Conjugate spacer effect on molecular structures and absorption spectra of triphenylamine dyes for sensitized solar cells: Density functional theory calculations

Spectrochimica Acta Part A 78 (2011) 287–293

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Conjugate spacer effect on molecular structures and absorption spectra of triphenylamine dyes for sensitized solar cells: Density functional theory calculations Jie Xu a,∗ , Lei Wang a , Guijie Liang a,b , Zikui Bai a , Luoxin Wang a , Weilin Xu a , Xiaolin Shen a a Key Lab of Green Processing & Functional Textiles of New Textile Materials, Ministry of Education, Wuhan Textile University, No. 1, Fangzhi Road, Hongshan District, 430073 Wuhan, China b College of Materials Science & Engineering, Xi’an Jiao tong University, 710049 Xi’an, China

a r t i c l e

i n f o

Article history: Received 21 May 2010 Received in revised form 13 September 2010 Accepted 1 October 2010 Keywords: Triphenylamine dyes Dye-sensitized solar cells Density functional theory Molecular structures Absorption spectra

a b s t r a c t The molecular structures and absorption spectra of triphenylamine dyes containing variable thiophene units as the spacers (TPA1–TPA3) were investigated by density functional theory (DFT) and timedependent DFT. The calculated results indicate that the strong conjugation is formed in the dyes and the length of conjugate bridge increases gradually with the increased thiophene spacers. The interfacial charge transfer between the TiO2 electrode and TPA1–TPA3 are electron injection processes from the excited dyes to the semiconductor conduction band. The simulated absorption bands are assigned to ␲ → ␲* transitions, which exhibit appreciable red-shift with respect to the experimental bands due to the lack of direct solute–solvent interaction and the inherent approximations in TD-DFT. The effect of thiophene spacers on the molecular structures, absorption spectra and photovoltaic performance were comparatively discussed and points out that the choice of appropriate conjugate bridge is very important for the design of new dyes with improved performance. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Converting solar energy into electricity is generally regarded as the most prospective method to solve the global energy crisis, owing to its huge reserves and pollution-free character. The commercially available solar cells are currently based on inorganic silicon semiconductors. However, their large-scale application has been limited due to the price of high-purity silicon. Organic solar cells, therefore, appear to be a highly promising and cost-effective candidate for the photovoltaic energy industry. In this context, dye-sensitized solar cells (DSSCs) have received widespread attention in recent years because of their high efficiency of converting solar energy into electricity and low cost [1–6]. The most successful sensitizers employed in these cells are ruthenium polypyridyl complexes, yielding solar-to-electric power conversion efficiencies up to 11–12% under air mass (AM) 1.5 irradiation [6]. However, Ru complexes contain a heavy metal, which is undesirable from point of view of the environmental aspects [7]. Moreover, the process to synthesize the complexes is complicated and costly. In addition to Ru complexes, metal-free organic dyes as sensitizers are also under intensive investigation due to their high molar extinction coeffi-

∗ Corresponding author. Tel.: +86 27 87426559; fax: +86 27 87426559. E-mail address: [email protected] (J. Xu). 1386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2010.10.008

cients, flexible structural modifications and low costs, and to date some of them have reached good efficiency [8–15]. In DSSCs, dye molecules first absorb visible or near infrared light, accompanied by the excitation of electrons to the excited states. The excited electrons subsequently are injected into the conduction band of TiO2 , and then transfer to the anode by electron diffusion through the disordered network of TiO2 nanoparticles. The oxidized dye molecules are regenerated by the iodide redox couple or holetransporter with the positive charge being transported from the electrolyte to the platinum counter electrode. Therefore, the performance of DSSC strongly depends upon the following factors: (1) absorption efficiency of the dye sensitizer for solar light spectrum; (2) electron injection probability from the excited state of the dye sensitizer to TiO2 (efficiency of the charge separation); (3) electron transfer probability from the electrolyte to the oxidized dye [3]. All these factors are closely associated with the ground and excited electronic states of the dye sensitizer. From this point of view, it is imperative to investigate the electronic structures of both ground and excited states of the dye molecule for understanding the mechanism of the charge separation and transfer, which are the key processes in this type of solar cells. In order to design and synthesize more efficient dyes, it is also necessary to understand the electronic structures of the existing efficient sensitizers. Density functional theory (DFT) has emerged as a reliable standard tool for the theoretical treatment of the structures as

