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Novel D-π-A organic sensitizers containing diarylmethylene-bridged triphenylamine and different spacers for solar cell application
Tetrahedron Letters 56 (2015) 1233–1238
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Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Novel D-p-A organic sensitizers containing diarylmethylene-bridged triphenylamine and different spacers for solar cell application Fei Wu a,b,⇑, Shangbi Zhao a,b, Lawrence Tien Lin Lee c, Min Wang a,b, Tao Chen c, Linna Zhu a,b,⇑ a
Institute for Clean Energy & Advanced Materials, Southwest University, Chongqing, China Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energy, China c Department of Physics, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China b
a r t i c l e
i n f o
Article history: Received 26 November 2014 Revised 14 January 2015 Accepted 23 January 2015 Available online 7 February 2015 Keywords: Bridged triphenylamine Dye-sensitized solar cell Thiophene Furan Photovoltaic properties
a b s t r a c t Bridged triphenylamine having almost planar core structure is a potential donor moiety for DSSCs. In this work, three novel D-p-A organic dyes TB-1, TB-2, and TB-3, containing diarylmethylene-bridged triphenylamine as the donor moiety, cyanoacrylic acid as the acceptor, and thiophene, benzene or furan as the p-spacers were synthesized and characterized for applications in dye-sensitized solar cells (DSSCs). Their photophysical, electrochemical, and photovoltaic properties were investigated, the performance of the triphenylamine donor was compared, and the effect of different p-spacers was evaluated. Results demonstrated that compared to the nonplanar triphenylamine donor, the more planar bridged TPA could offer better charge transfer process and as a result higher performances. On the other hand, in these compounds, both thiophene and furan linkages show better planarization and electron delocalization compared to the benzene linkage in this molecular system. Accordingly, dye TB-1 and TB-3 show higher IPCE and Jsc values. Considering the larger Voc of dye TB-3, therefore it outperforms the other two sensitizers, exhibiting power conversion efficiency of 3.81%, with Jsc of 7.54 mA cm 2 and Voc of 725 mV under simulated AM 1.5 irradiation (100 mW cm 2). The results are further confirmed by the EIS experiments. Ó 2015 Elsevier Ltd. All rights reserved.
Introduction For decades, dye-sensitized solar cells (DSSCs) have attracted wide interest due to their merits such as the low cost of production and ease of fabrication.1,2 As a key component of DSSC devices, the sensitizers of various kinds have been synthesized to improve the solar cell performance.3 In general, the metal-organic complexes and metal-free organic dyes could be classified. To date, DSSCs based on metal-organic complexes have been reported to achieve power conversion efficiency exceeding 12% under standard AM 1.5 solar light irradiation.4–6 Despite their leading performance in conversion efficiency of the metal–organic complexes, the rarity, toxicity, and high cost may limit their application in vast production of DSSCs. On the contrary, the organic dyes have many advantages such as high molar extinction coefficient, easy tuning of photophysical and electrochemical properties through molecular design, and low-cost synthetic routes in contrast to the metal– organic complexes.7 Enormous efforts have also been dedicated to developing efficient organic dyes for DSSCs, and great achievements are obtained. In recent years, various metal-free organic ⇑ Corresponding authors. Tel.: +86 23 68254957. E-mail address: [email protected] (L. Zhu). http://dx.doi.org/10.1016/j.tetlet.2015.01.156 0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.
dyes, such as suqaraine,8 carbazole,9 diketopyrrolopyrrole,10 isoindigo,11 and phenothiazine (PTZ)12 based organic dyes are devised and applied as efficient sensitizers for DSSCs, and good performances based on these dyes have also been achieved. For example, simple phenothiazine-based dyes have been reported to produce a power conversion efficiency of 8.18%, exceeding the reference N719 (7.73%) under identical fabrication conditions.12 D-p-A type organic dye incorporating first generation carbazole dendron as donor and cyanoacrylic acid as acceptor exhibited power conversion efficiency of 5.16% using I /I3 electrolyte.13 Donor-acceptor photosensitizers featuring modified triphenylamine and the tris(1,10-phenanthroline)cobalt(II/III) redox shuttle have achieved a solar-to-energy conversion efficiency of 9.4% when suitable bridge ligand was involved.14 Among the various organic dyes, triphenylamine (TPA) is still one of the most popular donor moieties and modification of TPA has caused wide interests.15–18 A successful approach was achieved by incorporating a planar amine with bulky substituents, which could not only increase the charge-separated state lifetime by the delocalization of the generated cation over a planar amine unit but also could avoid dye-aggregation.19 With two adjacent phenyl rings connected by a methylene linker, bridged TPA is a potential donor unit in constructing the donor-p-acceptor (D-p-A) type
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Scheme 1. Chemical structures of the reference compound L1 and dye TB-1, TB-2, and TB-3.
molecules, due to its much planar core structure compared to the conventional triphenylamine dyes, which is known to be in a propeller configuration. Bridged triphenylamine has a dihedral angle of 6.8 degree, which is much smaller as compared to that of triphenylamine ( 45 degree).20 The small dihedral angle between the plane of the phenyl ring and the plane of the N-bonded C atoms indicates that the electron delocalization in the bridged donor is much better than that for the conventional TPA, potentially leading to a red-shifted absorption spectrum.21 In 2012, organic sensitizers incorporating planar amine donor and bithiophene linker have shown high overall conversion efficiency of 8.71%.21 Later, B. Liu and coworkers further improved the efficiency by introducing the thiophene alkene p-spacer.22 Whereas as a whole, there is only very few study toward the bridged triphenylamine in solar cells. In this contribution, to further investigate the application of modified bridged-TPA for dye-sensitized solar cells, herein in this work we report the design, synthesis, and application of three new organic dyes based on diarylmethylene-bridged triphenylamine donor, cyanoacrylic acid as the acceptor and also the anchoring group, and thiophene, furan or benzene as the p-spacers, respectively. Their structures are shown in Scheme 1, together with the reference compound L1. Toluene groups on the periphery of the triphenylamine skeleton could prevent dye aggregation on the TiO2 surface, as well as help increase the solubility of the dye molecules.
