Influence of the antennas in starburst triphenylamine-based organic dye-sensitized solar cells: phenothiazine versus carbazole

Abstract

A new starburst triphenylamine-based organic dye (WD-1) with phenothiazine units as antenna groups was designed and synthesized and the corresponding dye (WD-4) containing carbazole antennas for the purpose of comparison was also synthesized. They were successfully applied in dye-sensitized solar cells (DSSCs). The photophysical and electrochemical properties of the dyes were investigated by UV-vis spectrometry and cyclic voltammetry. Under standard global AM 1.5 solar conditions, the WD-1 sensitized solar cell gave a short circuit photocurrent density (J_sc) of 9.2 mA cm⁻², an open circuit voltage (V_oc) of 625 mV, a fill factor (ff) of 0.79, corresponding to an overall conversion efficiency η of 4.54%. Under the same conditions, the WD-4 sensitized cell gave a J_sc of 7.3 mA cm⁻², an V_oc of 603 mV, and a ff of 0.74, corresponding to an overall conversion efficiency of 3.26%. An increase in η of about 39% was obtained from WD-4 to WD-1. Our findings demonstrate that the introduction of phenothiazine units as antennas in triphenylamine-based dyes could improve photovoltaic performance compared with carbazole units as antennas in DSSCs.

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Introduction

Dye-sensitized solar cells (DSSCs) have attracted significant attention as low-cost alternatives to conventional inorganic photovoltaic devices.¹ DSSCs based on the ruthenium sensitizers have shown very impressive solar-to-electric power conversion efficiencies, reaching 11% under standard AM 1.5G sunlight.² However, the large-scale application of ruthenium dyes is limited because of costs and environmental issues. More and more efforts have been dedicated to the development of pure organic dyes which exhibit not only higher molar extinction coefficients, but also simple preparation and purification procedures at lower cost. Coumarin,³ merocyanine,⁴ indoline,⁵ polyene,⁶ hemicyanine,⁷ triphenylamine,⁸ fluorene,⁹ carbazole¹⁰ and tetrahydroquinoline¹¹ based-organic dyes have been developed and showed good performance.

The sensitizer is a crucial element in DSSCs, exerting significant influence on the power conversion efficiency as well as the stability of the cells. Most organic sensitizers are constituted by donor, linker and acceptor moieties and usually have a rod-like configuration. However, these rod-like molecules are elongated, which may facilitate the recombination of electrons with the triiodide and the formation of aggregates between molecules.¹² Therefore, the dyes with a starburst conformation were designed and synthesized by introducing starburst donor units into the D-π-A molecule to form the D-D-π-A structure,¹³ which avoids the charge recombination processes of injected electrons with triiodide in the electrolyte. The improved performance of organic dyes with D-D-π-A structures over the simple D-π-A configuration can be achieved by molecular design.

Phenothiazine (PTZ) is a well-known heterocyclic compound with electron-rich sulfur and nitrogen heteroatoms, and the phenothiazine ring is nonplanar with a butterfly conformation in the ground state, which can impede the molecular aggregation and the formation of intermolecular excimers.¹⁴ There are few reports on the application of a dye containing a PTZ moiety in DSSCs so far.¹⁵

Based on these studies, a new starburst triphenylamine-based organic dye (WD-1) with phenothiazine units as antenna groups was designed and synthesized. The corresponding dye (WD-4) containing carbazole antennas for the purpose of comparison was also synthesized.^13c They were successfully applied in DSSCs. The effect of different antennas on the performance of the starburst triphenylamine-based dye-sensitized solar cells was studied. The corresponding molecular structures of the two dyes are shown in Scheme 1.

Scheme 1 Molecular structures of WD-1 and WD-4.

Experimental

Equipment

NMR spectra were recorded on a Bruker 400 MHz spectrometer. HRMS data were determined with an FTICR-APEX instrument. Absorption spectra were measured with a SHIMADZU (model UV1700) UV-vis spectrophotometer. The cyclic voltammograms of the dyes were obtained with a CH Instruments 660C electrochemical workstation using a normal three-electrode cell with a Pt working electrode, a Pt wire counter electrode, and Ag/AgCl as a reference electrode.

Materials

All solvents and other chemicals were reagent grade and used without further purification. 3-hexyl-1-methylimidazolium iodide (HMII) was prepared according to the literature.¹⁶ Lithium iodide (LiI) was purchased from Acros. 2-Cyanoacetic acid, 4-tert-butylpyridine (TBP) and 3-methoxypropionitrile (MPN) were purchased from Aldrich. Phenothiazine, carbazole and triphenylamine were purchased from Astatech.

