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A new organic dye bearing aldehyde electron-withdrawing group for dye-sensitized solar cell
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Solar Energy 86 (2012) 2306–2311 www.elsevier.com/locate/solener
A new organic dye bearing aldehyde electron-withdrawing group for dye-sensitized solar cell Jin Tang, Sanyin Qu, Juan Hu, Wenjun Wu, Jianli Hua ⇑ Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology, 130 Meilong Road, Shanghai 200237, PR China Received 16 November 2011; received in revised form 8 March 2012; accepted 1 May 2012 Available online 28 May 2012 Communicated by: Associate Editor Sam-Shajin Sun
Abstract A new metal-free dye (I) with a diketopyrrolopyrrole (DPP) core was synthesized, in which triphenylamine was used as electron donor, thiophene units as the p-conjugated bridge, aldehyde units as electron acceptor. The corresponding dye II containing carboxy group as the electron-withdrawing acceptor for the purpose of comparison was also synthesized. The absorption spectra, electrochemical and photovoltaic properties of I and II were extensively investigated. Electrochemical measurements data indicate that the tuning of HOMO and LUMO energy levels can be conveniently accomplished by alternating electron acceptor. The short-circuit photocurrent density and conversion efficiency of solar cell based on aldehyde-containing dye is more dominant than that bear a carboxy group as the electron withdrawing anchoring group. The new sensitizer I exhibited a photovoltaic performance: a short-circuit photocurrent density (Jsc) of 6.07 mA cm 2, an open-circuit photovoltage (Voc) of 568 mV, and a fill factor (FF) of 0.66, corresponding to an overall conversion efficiency of 2.27% under standard global AM 1.5 solar light condition. This work suggests that aldehyde units as new type of electron withdrawing anchoring group are promising candidates for improvement of the performance of DSSCs. Ó 2012 Elsevier Ltd. All rights reserved. Keywords: Dye-sensitized solar cells; Diketopyrrolopyrrole; Aldehyde electron acceptor
1. Introduction Dye-sensitized solar cells (DSSCs) based on nanocrystalline porous TiO2 films have been regarded as promising candidates for photovoltaic devices because of their relatively higher efficiency and low cost compared with conventional inorganic photovoltaic devices (Nazeeruddin et al., 1993). DSSCs based on the Ru-complex dyes can produce photoelectric conversion yield of 11.5% under standard AM 1.5 sun light irradiation (Chen et al., 2009). However, the large-scale application of ruthenium dyes is limited because of costs and environmental issues. So more and ⇑ Corresponding author. Tel.: +86 21 64250940; fax: +86 21 64252758.
E-mail address: [email protected] (J. Hua). 0038-092X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2012.05.003
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 procedure at lower cost (Wang et al., 2000, 2008; Sayama et al., 2000; Ning and Tian, 2009; Horiuchi et al., 2004; Li et al., 2009; Han and Islam, 2009; Marinado et al., 2009; Guo et al., 2010; Hara et al., 2005; Hagberg et al., 2006; Wu et al., 2011; Liu et al., 2009). Generally, D–p–A dyes with both the electron-donating (D) and electron-accepting (A) groups linked by a p-conjugation bridge that exhibits broad and intense absorption, were proposed as one of the most promising organic dye sensitizers. Nevertheless, most researches were dedicated to develop novel donor and p-conjugation bridge in D–p–A dyes and the accepting groups were relatively simple, such
J. Tang et al. / Solar Energy 86 (2012) 2306–2311
as carboxylic acid, cyanoacrylic acid, and rhodanine-3-acetic acid. The major factor is that the electron acceptors should also be anchoring groups to adsorb onto TiO2 surfaces and make sure that the eletrons could inject to conduction band of TiO2 electrode effectively. However, to develop more efficient pure organic dyes for DSSCs, it is demand to investigate novel and efficient accepting groups. Some effort have been made in these field, Youhei Numata et al. developed a new method to introduce various electron-withdrawing groups as the acceptor part of D–p–A dyes for free exploration of the acceptor designs (Numata et al., 2011). Yousuke Ooyama et al. proposed the use of a pyridine ring as an electron-withdrawing and demonstrated that pyridine ring can anchor to the surface by coordinate bonding with the Lewis acid sites of TiO2 (Ooyama et al., 2011). Nevertheless the low efficiency and complex synthesis is the major disadvantages in this field. DPP-containing materials are bright and strongly fluorescent with exceptional photochemical, mechanical, and thermal stability and are therefore used in industrial applications as high performance pigments in paints, plastics, and inks (Zhu et al., 2007). Recently, our group synthesized a series of compounds based on diketopyrrolopyrrole (DPP) dyes with triphenylamine as the donor and cyanoacrylic acid moiety as acceptor, showing an overall conversion efficiency of 5.65% to DSSCs (Qu et al., 2010, 2012). In the course of exploration of DPP dyes with cyanoacrylic acid as acceptor, we found that some DPP dyes with aldehyde as a electron acceptor can also been adsorbed to TiO2 surface effectively. Since the conventional sensitizers based on a carboxy acceptor as the electron-withdrawing group were synthesized via Knoevenagel condensation of respective aldehydes with cyanoacetic acid, the use of aldehyde as a new electron acceptor would predigest the synthetic route. In this work, we reported a new organic dye I bearing aldehyde electron-withdrawing group (Fig. 1), in which I can adsorbed to TiO2 surface effectively and applied it successfully to sensitization of nanocrystalline TiO2-based solar cells. This result indicated that the conversion efficiency of solar cell based on aldehyde-containing dye is more potential than conventional dye sensitizer bearing a carboxy group as the electron-withdrawing anchoring group.