The introduction of the thiophene, furan, and benzene linkage is expected to extend the whole conjugation system and is favorable for light harvesting. The novel triphenylamine-based sensitizers are successfully applied in the DSSCs and the corresponding photophysical, electrochemical, and photovoltaic properties are described in this work. The general synthetic routes for the intermediates and the final compounds are outlined in Scheme 2. Bridged-triphenylamine core structure was synthesized according to literature procedures.23,24 Mono-bromination of bridged TPA is a key step for synthesizing these molecules.20 The intermediates (compounds 2–4) were obtained by Suzuki coupling reaction between the mono-brominated bridged TPA and the corresponding arylboronic acid (5-formyl-2-thiopheneboronic acid for TB-1, 4-formylphenylboronic acid for TB-2, and 5-formyl-2-furylboronic acid for TB-3, respectively).25 The final products were fully characterized by 1H NMR spectroscopy, MALDI-TOF spectrometry, Elemental Analysis, and High Resolution Mass Spectrometry (HRMS) as well (for detailed synthetic routes, see ESI). Unfortunately, 13C NMR spectra of the final products were not able to be obtained due to their poor solubility. Detailed synthetic procedures, together with the Fabrication and Characterization of DSSCs are shown in the Supplementary data. Results and discussion Photophysical properties The UV–vis absorption of dyes TB-1, TB-2, and TB-3 in chloroform (CHCl3) is shown in Figure 1a. Each of these organic dyes exhibits major absorption bands at about 400–500 nm in chloroform, corresponding to the intramolecular charge transfer (ICT) transitions between the bridged-TPA donor and the cyanoacrylic acceptor. The absorption maxima (kmax) of TB-1 (488 nm) and TB-3 (461 nm) at the longer wavelength region are more redshifted in comparison to that of TB-2 (426 nm), which should be due to the better electron delocalization over the whole molecule in the case of the electron-rich thiophene and furan spacer. It has been reported that the traditional triphenylamine counterpart of dye TB-1 shows the maximum charge transfer absorption at 410 nm,15 suggesting improved ICT when bridged TPA is used as the donor. The result also indicates that the planar configuration in bridged TPA could offer more efficient charge transfer compared to the ordinary triphenylamine group. The molar extinction coefficients (e) at kmax of the ICT absorption peaks for TB-1, TB-2, and TB3 are calculated to be 3.23*104, 1.80*104, and 3.16*104 M 1 cm 1,
Scheme 2. Detailed synthetic procedures for TB-1, TB-2, and TB-3.
F. Wu et al. / Tetrahedron Letters 56 (2015) 1233–1238
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interface when they are adsorbed onto the TiO2 surface.26 Moreover, after the dyes are adsorbed on TiO2, the deprotonation of the carboxylate group due to the dye–TiO2 interaction will weaken the ability of the acceptor (carboxylate–TiO2 unit) compared to the carboxylic acid, which could also cause the blue-shifted absorption peak.27 Therefore, the reasons mentioned above may work together to explain the blue shift of absorption spectra of the dyes on TiO2 films in comparison to that in chloroform. Notably, the maximum absorption of TB-2 shifts to 409 nm when adsorbed on TiO2, corresponding to rather poor absorption range, indicating that benzene is not a favorable linker in this system. Electrochemical properties and theoretical calculations
Figure 1. Absorption of TB-1, TB-2, and TB-3 in (a) Chloroform (10 TiO2 film.