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well as the electronic and absorption spectra. Its time-dependent extension called time-dependent DFT (TD-DFT) can give reliable values for valence excitation energies with the standard exchange-correlation functionals. The computational cost of TDDFT calculation is comparative to that of a Hartree–Fock based single excitation theory, such as, configuration interaction singles (CIS) or time-dependent Hartree–Fock (TD-HF) method and maintains a uniform accuracy for open-shell and closed-shell systems. In recent years, DFT has been extensively used to study the structures and absorption spectra of sensitizers for DSSCs [4,16–32]. Triphenylamine has widely been used as an electron donor for metal-free organic sensitizers due to its excellent electrondonating capability and aggregation resistant nonplanar molecular configuration [33]. Aggregation can give rise to self-quenching, instability of the sensitizer, and reduce the electron injection efficiency, resulting in low conversion efficiency of the DSSCs. Recently, triphenylamine-based dyes containing thiophene as the spacers (TPA1–TPA3) were designed and synthesized, with power conversion efficiencies up to 6.15% under AM 1.5 irradiation [34,35]. To theoretically understand the ␲-spacer effect of various unit of thiophene and sensitized mechanism at a molecular level, the geometrical and electronic structures of the triphenylamine dyes TPA1–TPA3 were studied in detail using DFT, and the electronic absorption spectra were investigated based on the TD-DFT calculations. 2. Computational method All the calculations were performed with the Gaussian 03 program package [36]. The ground-state geometries were fully optimized without any symmetry constrains at the DFT level of theory with Becke’s [37] three parameters hybrid functional and Lee et al.’s correlational functional B3LYP [38] using a standard 6-31g(d) basis set on all atoms. A full natural bond orbital (NBO) analysis was obtained by using the POP = NBO keyword. The excitation energies and oscillator strengths for the lowest 30 singlet–singlet transitions at the optimized geometry in the ground state were obtained by TD-DFT calculations using the same basis set as for the ground state and three kinds of hybrid functional B3LYP, PBE1PBE and MPW1PW91, respectively. According to the calculated results, the UV–vis absorption spectra were simulated by means of the SWizard program (Revision 4.6) [39] using a Gaussian convolution with the full width at half-maximum of 3000 cm−1 . Solvation effects were introduced by the SCRF method, via the conductor polarizable continuum model (CPCM) [40,41] implemented in the Gaussian program, for both geometry optimizations and TD-DFT calculations.

3. Results and discussion 3.1. Geometrical structures The triphenylamine dyes TPA1–TPA3 in the present study are composed of an electron-accepting cyanoacrylic acid group, a ␲conjugate bridge with one to three units of thiophene rings, and an electron-donating diphenylaniline group in the form of donor␲-acceptor (D-␲-A). The optimized ground-state geometries of TPA1–TPA3 are shown in Fig. 1, and the selected bond lengths, bond angles and dihedral angles are listed in Table 1. Most of the corresponding parameters for TPA2 are in good agreement with the calculated results for the D5 dye (where the thiophene ring connected to the diphenylaniline in TPA2 is substituted by a vinylene) [42], indicating the reasonability of our results. All the CC lengths in the thiophene and phenyl rings are between the distance of a single bonded C–C and a double bonded C C, implying that there exists extensive delocalization throughout the molecule. The calculated distance between the C atom in carboxyl and the N atom in aniline are 15.538, 21.164 and 26.788 A˚ for TPA1–TPA3, respectively. Thus, the distance between the electron donor and semiconductor surface is increased with the increased thiophene unit because the thiophene group extends the conjugate bridge length. The cyanoacrylic acid group (acceptor) is located to be fully coplanar with the thiophene bridge, as represented by the C37/C53/C60-C39/C55/C62-C41-C43 dihedral angle; while the coplanarity between the diphenylaniline group (donor) and thiophene bridge is slightly destroyed due to steric repulsion, as shown by the C18-C20-C34-C35 angle. Thus, the donor and acceptor moieties are fully conjugated through the ␲-bridge. The delocalization in the conjugate bridge is beneficial to the intramolecular charge transfer and to the stability of the molecule. The values of C2-N12C13-C15 and C23-N12-C13-C14 angles are increased gradually from TPA1 to TPA3, indicating that the anilino groups are distorted significantly in response to the increased thiophene unit. 3.2. Electronic structures The dipole moments for TPA1–TPA3 are 15.71, 13.33 and 13.60 Debye, respectively, indicating that these triphenylamine dyes are highly polar. The values of quadrupole moments for TPA1–TPA3 are listed in Table 2, where the average of the diagonal quadrupole moment tensor elements Qii and unique quadrupole moment Q are defined as follows: Qii = (QXX + QYY + QZZ )/3 Q = QXX − QYY

