passivated in a future study. Ultrafast PL measurements and theoretical calculations have unveiled that torsional energy relaxation and electron injection occur from the multiple nonequilibrium excited states of organic dyes, resulting in a highly distributive kinetics of electron injection. KEYWORDS: solar cell, organic dye, excited state, torsional relaxation, charge transfer, ultrafast spectroscopy

I

electron acceptor for some zinc-porphyrin dyes and other metal-free organic dyes (for some of their structures see Figure S1), achieving higher PCEs up to 13% even without the use of any coadsorbate.17−22 To further improve the PCE of a DSC at the AM1.5G conditions, it will be imperative to develop narrow optical gap D−A dyes for a good harvesting of infrared photons. These dyes should also be able to inject charge carriers to transporting materials in almost quantitative yields.23 Moreover the selforganized dye layer on the surface of titania should serve as a highly effective blocking layer to reduce charge recombination for a large photovoltage output.24,25 Among many organic dyes reported in our group, the triphenylamine-dihexylbithiophene (TPA-DHBT)-based dye C239 (Figure 1a) is so far known to possess the highest capacity for the control of charge recombination of electrons in titania with electron-accepting species in an electrolyte.16 On the other hand, we have identified in our previous work that replacing the electron acceptor (CA) of a simple triphenylamine dye26 (G221, Figure

n the past years considerable research efforts on dyesensitized solar cells1 (DSCs) have been made toward the synthesis of donor−acceptor (D−A) organic dyes,2−8 on the principal grounds of abundant raw materials, flexible molecular design, and good visual effect. Great progress on the power conversion efficiencies (PCEs) has been attained by virtue of a logical conjugation of electron-rich and electron-deficient blocks and a sensible modulation of nonphotoactive auxiliary segments.9,10 In the former regard, the electron-acceptor cyanoacrylic acid (CA), which was initially introduced by Arakawa and his co-workers,11 has been applied for a large portion of many organic DSC dyes. Kakiage and his co-workers modified the CA-based MK-2 dye12 (Figure S1, Supporting Information) for ADEKA-1 (Figure S1) with an alkoxysilyl group13 to achieve a PCE record of 14.3% for DSCs in 2015.14 Note that this alkoxysilyl dye must be coupled with hierarchical photovoltage-enhancing coadsorbates to attain a high efficiency.14 In 2013, we employed benzothiadiazolebenzoic acid (BTBA) as the electron acceptor to build up a low-energy-gap organic dye, C258 (Figure S1),15 which can be coadsorbed with the high-photovoltage-dye C239 (Figure 1a)16 onto titania for a DSC reaching an over 11% PCE at the air mass 1.5 global (AM1.5G) conditions. Later, BTBA was also utilized as the © XXXX American Chemical Society

Received: October 8, 2016 Published: December 19, 2016 A

DOI: 10.1021/acsphotonics.6b00772 ACS Photonics XXXX, XXX, XXX−XXX

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Figure 1. (a) Molecular structures of D−A dyes C239 and SC-4, both of which utilize a triphenylamine-dihexylbithiophene (TPA-DHBT, black) electron donor. 2-Cyanoacrylic acid (CA, blue) and 4-(benzo[c][1,2,5]thiadiazol-4-yl)benzoic acid (BTBA, red) are implemented as the electron acceptors for C239 and SC-4, respectively. The chemical bond between electron donor and electron acceptor is marked in green. (b) Cyclic voltammograms (CVs) at a scan rate of 5 mV s−1 of the THF solutions of C239 and SC-4, with 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (EMITFSI, 0.1 M) as the supporting electrolyte. The electrochemical window (ECW, black dashed curve) with a glassy carbon working electrode is also displayed to support that the Faradaic currents in blue and red solid curves stem intrinsically from C239 and SC-4. Note that decamethylferrocene (DMFc) is employed as the internal reference, and all potentials are further adjusted with respect to the standard redox couple ferrocene/ferrocenium (Fc/Fc+). (c) Steady-state UV−vis absorption (solid lines) and PL (dash lines) spectroscopies of dyes dissolved in THF (50 μM). The excitation wavelength for PL is 490 nm. (d) Normalized ultrafast kinetic traces (scatter dots) at three selected PL wavelengths of SC-4 in THF (50 μM). The solid fitting lines are obtained via eq 1. Excitation wavelength: 490 nm. (e) Scatter plots of average time constants (τ)̅ at a series of PL wavelengths for dyes dissolved in THF. The solid lines are also displayed as a guide to the eyes.

Table 1. Experimental and Theoretical Data on Frontier Orbital Energy Levels, Electronic Absorption, and Photoluminescence of C239 and SC-4a ECV L

EB3LYP L

ECV H

EB3LYP H

ΔECV L/H

ΔEB3LYP L/H

λMEAS ABS,MAX

λTD‑MPW1K ABS,MAX

λMEAS PL,MAX

λTD‑MPW1K PL,MAX

Δν̅MEAS

Δν̅

dye

[eV]

[eV]

[eV]

[eV]

[eV]

[eV]

[nm]

[nm]

[nm]

[nm]

[103 cm−1]

[103 cm−1]

C239 SC-4

−3.28 −3.33

−2.74 −2.89

−5.22 −5.04

−5.24 −5.02

1.94 1.71

2.50 2.13

447 456

451 443

673 732

624 679

7.5 8.3

6.1 7.8

TD‑MPW1K

a

CV CV The LUMO and HOMO energy levels (ECV L and EH ) vs vacuum and LUMO/HOMO energy gaps (ΔEL/H) are estimated from CVs presented in B3LYP B3LYP and E ) and LUMO/HOMO energy gaps (ΔE Figure 1b. The LUMO and HOMO energy levels (EB3LYP L H L/H ) are calculated at the B3LYP/6311G(d,p) level of theory for dye molecules in THF. HOMO and LUMO are abbreviated as H and L. Maximum absorption wavelength (λMEAS ABS,MAX), MEAS ) are obtained from electronic absorption and PL spectra shown in Figure 1c. maximum PL wavelength (λMEAS PL,MAX), and Stokes shift (Δν̅ TD‑MPW1K ) are calculated at the TD-MPW1K/6-311G(d,p) level of Maximum absorption wavelength (λTD‑MPW1K ABS,MAX ) and maximum PL wavelength (λPL,MAX theory for dye molecules in THF.