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2.2. Materials Optically transparent FTO conducting glass (fluorine doped SnO2, transmission >90% in the visible, sheet resistance 15 X/square) was obtained from the Geao Science and Educational Co. Ltd. of China and cleaned by a standard procedure. Tetra-n-butylammonium hexafluorophosphate(TBAPF6), 4-tert-butylpyridine(4-TBP), and lithium iodide were bought from Fluka and iodine, 99.999%, was purchased from Alfa Aesar. All other chemicals were purchased from Aldrich and used as received without further purification. The sensitizers I and II were prepared according to published procedures (Qu et al., 2010), in which the structures of two DPP dyes were characterized by 1 HNMR, 13CNMR and HR-MS. I. 1H NMR (500 MHz, CDCl3) d: (ppm) 9.90 (s, 1H), 9.08 (d, J = 4.20 Hz, 1H), 8.85 (d, J = 4.17 Hz, 1H), 7.72 (d, J = 3.97 Hz, 1H), 7.49 (d, J = 2.58 Hz, 2H), 7.48 (d, J = 1.80, 1H), 7.39 (d, J = 3.94 Hz, 1H), 7.35 (d, J = 4.19 Hz, 1H), 7.10 (d, J = 8.88 Hz, 4H), 6.92 (d, J = 8.76 Hz, 2H), 6.86 (d, J = 8.91, 4H), 4.12 (m, 4H), 3.83 (s, 6H), 1.77 (m, 4H), 1.48 (m, 4H), 0.98 (m, 6H). 13 C NMR (125 MHz, CDCl3) d: 184.0, 163.2, 162.6, 158.2, 153.4, 151.4, 147.1, 144.5, 142.7, 141.6, 141.5, 139.8, 138.8, 138.6, 137.1, 128.8, 128.6, 127.0, 124.9, 121.2, 116.5, 57.2, 55.1, 43.8, 33.8, 33.7, 21.9, 15.4. HRMS (ESI) (m/z): [M] calcd for C47H43N3O5S3: 825.2365, found: 825.2372. II. 1H NMR (500 MHz, DMSO) d: (ppm) 8.92 (d, J = 2.92, 1H), 8.78 (d, J = 3.70, 1H), 8.13 (s, 1H), 7.72 (d, 1H), 7.68 (d, 1H), 7.62 (d, 1H), 7.56 (d, 1H), 7.53 (d, J = 8.78 Hz, 2H), 7.04 (d, J = 8.41 Hz, 4H), 6.93 (d, J = 8.57 Hz, 4H), 6.72 (d, J = 8.18 Hz, 2H), 4.02 (m, 4H), 3.76 (s, 6H), 1.64 (m, 4H), 1.38 (m, 4H), 0.94 (t, J = 7.13 Hz, 7.38 Hz, 6H). 13C NMR (125 MHz, DMSO) d: 162.8, 162.2, 158.0, 151.2, 151.0, 146.8, 144.5, 142.3, 141.9, 141.5, 137.9, 137.2, 136.2, 136.0, 134.3, 128.8, 128.5, 127.0, 124.9, 121.1, 116.4, 57.1, 56.5, 54.6, 43.3, 34.0, 33.4, 32.1, 31.8, 24.6, 24.3, 21.8, 15.9, 15.3. HRMS (m/z): [M + H]+ calcd. for (C50H44N4O6S3): 893.2501; found, 893.2502. 2.3. Preparation of dye-sensitized nanocrystalline TiO2 electrodes
2. Experimental 2.1. Equipments NMR spectra were obtained on Bru¨cker AM 500 spectrometer. The UV–Vis spectra were measured with model CARY 100 spectrophotometer. MS were recorded on ESI mass spectroscopy. The cyclic voltammograms of dyes were obtained with a Versastat II electrochemical workstation (Princeton applied research) using a normal three-electrode cell with a Pt working electrode, a Pt wire counter electrode, and a regular calomel reference electrode in saturated KCl solution.
A layer of ca. 5 mm TiO2 (13 nm paste, T/SP) was coated on the FTO conducting glass by screen printing and then dried for 6 min at 125 °C. This procedure was repeated 2 times (ca. 10 mm) and finally coated by a layer (ca. 4 mm) of TiO2 paste (Ti-nanoxide 300) as the scattering layer. The tri-layer TiO2 electrodes were gradually heated under an air flow at 275 °C for 5 min, 325 °C for 5 min, 375 °C for 5 min, 450 °C for 15 min, and 500 °C for 15 min. The sintered film was further treated with 0.2 M TiCl4 aqueous solution at room temperature for 12 h, then washed with water and ethanol, and annealed at 450 °C for 30 min. After the film was cooled to 50 °C, it was immersed
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Fig. 1. Molecular structures of I and II.
in a 3 10 4 M dye bath in CHCl3 solution and maintained in the dark for 24 h at room temperature. The electrode was then rinsed with CH2Cl2 and dried. The size of the TiO2 electrodes used was 0.28 cm2. To prepare the counter electrode, the Pt catalyst was deposited on cleaned FTO glass by coating with a drop of H2PtCl6 solution (0.02 M 2-propanol solution) with heat treatment at 400 °C for 15 min. A hole (0.8 mm diameter) was drilled on the counter electrode using a drill-press. The perforated sheet was cleaned with ultrasound in an ethanol bath for 10 min. For the assembly of DSSCs, the dye-covered TiO2 electrode and Pt-counter electrode were assembled into a sandwich type cell and sealed with a hot-melt gasket of 25 mm thickness made of the ionomer Surlyn 1702 (DuPont). The electrolyte consisting of 0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII), 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tertbutylpyridine (TBP) in a acetonitrile was introduced into the cell via vacuum backfilling from the hole in the back of the counter electrode. Finally, the hole was sealed using a UV-melt gum and a cover glass. 3. Results and discussion 3.1. Absorption properties in solutions and on TiO2 film Normalized absorption spectra of dyes I and II in CH2Cl2 and on TiO2 film are shown in Fig. 2, and their absorption data are listed in Table 1. In the UV–Vis
Fig. 2. Normalized absorption spectra of I and II in CH2Cl2 and on TiO2.
spectra, I and II exhibit two major prominent bands, appearing at 250–500 nm and at 500–750 nm, respectively. The former is ascribed to a localized aromatic p–p transition and the later is of charge-transfer character. The absorption maxima of the dyes I and II were appearing at 643 nm and 627 nm in diluted solution, respectively. The former was observed to be largely red shifted and the corresponding maximum extinction coefficient of I and II were 4.83 104 and 3.32 104 M 1 cm 1, respectively, which show that DPP dyes I with aldehyde units as electron acceptor can be used as a candidate sensitized dyes for DSSCs results from their light-harvesting range and relatively higher maximum extinction coefficient. When the two dyes were adsorbed on the TiO2 surface, the absorption spectra were generally broadened. Compared with the solution spectra (Fig. 2), the maximal absorption peak of TiO2 electrodes sensitized by I was blue-shifted by 35 nm and the p–p transition band was elevated. However, the absorption spectra of II on TiO2 film exhibited an opposite trend. The maximal absorption peak of TiO2 electrodes sensitized by II was red-shifted by 75 nm and the p–p transition band was depressed. The results may tentatively be rationalized by the difference of adsorption group that created different adsorption mode.