Table 1 Selected bond lengths (in Å), bond angles (in degree) and dihedral (in degree) of the triphenylamine dyes TPA1–TPA3. TPA1

TPA2

TPA3

C1–C2 C2–N12 N12–C23 N12–C13 C13–C14 C20–C34 C39–C41

1.4027 1.4290 1.4289 1.4023 1.4115 1.4563 1.4219

C1–C2 C2–N12 N12–C23 N12–C13 C13–C14 C20–C34 C39–C50 C55–C41

1.4035 1.4267 1.4276 1.4078 1.4097 1.4602 1.4370 1.4224

C13–N12–C2 C13–N12–C23 C20–C34–C35 C39–C41–C43 C2–N12–C13–C15 C23–N12–C13–C14 C18–C20–C34–C35 C37–C39–C41–C43

120.8 120.8 127.8 137.2 −29.5 −29.8 −163.3 179.4

C13–N12–C2 C13–N12–C23 C20–C34–C35 C55–C41–C43 C2–N12–C13–C15 C23–N12–C13–C14 C18–C20–C34–C35 C53–C55–C41–C43

120.8 120.5 128.6 129.8 −32.6 −32.0 162.7 −0.6

C1–C2 C2–N12 N12–C23 N12–C13 C13–C14 C20–C34 C39–C41 C56–C57 C62–C47 C13–N12–C2 C13–N12–C23 C20–C34–C35 C62–C47–C49 C2–N12–C13–C15 C23–N12–C13–C14 C18–C20–C34–C35 C60–C62–C47–C49

1.4036 1.4265 1.4260 1.4104 1.4087 1.4615 1.4403 1.4364 1.4228 120.4 120.6 128.8 129.8 −34.2 −34.1 165.3 0.1

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289

Fig. 1. Optimized ground-state geometries of the triphenylamine dyes TPA1–TPA3 with Cartesian axes.

All the diagonal elements of the quadrupole moment tensor for TPA1–TPA3 are negative, indicating that the negative charge distribution is farther removed from the molecular center of the nuclear charges. The off-diagonal tensor elements Qij vanish whenever the molecule has a plane of symmetry perpendicular to either one of Table 2 ˚ of the triphenylamine dyes TPA1–TPA3. Quadrupole moments (in Debye A) Dyes

QXX

QYY

QZZ

QXY

QXZ

QYZ

Qii

Q

TPA1 TPA2 TPA3

−214.5 −199.8 −251.2

−350.3 −406.7 −378.4

−211.6 −231.4 −457.3

−78.9 −21.0 −24.0

45.3 −15.5 57.0

119.6 124.0 202.3

−258.8 −279.3 −362.3

135.8 206.9 127.2

the coordinates i or j. The values of the off-diagonal elements QXY and QXZ of TPA2 are relatively lower, which can be attributed to its symmetric plane nearly perpendicular to the x-axis, as shown in Fig. 1. The NBO analysis was employed to characterize the intramolecular charge transfer of TPA1–TPA3. Table 3 shows the natural charges of cyanoacrylic acid, thiophene and diphenylaniline groups in TPA1–TPA3. There are some electrons transferred from the electron donor (diphenylaniline group) to the electron acceptor (cyanoacrylic acid) through the chemical bonds of the conjugate bridge for TPA1–TPA3. It is also found that the natural charges of the donor show a remarkable decrease with the increased thiophene unit while the natural charges of the conjugate bridge are increased.