Preparation of DSSCs

TiO₂ colloid for DSSCs was prepared according to the literature.¹⁷ The FTO glass substrates were immersed in 40 mM TiCl₄ aq. at 70 °C for 30 min and washed with water and ethanol. The 12 μm thick mesoporous nano-TiO₂ films composed of 15–20 nm anatase TiO₂ particles were coated on the FTO glass plates by a doctor blade. After drying the nanocrystalline TiO₂ layer at 125 °C, a 4 μm thick second layer of 300–500 nm sized light scattering anatase particles (Shanghai Cai Yu Nano Technology Co., Ltd) was deposited by a doctor blade onto the first layer. The TiO₂ electrodes were heated at 450 °C for 30 min. After the sintering, when the temperature cooled to about 90 °C, the electrodes were immersed in a dye bath containing 0.5 mM N3 in ethanol or 0.2 mM WD-1 and WD-4 in acetonitrile and left overnight. The films were then rinsed in ethanol to remove excess dye. Solar cells were assembled, using a 25 μm thick thermoplastic Surlyn frame, with a platinized counter electrode. An electrolyte solution was then introduced through the hole pre-drilled in the counter electrode, and the cell was sealed with thermoplastic Surlyn covers and a glass coverslip. The liquid electrolyte employed was a solution of 0.3 M HMII, 0.5 M LiI, 0.05 M I₂ and 0.5 M TBP in MPN.

Photovoltaic characterization

The irradiation source for the photocurrent density–voltage (J–V) measurement is an AM 1.5 solar simulator (91160A, Newport Co., USA). The incident light intensity was 100 mW cm⁻² calibrated with a standard Si solar cell. The tested solar cells were masked to a working area of 0.2 cm². Volt–current characteristics were performed on a Model 2611 Sourcemeter (Keithley Instruments, Inc., USA). A Keithley 2611 source meter and a Model spectrapro 300i monochromator (Acton research, USA) equipped with a 500 W xenon lamp (Aosiyuan Technology & Science Co., Ltd, China) were used for photocurrent action spectrum measurements. Electrochemical impedance spectroscopy (EIS) data were obtained under 100 mW cm⁻² illuminations, using a perturbation of ± 10 mV over the open-circuit potential by using Solartron 1255B frequency analyzer and Solartron SI 1287 electrochemical interface system.

Synthesis

Reagents: (a) POCl₃, DMF, 1,2-dichloroethane, reflux, 12 h; (b) KI, KIO₃, CH₃COOH, 80 °C, 4 h; (c) phenothiazine or carbazole, 1,2-dichlorobenzene, Cu, K₂CO₃, 18-crown-6, reflux, 48 h; (d) 2-cyanoacetic acid, piperidine, acetonitrile, reflux, 10 h.

The synthetic routes of the WD-1 and WD-4 were shown in Scheme 2. Compound 1 was prepared according to the reported procedures.¹⁸

Scheme 2 The synthetic procedure of the dyes.

4-[N,N-Di(4-iodophenyl)amino]benzaldehyde (2). 4-(N,N-diphenylamino)benzaldehyde (1.36 g, 5 mmol), potassium iodide (1.11 g, 6.7 mmol), acetic acid (20 mL) and water (2 mL) were heated to 80 °C, potassium iodate (1.07 g, 5 mmol) was added and the reaction was stirred for 4 h. The solution was allowed to cool and the solid was collected, washed with water and recrystallised from CH₂Cl₂ : ethanol (1 : 5) giving the product as a yellow powder (1.76 g, 70%). HRMS-EI (m/z): calcd for C₁₉H₁₃I₂NO, 524.9087; found, 524.9102.

4-{N,N-bis[4-(N-phenothiazinyl)phenyl]amino}benzaldehyde (3). Compound 2 (1.2 g, 2.28 mmol), phenothiazine (1.39 g, 6.96 mmol), potassium carbonate (2.6 g, 18.83 mmol), activated copper bronze (0.6 g, 9.46 mmol) and 18-crown-6 (0.142 g, 0.54 mmol) were refluxed in 1,2-dichlorobenzene (15 mL). After 48 h, the product was obtained as a yellow powder (0.75 g, 50%) after purification by column chromatography (toluene–hexane). ¹H NMR (400 MHz, DMSO): δ (ppm) = 9.88 (s, 1H), 7.88 (d, 2H), 7.47 (d, 8H), 7.28 (d, 2H), 7.14 (dd, 4H), 7.04 (dt, 4H), 6.93 (dt, 4H), 6.44 (dd, 4H). HRMS-EI (m/z): calcd for C₄₃H₂₉N₃OS₂, 667.1752; found, 667.1763.