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Table 1 Optical properties and redox potential of I and II. Dye
kmaxa (nm) (e 10
I II
643 (4.83) 627 (3.32)
4
M
1
cm 1)
kmaxb (nm)
HOMOc (V) (vs. NHE)
E0–0d (eV)
608 702
0.89 0.99
1.67 1.55
LUMOe (V) (vs. NHE) 0.78 0.56
a
Absorption maximum in solution (3 10 5 M). Absorption maximum on TiO2 film. c HOMO were measured in acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as electrolyte (working electrode: FTO/TiO2/ dye; reference electrode: SCE; calibrated with ferrocene/ferrocenium (Fc/Fc+) as an external reference. Counter electrode: Pt). d E0–0 was estimated from the absorption thresholds from absorption spectra of dyes adsorbed on the TiO2 film. e LUMO is estimated by subtracting E0–0 to HOMO. b
3.2. Electrochemical properties To evaluate the possibility of electron transfer from the excited dye to the conduction band of TiO2, cyclic voltammetry were performed in CH2Cl2 solution, using 0.1 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte. The examined highest occupied molecular orbital (HOMO) levels and the lowest unoccupied molecular orbital (LUMO) levels are collected in Table 1. The SCE reference electrode was calibrated using a ferrocene/ferrocenium (Fc/Fc+) redox couple as an external standard. The excitation transition energy (E0–0) of I and II were estimated from their absorption thresholds of dye-sensitized TiO2 films to be 1.67 eV and 1.55 eV, respectively. The HOMO of I and II corresponding to their first redox potential are 0.89 V and 0.99 V vs. NHE, which are much more positive than the iodine/iodide redox potential value (0.4 V), ensuring that there is enough driving force for the dye regeneration efficiently through the recapture of the injected electrons from I by the dye cation radical. The estimated excited state potential corresponding to the LUMO levels of I and II, calculated from EHOMO – E0–0, are 0.78 V and 0.56 V vs. NHE, respectively. Judging from the LUMO value, the two dyes are more negative than the bottom of the conduction band of TiO2 ( 0.5 V), indicating that the electron injection process from the excited dye molecule to TiO2 conduction band is energetically permitted.15 Noticeably, the dyes with aldehyde as the electron-withdrawing anchoring group has the relatively higher LUMO level ( 0.78 V) than corresponding carboxylic dye ( 0.56 V). When the LUMO level is improved, the driving force of electron injection increased, which was a positive effect for the performance of the DSSC. The large energy gaps between the LUMO of the dye and Ecb of semiconductor allow for the treatment the 4-tert-butylpyridine (TBP) to the electrolyte, which shift the Ecb of the TiO2 negatively and consequently improve the open circuit voltage and total conversion efficiency (Ning et al., 2008). 3.3. Photovoltaic performance of DSSCs Fig. 3 shows the incident monochromatic photon-tocurrent conversion efficiency (IPCE) obtained with a sandwich-type two electrode cells. The dye-coated TiO2 film was
Fig. 3. Photocurrent action spectra of the TiO2 electrodes sensitized by I and II.
used as working electrode, platinized FTO glass as counter electrodes and 0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII), 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tertbutylpyridine (TBP) in a acetonitrile solution as redox electrolyte. The solar cell based on I shows the highest value of 48.3% at 587 nm and the maximum IPCE value of II is 53.6% at 609 nm. On the other hand, the IPCE value of I in the range 452–582 nm is higher than that for II though that of II in the range 582–800 nm is higher than that for I. This result was in accordance with their absorption spectra on the TiO2 film. The photovoltaic performance of the DSSCs was measured at 100 mW cm 2 under simulated AM 1.5G solar light conditions. I sensitized cell gave a short circuit photocurrent density (Jsc) of 7.47 mA cm 2, an open circuit voltage (Voc) of 0.432 V and a fill factor (FF) of 0.58, corresponding to an overall conversion efficiency (g) of 1.87%. Under the same conditions, II sensitized cell gave a Jsc value of 7.65 mA cm 2, a Voc of 0.346 V and an FF of 0.53, corresponding to the g value of 1.45% (see Table 2). The increase in Voc could be caused by two different mechanisms: one is a retardation of recombination between injected electrons and oxidized species in the electrolyte, the other is a band edge movement of TiO2. The relatively higher LUMO of I than that of II mean that the driving force of electron injection increased. That may lead to an increase of electrons in TiO2 surface which shifted the
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Table 2 Influence of 4-TBP on the photovoltaic performance parameters.a Dye
4-TBP
Jsc (mA cm 2)
Voc (V)
FF
g (%)
I
0 0.25 0.50 0.75 0 0.5 M
7.47 6.40 6.07 4.03 7.65 1.71
0.432 0.536 0.568 0.570 0.346 0.419
0.58 0.64 0.66 0.54 0.55 0.64
1.87 2.18 2.27 1.23 1.45 0.46
II a
Illumination: 100 mW cm
2
simulated AM 1.5 G solar light.
conduction band edge negatively and obtained higher Voc. This result indicated that the aldehyde as novel acceptor improved the energy level and benefited the development of dye-sensitized solar cells. 4-Tertbutylpyridine (TBP) is a common additive in the electrolyte of DSSCs. It can retard the charge recombination between the electrons in the conductive band of TiO2 and I =I 3 in the electrolyte by interacting with the TiO2 film. The addition of TBP to the electrolyte could increase the Voc but decrease Jsc simultaneously, resulting in significant improvement of an overall conversion efficiency (g). As shown in Fig. 4, the cell voltage in the DSSCs with I was largely improved with an addition of 4-TBP in the electrolyte while the photocurrent density was decreased. The best data was obtained with 0.5 M 4-TBP, I sensitized cell gave a short circuit photocurrent density (Jsc) of 6.07 mA cm 2, an open circuit voltage (Voc) of 0.568 V and a fill factor (FF) of 0.66, corresponding to an overall conversion efficiency (g) of 2.27% at 100 mW cm 2 under simulated AM 1.5G solar light conditions. Under the same conditions, II sensitized cell gave a Jsc value of 1.71 mA cm 2 , a Voc of 0.419 V and an FF of 0.64, corresponding to the g value of 0.46% (see Table 2). However, further increase the 4-TBP concentration to 0.75 M leads to a dramatic decrease in the Jsc value, as a consequence, the overall conversion efficiency (g) decrease. The reduction
Fig. 4. Light and dark current–voltage characteristics of DSSCs based on I with various concentration of 4-TBP in the redox electrolyte under irradiation of AM 1.5G simulated solar light (100 mW cm 2).
potential of I obtained in CH2Cl2 is 0.76 V (Table 1), which is largely more negative than the value of 0.5 V for TiO2 electrode, indicating an improvement of total efficiency when the use of 4-TBP at a suitable concentration. In the contrary, the TiO2 conductive band levels are relatively close in energy to the excited state levels of the dye II (Table 1) when the concentration of 4-TBP is 0.5 M in the electrolyte, the Jsc value of II is drastically decreased. As a consequence, aldehyde-containing dyes are more potential than those for conventional dye sensitizers that bear a carboxyl group in DSSCs. Fig. 4 also shows the dark current-voltage characteristics of the mesoscopic dye sensitized solar cells employing various concentration of 4-TBP in the redox electrolyte. The dark current decreases in the order 0, 0.25, 0.5 and 0.75 M, which agrees well with the Voc values. This dark current change indicates that TBP can suppress charge recombination between injected electrons and I 3 ions in the electrolyte. CDCA is known to improve DSSC efficiency by solving the problem of dye aggregation. In order to optimize dye absorption and sensitization, the effect of CDCA as coadsorbents was investigated. However, CDCA is not suit for these aldehyde anchoring dyes which would decrease the adsorbance dramaticly. That may be due to the binding force between aldehyde and TiO2 particles are weaker than that of carboxyl group. 3.4. Electrochemical impedance spectroscopy A DSSC can be considered as a leaking capacitor in dark conditions. As a result, the resistance of the back reaction from TiO2 to the I 3 ions in the electrolyte was analyzed through electrochemical impedance technique under dark conditions. Fig. 5 compares the impedance spectra for I and II-sensitized cells measured in the dark under a forward bias of 0.50 V with a frequency range of 0.1 Hz to 100 KHz, respectively. The electron lifetime values derived from curve fitting of I and II are 24.3 ms and
Fig. 5. Electrochemical impedance for DSSCs sensitized by I and II, respectively.