version, nanosecond laser flash photolysis, and transient photovoltage decay. To inspect the impact of electron acceptors (BTBA vs CA) on the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of D−A dyes, we first carried out cyclic voltammetric determination (Figure 1b) of C239 and SC-4 with tetrahydrofuran (THF) serving as the solvent and acquired some key electrochemical parameters as collected in Table 1. Dye SC-4 using BTBA as the electron acceptor is characteristic of a destabilized HOMO

S1) with BTBA for C264 (Figure S1) can also result in a photovoltage improvement.27 With these in mind, we will prepare SC-4 (an analogue of C239, Figure 1a) in this paper to look for a higher photovoltage dye. The initial photovoltaic performance of C239- and SC-4-based DSCs with a low-cost ionic liquid composite electrolyte28 as well as the long-term evolution of cell parameters under 85 °C thermal stress will be carefully analyzed by joint use of several photophysical and electrical methods, including femtosecond fluorescence upconB


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the deactivation time constants of the equilibrium first excited eq singlet state (Seq 1 ). We have also noted that the lifetime of S1 for SC-4 is 284 ps, which is longer than that of 132 ps for C239, in spite of its smaller HOMO/LUMO energy gap. Knowing that the integral of PL kinetic trace at a certain wavelength (Figures S7 and S8) is actually propotional to the counts of emiting photons at a corresponding wavelength in the stationary PL spectroscopies (Figure 1c), we reconstructed time-resolved PL spectra (TRPL) as depcited in Figure 2a and

(−5.04 eV vs vacuum) and a stabilized LUMO (−3.33 eV), in contrast to those (HOMO: −5.22 eV; LUMO: −3.28 eV) of C239 with the electron acceptor CA. Our electrochemical measurements have also revealed that the usage of BTBA in place of the electron acceptor CA can give rise to a compressed HOMO/LUMO energy gap (ΔECV L/H). Meanwhile, density functional theory (DFT) calculations can roughly replicate the HOMO and LUMO energy level alignments and relative HOMO/LUMO energy gaps of these two dyes (see Table 1). Next, we measured UV−vis absorption spectroscopies (Figure 1c) of 50 μM dyes in THF. Dye SC-4 displays a maximum absorption wavelength (λMEAS ABS,MAX) of 456 nm (Table 1), which is red-shifted by 9 nm in comparison with that of 447 nm for C239. Simulation of absorption spectra based upon time-dependent DFT (TDDFT) computations gives an inconsistent tendency, the λTD‑MPW1K values of C239 and SCABS,MAX 4 being 451 and 443 nm, respectively. In general, we could conclude that an over 0.2 eV reduction of HOMO/LUMO B3LYP energy gap (ΔECV L/H or ΔEL/H ) does not bring forth a remarkable change of maximum absorption wavelength TD‑MPW1K (λMEAS ABS,MAX or λABS,MAX ). By taking a closer look at the TDDFT results (Table S9), we have noted that the low-energy absorption bands of both dyes are attributed to electronic transitions from HOMO to LUMO and from HOMO−1 to LUMO; however, with respect to the HOMO−1 to LUMO transition, the excitation from HOMO to LUMO is evidently weighted for C239 in contrast to SC-4, giving an explicit clue TD‑MPW1K on the tiny difference in their λMEAS ABS,MAX (or λABS,MAX ) values. The intramolecular charge transfer transition for the visible absorption of these two dyes could be clearly visualized from the isodensity surface plots of HOMO, HOMO−1, and LUMO presented in Figure S4. We also recorded the static photoluminescence (PL) spectroscopies (Figure 1c) of diluted solutions of dye molecules in THF and inferred large Stokes shifts of 7.5 × 103 cm−1 (0.93 eV) for C239 and 8.3 × 103 cm−1 (1.03 eV) for SC-4. As listed in Table 1, the relative trends of theoretical TD‑MPW1K maximum PL wavelengths (λPL,MAX ) and Stokes shifts TD‑MPW1K (Δν̅ ) are well consistent with those of their experimental results. We further monitored ultrafast PL traces of C239 and SC-4 in THF (Figures S7 and S8) in a very broad wavelength range, by use of the femtosecond fluorescence upconversion technique.29−33 PL traces at three selected wavelengths of 560, 720, and 900 nm for SC-4 are apparently very much disparate, as presented in Figure 1d. Nonetheless, we have discovered that the time-dependent PL intensities (IPL) at all wavelengths for either dye can be globally fitted with four time constants as compiled in Tables S18 and S19. This analysis is readily fulfilled by using the Surface Xplorer software (version 2.3), via a four-exponential function convoluted with a Gaussian instrument response function (IRF),

Figure 2. (a, b) Contour plots of time-resolved PL (TRPL) spectroscopies and (c−f) evolution-associated PL (EAPL) spectroscopies of (a, c, e) C239 and (b, d, f) SC-4 in THF. The logarithmic and linear plots of normalized PL counts are applied in the middle row and bottom row, respectively. In panels c−f, the time constants of spectroscopic evolution derived from global fitting are included, and the solid lines are obtained via log-normal function.

b. Evolution-associated PL spectra (EAPL) were further extracted by use of the Glotaran software.34 The PL intensities logarithmically shown in Figure 2c and d were replotted linearly in Figure 2e and f, respectively. Dynamic Stokes shifts happen to both C239 and SC-4. We have excluded that this scenario is relevant to the emission of excimers and high-level aggregates, in view of the fact that 5 times dilution of dye solutions leads only to a reduction of PL signal but does not change normalized PL kinetics at all. Instead, this could be understood by imaging the following picture: the absorption of light by dye molecules at the ground state (S0) promotes their transformation within 1 fs to the vertically excited state (Svert 1 ) via has the same geometry as intramolecular charge transfer; Svert 1 S0, but undergoes barrierless vibrational relaxations35 within ∼100 fs and stepwise torsional relaxations36−38 within longer time delays, evolving into the Seq 1 state. As a result of the significant conformational difference of S0 and Seq 1 (Figure S9), we believe that the occurrence of torsional relaxations is highly conceivable. This conclusion is also supported by the