4-{N,N-bis[4-(N-carbazolyl)phenyl]amino}benzaldehyde (4). Compound 4 was prepared using the same procedure as for compound 3. Compound 2 (1.2 g, 2.28 mmol), carbazole (1.18 g, 7.04 mmol), potassium carbonate (4 g, 29 mmol), activated copper bronze (0.93 g, 14.58 mmol) and 18-crown-6 (0.212 g, 0.8 mmol) gave, after column chromatography (toluene), the product as a yellow powder (0.83 g, 60%). ¹H NMR (400 MHz, DMSO): δ (ppm) = 9.90 (s, 1H), 8.29 (d, 4H), 7.90 (d, 2H), 7.75 (d, 4H), 7.60 (d, 4H), 7.51 (m, 8H), 7.32 (m, 6H). HRMS-EI (m/z): calcd for C₄₃H₂₉N₃O, 603.2311; found, 603.2348.

3-(4-(bis(4-(10H-phenothiazin-10-yl)phenyl)amino)phenyl)-2-cyanoacrylic acid (WD-1). A CH₃CN (10 mL) solution of compound 3 (167 mg, 0.25 mmol), 2-cyanoacetic acid (43 mg, 0.5 mmol) and a few drops of piperidine was charged sequentially in a three-necked flask and heated to reflux for 10 h. After cooling to room temperature, solvents were removed by rotary evaporation, and the residue was purified by silica gel column chromatography with CH₂Cl₂ : CH₃OH (10 : 1, v : v) as eluent to afford the dye WD-1 as a dark red solid (125 mg, yield 66%). ¹H NMR (400 MHz, DMSO): δ (ppm) = 8.23 (s, 1H), 8.05 (d, 2H), 7.45 (m, 8H), 7.25 (d, 2H), 7.12 (m, 4H), 7.03 (m, 4H), 6.91 (m, 4H), 6.43 (m, 4H). HRMS-EI (m/z): calcd for C₄₆H₃₀N₄O₂S₂, 734.1810; found, 734.1790.

3-(4-(bis(4-(9H-carbazol-9-yl)phenyl)amino)phenyl)-2-cyanoacrylic acid (WD-4). A procedure similar to that for the dye WD-1 but with compound 4 (180 mg, 0.3 mmol) instead of compound 3. The dye WD-4 was isolated as a red solid (144 mg, yield 71%). ¹H NMR (400 MHz, acetone): δ (ppm) = 8.21 (m, 5H), 7.73 (d, 4H), 7.69 (d, 4H), 7.51 (d, 4H), 7.46 (t, 6H), 7.35 (d, 2H), 7.29 (t, 4H). HRMS-EI (m/z): calcd for C₄₆H₃₀N₄O₂, 670.2369 found, 670.2375.

Results and discussion

Absorption properties in solutions and on TiO₂ films

The UV-vis spectra of the dyes WD-1 and WD-4 in acetonitrile were measured (Fig. 1) and the characterisation data are collated in Table 1. The two dyes both exhibit one intense absorption band in the visible region. The absorption spectrum of WD-1 has one absorption band centerd at 422 nm with a molar extinction coefficient of 29 367 M⁻¹ cm⁻¹. On the other hand, WD-4 exhibits an absorption band centerd at 424 nm with a molar extinction coefficient of 13 834 M⁻¹ cm⁻¹. The maxima absorption peak of WD-1 has a little blue shift compared to that of WD-4. It has been speculated that it may be caused by the decrease of co-planarity between the electron donor and the electron acceptor in the ground-state due to the introduction of phenothiazine unit. Notably, the molar extinction coefficient (29 367 M⁻¹ cm⁻¹) of WD-1 is higher than that of WD-4 (13 834 M⁻¹ cm⁻¹), indicating a good ability for light harvesting in the DSSCs.

Fig. 1 Absorption spectra of WD-1 and WD-4 in acetonitrile.