J. Tang et al. / Solar Energy 86 (2012) 2306–2311
12.29 ms, respectively. The longer electron lifetime observed with I relative to II indicates more effective suppression of the back reaction of the injected electron with I 3 in the electrolyte and is reflected in the improvements seen in the open circuit voltage, yielding substantially enhanced device efficiency. 4. Conclusion In summary, we have reported a new organic sensitizer (I) with a diketopyrrolopyrrole (DPP) as core, aldehyde unit as electron-withdrawing group and its application in DSSCs. The cell based on this new sensitizer exhibits a 2.27% conversion efficiency under AM 1.5G sunlight. The addition of TBP to electrolyte could improve Voc and decrease Jsc slightly, resulting in improvement of an overall conversion efficiency (g). Our finding demonstrates that the introduction of aldehyde unit as electron-withdrawing group can improve the LUMO energy level and conversion efficiency of dye-sensitized solar cells. Most importantly, it opens up the possibility of preparing new electron-withdrawing group for DSSCs. Acknowledgements This work was supported by NSFC/China (21161160444 and 20772031), National Basic Research 973 Program (2011CB808400), Ph.D. Programs Foundation of Ministry of Education of China (20090074110004), the Fundamental Research Funds for the Central Universities (WJ0913001) and Scientific Committee of Shanghai (10520709700). References Chen, C.Y., Wang, M.K., Li, J.Y., Pootrakulchote, N., Alibabaei, L., Ngoc-le, C., Decoppet, J.D., Tsai, J.H., Gra¨tzel, C., Wu, C.G., Zakeeruddin, S.M., Gra¨tzel, M., 2009. Highly efficient light-harvesting ruthenium sensitizer for thin-film dye-sensitized solar cells. ACS Nano. 3 (10), 3103–3109. Guo, L., Pan, X., Zhang, C.N., Liu, W.Q., Wang, M., Fang, X.Q., Dai, S.Y., 2010. Ionic liquid electrolyte based on s-propyltetrahydrothiophenium iodide for dye-sensitized solar cells. Sol. Energy 84, 373–378. Hagberg, D.P., Edvinsson, T., Marinado, T., Boschloo, G., Hagfeldt, A., Sun, L., 2006. A novel organic chromophore for dye-sensitized nanostructured solar cells. Chem. Commun., 2245. Han, L., Islam, A., 2009. Integrated dye-sensitized solar cell module with conversion efficiency of 8.2%. Appl. Phys. Lett. 94 (1), 013305. Hara, K., Sato, T., Katoh, R., Furube, A., Yoshihara, T., Murai, M., Kurashige, M., Shinpo, A., Ito, S., Suga, S., Arakawa, H., 2005. Novel conjugated organic dyes for efficient dye-sensitized solar cells. Adv. Funct. Mater. 15, 246–252. Horiuchi, T., Miura, H., Sumioka, K., Uchida, S., 2004. High efficiency of dye-sensitized solar cells based on metal-free indoline dyes. J. Am. Chem. Soc. 126, 12218–12219.
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Li, Q.Q., Lu, L.L., Zhong, C., Huang, J., Huang, Q., Shi, J., Jin, X.B., Peng, T.Y., Qin, J.G., Li, Z., 2009. New pyrrole-based organic dyes for dye-sensitized solar cells: convenient syntheses and high efficiency. Chem. A Eur. J. 15, 9664–9668. Liu, B., Zhu, W., Zhang, Q., Wu, W., Xu, M., Ning, Z., 2009. Conveniently synthesized isophorone dyes for high efficiency dyesensitized solar cells: tuning photovoltaic performance by structural modification of donor group in donor–acceptor system. Chem. Commun., 1766–1768. Marinado, T., Hagberg, D.P., Hedlund, M., Edvinsson, T., Johansson, E.M.J., Boschloo, G., Rensmo, H., Brinck, T., Sun, L.C., Hagfeldty, 2009. Rhodanine dyes for dye-sensitized solar cells: spectroscopy, energy levels and photovoltaic performance. A. Phys. Chem. Chem. Phys 11, 133–141. Nazeeruddin, M.K., Kay, A., Rodicio, I., Humphry, R., Muller, E., Gra¨tzel, M., 1993. Conversion of light to electricity by cis-X2 bis(2,20bipyridyl-4,40-dicarboxylate) ruthenium (II) charge-transfer sensitizers (X = Cl , Br , I , CN and SCN ) on nanocrystalline titanium dioxide electrodes. J. Am. Chem. Soc. 115, 6382–6391. Ning, Z., Tian, H., 2009. Triarylamine: a promising core unit for efficient photovoltaic materials. Chem. Commun. 5483–5495. Ning, Z., Zhang, Q., Wu, W., Pei, H., Liu, B., Tian, H., 2008. Starburst triarylamine based dyes for efficient dye-sensitized solar cells. J. Org. Chem. 73, 3791–3797. Numata, Y., Ashraful, I., Shirai, Y., Han, L.Y., 2011. Preparation of donor-acceptor type organic dyes bearing various electron-withdrawing groups for dye-sensitized solar cell application. Chem. Commun., 6159–6161. Ooyama, Y., Inoue, S., Nagano, T., Kushimoto, K., Ohshita, J., Imae, I., Komaguchi, K.J., Harima, Y., 2011. Dye-sensitized solar cells based on donor-acceptor p-conjugated fluorescent dyes with a pyridine ring as an electron-withdrawing anchoring group. Angew. Chem. Int. Ed. 50, 1–6. Qu, S.Y., Wu, W.J., Hua, J.L., Kong, C., Long, Y.T., Tian, H., 2010. New diketo-pyrrolo-pyrrole (DPP) dyes for efficient dye-sensitized solar cells. J. Phys. Chem. C 114, 1343–1349. Qu, S.Y., Wang, B., Guo, F.L., Li, J., Wu, W.J., Kong, C., Long, Y.T., Hua, J.L., 2012. New diketo-pyrrolo-pyrrole (DPP) sensitizer containing furan moiety for efficient and stable dye-sensitized solar cells. Dyes Pigments 92, 1384–1393. Sayama, K., Hara, K., Mori, N., Satsuki, M., Suga, S., Tsukagoshi, S., Abe, Y., Sugihara H., Arakawa, H., 2000. Photosensitization of a porous TiO2 electrode with merocyanine dyes containing a carboxyl group and a long alkyl chain. Chemical Communications 1173–1174. Wang, Z.S., Li, F.Y., Huang, C.H., 2000. Highly efficient sensitization of nanocrystalline TiO2 films with styryl benzothiazolium propylsulfonate. Chem. Commun., 2063–2064. Wang, Z.S., Cui, Y., Dan-oh, Y., Kasada, C., Shinpo, A., Hara, K., 2008. Molecular design of coumarin dyes for stable and efficient organic dyesensitized solar cells. J. Phys. Chem. C 112, 17011–17017. Wu, Q.P., Zhang, L., Liang, M., Sun, Z., Xue, S., 2011. Sensitizers containing donor cascade and rhodanine-3-acetic acid moieties for dye-sensitized solar cells. Sol. Energy 85, 1–6. Zhu, Y., Rabindranath, A.R., Beyerlein, T., Tieke, B., 2007. Highly luminescent 1,4-diketo-3,6-diphenylpyrrolo[3,4-c]pyrrole-(DPP-) based conjugated polymers prepared upon Suzuki coupling. Macromolecules 40, 6981–6989.