4

IPL ∝

∑ A i exp(−t /τi) ⊗ IRF i=1

(1)

wherein t, Ai, and τi stand for the delay time, fractional amplitude, and time constant, respectively. The values of average lifetime (τ)̅ of PL decays were further determined with n n ∑i = 1 Ai τi /∑i = 1 Ai (Ai > 0) and were also tabulated in Tables S18 and S19. From Figure 1e one can detect that, along with a red-shifting of PL wavelengths, τ ̅ increases progressively for over 2 orders of magnitude, amounting to a constant value at a certain long wavelength. We consider the invariable τ ̅ value as C


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eventually reaching an invariable value at long wavelengths. The Seq 1 lifetimes of C239 and SC-4 on alumina are 303 and 552 ps, respectively, both of which are even longer than those of dye molecules in THF. The aggregation-induced lifetime elongation could be ascribed to the inhibition of some radiationless excited-state deactivation channels, the formation of delocalized intermolecular excitons, or both. In addition, these two TPADHBT dyes both on alumina and in THF feature consistent relative lifetimes. TRPL and EAPL of dyed oxide films were also collected in Figure 4. Dynamic Stokes shifts can be clearly witnessed, proposing the existence of intramolecular torsional relaxation, intermolecular energy transfer, and generation of intermolecular excitons in the self-organized dye layers. The dynamic quenching yields (QYD) at a suite of PL wavelengths were calculated with the following equation:

considerably slow arisings (Figures S7 and S8) of kinetic traces at the long-PL wavelengths and their negative amplitudes of the small time constants listed in Tables S18 and S19. Likewise, we measured the stationary PL spectroscopies (Figure 3a) and dynamic PL traces (Figures S10−S13) of dye

QYD(λ) = 1 −

τ ̅(T, λ) τ ̅(A, λ)

(2)

where τ(T, λ) and τ(A, λ) denote the wavelength-dependent ̅ ̅ PL lifetimes of dyes on titania and alumina, respectively. For either dye, the lower QYD values of high-energy excited states presented in Figure 5a could be elucidated in line with their competitive transformations, to the low-energy excited states, which actually retain high PL quenching yields (Figure 5a). From QYD measurements, we could thereby reckon a higher electron injection yield (ϕei) for SC-4 in comparison with C239. Moreover, we inferred the time constant of electron injection (τe̅ i) by use of the equation τei̅ (λ) =

1 1 τ ̅(T, λ)

−

1 τ ̅(A, λ)

(3)

Figure 5b shows the values of τe̅ i of C239 and SC-4 at the excited states emitting photons with different energies. For both dyes, elongation of τe̅ i for over 1 order of magnitude was detected along with decreasing the PL photon energy from 2.2 eV to 1.5 eV. Note that the photon energy to produce a vertical excitation of C239 and SC-4 is ∼2.5 eV. The values of τe̅ i are very scattered owing to the broad energy profile of excited states, although the τe̅ i values at the Seq 1 state are not changed anymore, being 27 and 35 ps for C239 and SC-4, respectively. For excited states of C239 and SC-4, which can emit the same photons, the free energy of SC-4 should be higher than that of C239, in consideration of the higher HOMO energy level of SC-4 compared to C239 (Table 1). Thus, it is not sensible to probe a larger τe̅ i for SC-4 at a given energy of PL, in terms of the driving force of electron injection. We consider that the larger τe̅ i for SC-4 could be attributed to the weaker electronic coupling of dye molecules in the excited state with titania and/ or to the larger reorganization energy of electron injection. Subsequently, we made use of a nanosecond laser flash photolysis spectrometer39−41 to assess the charge transfer kinetics of reactions of photo-oxidized dye molecules (D+), only with either electrons in the conduction band (CB) and traps below the CB of titania, or with both the titania electrons and iodide anions in the ionic liquid composite electrolyte. Resorting to spectroelectrochemical measurements on dyegrafted mesoporous titania films with EMITFSI as the electrolyte, we recognized pronounced electronic absorptions of dye molecules at the oxidized state in the near-infrared range. The transient absorption traces at 785 nm shown in Figure 6 could be nicely fitted with multiexponential functions for an easy determination of half-reaction times (t1/2). When an inert

Figure 3. (a) Steady-state PL spectroscopies of dye grafted alumina and titania films in contact with an ionic liquid composite electrolyte. Excitation wavelength: 490 nm. (b) Normalized ultrafast kinetic traces (scatter dots) at three selected PL wavelengths of the SC-4 grafted alumina film in contact with an ionic liquid composite electrolyte. The solid fitting lines are obtained via eq 1. Excitation wavelength: 490 nm. (c) Wavelength-dependent average time constants (τ)̅ of dye grafted oxide films in contact with an ionic liquid composite electrolyte. The solid lines are also displayed as a guide to the eyes.

molecules self-organized on the surface of alumina and titania from a solvent mixture of chloroform and ethanol (volume ratio, 1:9). Herein the dyed transparent oxide films are also in contact with an ionic liquid composite electrolyte for DSC fabrication. For the electrolyte composition, see the Methods section. On the same oxide matrix (alumina or titania), dye SC4 shows a red-shifted PL peak compared to C239. For a given dye, the dyed titania film presents a blue-shifted PL peak in contrast to the alumina counterpart, which we think is associated with the partial electron injection to titania from the nonequilibrium excited states. The PL traces at three selected wavelengths of 560, 720, and 900 nm for the SC-4 grafted alumina film are shown in Figure 3b. By use of the same protocols for solutions, we globally fitted the PL traces of dyed oxide films and calculated the τ ̅ values of PL decays. It is noticeable that the τ ̅ value of C239 (or SC-4) on the surface of alumina (or titania) also increases continuously with the redshifting of PL wavelengths, by 1−2 orders of magnitude, D


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Figure 4. (a−d) Contour plots of TRPL spectroscopies and (e−h) EAPL spectroscopies of (a, e, i) C239 grafted on alumina, (b, f, j) SC-4 grafted on alumina, (c, g, k) C239 grafted on titania, and (d, h, l) SC-4 grafted on titania. Note that the logarithmic and linear plots of normalized PL counts are applied in the middle row and bottom row, respectively. All films are immersed in an ionic liquid composite electrolyte. The solid lines in panels e−l are fittings obtained via a log-normal function. Time constants of spectroscopic evolution are also included.