5
M), and (b) on
respectively (Table 1). The quite low molar extinction coefficient as well as the more blue shifted absorption band in TB-2 are very disadvantageous for light harvesting and thus should have adverse effect on the corresponding device performance. When these dyes are adsorbed onto the TiO2 films, their absorption maxima are blue-shifted more or less as compared to that measured in CHCl3 (Fig. 1b). The maximum absorption peaks for these dyes on TiO2 films are outlined in Table 1. According to literature report, the sensitizers have a tendency to aggregate at the solid–liquid
Table 1 Electrochemical and photophysical properties of dyes TB-1, TB-2, and TB-3 Dyes
TB-1 TB-2 TB-3 a
kmax (nm) (e(104 M 1 cm
488 (3.23) 426 1.80) 461 (3.16)
1
))a
kabs (nm) TiO2 film
Eopt g (eV)b
455 409 440
2.08 2.26 2.21
EHOMO (eV)c
5.22 5.15 5.08
ELUMO (eV)d
3.14 2.89 2.87
Absorption spectra were measured in CH2Cl2 solution. Calculated from Eopt g = 1240/konset. EHOMO was calculated by (4.74+E(onset.ox vs FC+/FC)), ELUMO was calculated by opt EHOMO + Eg . d EHOMO was calculated by (4.74+E(onset.ox vs FC+/FC)), ELUMO was calculated by EHOMO + Eopt g . b
c
The electrochemical behaviors of these dyes were measured by cyclic voltammetry. And the relevant CV data are presented in Table 1. Cyclic voltammetry in CH2Cl2 solution was used to determine the highest occupied molecular orbital (HOMO) of the dyes. The ground oxidation potentials (Eox) correspond to the highest occupied molecular orbitals (HOMO) while the lowest unoccupied molecular orbitals (LUMO) were obtained from the values of Eox and the zero–zero band gaps (E0–0) estimated from the onset of the UV–visible absorption spectra. The HOMO and LUMO levels of the dyes are collected in Table 1. Apparently, the HOMO levels of all dye molecules are more positive than the iodide/tri-iodide redox potential value (0.4 V vs NHE), indicating that the oxidized dyes could be efficiently regenerated by the electrolyte. Their LUMO levels are estimated from the HOMOs and the energy gaps (Eopt g ), and the values are higher than the conduction band (CB) of the TiO2 anode (0.5 V vs NHE), suggesting efficient driving force for electron injection from the LUMO of the dyes to the CB band of TiO2 semiconductor. Therefore, the three dyes are feasible for application in DSSCs. The structures of the three dyes are also analyzed by Gaussian 09 at B3LYP/6-31+G(d) level for full geometrical optimization (Fig. 2).28 Apparently, the HOMO levels are delocalized evenly throughout the entire core structure of bridged TPA and the adjacent spacer, while the LUMO levels are mainly located on the cyanoacrylic acid acceptor and the neighboring spacer, as well as partly on the TPA skeleton. The molecular orbital distribution could exert a favorable effect on the electronic transition from HOMO to LUMO. The orbital overlap between the donor and the acceptor is able to afford fast charge transition. When these molecules are adsorbed onto the TiO2 surface, the photogenerated electron can be effectively injected into the conduction band of the semiconductor from the acceptor part via the terminal cyanoacrylic acid anchoring group. Moreover, the dihedral angles formed between the phenyl rings in TPA and the p-spacer nearing the acceptor moiety in TB-1–TB-3 are calculated to be 19°, 31°, and 17°, respectively. The relatively larger dihedral angle between bridged TPA and the benzene spacer may explain the larger blue-shifted absorption band of TB-2 both in solution and on the TiO2 film in comparison with the other two dyes. Photovoltaic performances The typical current-voltage characteristics of DSSCs sensitized with the three dyes on TiO2 films using I /I3 electrolyte are measured under standard AM 1.5G conditions (100 mW cm 2). The results are depicted in Figure 3a. The performance of the triphenylamine based sensitizer L1 with thiophene linkage was also investigated under the same conditions for comparison. Among the three sensitizers, dye TB-3 with furan linkage exhibits the highest power conversion efficiency of 3.81%, with a short circuit current density (Jsc) of 7.54 mA cm 2, and the open circuit potential (Voc) of 725 mV; TB-1 under the same condition shows g of 3.59%, with
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Figure 2. Frontier molecular orbitals of the HOMO and LUMO of the three dyes calculated by DFT on a B3LYP/6-31 + G (d)⁄ level.
Figure 3. (a) Photocurrent density-photovoltage curves of TB-1, TB-2, TB-3, and L1 under AM 1.5 G simulated sunlight (100 mW cm 2) illumination and (b) IPCE spectra of TB-1, TB-2, TB-3, and L1 based devices.
Jsc of 7.44 mA cm-2 and Voc of 694 mV. However, the TB-2 dye with benzene linkage delivers the lowest g of 3.01%, with Jsc of 5.81 mA cm 2 and Voc of 736 mV. Under the same circumstance, dye L1 shows g of 2.96%, with Jsc of 5.57 mA cm 2 and Voc of 734 mV. The result of L1 is comparable to the literature reported value.29,30 Comparing the performances of TB-1 to L1, it is apparent
that the introduction of bridged TPA could enhance the power conversion efficiency by 21%, which confirms better choice of the more planar structure as the donor. However, when compared to the previous reports on dimethylmethylene-bridged TPA dyes,21,22 our diarylmethylene-bridged dyes show much lower efficiencies. On the one hand, in our bridged TPA structures, the aryl-bridge is much larger than the methyl-bridge reported in literatures, which may cause steric bulk effect on our dyes. On the other hand, the disparity should also be ascribed to the different p-spacers incorporated. We may estimate that the use of longer thiophene spacers or the EDOT units should enhance the solar cell performance. The DSSC performance parameters of these sensitizers are presented in Table 2. Evidently, the photocurrents of the dyes TB-1 and TB-3 with thiophene and furan linkages are higher than the dye TB-2 containing benzene linkage. This may be caused by the higher molar extinction coefficient of TB-1 and TB-3, which yields better light harvesting and higher short circuit current. The incident photo-to-current conversion efficiencies (IPCEs) as