Solar Energy 86 (2012) 2306–2311 www.elsevier.com/locate/solener
A new organic dye bearing aldehyde electron-withdrawing group for dye-sensitized solar cell Jin Tang, Sanyin Qu, Juan Hu, Wenjun Wu, Jianli Hua ⇑ Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology, 130 Meilong Road, Shanghai 200237, PR China Received 16 November 2011; received in revised form 8 March 2012; accepted 1 May 2012 Available online 28 May 2012 Communicated by: Associate Editor Sam-Shajin Sun
Abstract A new metal-free dye (I) with a diketopyrrolopyrrole (DPP) core was synthesized, in which triphenylamine was used as electron donor, thiophene units as the p-conjugated bridge, aldehyde units as electron acceptor. The corresponding dye II containing carboxy group as the electron-withdrawing acceptor for the purpose of comparison was also synthesized. The absorption spectra, electrochemical and photovoltaic properties of I and II were extensively investigated. Electrochemical measurements data indicate that the tuning of HOMO and LUMO energy levels can be conveniently accomplished by alternating electron acceptor. The short-circuit photocurrent density and conversion efficiency of solar cell based on aldehyde-containing dye is more dominant than that bear a carboxy group as the electron withdrawing anchoring group. The new sensitizer I exhibited a photovoltaic performance: a short-circuit photocurrent density (Jsc) of 6.07 mA cm 2, an open-circuit photovoltage (Voc) of 568 mV, and a fill factor (FF) of 0.66, corresponding to an overall conversion efficiency of 2.27% under standard global AM 1.5 solar light condition. This work suggests that aldehyde units as new type of electron withdrawing anchoring group are promising candidates for improvement of the performance of DSSCs. Ó 2012 Elsevier Ltd. All rights reserved. Keywords: Dye-sensitized solar cells; Diketopyrrolopyrrole; Aldehyde electron acceptor
1. Introduction Dye-sensitized solar cells (DSSCs) based on nanocrystalline porous TiO2 films have been regarded as promising candidates for photovoltaic devices because of their relatively higher efficiency and low cost compared with conventional inorganic photovoltaic devices (Nazeeruddin et al., 1993). DSSCs based on the Ru-complex dyes can produce photoelectric conversion yield of 11.5% under standard AM 1.5 sun light irradiation (Chen et al., 2009). However, the large-scale application of ruthenium dyes is limited because of costs and environmental issues. So more and ⇑ Corresponding author. Tel.: +86 21 64250940; fax: +86 21 64252758.
E-mail address: [email protected] (J. Hua). 0038-092X/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.solener.2012.05.003
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 procedure at lower cost (Wang et al., 2000, 2008; Sayama et al., 2000; Ning and Tian, 2009; Horiuchi et al., 2004; Li et al., 2009; Han and Islam, 2009; Marinado et al., 2009; Guo et al., 2010; Hara et al., 2005; Hagberg et al., 2006; Wu et al., 2011; Liu et al., 2009). Generally, D–p–A dyes with both the electron-donating (D) and electron-accepting (A) groups linked by a p-conjugation bridge that exhibits broad and intense absorption, were proposed as one of the most promising organic dye sensitizers. Nevertheless, most researches were dedicated to develop novel donor and p-conjugation bridge in D–p–A dyes and the accepting groups were relatively simple, such
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as carboxylic acid, cyanoacrylic acid, and rhodanine-3-acetic acid. The major factor is that the electron acceptors should also be anchoring groups to adsorb onto TiO2 surfaces and make sure that the eletrons could inject to conduction band of TiO2 electrode effectively. However, to develop more efficient pure organic dyes for DSSCs, it is demand to investigate novel and efficient accepting groups. Some effort have been made in these field, Youhei Numata et al. developed a new method to introduce various electron-withdrawing groups as the acceptor part of D–p–A dyes for free exploration of the acceptor designs (Numata et al., 2011). Yousuke Ooyama et al. proposed the use of a pyridine ring as an electron-withdrawing and demonstrated that pyridine ring can anchor to the surface by coordinate bonding with the Lewis acid sites of TiO2 (Ooyama et al., 2011). Nevertheless the low efficiency and complex synthesis is the major disadvantages in this field. DPP-containing materials are bright and strongly fluorescent with exceptional photochemical, mechanical, and thermal stability and are therefore used in industrial applications as high performance pigments in paints, plastics, and inks (Zhu et al., 2007). Recently, our group synthesized a series of compounds based on diketopyrrolopyrrole (DPP) dyes with triphenylamine as the donor and cyanoacrylic acid moiety as acceptor, showing an overall conversion efficiency of 5.65% to DSSCs (Qu et al., 2010, 2012). In the course of exploration of DPP dyes with cyanoacrylic acid as acceptor, we found that some DPP dyes with aldehyde as a electron acceptor can also been adsorbed to TiO2 surface effectively. Since the conventional sensitizers based on a carboxy acceptor as the electron-withdrawing group were synthesized via Knoevenagel condensation of respective aldehydes with cyanoacetic acid, the use of aldehyde as a new electron acceptor would predigest the synthetic route. In this work, we reported a new organic dye I bearing aldehyde electron-withdrawing group (Fig. 1), in which I can adsorbed to TiO2 surface effectively and applied it successfully to sensitization of nanocrystalline TiO2-based solar cells. This result indicated that the conversion efficiency of solar cell based on aldehyde-containing dye is more potential than conventional dye sensitizer bearing a carboxy group as the electron-withdrawing anchoring group.