Table 1 UV-vis and electrochemical data

Dye	λ max ^a/nm (ε^b/M^−1 cm^−1)	λ max ^c/nm	E ox ^d/V (vs. NHE)	E g ^e/eV	E ox* ^f/V(vs. NHE)
a Absorption spectra were measured in acetonitrile (2.5 × 10^−5 M). b The molar extinction coefficient at λmax of the absorption spectra. c Absorption spectra of the dyes adsorbed on TiO2 electrodes. d E ox was measured in DMF with 0.1 M n-Bu4NPF6 as electrolyte (scanning rate: 100 mV s^−1, working electrode and counter electrode: Pt wires, and reference electrode: Ag/AgCl), potentials measured vs. Ag/AgCl were converted to normal hydrogen electrode (NHE) by the addition of +0.2 V. e E g was estimated from the absorption spectra of TiO2 electrodes sensitized by the dyes. f E ox* was calculated from Eox−Eg.
WD-1	422 (29 367)	416	0.64	2.63	−1.99
WD-4	424 (13 834)	424	0.45	2.55	−2.10

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Fig. 2 shows the absorption spectra of WD-1 and WD-4 on 3 μm thick TiO₂ films after 12 h adsorption. The maximal absorption peaks for WD-1 and WD-4 on the TiO₂ films are at λ = 416 nm and 424 nm, respectively. The absorption spectra of the two dyes adsorbed onto TiO₂ electrodes are similar to those of the corresponding solution spectra. In addition, the absorption spectra of the dyes were broadened after adsorption on the TiO₂ surface, which should favor the light harvesting of the solar cells and thus increase the photocurrent response region, leading to the increase of J_sc. More importantly, the maximum absorption wavelengths of the dyes on TiO₂ films did not show a big difference in comparison with those in acetonitrile, this may be ascribed to the starburst antenna groups linked to the triphenylamine, which prevented the strong tendency of the dyes to aggregate on the TiO₂ film in a large degree. This method for the prevention of aggregation might be better than using co-absorption of the dyes with deoxycholic acid, which was also able to prevent aggregation and hence improve the photovoltaic performance by means of improving both photocurrent and photovoltage, but the unavoidably significant decreased dye adsorption and limited the photovoltaic performance. It is confirmed that this is an effective way to lower the tendency to aggregate by introducing a starburst antenna group into triphenylamine dye.

Fig. 2 Absorption spectra of TiO₂ electrodes sensitized by WD-1 and WD-4.

Based on the Tauc relation, the energy gap (E_g) can be obtained by plotting (ahv)²vs. hv and extrapolating the linear portion of (ahv)² to zero as shown in Fig. 3.¹⁹ The blue shift of WD-1 can be evidenced by the higher energy band-gap as compared WD-4. The E_g of WD-1 and WD-4 are estimated to be 2.63 eV and 2.55 eV, respectively.

Fig. 3 Plot of (ahv)²vs. hv.

Electrochemical properties

Cyclic voltammetry measurements were performed in dimethylformamide (DMF) solution using 0.1 M tetrabutyl ammonium tetrafluoroborate as supporting electrolyte (Fig. 4), and the corresponding data are summarized in Table 1.

Fig. 4 Cyclic voltammograms of WD-1 and WD-4 dissolved in DMF.

The oxidation potential vs. NHE (E_ox) corresponded to the highest occupied molecular orbital (HOMO), while the excited state oxidation potential vs. NHE (E_ox*), which corresponded to the lowest unoccupied molecular orbital (LUMO). In this study, the first oxidation potentials (E_ox) of WD-1 and WD-4 are 0.64 V and 0.45 V vs. NHE, respectively. The E_ox of the two dyes are more positive than the iodine/iodide redox potential (0.4 V vs. NHE), and indicate that the oxidized dyes formed from respective electron injection into conduction band of TiO₂ will favorably accept electrons from I⁻ ions in thermodynamic property. The excited state oxidation potentials (E_ox*) of the dyes can be obtained by the first oxidation potentials (E_ox) and the energy gaps (E_g) of the dyes, namely, E_ox−E_g. The E_ox* of the WD-1 and WD-4 are −1.99 V and −2.10 V vs. NHE, respectively. The E_ox* of the dyes are far more negative than the band edge energy of the nanocrystalline TiO₂ electrode (−0.5 V vs. NHE),²⁰ indicating that the electron injection process from the excited dye molecule to the TiO₂ conduction band is energetically permitted.

Theoretical calculations

To get further insight into the molecular structure and electron distribution of the two dyes, their geometries were optimized by density functional theory (DFT) calculations at the B3LYP/6-31G(d,p) level with Gaussian 03.²¹ From the optimized geometries of the dyes (Fig. 5) and the selected dihedral angles (Table 2), we can know that all dihedral angles between phenothiazine/carbazole and benzene are all non-coplanar, which can help to inhibit the close π–π aggregation effectively between the starburst structures. This result is consistent with the absorption spectra of TiO₂ electrodes sensitized by the dyes.