a function of incident wavelength for DSSCs based on these dyes are plotted in Figure 3b.31 Their IPCE spectra are significantly broadened compared to their absorption in chloroform. The IPCE spectra of the bridged TPA based dyes exhibit higher IPCE values from 400 to 500 nm than that of dye L1, which agree well with their Jsc values. Especially, TB-1 and TB-3 have broader band than TB-2, indicating that the solar cells sensitized with dyes TB-1 and TB-3 have better performances at the range of 300–600 nm. The broader and higher IPCE values of TB-1 and TB-3 are consistent with their higher Jsc values measured from the J-V curves. However, dye TB-2 with benzene linkage shows the most blue-shifted IPCE spectrum and relatively narrower spectrum compared to the other two dyes, corresponding to its lower Jsc value, which is also concerned with its more blue shifted absorption as well as its lower light harvesting ability. The IPCE spectra are consistent with the UV-vis absorption spectra of organic dyes adsorbed on TiO2 films. Although the Jsc values of dye TB-1 and TB-3 are similar, the larger Voc in TB-3 provides better device performance in TB-3 based DSSCs. Since the number of anchored sensitizers on the TiO2 film critically influences the Jsc as well, the dye loading amount of the three
Table 2 Photovoltaic performance of DSSCs based on TB-1, TB-2, TB-3, and L1 dyes Dye
Jsc (mA cm-2)
Voc (mV)
Fill factor (ff)
PCE g (%)
TB-1 TB-2 TB-3 L1
7.44 5.81 7.54 5.57
694 736 725 734
69.7 70.6 69.8 72.5
3.59 3.01 3.81 2.96
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sensitizers was determined. The TiO2 film was soaked in 0.3 mM organic dye in THF/ethanol (1:1 v/v) for 18 h in the dark, and the UV–vis absorbance of the remaining solution was measured. The dye loading of each device is then estimated from the Beer–Lambert’s law with the help of the dye’s respective extinction coefficient. The dye loading of each device is estimated to be 2.54 10 7, 2.51 10 7, and 2.90 10 7 mol/cm2 for TB-1, TB-2, and TB-3, respectively. The three dyes investigated show similar dye loading amount, perhaps owing to their similar molecular structures. This experiment further indicates that the low Jsc of dye TB-2 should be caused by the poor molar extinction coefficient. Electrochemical impedance spectroscopy (EIS) is performed to elucidate the interfacial charge recombination processes in DSSCs based on the three dyes under the dark conditions. The Nyquist and Bode plots for DSSCs based on the dyes are shown in Figure 4. The larger semicircle in the lower frequency range represents the resistance of the recombination between electrons on the TiO2 conduction band and I3 species in the electrolyte at the TiO2/dye/ electrolyte interface.32 The calculated resistance values (Rrec) are 97.69 X for TB-1, 243.4 X for TB-2, and 163.5 X for TB-3, respectively. The larger Rrec demonstrates slower recombination kinetics, thus resulting in smaller dark current and a larger value of opencircuit photovoltage.33 The trend of Rrec appears to be consistent with the values of the open circuit voltage of the three dye molecules (TB-2 >TB-3 >TB-1). Figure 4b shows the Bode plots of DSSCs based on dyes TB-1–TB-3. All EIS Bode plots exhibit two peaks
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featuring the frequency investigated. The one at higher frequencies corresponds to the charge transfer at the Pt/electrolyte interface and the other one at lower frequencies corresponds to the charge transfer at TiO2/dye/electrolyte interface, which is also related to the charge recombination rate, and moreover, its reciprocal is associated with the electron lifetime.34 The electron lifetime (s) was estimated to be in the order of TB-2 >TB-3 >TB-1. In general, a longer electron lifetime corresponds to a larger value of Voc.35 Therefore the lifetime result is also consistent with the Voc values measured from the J-V curves. Despite TB-2 has the largest Voc among the dyes studied, it still exhibits the lowest power conversion efficiency, probably come from the low molar extinction coefficient and the poor IPCE value. Conclusions In summary, we have demonstrated the design strategy and synthesis of novel D-p-A type organic sensitizers, containing diarylmethylene-bridged triphenylamine as the donor, cyanoacrylic acid as the acceptor, and thiophene, benzene or furan as the p-spacer, respectively. All of the sensitizers show considerable power conversion efficiency in the range of 300–600 nm. Higher performance (21% enhancement) of TB-1 relative to the triphenylamine counterpart L1 suggested rational design of the bridged TPA as the donor unit. Compared to the benzene linkage, both thiophene and furan linkages show better planarization and electron delocalization, exhibiting better absorption and broader IPCE spectra. In addition, the furan linkage provides dye TB-3 with larger Voc, which could also be verified by the EIS experiments. As a result, dye TB-3 delivers the best performance among the dyes studied, exhibiting power conversion efficiency of 3.81%, with Jsc of 7.54 mA cm 2 and Voc of 725 mV under simulated AM 1.5 irradiation (100 mW cm 2). However, although TB-2 has the largest Voc over the other two dyes, it still exhibits the lowest power conversion efficiency. This might be elucidated from its lower Jsc value verified from the more blue shifted absorption band as well as the IPCE spectra. This work indicates that the modified bridged triphenylamine could work as an efficient donor group to construct organic sensitizers for DSSCs. Further studies for modified bridged-TPA based sensitizers toward higher efficiency through molecular modifications are ongoing in our laboratory. Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 51203046), ‘Fundamental Research Funds for the Central Universities’ (XDJK2014C145 and XDJK2014C052), and the Starting Foundation of Southwest University (Nos. SWU113076 and SWU113078). Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.01. 156. References and notes
Figure 4. (a) Nyquist plots and (b) Bode-phase plots of the DSSCs fabricated using dyes TB-1, TB-2, and TB-3.