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2.2. Materials Optically transparent FTO conducting glass (fluorine doped SnO2, transmission >90% in the visible, sheet resistance 15 X/square) was obtained from the Geao Science and Educational Co. Ltd. of China and cleaned by a standard procedure. Tetra-n-butylammonium hexafluorophosphate(TBAPF6), 4-tert-butylpyridine(4-TBP), and lithium iodide were bought from Fluka and iodine, 99.999%, was purchased from Alfa Aesar. All other chemicals were purchased from Aldrich and used as received without further purification. The sensitizers I and II were prepared according to published procedures (Qu et al., 2010), in which the structures of two DPP dyes were characterized by 1 HNMR, 13CNMR and HR-MS. I. 1H NMR (500 MHz, CDCl3) d: (ppm) 9.90 (s, 1H), 9.08 (d, J = 4.20 Hz, 1H), 8.85 (d, J = 4.17 Hz, 1H), 7.72 (d, J = 3.97 Hz, 1H), 7.49 (d, J = 2.58 Hz, 2H), 7.48 (d, J = 1.80, 1H), 7.39 (d, J = 3.94 Hz, 1H), 7.35 (d, J = 4.19 Hz, 1H), 7.10 (d, J = 8.88 Hz, 4H), 6.92 (d, J = 8.76 Hz, 2H), 6.86 (d, J = 8.91, 4H), 4.12 (m, 4H), 3.83 (s, 6H), 1.77 (m, 4H), 1.48 (m, 4H), 0.98 (m, 6H). 13 C NMR (125 MHz, CDCl3) d: 184.0, 163.2, 162.6, 158.2, 153.4, 151.4, 147.1, 144.5, 142.7, 141.6, 141.5, 139.8, 138.8, 138.6, 137.1, 128.8, 128.6, 127.0, 124.9, 121.2, 116.5, 57.2, 55.1, 43.8, 33.8, 33.7, 21.9, 15.4. HRMS (ESI) (m/z): [M] calcd for C47H43N3O5S3: 825.2365, found: 825.2372. II. 1H NMR (500 MHz, DMSO) d: (ppm) 8.92 (d, J = 2.92, 1H), 8.78 (d, J = 3.70, 1H), 8.13 (s, 1H), 7.72 (d, 1H), 7.68 (d, 1H), 7.62 (d, 1H), 7.56 (d, 1H), 7.53 (d, J = 8.78 Hz, 2H), 7.04 (d, J = 8.41 Hz, 4H), 6.93 (d, J = 8.57 Hz, 4H), 6.72 (d, J = 8.18 Hz, 2H), 4.02 (m, 4H), 3.76 (s, 6H), 1.64 (m, 4H), 1.38 (m, 4H), 0.94 (t, J = 7.13 Hz, 7.38 Hz, 6H). 13C NMR (125 MHz, DMSO) d: 162.8, 162.2, 158.0, 151.2, 151.0, 146.8, 144.5, 142.3, 141.9, 141.5, 137.9, 137.2, 136.2, 136.0, 134.3, 128.8, 128.5, 127.0, 124.9, 121.1, 116.4, 57.1, 56.5, 54.6, 43.3, 34.0, 33.4, 32.1, 31.8, 24.6, 24.3, 21.8, 15.9, 15.3. HRMS (m/z): [M + H]+ calcd. for (C50H44N4O6S3): 893.2501; found, 893.2502. 2.3. Preparation of dye-sensitized nanocrystalline TiO2 electrodes
2. Experimental 2.1. Equipments NMR spectra were obtained on Bru¨cker AM 500 spectrometer. The UV–Vis spectra were measured with model CARY 100 spectrophotometer. MS were recorded on ESI mass spectroscopy. The cyclic voltammograms of dyes were obtained with a Versastat II electrochemical workstation (Princeton applied research) using a normal three-electrode cell with a Pt working electrode, a Pt wire counter electrode, and a regular calomel reference electrode in saturated KCl solution.
A layer of ca. 5 mm TiO2 (13 nm paste, T/SP) was coated on the FTO conducting glass by screen printing and then dried for 6 min at 125 °C. This procedure was repeated 2 times (ca. 10 mm) and finally coated by a layer (ca. 4 mm) of TiO2 paste (Ti-nanoxide 300) as the scattering layer. The tri-layer TiO2 electrodes were gradually heated under an air flow at 275 °C for 5 min, 325 °C for 5 min, 375 °C for 5 min, 450 °C for 15 min, and 500 °C for 15 min. The sintered film was further treated with 0.2 M TiCl4 aqueous solution at room temperature for 12 h, then washed with water and ethanol, and annealed at 450 °C for 30 min. After the film was cooled to 50 °C, it was immersed
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Fig. 1. Molecular structures of I and II.
in a 3 10 4 M dye bath in CHCl3 solution and maintained in the dark for 24 h at room temperature. The electrode was then rinsed with CH2Cl2 and dried. The size of the TiO2 electrodes used was 0.28 cm2. To prepare the counter electrode, the Pt catalyst was deposited on cleaned FTO glass by coating with a drop of H2PtCl6 solution (0.02 M 2-propanol solution) with heat treatment at 400 °C for 15 min. A hole (0.8 mm diameter) was drilled on the counter electrode using a drill-press. The perforated sheet was cleaned with ultrasound in an ethanol bath for 10 min. For the assembly of DSSCs, the dye-covered TiO2 electrode and Pt-counter electrode were assembled into a sandwich type cell and sealed with a hot-melt gasket of 25 mm thickness made of the ionomer Surlyn 1702 (DuPont). The electrolyte consisting of 0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII), 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tertbutylpyridine (TBP) in a acetonitrile was introduced into the cell via vacuum backfilling from the hole in the back of the counter electrode. Finally, the hole was sealed using a UV-melt gum and a cover glass. 3. Results and discussion 3.1. Absorption properties in solutions and on TiO2 film Normalized absorption spectra of dyes I and II in CH2Cl2 and on TiO2 film are shown in Fig. 2, and their absorption data are listed in Table 1. In the UV–Vis
Fig. 2. Normalized absorption spectra of I and II in CH2Cl2 and on TiO2.
spectra, I and II exhibit two major prominent bands, appearing at 250–500 nm and at 500–750 nm, respectively. The former is ascribed to a localized aromatic p–p transition and the later is of charge-transfer character. The absorption maxima of the dyes I and II were appearing at 643 nm and 627 nm in diluted solution, respectively. The former was observed to be largely red shifted and the corresponding maximum extinction coefficient of I and II were 4.83 104 and 3.32 104 M 1 cm 1, respectively, which show that DPP dyes I with aldehyde units as electron acceptor can be used as a candidate sensitized dyes for DSSCs results from their light-harvesting range and relatively higher maximum extinction coefficient. When the two dyes were adsorbed on the TiO2 surface, the absorption spectra were generally broadened. Compared with the solution spectra (Fig. 2), the maximal absorption peak of TiO2 electrodes sensitized by I was blue-shifted by 35 nm and the p–p transition band was elevated. However, the absorption spectra of II on TiO2 film exhibited an opposite trend. The maximal absorption peak of TiO2 electrodes sensitized by II was red-shifted by 75 nm and the p–p transition band was depressed. The results may tentatively be rationalized by the difference of adsorption group that created different adsorption mode.
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Table 1 Optical properties and redox potential of I and II. Dye
kmaxa (nm) (e 10
I II
643 (4.83) 627 (3.32)
4
M
1
cm 1)
kmaxb (nm)
HOMOc (V) (vs. NHE)
E0–0d (eV)
608 702
0.89 0.99
1.67 1.55
LUMOe (V) (vs. NHE) 0.78 0.56
a
Absorption maximum in solution (3 10 5 M). Absorption maximum on TiO2 film. c HOMO were measured in acetonitrile with 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as electrolyte (working electrode: FTO/TiO2/ dye; reference electrode: SCE; calibrated with ferrocene/ferrocenium (Fc/Fc+) as an external reference. Counter electrode: Pt). d E0–0 was estimated from the absorption thresholds from absorption spectra of dyes adsorbed on the TiO2 film. e LUMO is estimated by subtracting E0–0 to HOMO. b
3.2. Electrochemical properties To evaluate the possibility of electron transfer from the excited dye to the conduction band of TiO2, cyclic voltammetry were performed in CH2Cl2 solution, using 0.1 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte. The examined highest occupied molecular orbital (HOMO) levels and the lowest unoccupied molecular orbital (LUMO) levels are collected in Table 1. The SCE reference electrode was calibrated using a ferrocene/ferrocenium (Fc/Fc+) redox couple as an external standard. The excitation transition energy (E0–0) of I and II were estimated from their absorption thresholds of dye-sensitized TiO2 films to be 1.67 eV and 1.55 eV, respectively. The HOMO of I and II corresponding to their first redox potential are 0.89 V and 0.99 V vs. NHE, which are much more positive than the iodine/iodide redox potential value (0.4 V), ensuring that there is enough driving force for the dye regeneration efficiently through the recapture of the injected electrons from I by the dye cation radical. The estimated excited state potential corresponding to the LUMO levels of I and II, calculated from EHOMO – E0–0, are 0.78 V and 0.56 V vs. NHE, respectively. Judging from the LUMO value, the two dyes are more negative than the bottom of the conduction band of TiO2 ( 0.5 V), indicating that the electron injection process from the excited dye molecule to TiO2 conduction band is energetically permitted.15 Noticeably, the dyes with aldehyde as the electron-withdrawing anchoring group has the relatively higher LUMO level ( 0.78 V) than corresponding carboxylic dye ( 0.56 V). When the LUMO level is improved, the driving force of electron injection increased, which was a positive effect for the performance of the DSSC. The large energy gaps between the LUMO of the dye and Ecb of semiconductor allow for the treatment the 4-tert-butylpyridine (TBP) to the electrolyte, which shift the Ecb of the TiO2 negatively and consequently improve the open circuit voltage and total conversion efficiency (Ning et al., 2008). 3.3. Photovoltaic performance of DSSCs Fig. 3 shows the incident monochromatic photon-tocurrent conversion efficiency (IPCE) obtained with a sandwich-type two electrode cells. The dye-coated TiO2 film was
Fig. 3. Photocurrent action spectra of the TiO2 electrodes sensitized by I and II.