Figure 6. Normalized absorption transients upon nanosecond laser pulse excitation of transparent titania films grafted with (a, c) C239 and (b, d) SC-4, which are also immersed in (a, b) an inert electrolyte composed of 0.5 M N-butylbenzoimidazole in EMITFSI and (c, d) an ionic liquid composite electrolyte. The excitation wavelength was selected according to a 0.5 optical density of dye-grafted titania films to yield an alike distribution profile of vertically excited states in our testing samples. Excitation wavelength: 555 nm for C239 and 584 nm for SC-4 in contact with an inert electrolyte, while 549 nm for C239 and 580 nm for SC-4 in contact with an ionic liquid composite electrolyte. Pulse fluence: 20 μJ cm−2. Probe wavelength: 785 nm. Multiexponential fittings are shown as solid gray lines.

Figure 5. (a) Plots of dynamic PL quenching yields (QYD) as a function of PL wavelengths for C239 and SC-4. (b) Plots of time constants of electron injection as a function of PL wavelengths. The solid lines are also displayed as a guide to the eyes.

that substituting BTBA for CA has triggered a shorter t1/2 as listed in Figure 6a and b. This phenomenon is in good accord with the result reported in our previous paper,27 on the comparison of two archetypal CA and BTBA dyes based on a simple triphenylamine (TPA) electron donor. Furthermore, when an ionic liquid composite electrolyte for DSC fabrication (for its recipe see the Methods section) was applied, we recorded remarkably quicker signal decays (Figure 6c and d)

electrolyte made by dissolving 0.5 M N-butylbenzoimidazole (NBB) in EMITFSI was in contact with dye-grafted titania films, we observed decay traces (Figure 6a and b) on a millisecond time domain, which were in nature connected with the back electron transfer reaction between dye molecules at the oxidized state and electrons injected into titania. Also note E


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(Figure 7c) were further recorded at an irradiance of 100 mW cm−2, simulated AM1.5 sunlight to evaluate the photovoltaic parameters (Table S24) of DSCs. Dye C239 exhibits a short-circuit photocurrent density (JSC) of 8.13 mA cm−2, a VOC of 738 mV, and a fill factor (FF) of 77.4%, yielding a PCE of 4.7%. Moreover, compared to C239, dye SC-4 not only has an improved JSC of 10.74 mA cm−2 but also possesses a significantly enhanced VOC of 825 mV, affording a good PCE of 6.7% for ionic liquid based DSCs. To understand the influence of electrolytes (a cobalt-based volatile electrolyte versus an iodine-based ionic liquid composite electrolyte) on the efficiencies of DSCs, we also used a standard tris(1,10phenanthroline)cobalt(II/III) electrolyte for device fabrication. J−V curves of the cobalt cells are shown in Figure S15, and the detailed photovoltaic parameters are compiled in Table S25. In general, both dyes can display higher PCEs owing to the increase of VOC, amounting to 1.005 V with dye SC-4. J−V curves of DSCs based on the ionic liquid composite electrolyte were also measured under a series of irradiances generated with neutral metal meshes. As presented in Figure 7d, VOC is plotted as a logarithmic function of JSC. Two nearly parallel fitting lines can be obtained. A 1 order magnitude increase of JSC leads to a 69 ± 4 mV enlargement of VOC. It is also noted that at a certain JSC the VOC of SC-4 is larger than that of C239. To understand the interfacial energetics and kinetics behind the VOC difference,42,43 we carried out charge extraction (CE) and transient photovoltage decay (TPD) experiments.44,45 As depicted in Figure 7e, charges (QCE) are plotted as a function of VOC. Obviously for both cells a higher VOC under a stronger light is intrinsically associated with more charges stored in titania. Note that the change of dye molecules does not affect the CB edge and the profile of trap states under the CB of titania. However, we have found that at a given QCE the half-lifetimes (tTPD 1/2 ) of electrons in titania for C239 are shorter than that of SC-4 (Figure 7f), explaining the preceding VOC trend. In terms of the close to unity ϕhi for both dyes, the tTPD 1/2 of electrons in titania can be ascribed to the interfacial charge recombination of titania electrons with triiodide ions in the redox electrolyte. Dye loading amounts (cm) were further measured to be 1.2 × 10−8 mol cm−2 μm−1 for C239 and 2.7 × 10−8 mol cm−2 μm−1 for SC-4, showing a noticeable correlation with VOC. Note that the contribution of other factors such as molecular length and packing mode could also affect VOC. The ionic liquid based cells covered with a 50 μm thick layer of polyester film (Preservation Equipment Ltd., UK) as a UV cutoff filter (up to 400 nm) were irradiated at 60 °C under a Suntest CPS Plus lamp (ATLAS GmbH, 100 mW/cm2). As shown in Figure S16, the SC-4 cell presents an excellent stability under the dual stress of heating and visible light soaking, retaining 95% of its initial PCE, much better than that of 79% for C239. Impressively, the measured JSC of 11.16 mA cm−2 for SC-4 after 1000 h aging is still higher than the initial value of 10.74 mA/cm2, while a 71 mV drop in VOC and a constant FF are observed. Hermetically sealed cells, stored in the oven at 85 °C, were used for long-term thermal stress test. Note that cell parameters were measured after equilibration at room temperature. Prior to measurements, the aged cells were irradiated under visible light (AM1.5G) for 30 min. As shown in Figure S17, the JSC of the C239 cell drops by 2.25 mA cm−2 during 1000 h of aging at 85 °C, while there is a 0.24 mA cm−2 increase for the SC-4 cell. The SC-4 cell maintains 84% of its initial PCE after thermal aging, which is higher than that of 60% for C239. As presented in Figure S19, no desorption of dye