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Table 3 Natural charges (e) of different groups in the triphenylamine dyes TPA1–TPA3. Dyes

Cyanoacrylic acid

Thiophene1 Thiophene2 Thiophene3 Diphenylaniline

TPA1 TPA2 TPA3

−0.167 −0.167 −0.164

0.086 0.080 0.081

0.034 0.027

0.016

0.081 0.053 0.039

However, the natural charges of the acceptor and the bridge unit connected to the acceptor (thiophene1) for TPA1–TPA3 are almost equal. These indicate that the insertion of more thiophene units has significant effect on the electron-donating capability of the donor, but does not significantly alter the electron-drawing capability of the acceptor. The frontier molecular orbital (MO) contribution is very important in determining the charge-separated states of dye sensitizers. To create an efficient charge-separated state, the highest occupied MO (HOMO) must be localized on the extended donor moiety and the lowest unoccupied MO (LUMO) on the acceptor moiety. The MO energies and MO isodensity plots of TPA1–TPA3 are shown in Figs. 2 and 3, respectively. For TPA1, the HOMO, lying at −5.12 eV, is a ␲ orbital that delocalized over the cyano group through diphenylaniline. The HOMO-1, lying 1.06 eV below the HOMO, is a delocalized ␲ orbital over the entire molecule. While the HOMO-2 and HOMO-3, lying 1.71 and 1.76 eV below the HOMO, respectively, are ␲ orbitals that localized in phenyl group of diphenylaniline. The LUMO, lying at −2.56 eV, is a ␲* orbital that localized in cyanoacrylic acid, thiophene, and phenyl in diphenylaniline. The LUMO+1, lying 1.77 eV above the LUMO, is also a ␲* orbital similar to the LUMO. For TPA2 and TPA3, the HOMO is lying at −4.95 and −5.09 eV, while the LUMO is lying at −2.75 and 2.80 eV, respectively. Apparently, the HOMO–LUMO excitation on TPA1–TPA3 induced by light irradiation could move the electron distribution from the donor moieties to the anchoring/acceptor moieties, thus favoring electron injection from dye to TiO2 . The LUMO of TPA1 is computed at a significantly higher energy (0.19–0.24 eV) than those of TPA2 and TPA3. Meanwhile, the isodensity plots of LUMO for TPA2 and TPA3 exhibit a charge transfer to the anchoring acid group compared to that for TPA1, proving that the thiophene group is indeed an electron donor with respect to the cyanoacrylic acid group and the insertion of more thiophene units results in a decreased contribution of the diphenylaniline donor in the LUMO (in agreement with the previously discussed natural charge results). There is only a very small variation of the LUMO energy of TPA3 with respect to TPA2 (0.05 eV), pointing out that the addition of a third thiophene unit does not significantly increase the acceptor strength, but does

4 2 0

E (eV)

-2

LUMO

LUMO

LUMO

HOMO

HOMO

HOMO

-4 -6 -8 -10

TPA1

TPA2

TPA3

Fig. 2. Frontier molecular orbital energies of the triphenylamine dyes TPA1–TPA3.