Fig. 5 Optimized geometries of WD-1 (left) and WD-4 (right).

Table 2 Dihedral angle parameters of WD-1 and WD-4

Dihedral angles	B1–B2	B1–B3	B2–B4	B3–B5
WD-1	30.76°	30.82°	81.91°	82.21°
WD-4	29.51°	30.10°	54.35°	54.08°

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Fig. 6 shows the electron distribution of the HOMOs and LUMOs of WD-1 and WD-4. At the ground states, HOMO-1 and HOMO are mainly distributed on carbazole and carbazole/triphenylamine in WD-4, respectively, but electrons are only localized on a phenothiazine ring in WD-1. The difference in HOMOs between WD-1 and WD-4 might be attributed to the plane of phenothiazine almost being vertical to the plane of benzene in triphenylamine. Therefore, base on the theory of LCAO-MO, the orbitals of phenothiazine are localized on their own zone to form quasi-degenerate HOMO-1 and HOMO orbitals. At the excited state (LUMO), intramolecular charge-transfer occurs, thereby resulting in the electron movement to the electron-acceptor part (cyanoacrylic acid). The frontier molecular orbitals of the dyes reveals that HOMOs–LUMO excitation moves the electron-density distribution from the electron-donor moiety to the electron-acceptor moiety through π bridge. Thus, an efficient charge separation is obtained based on the change of electron distribution, which is induced by photoexcitation.

Fig. 6 Frontier molecular orbitals of WD-1 and WD-4.

Photovoltaic performances of DSSCs

Fig. 7 shows the incident monochromatic photon-to-current conversion efficiency (IPCE) obtained with a sandwich-type two electrode cell using 0.3 M HMII, 0.5 M LiI, 0.05 M I₂ and 0.5 M TBP in MPN as redox electrolyte. In the range of 400–500 nm, the solar cells based on WD-1 and WD-4 show high IPCE above 44% and with the highest value of 70% at 470 nm for WD-1 and 57% at 470 nm for WD-4, respectively. In comparison with WD-4, WD-1 gives the higher IPCE value, which implies the dye would show a relatively large photocurrent in DSSCs. The higher IPCE value of WD-1 might be attributed to the higher molar extinction coefficient. On the other hand, the IPCE values for WD-1 and WD-4 are obviously decreased above 500 nm in the long wavelength region, which can be attributed to the decrease in light harvesting for the dyes. These results are consistent with the absorption spectra in solution and on the TiO₂ films of the dyes.

Fig. 7 The IPCE spectra of solar cells based on WD-1, WD-4 and N3.

Fig. 8 shows the current–voltage characteristics of dye-sensitized solar cells employing WD-1, WD-4 and N3 as sensitizers under standard global AM 1.5G solar light. The WD-1-sensitized cell gave a short-circuit photocurrent density (J_sc) of 9.2 mA cm⁻², an open-circuit voltage (V_oc) of 625 mV, and a fill factor (ff) of 0.79, corresponding to an overall conversion efficiency of 4.54% (see Table 3). The WD-4-sensitized solar cell gave a J_sc of 7.3 mA cm⁻², an V_oc of 603 mV, and a ff of 0.74, corresponding to an overall conversion efficiency of 3.26%. These values were comparable to those of the cell based on N3 (J_sc = 15.6 mA cm⁻², V_oc = 656 mV, ff = 0.72, and η = 7.37%) measured in the same conditions.

Fig. 8 Current density–voltage curves of solar cells based on WD-1, WD-4 and N3.

Table 3 Photovoltaic performances of DSSCs

Dyes	J sc/mA cm^−2	V oc/mV	ff	η/%	τ e/ms
WD-1	9.2	625	0.79	4.54	70.15
WD-4	7.3	603	0.74	3.26	63.95
N3	15.6	656	0.72	7.37	87.01

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According to Fig. 8 and Table 3, it is clear that the photovoltaic performances of the DSSCs can be evidently affected by the phenothiazine/carbazole antennas in the dye molecules. In comparison with WD-4, the J_sc and V_oc values of WD-1 are improved by introducing phenothiazine antennas. The higher J_sc value of WD-1 might be due to the IPCE of WD-1 being higher than that of WD-4. The higher V_oc of WD-1 than that of WD-4 might be due to the comparatively lower E_ox (HOMO) of the former, which offers enough of a larger driving force for the reduction of the oxidized dye. This, in turn, will lead to a slower back electron transfer from TiO₂ to the dye and result in a lager V_oc value.²² The higher J_sc and V_oc for the DSSCs of WD-1 lead to the higher efficiency finally, and an increase in η of about 39% was obtained from WD-4 to WD-1.