occurring in the microsecond time domain, suggesting that hole injection from D+ to iodide ions should be switched on. An adverse elongation of t1/2 for SC-4 as marked in Figure 6d, in contrast to that for C239 in Figure 6c, should result from a reduction of driving force owing to its destabilized HOMO energy level. As a whole, the kinetics of these two competitive charge transfer reactions for both C239 and SC-4 differentiate for over 2 orders of magnitude, generating close to unity hole injection yields (ϕhi). Thereby the recombination between the titania electrons and photo-oxidized dye molecules should not affect the photovoltaic performance of DSCs, using the dyes and redox electrolyte presented in this work. To fabricate DSCs with an ionic liquid composite electrolyte, a bilayer porous titania film that was presintered on a fluorinedoped tin oxide (FTO) glass was employed to adsorb dye molecules. For the electrolyte composition and manufacturing details, see the Methods section. We recorded external quantum efficiencies (EQEs) of DSCs made with these two TPA-DHBT dyes at short circuit, under an irradiance of monochromatic light at the 10 nm wavelength interval. As Figure 7a shows, with respect to C239, a broader EQE response of SC-4 is firmly relevant to a narrower optical energy gap (Figure 7b) of a translucent titania film grafted with this dye. In addition, dye SC-4 with BTBA as the electron acceptor acquires a larger EQE peak value of 80%, which is in good accord with the higher QYD (Figure 5b) values of its relaxed excited states. The photocurrent density−voltage (J−V) characteristics

Figure 7. (a) External quantum efficiencies (EQEs) plotted at a set of wavelengths (λ) of incident monochromatic light for DSCs made with dye-grafted bilayer [(5.0 + 5.0)-μm-thick] titania films in contact with an ionic liquid composite electrolyte. (b) Wavelength-dependent lightharvesting efficiencies (LHE) of 10.0-μm-thick mesoporous titania films grafted with dye molecules and also immersed in an ionic liquid composite electrolyte. (c) Current−voltage (J−V) curves recorded under simulated AM1.5G sunlight (100 mW cm−2). The aperture area of our metal mask was 0.160 cm2. (d) Relationship between opencircuit photovoltages (VOC) and short-circuit photocurrent densities (JSC). The solid lines are linear fittings. (e) Plot of the amount of charges extracted from a dye-grafted titania film (QCE) as a function of CE VOC. (f) Dependence of electron half-lifetimes (tTDP 1/2 ) on Q . F


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molecules is probed for the SC-4 cell after aging at 85 °C, suggesting the strong binding of dye molecules on the surface of titania. The drop of JSC observed for the C239 cell can be attributed to the dye desorption and shortened electron diffusion length (Figure S19). Also note from Figure S19 that the thermal stress does not change the kinetics of electron and hole injections for both dyes. The PCE degradation at 85 °C is mainly related to the remarkable drop of VOC, being 119 and 152 mV for C239 and SC-4, respectively. The VOC drop is mainly caused by significant acceleration of interfacial charge recombination, as unveiled by comparing the tTPD 1/2 values (Figure S18). Note that so far only one paper46 reported that an organic dye-sensitized solar cell with ∼6% PCE presented 80% stability after 1000 h aging at 80 °C. By measuring the dark J−V curves and impedance spectroscopies (Figure S20) of a control cell without dye molecules, we have found that the interface between the ionic liquid composite electrolyte and titania is not stable under the thermal stress at 85 °C. This crucial problem needs to be addressed in the coming studies. To summarize, we have employed the electron acceptor BTBA in combination with a TPA-DHBT electron donor to make a D−A dye for sensitized solar cells with either an ionic liquid composite electrolyte or a cobalt electrolyte, displaying excellent open-circuit photovoltages. We anticipate that this dye could be cografted with other low-energy-gap dyes for highly efficient solar cells. We have demonstrated that with respect to the conventional electron acceptor, BTBA is more suitable for making a D−A dye for sensitized solar cells that can endure a high thermal stress. This encouraging finding could guide a further design of efficient organic dyes for solar cells with a practical use. Ultrafast PL measurements have disclosed that with respect to traditional CA, BTBA can be used for making a dye possessing a longer lifetime of the equilibrium excited state. In addition, dynamic Stokes shifts have been probed for dye molecules both dissolved in THF and grafted on the surface of oxides, suggesting the occurrence of stepwise intramolecular relaxations of vertically excited states. In addition, nonequilibrium excited states can also inject electrons to titania, although their electron injection yields are not as high as the equilibrium excited state. Our study has suggested that in future dye designs the search for a good strategy to control the energy loss caused by excited-state relaxation should be taken into account.

Voltammetric, UV−Vis, and PL Measurements. Cyclic voltammograms were recorded on a 660C electrochemical workstation (CH Instruments, Inc.) in connection with a minielectrolytic cell, which was equipped with a glass carbon working electrode, a platinum counter electrode, and a silver quasi-reference electrode. UV−vis spectroscopies were measured with a G1103A spectrometer (Agilent). Steady-state PL spectroscopies were tested by use of a SpectraSuite spectrometer (Ocean Optics). Femtosecond Fluorescence Upconversion and Nanosecond Laser Flash Photolysis. The particulars of ultrafast PL measurements were reported in our earlier paper.53 In short, the 130 fs, 800 nm pulses produced from a regenerative amplifier (Spitfire, Spectra Physics) were split into two beams with an energy ratio of 9:1. The major beam was supplied to an optical parametric amplifier (TOPAS-C, Light Conversion) to generate an excitation light at 490 nm. Both the light transmitted from a rotatory sample and the minor beam from the Spitfire were focused on a 0.3-mm-thick BBO crystal to afford a sum frequency light. Nanosecond laser flash photolysis measurements were carried out with an LP920 laser flash spectrometer. The probe light at 785 nm emitted from an LDM 785 laser diode module (Thorlabs Corp.) passed through the testing samples and was then measured by a silicon detector (PDA10A-EC). Cell Fabrication and Measurements. A bilayer titania film deposited on FTO glass (NSG, Solar) was used as the negative electrode of a DSC. The bilayer film has a 5.0-μmthick transparent layer composed of small anatase particles (25 nm) and a 5.0-μm-thick light-scattering layer composed of large anatase particles (350−450 nm). Film preparation was detailed in a former paper.54 Dye loading was done by dipping a titania film in a dye solution overnight. The 150 μM dye solution was formulated by dissolving dye powder in a solvent mixture of chloroform and ethanol at a volume ratio of 1:9. We utilized a Surlyn O-ring (thickness, 25 μm) for the sticking of a dyed titania electrode to a platinized FTO electrode, at 125 °C under a small mechanical pressure. The ionic liquid composite electrolyte with iodide/triiodide as the redox couple contains DMII, EMIII, sulfolane, iodine, NBB, and GNCS at a molar ratio of 12:12:16:1.67:3.33:0.67. Electrical measurements (EQE, J−V, CE, TPD, and impedance spectroscopy) were performed as reported in our prior papers.55,56