indeed enhance the spatial extent of the ␲ conjugate system and is expected to generate sizable effects on the absorption spectra. The HOMO–LUMO gap of TPA1–TPA3 is 2.56, 2.20 and 2.29 eV, respectively. The calculated HOMO and LUMO energies of the bare Ti38 O76 cluster as a model for nanocrystalline are −6.55 and −2.77 eV, respectively, resulting in a HOMO–LUMO gap of 3.78 eV, the lowest transition is reduced to 3.20 eV according to TD-DFT, which is slightly smaller than typical band gap of TiO2 nanoparticles [4]. Furthermore, the HOMO, LUMO and HOMO–LUMO gap of (TiO2 )60 cluster is −7.52, −2.97, and 4.55 eV (B3LYP/VDZ), respectively [43]. Usually an energy gap more than 0.2 eV between the LUMO of the dye and the conduction band of the TiO2 is necessary for effective electron injection [9]. Taking into account of the cluster size effects and the calculated HOMO, LUMO, HOMO–LUMO gap of the dyes TPA1–TPA3, Ti38 O76 and (TiO2 )60 clusters, it can be found that the HOMO energies of these dyes fall within the TiO2 gap. The above data also reveal the sensitized mechanism: the interfacial electron transfer between semiconductor TiO2 electrode and the dye sensitizers TPA1–TPA3 are electron injection processes from excited dyes to the semiconductor conduction band. This is a kind of typical interfacial electron transfer reaction [44]. Relatively large energy gaps between the LUMO energies of these dyes and the semiconductor conduction band would be beneficial to the photovoltaic conversion properties. 3.3. Absorption spectra The UV–vis absorption spectra of TPA1–TPA3 were measured in THF solution, and consist of a very intense transition at 432, 473 and 480 nm and of two less intense bands in the 250–400 nm region, respectively [34,35]. As both the HOMO and the LUMO are of the ␲ and ␲* type, the HOMO–LUMO transition can be classified as a ␲–␲* intramolecular charge transfer. All bands are red-shifted when going from TPA1 to TPA2 and TPA3. In order to understand electronic transitions, TD-DFT calculations in THF solution were performed with three kinds of hybrid functional B3LYP, MPW1PW91 and PEB1PBE based on the optimized geometries of B3LYP, taking the 30 lowest spin-allowed singlet–singlet transitions into account. The simulated UV–vis absorption spectra of TPA1 using the hybrid functional B3LYP, PBE1PBE and MPW1PW91 are shown in Fig. 4 as the representation. It is found that the calculated line shape and relative strength are in good agreement with those of the experiment, and the overall spectral evolution (red-shift) when going from TPA1 to TPA2 and TPA3 is also correctly reproduced. However, the hybrid functional PBE1PBE and MPW1PW91 are more suitable than B3LYP for calculating absorption spectra of the triphenylamine dyes TPA1–TPA3. The vertical excitation energy and oscillator strength along with the main excitation configuration calculated by PBE1PBE are listed in Table 4. The simulated UV–vis absorption spectra of TPA1–TPA3 using PBE1PBE1 are given in Fig. 5. The first band clearly corresponds to the previously described HOMO–LUMO transition, and is of charge transfer character thus possessing high transition intensity. The second and less intense band also corresponds to a ␲–␲* transition with a strong HOMO-1 to LUMO character. Since the HOMO-1 is delocalized on the entire molecule but with a larger contribution from atomic orbitals of cyanoacrylic acid group, this band also has a charge transfer character and consequently is computed to be red-shifted going from TPA1 to TPA2 (of 0.58 eV) and from TPA2 to TPA3 (of 0.30 eV). The results of TD-DFT calculations exhibit notable red-shift with respect to the experimental data, which are also found for several dye sensitizers [16,17,42]. This discrepancy can be ascribed to the absence of explicit solvent molecules around the cyanoacrylic function and the inherent approximations in the TD-DFT. The LUMOs of TPA1–TPA3 significantly extend over the cyanoacrylic groups

J. Xu et al. / Spectrochimica Acta Part A 78 (2011) 287–293

291

Fig. 3. Isodensity plots (isodensity contour = 0.02 a.u.) of the frontier orbitals of the triphenylamine dyes TPA1–TPA3.

and are expected to result in direct solute–solvent interaction with the THF molecules through hydrogen bonding. Thus, since in the implicit solvent model used, direct solute–solvent interactions are fully neglected, the large red-shift in the simulated absorption spectra for TPA1–TPA3 can be fully justified. In addition, the TD-DFT always give smaller gap of materials [45], and thus give rise to smaller excited energies, especially for larger conjugate system and charge transfer complex in excited states [46]. Therefore, it can be concluded that the TD-DFT calculations are capable of describing the spectral features because of the qualitative agreement of line shape, relative strength and overall spectral red-shift evolution from TPA1 to TPA2 and TPA3 with respect to the experiment, although the discrepancy in absorption wavelengths exists. Generally, the dyes with broader absorption bands and larger extinction coefficients are expected to have higher photo-

to-current efficiency. But the experiment reported that the conversion efficiency of TPA1–TPA3 is in the following order: TPA2 > TPA1 > TPA3 [34,35]. The insertion of more thiophene units in the conjugate bridge alters the absorption properties of the triphenylamine dyes more suitable for solar spectrum, but it is also enlarged the distance between electron donor group and semiconductor surface, and therefore reduces the probability of a dye ␲ orbital delocalized onto the nanocrystalline surface, and decreases the electronic coupling between the chromophore and TiO2 substrates [47]. The weak electronic coupling may slow down the electron injection rate from the excited dyes to the semiconductor conduction band. On the other hand, the LUMO levels of the dyes decrease with the introduction of more thiophene units, which also make against the electron injection process. Thus, we can conclude that the appropriate conjugate bridge in dye sensitizer is also of great importance to improve the performance of DSSC.

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Table 4 Electronic transition configurations, computed excitation energies and oscillator strengths (f) for the optical transitions with f > 0.01 of the absorption bands in visible and near-UV region for the triphenylamine dyes in THF solution (H = HOMO, L = LUMO, L + 1 = LUMO + 1, etc.). Dye

Configuration

Excitation energy (eV/nm)

f

Assign.