Electrochemical impedance spectroscopic (EIS) analysis was performed to elucidate the photovoltaic findings further. EIS is a useful tool for characterizing important interfacial charge transfer processes in DSSCs.²³ In this study, impedance spectra of the solar cells based on the dyes were used to investigate mechanisms at the internal interfaces of the DSSCs. This examination was conducted by subjecting the cells to constant AM 1.5G 100 mW cm⁻² illumination and to bias at the open-circuit voltage (V_oc) of the cells.

Fig. 9a shows the Nyquist plots for solar cells based on the dyes. For the frequency range investigated (0.1 Hz to 100 KHz), a larger semicircle occurs in the lower-frequency range and a smaller semicircle occurs in the higher-frequency range. With the bias illumination and voltage applied, the larger semicircle at lower frequencies corresponded to the charge transfer processes at the TiO₂/dye/electrolyte interface, while the smaller semicircle at higher frequencies corresponded to the charge transfer processes at the Pt/electrolyte interface. As shown in Fig. 9a, the order of the larger semicircle radius decreases from WD-4 to WD-1, thus indicating the more efficient electron generation and transport in the solar cell based on WD-1.

Fig. 9 Impedance spectra of DSSCs based on dyes. (a) Nyquist plots; (b) Bode phase plots.

In the Bode phase plots (Fig. 9b), the middle-frequency peak (in the 0.1–100 Hz range) is indicative of the charge-transfer process of injected electrons in TiO₂. This finding is related to the electron life time and is ultimately correlated with V_oc. The electron life time is obtained from the middle-frequency peak in the Bode phase plot using eqn (1), in which f is the frequency of the peak.

τ_e = 1/ω = 1/2πf (1)

The injected electron lifetime of the WD dyes increases from WD-4 (63.95 ms) to WD-1 (70.15 ms) (Table 3). This result is correlated with the fact that WD-1, which have the phenothiazine units as antennas, possess higher V_oc value (625 mV) than WD-4 (603 mV). The device based on WD-1 showed the longer electron life time. Thus, the presence of phenothiazine units in the WD dyes as antennas increases not only charge generation and injection but also the electron life time, which improves V_oc and J_sc, and ultimately the overall power conversion efficiency. The EIS results are in good agreement with results of the short-circuit currents, open circuit voltages and the overall power conversion efficiencies of the DSSCs based on WD-1 and WD-4.

Conclusion

In summary, a new starburst triphenylamine-based organic dye, WD-1, that contained phenothiazine units as antennas has been designed and synthesized. We also synthesized the corresponding dye, WD-4, containing carbazole units as antennas for the purpose of comparison. The effect of different antennas to the performance of the starburst triphenylamine-based dye-sensitized solar cells was studied. Under standard global AM 1.5 solar conditions, the WD-1 sensitized solar cell gave a short circuit photocurrent density (J_sc) of 9.2 mA cm⁻², an open circuit voltage (V_oc) of 625 mV, a fill factor (ff) of 0.79, corresponding to an overall conversion efficiency η of 4.54%. Under the same conditions, the WD-4 sensitized cell gave a J_sc of 7.3 mA cm⁻², an V_oc of 603 mV, and a ff of 0.74, corresponding to an overall conversion efficiency of 3.26%. An increase in η of about 39% was obtained from WD-4 to WD-1. EIS analysis was performed to elucidate the photovoltaic findings. The EIS results are in good agreement with results of the short-circuit currents, open circuit voltages and the overall power conversion efficiencies of the DSSCs based on WD-1 and WD-4. Our findings demonstrate that the introduction of phenothiazine units as antennas in triphenylamine-based dyes could improve photovoltaic performances compared with the carbazole units as antennas in DSSCs. Because the spectral response of WD-1 is not enough, the work to extend the spectral response of the dye aiming for even better performance of DSSCs is in progress. It was expected that, by adjusting the molecular structure of this kind of starburst triphenylamine-based dyes, higher efficiency could be achieved.

Acknowledgements

We thank the National Natural Science Foundation of China (Grant Nos. 20602005, 20873015), the Fundamental Research Funds for the Central Universities (Grant No. ZYGX2010J035), the Innovation Funds of State key Laboratory of Electronic Thin Films and Integrated Device (Grant No. CXJJ201104) and the Beijing National Laboratory for Molecular Sciences (BNLMS) for financial support.