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Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet
Novel D-p-A organic sensitizers containing diarylmethylene-bridged triphenylamine and different spacers for solar cell application Fei Wu a,b,⇑, Shangbi Zhao a,b, Lawrence Tien Lin Lee c, Min Wang a,b, Tao Chen c, Linna Zhu a,b,⇑ a
Institute for Clean Energy & Advanced Materials, Southwest University, Chongqing, China Chongqing Key Laboratory for Advanced Materials and Technologies of Clean Energy, China c Department of Physics, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China b
a r t i c l e
i n f o
Article history: Received 26 November 2014 Revised 14 January 2015 Accepted 23 January 2015 Available online 7 February 2015 Keywords: Bridged triphenylamine Dye-sensitized solar cell Thiophene Furan Photovoltaic properties
a b s t r a c t Bridged triphenylamine having almost planar core structure is a potential donor moiety for DSSCs. In this work, three novel D-p-A organic dyes TB-1, TB-2, and TB-3, containing diarylmethylene-bridged triphenylamine as the donor moiety, cyanoacrylic acid as the acceptor, and thiophene, benzene or furan as the p-spacers were synthesized and characterized for applications in dye-sensitized solar cells (DSSCs). Their photophysical, electrochemical, and photovoltaic properties were investigated, the performance of the triphenylamine donor was compared, and the effect of different p-spacers was evaluated. Results demonstrated that compared to the nonplanar triphenylamine donor, the more planar bridged TPA could offer better charge transfer process and as a result higher performances. On the other hand, in these compounds, both thiophene and furan linkages show better planarization and electron delocalization compared to the benzene linkage in this molecular system. Accordingly, dye TB-1 and TB-3 show higher IPCE and Jsc values. Considering the larger Voc of dye TB-3, therefore it outperforms the other two sensitizers, exhibiting power conversion efficiency of 3.81%, with Jsc of 7.54 mA cm 2 and Voc of 725 mV under simulated AM 1.5 irradiation (100 mW cm 2). The results are further confirmed by the EIS experiments. Ó 2015 Elsevier Ltd. All rights reserved.
Introduction For decades, dye-sensitized solar cells (DSSCs) have attracted wide interest due to their merits such as the low cost of production and ease of fabrication.1,2 As a key component of DSSC devices, the sensitizers of various kinds have been synthesized to improve the solar cell performance.3 In general, the metal-organic complexes and metal-free organic dyes could be classified. To date, DSSCs based on metal-organic complexes have been reported to achieve power conversion efficiency exceeding 12% under standard AM 1.5 solar light irradiation.4–6 Despite their leading performance in conversion efficiency of the metal–organic complexes, the rarity, toxicity, and high cost may limit their application in vast production of DSSCs. On the contrary, the organic dyes have many advantages such as high molar extinction coefficient, easy tuning of photophysical and electrochemical properties through molecular design, and low-cost synthetic routes in contrast to the metal– organic complexes.7 Enormous efforts have also been dedicated to developing efficient organic dyes for DSSCs, and great achievements are obtained. In recent years, various metal-free organic ⇑ Corresponding authors. Tel.: +86 23 68254957. E-mail address: [email protected] (L. Zhu). http://dx.doi.org/10.1016/j.tetlet.2015.01.156 0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.
dyes, such as suqaraine,8 carbazole,9 diketopyrrolopyrrole,10 isoindigo,11 and phenothiazine (PTZ)12 based organic dyes are devised and applied as efficient sensitizers for DSSCs, and good performances based on these dyes have also been achieved. For example, simple phenothiazine-based dyes have been reported to produce a power conversion efficiency of 8.18%, exceeding the reference N719 (7.73%) under identical fabrication conditions.12 D-p-A type organic dye incorporating first generation carbazole dendron as donor and cyanoacrylic acid as acceptor exhibited power conversion efficiency of 5.16% using I /I3 electrolyte.13 Donor-acceptor photosensitizers featuring modified triphenylamine and the tris(1,10-phenanthroline)cobalt(II/III) redox shuttle have achieved a solar-to-energy conversion efficiency of 9.4% when suitable bridge ligand was involved.14 Among the various organic dyes, triphenylamine (TPA) is still one of the most popular donor moieties and modification of TPA has caused wide interests.15–18 A successful approach was achieved by incorporating a planar amine with bulky substituents, which could not only increase the charge-separated state lifetime by the delocalization of the generated cation over a planar amine unit but also could avoid dye-aggregation.19 With two adjacent phenyl rings connected by a methylene linker, bridged TPA is a potential donor unit in constructing the donor-p-acceptor (D-p-A) type
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Scheme 1. Chemical structures of the reference compound L1 and dye TB-1, TB-2, and TB-3.
molecules, due to its much planar core structure compared to the conventional triphenylamine dyes, which is known to be in a propeller configuration. Bridged triphenylamine has a dihedral angle of 6.8 degree, which is much smaller as compared to that of triphenylamine ( 45 degree).20 The small dihedral angle between the plane of the phenyl ring and the plane of the N-bonded C atoms indicates that the electron delocalization in the bridged donor is much better than that for the conventional TPA, potentially leading to a red-shifted absorption spectrum.21 In 2012, organic sensitizers incorporating planar amine donor and bithiophene linker have shown high overall conversion efficiency of 8.71%.21 Later, B. Liu and coworkers further improved the efficiency by introducing the thiophene alkene p-spacer.22 Whereas as a whole, there is only very few study toward the bridged triphenylamine in solar cells. In this contribution, to further investigate the application of modified bridged-TPA for dye-sensitized solar cells, herein in this work we report the design, synthesis, and application of three new organic dyes based on diarylmethylene-bridged triphenylamine donor, cyanoacrylic acid as the acceptor and also the anchoring group, and thiophene, furan or benzene as the p-spacers, respectively. Their structures are shown in Scheme 1, together with the reference compound L1. Toluene groups on the periphery of the triphenylamine skeleton could prevent dye aggregation on the TiO2 surface, as well as help increase the solubility of the dye molecules.