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METHODS Materials. THF, ethanol, chloroform, DMFc, EMITFSI, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), guanidinium thiocyanate (GNCS), sulfolane, 4-tert-butylpyridine (TBP), and acetonitrile were purchased from Sigma-Aldrich. 1,3-Dimethylimidazolium iodide (DMII),47 1-ethyl-3-methylimidazolium iodide (EMII),47 N-butylbenzimidazole (NBB),48 and C23916 were prepared as reported by the related literature. The preparation specifics of SC-4 are illustrated in the Supporting Information. Theoretical Calculations. For quantum calculations we adopted the 6-311G(d,p) basis set in the Gaussian 09 software package. We applied the conductor-like polarized continuum model (C-PCM) for the modeling of solvent effects.49 We utilized the famous B3LYP exchange−correlation functional to optimize the ground-state geometries.50 For the vertical electron transitions and excited-state geometries, we employed the TD-MPW1K hybrid functional, which consists of 42% Hartree−Fock exchange.51,52

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.6b00772. Additional data on synthetic details, theoretical calculations, spectroscopies, and photovoltaic parameters (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail (P. Wang): [email protected]. ORCID

Peng Wang: 0000-0002-6018-1515 Notes

The authors declare no competing financial interest. G


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Porphyrin Dyes for Highly Efficient Dye-Sensitized Solar Cells: The Role of Benzene Spacers. Angew. Chem., Int. Ed. 2014, 53, 2973−2977. (18) Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; Curchod, B. F. E.; Ashari-Astani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin, M. K.; Grätzel, M. Dye-Sensitized Solar Cells with 13% Efficiency Achieved Through the Molecular Engineering of Porphyrin Sensitizers. Nat. Chem. 2014, 6, 242−247. (19) Luo, J.; Zhang, J.; Huang, K.-W.; Qi, Q.; Dong, S.; Zhang, J.; Wang, P.; Wu, J. N-Annulated perylene substituted zinc−porphyrins with different linking modes and electron acceptors for dye sensitized solar cells. J. Mater. Chem. A 2016, 4, 8428−8434. (20) Yao, Z.; Zhang, M.; Li, R.; Yang, L.; Qiao, Y.; Wang, P. A MetalFree N-Annulated Thienocyclopentaperylene Dye: Power Conversion Efficiency of 12% for Dye-Sensitized Solar Cells. Angew. Chem., Int. Ed. 2015, 54, 5994−5998. (21) Yao, Z.; Zhang, M.; Wu, H.; Yang, L.; Li, R.; Wang, P. Donor/ Acceptor Indenoperylene Dye for Highly Efficient Organic DyeSensitized Solar Cells. J. Am. Chem. Soc. 2015, 137, 3799−3802. (22) Qi, Q.; Zhang, J.; Das, S.; Zeng, W.; Luo, J.; Zhang, J.; Wang, P.; Wu, J. Push−pull type alkoxy-wrapped N-annulated perylenes for dyesensitized solar cells. RSC Adv. 2016, 6, 81184−81190. (23) Ardo, S.; Meyer, G. J. Photodriven Heterogeneous Charge Transfer with Transition-Metal Compounds Anchored to TiO2 Semiconductor Surfaces. Chem. Soc. Rev. 2009, 38, 115−164. (24) Listorti, A.; O’Regan, B.; Durrant, J. R. Electron Transfer Dynamics in Dye-Sensitized Solar Cells. Chem. Mater. 2011, 23, 3381−3399. (25) Griffith, M. J.; Sunahara, K.; Wagner, P.; Wagner, K.; Wallace, G. G.; Officer, D. L.; Furube, A.; Katoh, R.; Mori, S.; Mozer, A. J. Porphyrins for Dye-Sensitized Solar Cells: New Insights into Efficiency-Determining Electron Transfer Steps. Chem. Commun. 2012, 48, 4145−4162. (26) Gao, P.; Kim, Y. J.; Yum, J.-H.; Holcombe, T. W.; Nazeeruddin, M. K.; Grätzel, M. Facile Synthesis of A Bulky BPTPA Donor Group Suitable for Cobalt Electrolyte Based Dye Sensitized Solar Cells. J. Mater. Chem. A 2013, 1, 5535−5544. (27) Li, R.; Zhang, M.; Yan, C.; Yao, Z.; Zhang, J.; Wang, P. ElectronAcceptor Dependent Light Absorption, Excited State Relaxation, and Charge Generation in Triphenylamine Dye-Sensitized Solar Cells. ChemSusChem 2015, 8, 97−104. (28) Yang, L.; Yao, Z.; Liu, J.; Wang, J.; Wang, P. A Systematic Study on the Influence of Electron−Acceptors in Phenanthrocarbazole DyeSensitized Solar Cells. ACS Appl. Mater. Interfaces 2016, 8, 9839−9848. (29) Rehm, J. M.; McLendon, G. L.; Nagasawa, Y.; Yoshihara, K.; Moser, J.; Grätzel, M. Femtosecond Electron-Transfer Dynamics at a Sensitizing Dye−Semiconductor (TiO2) Interface. J. Phys. Chem. 1996, 100, 9577−9578. (30) Tachibana, Y.; Rubtsov, I. V.; Montanari, I.; Yoshihara, K.; Klug, D. R.; Durrant, J. R. Transient Luminescence Studies of Electron Injection in dye Sensitised Nanocrystalline TiO2 Films. J. Photochem. Photobiol., A 2001, 142, 215−220. (31) Luo, L.; Lo, C.-F.; Lin, C.-Y.; Chang, I.-J.; Diau, W.-G. Femtosecond Fluorescence Dynamics of Porphyrin in Solution and Solid Films: The Effects of Aggregation and Interfacial Electron Transfer between Porphyrin and TiO2. J. Phys. Chem. B 2006, 110, 410−419. (32) Martín, C.; Ziółek, M.; Marchena, M.; Douhal, A. Interfacial Electron Transfer Dynamics in a Solar Cell Organic Dye Anchored to Semiconductor Particle and Aluminum-Doped Mesoporous Materials. J. Phys. Chem. C 2011, 115, 23183−23191. (33) Fakis, M.; Hrobárik, P.; Yushchenko, O.; Sigmundová, I.; Koch, M.; Rosspeintner, A.; Stathatos, E.; Vauthey, E. Excited State and Injection Dynamics of Triphenylamine Sensitizers Containing a Benzothiazole Electron-Accepting Group on TiO2 and Al2O3 Thin Flims. J. Phys. Chem. C 2014, 118, 28509−28519. (34) Snellenburg, J. J.; Laptenok, S. P.; Seger, R.; Mullen, K. M.; van Stokkum, I. H. M. Glotaran: A Java-Based Graphical User Interface for the R Package TIMP. J. Stat. Softw. 2012, 49, 1−22.