used as working electrode, platinized FTO glass as counter electrodes and 0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII), 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tertbutylpyridine (TBP) in a acetonitrile solution as redox electrolyte. The solar cell based on I shows the highest value of 48.3% at 587 nm and the maximum IPCE value of II is 53.6% at 609 nm. On the other hand, the IPCE value of I in the range 452–582 nm is higher than that for II though that of II in the range 582–800 nm is higher than that for I. This result was in accordance with their absorption spectra on the TiO2 film. The photovoltaic performance of the DSSCs was measured at 100 mW cm 2 under simulated AM 1.5G solar light conditions. I sensitized cell gave a short circuit photocurrent density (Jsc) of 7.47 mA cm 2, an open circuit voltage (Voc) of 0.432 V and a fill factor (FF) of 0.58, corresponding to an overall conversion efficiency (g) of 1.87%. Under the same conditions, II sensitized cell gave a Jsc value of 7.65 mA cm 2, a Voc of 0.346 V and an FF of 0.53, corresponding to the g value of 1.45% (see Table 2). The increase in Voc could be caused by two different mechanisms: one is a retardation of recombination between injected electrons and oxidized species in the electrolyte, the other is a band edge movement of TiO2. The relatively higher LUMO of I than that of II mean that the driving force of electron injection increased. That may lead to an increase of electrons in TiO2 surface which shifted the
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Table 2 Influence of 4-TBP on the photovoltaic performance parameters.a Dye
4-TBP
Jsc (mA cm 2)
Voc (V)
FF
g (%)
I
0 0.25 0.50 0.75 0 0.5 M
7.47 6.40 6.07 4.03 7.65 1.71
0.432 0.536 0.568 0.570 0.346 0.419
0.58 0.64 0.66 0.54 0.55 0.64
1.87 2.18 2.27 1.23 1.45 0.46
II a
Illumination: 100 mW cm
2
simulated AM 1.5 G solar light.
conduction band edge negatively and obtained higher Voc. This result indicated that the aldehyde as novel acceptor improved the energy level and benefited the development of dye-sensitized solar cells. 4-Tertbutylpyridine (TBP) is a common additive in the electrolyte of DSSCs. It can retard the charge recombination between the electrons in the conductive band of TiO2 and I =I 3 in the electrolyte by interacting with the TiO2 film. The addition of TBP to the electrolyte could increase the Voc but decrease Jsc simultaneously, resulting in significant improvement of an overall conversion efficiency (g). As shown in Fig. 4, the cell voltage in the DSSCs with I was largely improved with an addition of 4-TBP in the electrolyte while the photocurrent density was decreased. The best data was obtained with 0.5 M 4-TBP, I sensitized cell gave a short circuit photocurrent density (Jsc) of 6.07 mA cm 2, an open circuit voltage (Voc) of 0.568 V and a fill factor (FF) of 0.66, corresponding to an overall conversion efficiency (g) of 2.27% at 100 mW cm 2 under simulated AM 1.5G solar light conditions. Under the same conditions, II sensitized cell gave a Jsc value of 1.71 mA cm 2 , a Voc of 0.419 V and an FF of 0.64, corresponding to the g value of 0.46% (see Table 2). However, further increase the 4-TBP concentration to 0.75 M leads to a dramatic decrease in the Jsc value, as a consequence, the overall conversion efficiency (g) decrease. The reduction
Fig. 4. Light and dark current–voltage characteristics of DSSCs based on I with various concentration of 4-TBP in the redox electrolyte under irradiation of AM 1.5G simulated solar light (100 mW cm 2).
potential of I obtained in CH2Cl2 is 0.76 V (Table 1), which is largely more negative than the value of 0.5 V for TiO2 electrode, indicating an improvement of total efficiency when the use of 4-TBP at a suitable concentration. In the contrary, the TiO2 conductive band levels are relatively close in energy to the excited state levels of the dye II (Table 1) when the concentration of 4-TBP is 0.5 M in the electrolyte, the Jsc value of II is drastically decreased. As a consequence, aldehyde-containing dyes are more potential than those for conventional dye sensitizers that bear a carboxyl group in DSSCs. Fig. 4 also shows the dark current-voltage characteristics of the mesoscopic dye sensitized solar cells employing various concentration of 4-TBP in the redox electrolyte. The dark current decreases in the order 0, 0.25, 0.5 and 0.75 M, which agrees well with the Voc values. This dark current change indicates that TBP can suppress charge recombination between injected electrons and I 3 ions in the electrolyte. CDCA is known to improve DSSC efficiency by solving the problem of dye aggregation. In order to optimize dye absorption and sensitization, the effect of CDCA as coadsorbents was investigated. However, CDCA is not suit for these aldehyde anchoring dyes which would decrease the adsorbance dramaticly. That may be due to the binding force between aldehyde and TiO2 particles are weaker than that of carboxyl group. 3.4. Electrochemical impedance spectroscopy A DSSC can be considered as a leaking capacitor in dark conditions. As a result, the resistance of the back reaction from TiO2 to the I 3 ions in the electrolyte was analyzed through electrochemical impedance technique under dark conditions. Fig. 5 compares the impedance spectra for I and II-sensitized cells measured in the dark under a forward bias of 0.50 V with a frequency range of 0.1 Hz to 100 KHz, respectively. The electron lifetime values derived from curve fitting of I and II are 24.3 ms and
Fig. 5. Electrochemical impedance for DSSCs sensitized by I and II, respectively.