References

1. (a) B. O'Regan and M. Grätzel, Nature, 1991, 353, 737; (b) M. Grätzel, Acc. Chem. Res., 2009, 42, 1788.
2. (a) M. K. Nazeeruddin, F. De Angelis, S. Fantacci, A. Selloni, G. Viscardi, P. Liska, S. Ito, B. Takeru and M. Grätzel, J. Am. Chem. Soc., 2005, 127, 16835; (b) F. Gao, Y. Wang, D. Shi, J. Zhang, M. Wang, X. Jing, R. Humphry-Baker, P. Wang, S. M. Zakeeruddin and M. Grätzel, J. Am. Chem. Soc., 2008, 130, 10720; (c) C. Chen, M. Wang, J. Li, N. Pootrakulchote, C. Alibabaei, J. Decoppet, J. Tsai, C. Grätzel, C. Wu, S. M. Zakeeruddin and M. Grätzel, ACS Nano, 2009, 3, 3103.
3. (a) K. Hara, K. Sayama, Y. Ohga, A. Shinpo, S. Suga and H. Arakawa, Chem. Commun., 2001, 569; (b) Z. S. Wang, Y. Cui, Y. Dan-Oh, C. Kasada, A. Shinpo and K. Hara, J. Phys. Chem. C, 2008, 112, 17011; (c) K. D. Seo, H. M. Song, M. J. Lee, M. Pastore, C. Anselmi, F. D. Angelis, M. K. Nazeeruddin, M. Gräetzel and H. K. Kim, Dyes Pigm., 2011, 90, 304.
4. (a) K. Sayama, K. Hara, N. Mori, M. Satsuki, S. Suga, S. Tsukagoshi, Y. Abe, H. Sugihara and H. Arakawa, Chem. Commun., 2000, 1173; (b) W. J. Wu, J. L. Hua, Y. H. Jin, W. H. Zhan and H. Tian, Photochem. Photobiol. Sci., 2008, 7, 63.
5. (a) T. Horiuchi, H. Miura, K. Sumioka and S. Uchida, J. Am. Chem. Soc., 2004, 126, 12218; (b) T. Dentani, Y. Kubota, K. Funabiki, J. Y. Jin, T. Yoshida, H. Minoura, H. Miura and M. Matsui, New J. Chem., 2009, 33, 93; (c) M. Matsui, M. Kotani, Y. Kubota, K. Funabiki, J. Y. Jin, T. Yoshida, S. Higashijima and H. Miura, Dyes Pigm., 2011, 91, 145.
6. K. Hara, T. Sato, R. Katoh, A. Furube, T. Yoshihara, M. Murai, M. Kurashige, S. Ito, A. Shinpo, S. Suga and H. Arakawa, Adv. Funct. Mater., 2005, 15, 246.
7. (a) Z. S. Wang, F. Y. Li and C. H. Huang, Chem. Commun., 2000, 2063; (b) Q. H. Yao, F. S. Meng, F. Y. Li, H. Tian and C. H. Huang, J. Mater. Chem., 2003, 13, 1048.
8. (a) D. P. Hagberg, T. Edvinsson, T. Marinado, G. Boschloo, A. Hagfeldt and L. C. Sun, Chem. Commun., 2006, 2245; (b) H. N. Tian, J. X. Pan, R. K. Chen, M. Liu, Q. Y. Zhang, A. Hagfeldt and L. C. Sun, Adv. Funct. Mater., 2008, 18, 3461; (c) P. Qin, H. J. Zhu, T. Edvinsson, G. Boschloo, A. Hagfeldt and L. C. Sun, J. Am. Chem. Soc., 2008, 130, 8570; (d) D. H. Lee, M. J. Lee, H. M. Song, B. J. Song, K. D. Seo, M. Pastore, C. Anselmi, S. Fantacci, F. D. Angelis, M. K. Nazeeruddin, M. Gräetzel and H. K. Kim, Dyes Pigm., 2011, 91, 192.
9. (a) S. Kim, J. K. Lee, S. O. Kang, J. Ko, J. H. Yum, S. Fantacci, F. D. Angelis, D. D. Censo, M. K. Nazeeruddin and M. Grätzel, J. Am. Chem. Soc., 2006, 128, 16701; (b) D. Kim, J. K. Lee, S. O. Kang and J. Ko, Tetrahedron, 2007, 63, 1913; (c) H. Choi, J. K. Lee, K. Song, S. O. Kang and J. Ko, Tetrahedron, 2007, 63, 3115; (d) W. Q. Li, Y. Z. Wu, X. Li, Y.S. Xie and W. H. Zhu, Energy Environ. Sci., 2011, 4, 1830.