The introduction of the thiophene, furan, and benzene linkage is expected to extend the whole conjugation system and is favorable for light harvesting. The novel triphenylamine-based sensitizers are successfully applied in the DSSCs and the corresponding photophysical, electrochemical, and photovoltaic properties are described in this work. The general synthetic routes for the intermediates and the final compounds are outlined in Scheme 2. Bridged-triphenylamine core structure was synthesized according to literature procedures.23,24 Mono-bromination of bridged TPA is a key step for synthesizing these molecules.20 The intermediates (compounds 2–4) were obtained by Suzuki coupling reaction between the mono-brominated bridged TPA and the corresponding arylboronic acid (5-formyl-2-thiopheneboronic acid for TB-1, 4-formylphenylboronic acid for TB-2, and 5-formyl-2-furylboronic acid for TB-3, respectively).25 The final products were fully characterized by 1H NMR spectroscopy, MALDI-TOF spectrometry, Elemental Analysis, and High Resolution Mass Spectrometry (HRMS) as well (for detailed synthetic routes, see ESI). Unfortunately, 13C NMR spectra of the final products were not able to be obtained due to their poor solubility. Detailed synthetic procedures, together with the Fabrication and Characterization of DSSCs are shown in the Supplementary data. Results and discussion Photophysical properties The UV–vis absorption of dyes TB-1, TB-2, and TB-3 in chloroform (CHCl3) is shown in Figure 1a. Each of these organic dyes exhibits major absorption bands at about 400–500 nm in chloroform, corresponding to the intramolecular charge transfer (ICT) transitions between the bridged-TPA donor and the cyanoacrylic acceptor. The absorption maxima (kmax) of TB-1 (488 nm) and TB-3 (461 nm) at the longer wavelength region are more redshifted in comparison to that of TB-2 (426 nm), which should be due to the better electron delocalization over the whole molecule in the case of the electron-rich thiophene and furan spacer. It has been reported that the traditional triphenylamine counterpart of dye TB-1 shows the maximum charge transfer absorption at 410 nm,15 suggesting improved ICT when bridged TPA is used as the donor. The result also indicates that the planar configuration in bridged TPA could offer more efficient charge transfer compared to the ordinary triphenylamine group. The molar extinction coefficients (e) at kmax of the ICT absorption peaks for TB-1, TB-2, and TB3 are calculated to be 3.23*104, 1.80*104, and 3.16*104 M 1 cm 1,
Scheme 2. Detailed synthetic procedures for TB-1, TB-2, and TB-3.
F. Wu et al. / Tetrahedron Letters 56 (2015) 1233–1238
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interface when they are adsorbed onto the TiO2 surface.26 Moreover, after the dyes are adsorbed on TiO2, the deprotonation of the carboxylate group due to the dye–TiO2 interaction will weaken the ability of the acceptor (carboxylate–TiO2 unit) compared to the carboxylic acid, which could also cause the blue-shifted absorption peak.27 Therefore, the reasons mentioned above may work together to explain the blue shift of absorption spectra of the dyes on TiO2 films in comparison to that in chloroform. Notably, the maximum absorption of TB-2 shifts to 409 nm when adsorbed on TiO2, corresponding to rather poor absorption range, indicating that benzene is not a favorable linker in this system. Electrochemical properties and theoretical calculations
Figure 1. Absorption of TB-1, TB-2, and TB-3 in (a) Chloroform (10 TiO2 film.