ACKNOWLEDGMENTS This work was supported by the National 973 Program (2015CB932204), the National Science Foundation of China (No. 91233206 and No. 51673165), and the Key Technology R&D Program (BE2014147-1) of Science and Technology Department of Jiangsu Province.

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REFERENCES

(1) O’Regan, B.; Grätzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737−740. (2) Mishra, A.; Fischer, M. K. R.; Bäuerle, P. Metal-Free Organic Dyes for Dye-Sensitized Solar Cells: from Structure: Property Relationships to Design Rules. Angew. Chem., Int. Ed. 2009, 48, 2474−2499. (3) Imahori, H.; Umeyama, T.; Ito, S. Large π-Aromatic Molecules as Potential Sensitizers for Highly Efficient Dye-Sensitized Solar Cells. Acc. Chem. Res. 2009, 42, 1809−1818. (4) Clifford, J. N.; Martínez-Ferrero, E.; Viterisi, A.; Palomares, E. Sensitizer Molecular Structure-Device Efficiency Relationship in Dye Sensitized Solar Cells. Chem. Soc. Rev. 2011, 40, 1635−1646. (5) Li, C.; Wonneberger, H. Perylene Imides for Organic Photovoltaics: Yesterday, Today, and Tomorrow. Adv. Mater. 2012, 24, 613−636. (6) Yen, Y.-S.; Chou, H.-H.; Chen, Y.-C.; Hsu, C.-Y.; Lin, J. T. Recent Developments in Molecule-Based Organic Materials for DyeSensitized Solar Cells. J. Mater. Chem. 2012, 22, 8734−8747. (7) Wu, Y.; Zhu, W. Organic Sensitizers from D-π-A to D-A-π-A: Effect of the Internal Electron-Withdrawing Units on Molecular Absorption, Energy Levels and Photovoltaic Performances. Chem. Soc. Rev. 2013, 42, 2039−2058. (8) Liang, M.; Chen, J. Arylamine Organic Dyes for Dye-Sensitized Solar Cells. Chem. Soc. Rev. 2013, 42, 3453−3488. (9) Yang, J.; Ganesan, P.; Teuscher, J.; Moehl, T.; Kim, Y. J.; Yi, C.; Comte, P.; Pei, K.; Holcombe, T. W.; Nazeeruddin, M. K.; Hua, J.; Zakeeruddin, S. M.; Tian, H.; Grätzel, M. Influence of the Donor Size in D−π−A Organic Dyes for Dye-Sensitized Solar Cells. J. Am. Chem. Soc. 2014, 136, 5722−5730. (10) Yao, Z.; Wu, H.; Li, Y.; Wang, J.; Zhang, J.; Zhang, M.; Guo, Y.; Wang, P. Dithienopicenocarbazole as the Kernel Module of LowEnergy-Gap Organic Dyes for Efficient Conversion of Sunlight to Electricity. Energy Environ. Sci. 2015, 8, 3192−3197. (11) Hara, K.; Sayama, K.; Ohga, Y.; Shinpo, A.; Suga, S.; Arakawa, H. A Coumarin-Derivative Dye Sensitized Nanocrystalline TiO2 Solar Cell Having A High Solar-Energy Conversion Efficiency up to 5.6%. Chem. Commun. 2001, 569−570. (12) Koumura, N.; Wang, Z.-S.; Mori, S.; Miyashita, M.; Suzuki, E.; Hara, K. Alkyl-Functionalized Organic Dyes for Efficient Molecular Photovoltaics. J. Am. Chem. Soc. 2006, 128, 14256−14257. (13) Kakiage, K.; Aoyama, Y.; Yano, T.; Otsuka, T.; Kyomen, T.; Unno, M.; Hanaya, M. An Achievement of Over 12% Efficiency in an Orgainc Dye-Sensitized Solar Cells. Chem. Commun. 2014, 50, 6379− 6381. (14) Kakiage, K.; Aoyama, Y.; Yano, T.; Oya, K.; Fujisawa, J.; Hanaya, M. Highly-Efficient Dye-Sensitized Solar Cells with Collaborative Sensitization by Silyl-Anchor and Carboxy-Anchor Dyes. Chem. Commun. 2015, 51, 15894−15897. (15) Zhang, M.; Wang, Y.; Xu, M.; Ma, W.; Li, R.; Wang, P. Design of High-Efficiency Organic Dyes for Titania Solar Cells Based on the Chromophoric Core of Cyclopentadithiophene-Benzothiadiazole. Energy Environ. Sci. 2013, 6, 2944−2949. (16) Xu, M.; Zhang, M.; Pastore, M.; Li, R.; De Angelis, F.; Wang, P. Joint Electrical, Photophysical and Computational Studies on D-π-A Dye Sensitized Solar Cells: the Impacts of Dithiophene Rigidification. Chem. Sci. 2012, 3, 976−983. (17) Yella, A.; Mai, C.-L.; Zakeeruddin, S. M.; Chang, S.-N.; Hsieh, C.-H.; Yeh, C.-Y.; Grätzel, M. Molecular Engineering of Push−Pull H