10. (a) C. Zafer, B. Gultekin, C. Ozsoy, C. Tozlu, B. Aydin and S. Icli, Sol. Energy Mater. Sol. Cells, 2010, 94, 655; (b) C. Teng, X. C. Yang, C. Z. Yuan, C. Y. Li, R. K. Chen, H. N. Tian, S. F. Li, A. Hagfeldt and L. C. Sun, Org. Lett., 2009, 11, 5542; (c) K. Hara, Z. S. Wang, Y. Cui, A. Furube and N. Koumura, Energy Environ. Sci., 2009, 2, 1109; (d) Z. S. Wang, N. Koumura, Y. Cui, M. Takahashi, H. Sekiguchi, A. Mori, T. Kubo, A. Furube and K. Hara, Chem. Mater., 2008, 20, 3993.
11. R. Q. Chen, X. C. Yang, H. N. Tian and L. C. Sun, J. Photochem. Photobiol., A, 2007, 189, 295.
12. J. Tang, W. J. Wu, J. L. Hua, J. Li and H. Tian, Energy Environ. Sci., 2009, 2, 982.
13. (a) J. Tang, J. L. Hua, W. J. Wu, J. Li, Z. G. Jin, Y. T. Long and H. Tian, Energy Environ. Sci., 2010, 3, 1736; (b) Z. J. Ning, Q. Zhang, W. J. Wu, H. C. Pei, B. Liu and H. Tian, J. Org. Chem., 2008, 73, 3791; (c) C. Teng, X. C. Yang, S. F. Li, M. Cheng, A. Hagfeldt, L. Z. Wu and L. C. Sun, Chem.–Eur. J., 2010, 16, 13127; (d) L. Alibabaei, J. H. Kim, M. Wang, N. Pootrakulchote, J. Teuscher, D. Di Censo, R. Humphry-Baker, J.-E. Moser, Y.-J. Yu, K.-Y. Kay, S. M. Zakeeruddin and M. Grätzel, Energy Environ. Sci., 2010, 3, 1757.
14. X. X. Kong, A. P. Kulkarni and S. A. Jenekhe, Macromolecules, 2003, 36, 8992.
15. (a) Z. Q. Wan, C. Y. Jia, J. Q. Zhang, Y. D. Duan, Y. Lin and Y. Shi, J. Power Sources, 2012, 199, 426; (b) H. N. Tian, X. C. Yang, J. Y. Cong, R. K. Chen, C. Teng, J. Liu, Y. Hao, L. Wang and L. C. Sun, Dyes Pigm., 2010, 84, 62; (c) S. S. Park, Y. S. Won, Y. C. Choi and J. H. Kim, Energy Fuels, 2009, 23, 3732; (d) Z. B. Xie, A. Midya, K. P. Loh, S. Adams, D. J. Blackwood, J. Wang, X. J. Zhang and Z. K. Chen, Prog. Photovoltaics, 2010, 18, 573; (e) W. J. Wu, J. B. Yang, J. L. Hua, J. Tang, L. Zhang, Y. T. Long and H. Tian, J. Mater. Chem., 2010, 20, 1772.
16. P. Bonhôte, A. P. Dias, N. Papageorgiou, K. Kalyanasundaram and M. Grätzel, Inorg. Chem., 1996, 35, 1168.
17. J. Liu, H. T. Yang, W. W. Tan, X. W. Zhou and Y. Lin, Electrochim. Acta, 2010, 56, 396.
18. W. Xu, B. Peng, J. Chen, M. Liang and F. S. Cai, J. Phys. Chem. C, 2008, 112, 874.
19. C. H. Yang, H. L. Chen, Y. Y. Chuang, C. G. Wu, C. P. Chen, S. H. Liao and T. L. Wang, J. Power Sources, 2009, 188, 627.
20. A. Hagfeldt and M. Grätzel, Chem. Rev., 1995, 95, 49.
21. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Jr. Montgomery, T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez and J. A. Pople, Gaussian 03, revision B.05; Gaussian, Inc.: Wallingford, CT, 2004.
22. K. R. J. Thomas, Y. Hsu, J. T. Lin, K. Lee, K. Ho, C. Lai, Y. Cheng and P. Chou, Chem. Mater., 2008, 20, 1830.
23. B. J. Song, H. M. Song, I. T. Choi, S. K. Kim, K. D. Seo, M. S. Kang, M. J. Lee, D. W. Cho, M. J. Ju and H. K. Kim, Chem.–Eur. J., 2011, 17, 11115.

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