5
M), and (b) on
respectively (Table 1). The quite low molar extinction coefficient as well as the more blue shifted absorption band in TB-2 are very disadvantageous for light harvesting and thus should have adverse effect on the corresponding device performance. When these dyes are adsorbed onto the TiO2 films, their absorption maxima are blue-shifted more or less as compared to that measured in CHCl3 (Fig. 1b). The maximum absorption peaks for these dyes on TiO2 films are outlined in Table 1. According to literature report, the sensitizers have a tendency to aggregate at the solid–liquid
Table 1 Electrochemical and photophysical properties of dyes TB-1, TB-2, and TB-3 Dyes
TB-1 TB-2 TB-3 a
kmax (nm) (e(104 M 1 cm
488 (3.23) 426 1.80) 461 (3.16)
1
))a
kabs (nm) TiO2 film
Eopt g (eV)b
455 409 440
2.08 2.26 2.21
EHOMO (eV)c
5.22 5.15 5.08
ELUMO (eV)d
3.14 2.89 2.87
Absorption spectra were measured in CH2Cl2 solution. Calculated from Eopt g = 1240/konset. EHOMO was calculated by (4.74+E(onset.ox vs FC+/FC)), ELUMO was calculated by opt EHOMO + Eg . d EHOMO was calculated by (4.74+E(onset.ox vs FC+/FC)), ELUMO was calculated by EHOMO + Eopt g . b
c
The electrochemical behaviors of these dyes were measured by cyclic voltammetry. And the relevant CV data are presented in Table 1. Cyclic voltammetry in CH2Cl2 solution was used to determine the highest occupied molecular orbital (HOMO) of the dyes. The ground oxidation potentials (Eox) correspond to the highest occupied molecular orbitals (HOMO) while the lowest unoccupied molecular orbitals (LUMO) were obtained from the values of Eox and the zero–zero band gaps (E0–0) estimated from the onset of the UV–visible absorption spectra. The HOMO and LUMO levels of the dyes are collected in Table 1. Apparently, the HOMO levels of all dye molecules are more positive than the iodide/tri-iodide redox potential value (0.4 V vs NHE), indicating that the oxidized dyes could be efficiently regenerated by the electrolyte. Their LUMO levels are estimated from the HOMOs and the energy gaps (Eopt g ), and the values are higher than the conduction band (CB) of the TiO2 anode (0.5 V vs NHE), suggesting efficient driving force for electron injection from the LUMO of the dyes to the CB band of TiO2 semiconductor. Therefore, the three dyes are feasible for application in DSSCs. The structures of the three dyes are also analyzed by Gaussian 09 at B3LYP/6-31+G(d) level for full geometrical optimization (Fig. 2).28 Apparently, the HOMO levels are delocalized evenly throughout the entire core structure of bridged TPA and the adjacent spacer, while the LUMO levels are mainly located on the cyanoacrylic acid acceptor and the neighboring spacer, as well as partly on the TPA skeleton. The molecular orbital distribution could exert a favorable effect on the electronic transition from HOMO to LUMO. The orbital overlap between the donor and the acceptor is able to afford fast charge transition. When these molecules are adsorbed onto the TiO2 surface, the photogenerated electron can be effectively injected into the conduction band of the semiconductor from the acceptor part via the terminal cyanoacrylic acid anchoring group. Moreover, the dihedral angles formed between the phenyl rings in TPA and the p-spacer nearing the acceptor moiety in TB-1–TB-3 are calculated to be 19°, 31°, and 17°, respectively. The relatively larger dihedral angle between bridged TPA and the benzene spacer may explain the larger blue-shifted absorption band of TB-2 both in solution and on the TiO2 film in comparison with the other two dyes. Photovoltaic performances The typical current-voltage characteristics of DSSCs sensitized with the three dyes on TiO2 films using I /I3 electrolyte are measured under standard AM 1.5G conditions (100 mW cm 2). The results are depicted in Figure 3a. The performance of the triphenylamine based sensitizer L1 with thiophene linkage was also investigated under the same conditions for comparison. Among the three sensitizers, dye TB-3 with furan linkage exhibits the highest power conversion efficiency of 3.81%, with a short circuit current density (Jsc) of 7.54 mA cm 2, and the open circuit potential (Voc) of 725 mV; TB-1 under the same condition shows g of 3.59%, with
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Figure 2. Frontier molecular orbitals of the HOMO and LUMO of the three dyes calculated by DFT on a B3LYP/6-31 + G (d)⁄ level.
Figure 3. (a) Photocurrent density-photovoltage curves of TB-1, TB-2, TB-3, and L1 under AM 1.5 G simulated sunlight (100 mW cm 2) illumination and (b) IPCE spectra of TB-1, TB-2, TB-3, and L1 based devices.
Jsc of 7.44 mA cm-2 and Voc of 694 mV. However, the TB-2 dye with benzene linkage delivers the lowest g of 3.01%, with Jsc of 5.81 mA cm 2 and Voc of 736 mV. Under the same circumstance, dye L1 shows g of 2.96%, with Jsc of 5.57 mA cm 2 and Voc of 734 mV. The result of L1 is comparable to the literature reported value.29,30 Comparing the performances of TB-1 to L1, it is apparent
that the introduction of bridged TPA could enhance the power conversion efficiency by 21%, which confirms better choice of the more planar structure as the donor. However, when compared to the previous reports on dimethylmethylene-bridged TPA dyes,21,22 our diarylmethylene-bridged dyes show much lower efficiencies. On the one hand, in our bridged TPA structures, the aryl-bridge is much larger than the methyl-bridge reported in literatures, which may cause steric bulk effect on our dyes. On the other hand, the disparity should also be ascribed to the different p-spacers incorporated. We may estimate that the use of longer thiophene spacers or the EDOT units should enhance the solar cell performance. The DSSC performance parameters of these sensitizers are presented in Table 2. Evidently, the photocurrents of the dyes TB-1 and TB-3 with thiophene and furan linkages are higher than the dye TB-2 containing benzene linkage. This may be caused by the higher molar extinction coefficient of TB-1 and TB-3, which yields better light harvesting and higher short circuit current. The incident photo-to-current conversion efficiencies (IPCEs) as a function of incident