ACS Photonics

Article

(35) Bloembergen, N.; Zewail, A. H. Energy Redistribution in Isolated Molecules and the Question of Mode-Selective Laser Chemistry Revisited. J. Phys. Chem. 1984, 88, 5459−5465. (36) Kukura, P.; McCamant, D. W.; Yoon, S.; Wandschneider, D. B.; Mathies, R. A. Structural Observation of the Primary Isomerization in Vision with Femtosecond-Stimulated Raman. Science 2005, 310, 1006−1009. (37) Clark, J.; Nelson, T.; Tretiak, S.; Cirmi, G.; Lanzani, G. Femtosecond Torsional Relaxation. Nat. Phys. 2012, 8, 225−231. (38) Zhou, J.; Yu, W.; Bragg, A. E. Structural Relaxation of Photoexcited Quaterthiophenes Probed with Vibrational Specificity. J. Phys. Chem. Lett. 2015, 6, 3496−3502. (39) Tachibana, Y.; Moser, J. E.; Grätzel, M.; Klug, D. R.; Durrant, J. R. Subpicosecond Interfacial Charge Separation in Dye-Sensitized Nanocrystalline Titanium Dioxide Films. J. Phys. Chem. 1996, 100, 20056−20062. (40) Pelet, S.; Moser, J.-E.; Grätzel, M. Cooperative Effect of Adsorbed Cations and Iodide on the Interception of Back Electron Transfer in the Dye Sensitization of Nanocrystalline TiO2. J. Phys. Chem. B 2000, 104, 1791−1795. (41) Swords, W. B.; Simon, S. J. C.; Parlane, F. G. L.; Dean, R. K.; Kellett, C. W.; Hu, K.; Meyer, G. J.; Berlinguette, C. P. Evidence for Interfacial Halogen Bonding. Angew. Chem., Int. Ed. 2016, 55, 5956− 5960. (42) Bisquert, J. Chemical Capacitance of Nanostructured Semiconductors: Its Origin and Significance for Nanocomposite Solar Cells. Phys. Chem. Chem. Phys. 2003, 5, 5360−5364. (43) O’Regan, B. C.; Durrant, J. R. Kinetic and Energetic Paradigms for Dye-Sensitized Solar Cells: Moving from the Ideal to the Real. Acc. Chem. Res. 2009, 42, 1799−1808. (44) Duffy, N. W.; Peter, L. M.; Rajapakse, R. M. G.; Wijayant, K. G. U. A Novel Charge Extraction Method for the Study of Electron Transport and Interfacial Transfer in Dye Sensitised Nanocrystalline Solar Cells. Electrochem. Commun. 2000, 2, 658−662. (45) O’Regan, B. C.; Bakker, K.; Kroeze, J.; Smit, H.; Sommeling, P.; Durrant, J. R. Measuring Charge Transport from Transient Photovoltage Rise Times. A New Tool to Investigate Electron Transport in Nanoparticle Films. J. Phys. Chem. B 2006, 110, 17155−17160. (46) Yum, J.-H.; Humphry-Baker, R.; Zakeeruddin, S. M.; Nazeeruddin, M. K.; Grätzel, M. Effect of Heat and Light on the Performance of Dye-Sensitized Solar Cells Based on Organic Sensitizers and Nanostructured TiO2. Nano Today 2010, 5, 91−98. (47) Cao, Y.; Zhang, J.; Bai, Y.; Li, R.; Zakeeruddin, S. M.; Gätzel, M.; Wang, P. Dye-Sensitized Solar Cells with Solvent-Free Ionic Liquid Electrolytes. J. Phys. Chem. C 2008, 112, 13775−13781. (48) Pilarski, B. A New Method for N-Alkylation of Imidazoles and Benzimidazoles. Liebigs Ann. Chem. 1983, 1983, 1078−1080. (49) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. Energies, Structures, and Electronic Properties of Molecules in Solution with the C-PCM Solvation Model. J. Comput. Chem. 2003, 24, 669−681. (50) Becke, A. D. A New Mixing of Hartree-Fock and Local DensityFunctional Theories. J. Chem. Phys. 1993, 98, 1372−1377. (51) Lynch, B. J.; Fast, P. L.; Harris, M.; Truhlar, D. G. Adiabatic Connection for Kinetics. J. Phys. Chem. A 2000, 104, 4811−4815. (52) Pastore, M.; Mosconi, E.; De Angelis, F.; Gätzel, M. A Computational Investigation of Organic Dyes for Dye-Sensitized Solar Cells: Benchmark, Strategies, and Open Issues. J. Phys. Chem. C 2010, 114, 7205−7212. (53) Zhang, J.; Yao, Z.; Cai, Y.; Yang, L.; Xu, M.; Li, R.; Zhang, M.; Dong, X.; Wang, P. Conjugated Linker Correlated Energetics and Kinetics in Dithienopyrrole Dye-Sensitized Solar Cells. Energy Environ. Sci. 2013, 6, 1604−1614. (54) Wang, P.; Zakeeruddin, S. M.; Comte, P.; Charvet, R.; Humphry-Baker, R.; Grätzel, M. Enhance the Performance of DyeSensitized Solar Cells by Co-grafting Amphiphilic Sensitizer and Hexadecylmalonic Acid on TiO2 Nanocrystals. J. Phys. Chem. B 2003, 107, 14336−14341.

(55) Liu, J.; Li, R.; Si, X.; Zhou, D.; Shi, Y.; Wang, Y.; Jing, X.; Wang, P. Oligothiophene Dye-Sensitized Solar Cells. Energy Environ. Sci. 2010, 3, 1924−1928. (56) Cai, N.; Wang, Y.; Xu, M.; Fan, Y.; Li, R.; Zhang, M.; Wang, P. Engineering of Push-Pull Thiophene Dyes to Enhance Light Absorption and Modulate Charge Recombination in Mesoscopic Solar Cells. Adv. Funct. Mater. 2013, 23, 1846−1854.

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