Exp. (nm)

TPA1

H → L (+88%) H − 1 → L (+84%) H → L + 1 (+62%); H → L + 2 (18%) H − 2 → L (+75%); H − 3 → L (+14%) H → L + 3 (+65%); H − 6 → L (12%) H → L (+90%) H − 1 → L (+84%) H → L + 1 (+86%) H − 2 → L (+63%); H − 4 → L (+15%) H → L + 4(+49%); H → L + 3(38%) H → L (+90%) H − 1 → L (+84%) H → L + 1 (+86%) H − 2 → L (+75%); H − 1 → L + 1 (+17%) H → L + 2 (+81%) H → L + 3 (+59%); H − 4 → L (+20%) H → L + 4 (+86%); H − 1 → L + 4 (+5%)

2.38/521.2 3.47/357.0 4.02/308.5 4.17/297.6 4.22/293.8 2.08/596.5 2.89/428.9 3.34/371.4 3.88/319.8 4.15/299.1 1.95/634.5 2.59/477.9 2.95/420.5 3.44/360.2 3.83/324.1 3.91/317.1 4.11/301.8

1.092 0.492 0.114 0.025 0.093 1.123 0.688 0.243 0.040 0.216 1.290 0.763 0.350 0.068 0.139 0.023 0.221

␲ → ␲*

432

␲ → ␲*

473 351

TPA2

TPA3

90000

Absorbance (a.u.)

70000 60000 50000 40000 30000 20000 10000

300

400

500

600

700

Wavelength (nm)

800

Fig. 4. Simulated absorption spectra of TPA1 using the hybrid functional B3LYP, PBE1PBE and MPW1PW91.

110000 100000

TPA1 TPA2 TPA3

90000

632.9 520.8

80000

Absorbance (a.u.)

302 480 386

302

4. Conclusions B3LYP PBE1PBE MPW1PW91

80000

0

␲ → ␲*

595.2

70000

476.2 429.2

60000 50000

357.1

40000 30000

The molecular and electronic structures of triphenylamine dyes containing different thiophene units (TPA1–TPA3) in THF solution have been calculated using 6-31g(d) basis set at density functional B3LYP level. The simulated UV–vis absorption spectra have been provided by TD-DFT calculations. The calculated geometric characters indicate that the strong conjugate effects are formed in the dyes TPA1–TPA3, which is beneficial to the intramolecular charge transfer. The NBO results suggest that there are some electrons transferred from the electron donor (diphenylaniline group) to the electron acceptor (cyanoacrylic acid) through the chemical bonds of the conjugate thiophene bridge in TPA1–TPA3. The insertion of more thiophene units significantly decreases the donor strength, but has no significant effect on the electron-drawing ability of the acceptor. The HOMO energy levels are calculated to be −5.12, −4.95 and −5.09 eV, while the LUMOs are −2.56, −2.75 and −2.80 eV for TPA1–TPA3, respectively, indicating that the electron transfer from the excited dyes to the TiO2 conduction band is available. The absorption bands of TPA1–TPA3 are assigned to ␲ → ␲* transitions according to the qualitative agreement between the experimental and calculated results. The red-shift in the simulated absorption spectra with respect to the experiment is attributed to the lack of direct solute–solvent interaction and the inherent approximations in the TD-DFT. The comparative analysis of the molecular and electronic structures as well as absorption spectra between TPA1–TPA3 points out that the introduction of more thiophene units led to a better match between the absorption properties of dyes and solar spectrum, but also enlarged the distance between electron donor group and semiconductor surface, reduced the electronic coupling, decreased the electron injection rate and then gave rise to lower overall conversion efficiency. Therefore, the choice of the appropriate conjugate bridge in dye sensitizer is very important for the design of new dyes with improved performance. Acknowledgements

20000 10000 0

300

400

500

600

700

800

Wavelength (nm) Fig. 5. Simulated absorption spectra of the triphenylamine dyes TPA1–TPA3 at the PBE1PBE/6-31g(d) level.

This work was supported by the Foundation of Wuhan University of Science & Engineering (No. 2009003), the Natural Science Foundation of Hubei Province (No. 2008CDB261), the Key Project of Science and Technology Research of Ministry of Education (No. 208089), the Educational Commission of Hubei Province (Q20101606) and the Natural Science Foundation of China (No. 51003082). The authors gratefully wish to express their thanks to

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