wavelength for DSSCs based on these dyes are plotted in Figure 3b.31 Their IPCE spectra are significantly broadened compared to their absorption in chloroform. The IPCE spectra of the bridged TPA based dyes exhibit higher IPCE values from 400 to 500 nm than that of dye L1, which agree well with their Jsc values. Especially, TB-1 and TB-3 have broader band than TB-2, indicating that the solar cells sensitized with dyes TB-1 and TB-3 have better performances at the range of 300–600 nm. The broader and higher IPCE values of TB-1 and TB-3 are consistent with their higher Jsc values measured from the J-V curves. However, dye TB-2 with benzene linkage shows the most blue-shifted IPCE spectrum and relatively narrower spectrum compared to the other two dyes, corresponding to its lower Jsc value, which is also concerned with its more blue shifted absorption as well as its lower light harvesting ability. The IPCE spectra are consistent with the UV-vis absorption spectra of organic dyes adsorbed on TiO2 films. Although the Jsc values of dye TB-1 and TB-3 are similar, the larger Voc in TB-3 provides better device performance in TB-3 based DSSCs. Since the number of anchored sensitizers on the TiO2 film critically influences the Jsc as well, the dye loading amount of the three
Table 2 Photovoltaic performance of DSSCs based on TB-1, TB-2, TB-3, and L1 dyes Dye
Jsc (mA cm-2)
Voc (mV)
Fill factor (ff)
PCE g (%)
TB-1 TB-2 TB-3 L1
7.44 5.81 7.54 5.57
694 736 725 734
69.7 70.6 69.8 72.5
3.59 3.01 3.81 2.96
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sensitizers was determined. The TiO2 film was soaked in 0.3 mM organic dye in THF/ethanol (1:1 v/v) for 18 h in the dark, and the UV–vis absorbance of the remaining solution was measured. The dye loading of each device is then estimated from the Beer–Lambert’s law with the help of the dye’s respective extinction coefficient. The dye loading of each device is estimated to be 2.54 10 7, 2.51 10 7, and 2.90 10 7 mol/cm2 for TB-1, TB-2, and TB-3, respectively. The three dyes investigated show similar dye loading amount, perhaps owing to their similar molecular structures. This experiment further indicates that the low Jsc of dye TB-2 should be caused by the poor molar extinction coefficient. Electrochemical impedance spectroscopy (EIS) is performed to elucidate the interfacial charge recombination processes in DSSCs based on the three dyes under the dark conditions. The Nyquist and Bode plots for DSSCs based on the dyes are shown in Figure 4. The larger semicircle in the lower frequency range represents the resistance of the recombination between electrons on the TiO2 conduction band and I3 species in the electrolyte at the TiO2/dye/ electrolyte interface.32 The calculated resistance values (Rrec) are 97.69 X for TB-1, 243.4 X for TB-2, and 163.5 X for TB-3, respectively. The larger Rrec demonstrates slower recombination kinetics, thus resulting in smaller dark current and a larger value of opencircuit photovoltage.33 The trend of Rrec appears to be consistent with the values of the open circuit voltage of the three dye molecules (TB-2 >TB-3 >TB-1). Figure 4b shows the Bode plots of DSSCs based on dyes TB-1–TB-3. All EIS Bode plots exhibit two peaks
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featuring the frequency investigated. The one at higher frequencies corresponds to the charge transfer at the Pt/electrolyte interface and the other one at lower frequencies corresponds to the charge transfer at TiO2/dye/electrolyte interface, which is also related to the charge recombination rate, and moreover, its reciprocal is associated with the electron lifetime.34 The electron lifetime (s) was estimated to be in the order of TB-2 >TB-3 >TB-1. In general, a longer electron lifetime corresponds to a larger value of Voc.35 Therefore the lifetime result is also consistent with the Voc values measured from the J-V curves. Despite TB-2 has the largest Voc among the dyes studied, it still exhibits the lowest power conversion efficiency, probably come from the low molar extinction coefficient and the poor IPCE value. Conclusions In summary, we have demonstrated the design strategy and synthesis of novel D-p-A type organic sensitizers, containing diarylmethylene-bridged triphenylamine as the donor, cyanoacrylic acid as the acceptor, and thiophene, benzene or furan as the p-spacer, respectively. All of the sensitizers show considerable power conversion efficiency in the range of 300–600 nm. Higher performance (21% enhancement) of TB-1 relative to the triphenylamine counterpart L1 suggested rational design of the bridged TPA as the donor unit. Compared to the benzene linkage, both thiophene and furan linkages show better planarization and electron delocalization, exhibiting better absorption and broader IPCE spectra. In addition, the furan linkage provides dye TB-3 with larger Voc, which could also be verified by the EIS experiments. As a result, dye TB-3 delivers the best performance among the dyes studied, exhibiting power conversion efficiency of 3.81%, with Jsc of 7.54 mA cm 2 and Voc of 725 mV under simulated AM 1.5 irradiation (100 mW cm 2). However, although TB-2 has the largest Voc over the other two dyes, it still exhibits the lowest power conversion efficiency. This might be elucidated from its lower Jsc value verified from the more blue shifted absorption band as well as the IPCE spectra. This work indicates that the modified bridged triphenylamine could work as an efficient donor group to construct organic sensitizers for DSSCs. Further studies for modified bridged-TPA based sensitizers toward higher efficiency through molecular modifications are ongoing in our laboratory. Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 51203046), ‘Fundamental Research Funds for the Central Universities’ (XDJK2014C145 and XDJK2014C052), and the Starting Foundation of Southwest University (Nos. SWU113076 and SWU113078). Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.01. 156. References and notes
Figure 4. (a) Nyquist plots and (b) Bode-phase plots of the DSSCs fabricated using dyes TB-1, TB-2, and TB-3.
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