J. Tang et al. / Solar Energy 86 (2012) 2306–2311
12.29 ms, respectively. The longer electron lifetime observed with I relative to II indicates more effective suppression of the back reaction of the injected electron with I 3 in the electrolyte and is reflected in the improvements seen in the open circuit voltage, yielding substantially enhanced device efficiency. 4. Conclusion In summary, we have reported a new organic sensitizer (I) with a diketopyrrolopyrrole (DPP) as core, aldehyde unit as electron-withdrawing group and its application in DSSCs. The cell based on this new sensitizer exhibits a 2.27% conversion efficiency under AM 1.5G sunlight. The addition of TBP to electrolyte could improve Voc and decrease Jsc slightly, resulting in improvement of an overall conversion efficiency (g). Our finding demonstrates that the introduction of aldehyde unit as electron-withdrawing group can improve the LUMO energy level and conversion efficiency of dye-sensitized solar cells. Most importantly, it opens up the possibility of preparing new electron-withdrawing group for DSSCs. Acknowledgements This work was supported by NSFC/China (21161160444 and 20772031), National Basic Research 973 Program (2011CB808400), Ph.D. Programs Foundation of Ministry of Education of China (20090074110004), the Fundamental Research Funds for the Central Universities (WJ0913001) and Scientific Committee of Shanghai (10520709700). References Chen, C.Y., Wang, M.K., Li, J.Y., Pootrakulchote, N., Alibabaei, L., Ngoc-le, C., Decoppet, J.D., Tsai, J.H., Gra¨tzel, C., Wu, C.G., Zakeeruddin, S.M., Gra¨tzel, M., 2009. Highly efficient light-harvesting ruthenium sensitizer for thin-film dye-sensitized solar cells. ACS Nano. 3 (10), 3103–3109. Guo, L., Pan, X., Zhang, C.N., Liu, W.Q., Wang, M., Fang, X.Q., Dai, S.Y., 2010. Ionic liquid electrolyte based on s-propyltetrahydrothiophenium iodide for dye-sensitized solar cells. Sol. Energy 84, 373–378. Hagberg, D.P., Edvinsson, T., Marinado, T., Boschloo, G., Hagfeldt, A., Sun, L., 2006. A novel organic chromophore for dye-sensitized nanostructured solar cells. Chem. Commun., 2245. Han, L., Islam, A., 2009. Integrated dye-sensitized solar cell module with conversion efficiency of 8.2%. Appl. Phys. Lett. 94 (1), 013305. Hara, K., Sato, T., Katoh, R., Furube, A., Yoshihara, T., Murai, M., Kurashige, M., Shinpo, A., Ito, S., Suga, S., Arakawa, H., 2005. Novel conjugated organic dyes for efficient dye-sensitized solar cells. Adv. Funct. Mater. 15, 246–252. Horiuchi, T., Miura, H., Sumioka, K., Uchida, S., 2004. High efficiency of dye-sensitized solar cells based on metal-free indoline dyes. J. Am. Chem. Soc. 126, 12218–12219.
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Li, Q.Q., Lu, L.L., Zhong, C., Huang, J., Huang, Q., Shi, J., Jin, X.B., Peng, T.Y., Qin, J.G., Li, Z., 2009. New pyrrole-based organic dyes for dye-sensitized solar cells: convenient syntheses and high efficiency. Chem. A Eur. J. 15, 9664–9668. Liu, B., Zhu, W., Zhang, Q., Wu, W., Xu, M., Ning, Z., 2009. Conveniently synthesized isophorone dyes for high efficiency dyesensitized solar cells: tuning photovoltaic performance by structural modification of donor group in donor–acceptor system. Chem. Commun., 1766–1768. Marinado, T., Hagberg, D.P., Hedlund, M., Edvinsson, T., Johansson, E.M.J., Boschloo, G., Rensmo, H., Brinck, T., Sun, L.C., Hagfeldty, 2009. Rhodanine dyes for dye-sensitized solar cells: spectroscopy, energy levels and photovoltaic performance. A. Phys. Chem. Chem. Phys 11, 133–141. Nazeeruddin, M.K., Kay, A., Rodicio, I., Humphry, R., Muller, E., Gra¨tzel, M., 1993. Conversion of light to electricity by cis-X2 bis(2,20bipyridyl-4,40-dicarboxylate) ruthenium (II) charge-transfer sensitizers (X = Cl , Br , I , CN and SCN ) on nanocrystalline titanium dioxide electrodes. J. Am. Chem. Soc. 115, 6382–6391. Ning, Z., Tian, H., 2009. Triarylamine: a promising core unit for efficient photovoltaic materials. Chem. Commun. 5483–5495. Ning, Z., Zhang, Q., Wu, W., Pei, H., Liu, B., Tian, H., 2008. Starburst triarylamine based dyes for efficient dye-sensitized solar cells. J. Org. Chem. 73, 3791–3797. Numata, Y., Ashraful, I., Shirai, Y., Han, L.Y., 2011. Preparation of donor-acceptor type organic dyes bearing various electron-withdrawing groups for dye-sensitized solar cell application. Chem. Commun., 6159–6161. Ooyama, Y., Inoue, S., Nagano, T., Kushimoto, K., Ohshita, J., Imae, I., Komaguchi, K.J., Harima, Y., 2011. Dye-sensitized solar cells based on donor-acceptor p-conjugated fluorescent dyes with a pyridine ring as an electron-withdrawing anchoring group. Angew. Chem. Int. Ed. 50, 1–6. Qu, S.Y., Wu, W.J., Hua, J.L., Kong, C., Long, Y.T., Tian, H., 2010. New diketo-pyrrolo-pyrrole (DPP) dyes for efficient dye-sensitized solar cells. J. Phys. Chem. C 114, 1343–1349. Qu, S.Y., Wang, B., Guo, F.L., Li, J., Wu, W.J., Kong, C., Long, Y.T., Hua, J.L., 2012. New diketo-pyrrolo-pyrrole (DPP) sensitizer containing furan moiety for efficient and stable dye-sensitized solar cells. Dyes Pigments 92, 1384–1393. Sayama, K., Hara, K., Mori, N., Satsuki, M., Suga, S., Tsukagoshi, S., Abe, Y., Sugihara H., Arakawa, H., 2000. Photosensitization of a porous TiO2 electrode with merocyanine dyes containing a carboxyl group and a long alkyl chain. Chemical Communications 1173–1174. Wang, Z.S., Li, F.Y., Huang, C.H., 2000. Highly efficient sensitization of nanocrystalline TiO2 films with styryl benzothiazolium propylsulfonate. Chem. Commun., 2063–2064. Wang, Z.S., Cui, Y., Dan-oh, Y., Kasada, C., Shinpo, A., Hara, K., 2008. Molecular design of coumarin dyes for stable and efficient organic dyesensitized solar cells. J. Phys. Chem. C 112, 17011–17017. Wu, Q.P., Zhang, L., Liang, M., Sun, Z., Xue, S., 2011. Sensitizers containing donor cascade and rhodanine-3-acetic acid moieties for dye-sensitized solar cells. Sol. Energy 85, 1–6. Zhu, Y., Rabindranath, A.R., Beyerlein, T., Tieke, B., 2007. Highly luminescent 1,4-diketo-3,6-diphenylpyrrolo[3,4-c]pyrrole-(DPP-) based conjugated polymers prepared upon Suzuki coupling. Macromolecules 40, 6981–6989.