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Indolo[3,2-b]carbazole-based multi-donor–π–acceptor type organic dyes for highly efficient dye-sensitized solar cells
Journal of Power Sources 319 (2016) 39e47
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Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Indolo[3,2-b]carbazole-based multi-donorepeacceptor type organic dyes for highly efficient dye-sensitized solar cells Xing Qian, Li Shao, Hongmei Li, Rucai Yan, Xiaoying Wang, Linxi Hou* School of Chemical Engineering, Fuzhou University, Xueyuan Road No. 2, Fuzhou 350116, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Four novel indolo[3,2-b]carbazolebased organic dyes have been synthesized. These dyes are designed as a multidonorepeacceptor type. The secondary donors led to striking changes of the photoelectronic properties. QX02 with diethylaniline as the secondary donor achieved a PCE of 8.09%.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 January 2016 Received in revised form 23 March 2016 Accepted 8 April 2016
Four novel indolo[3,2-b]carbazole-based multi-donorepeacceptor type organic dyes QX01e04 have been designed, synthesized, and applied for dye-sensitized solar cells. These dyes consist of an indolo [3,2-b]carbazole core acting as the main donor group, a couple of groups such as ethylbenzene, N,Ndiethylaniline, ethyloxylbenzene, and octyloxylbenzene acting as the secondary donors. The photophysical, electrochemical, and theoretical studies indicate that the four dyes are all capable as the photosensitizers. When introducing N,N-diethylaniline as the secondary donor, QX02 exhibits a broader absorption region and significantly improved IPCE values, which ensured a good light-harvesting ability and a high Jsc of 15.2 mA cm2. Finally, QX02-based cell achieved a high efficiency of 8.09% which is very close to that of the commercial N719-based cell (8.26%) under 100 mW cm2 irradiation. © 2016 Elsevier B.V. All rights reserved.
Kyewords: Dye-sensitized solar cells Organic dyes Indolo[3,2-b]carbazole Secondary donor Intramolecular charge transfer Photovoltaic performances
1. Introduction Dye-sensitized solar cells (DSSCs) are promising alternatives to traditional inorganic semiconductor-based solar cells because of their easy manufacturing processes, low cost, less environmental issues, and high power conversion efficiency (PCE) [1e3]. A typical DSSC has four main components: a semiconductor photoanode, an electrolyte, a counter electrode, and a dye sensitizer [4]. The
* Corresponding author. E-mail address: [email protected] (L. Hou). http://dx.doi.org/10.1016/j.jpowsour.2016.04.043 0378-7753/© 2016 Elsevier B.V. All rights reserved.
sensitizer, as a critical component in a DSSC, plays a crucial role in the light-harvesting, electron generation, electron transfer/injection [5e10]. Recently, numerous research efforts have been devoted to explore new photosensitizers in the development of DSSCs [11e17]. More and more attention have been paid to metal-free organic dye sensitizers because of their low cost, easy synthesis, high molar extinction coefficients, as well as low toxicity [18e21]. Indole and carbazole are both well-known heterocyclic compounds with strong electron-rich properties. Thus, indole and carbazole derivatives are widely used as the electron donor units for organic dyes which are applied in DSSCs due to their significant
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electronic and optical properties [22e27]. Indolo[3,2-b]carbazole contains an electron-rich indole unit fused with another electronrich carbazole unit and presents a planar structure similar to pentacene. Moreover, the solubility of the indolo[3,2-b]carbazole derivatives can be enhanced with the addition of long alkyl chains on the nitrogen atoms [28,29]. This unique structure endows the resultant indolo[3,2-b]carbazole with excellent potentials of strong electron donating ability and excellent hole-transporting ability, so indolo[3,2-b]carbazole derivatives have been applied in fields of organic light-emitting diodes (OLEDs) [30,31], organic field-effect transistors (OFETs) [32,33], hole transporting materials (HTM) [34], and photovoltaic cells [28,35,36]. Donorepeacceptor (DepeA) type have proven to be a nice design pattern for efficient metal-free organic dyes. In these dyes, cyanoacrylic acid is commonly applied as the acceptor/anchoring group because of the increased spectral response based on a strong intramolecular charge transfer (ICT) and its good electron injection property [37e39]. Generally, organic dyes used for efficient DSSCs should have a broad and intense spectral absorption in the UVevisible region. One of the most used method is to design a long rod-like structure by introducing more conjugated p-bridges. However, organic dyes with this structure may facilitate the recombination of electrons with the redox couple in the electrolyte and the intermolecular aggregations of dye molecules. Incorporating a nonplanar secondary electron donor unit into the organic donor framework has been proved to be an effective method to suppress intermolecular aggregation, retard charge recombination rate, and enhance the photovoltage [21,40]. On the other hand, incorporating extra electron donor units into the organic donor framework may increase the electron donating ability of the DepeA type dye and therefore enhance the ICT progress. Because of the highly p-conjugated planar structure, indolo[3,2-b]carbazole-based dyes may easily tend to aggregate when anchored on TiO2 films. So the suppression of dye aggregation is very important for designing efficient indolo[3,2-b]carbazole-based dyes. In this report, we present a new extension of multi-donorepeacceptor dyes by introducing secondary donor groups into the side of indolo [3,2-b]carbazole skeleton, with the aims of further expanding the possibility of improving the optical and ICT properties of the sensitizers as well as the suppression of dye aggregations. Therefore, four novel multi-donorepeacceptor type organic dyes QX01e04 have been designed and synthesized. These dyes consist of an indolo[3,2-b]carbazole core acting as the main donor group, a couple of groups such as ethylbenzene, N,N-diethylaniline, ethyloxylbenzene, and octyloxylbenzene acting as the secondary donor groups, a thiophene unit acting as the p-conjugated linker, and a 2cyanoacylic acid unit acting as the electron acceptor/anchoring group. The molecular structures of these dyes are shown in Fig. 1. 2. Experimental section 2.1. Materials and reagents All solvents were purified by common standard purification methods, and all NMR reagents were used as received. All reagents were purchased from Aldrich, TCI, and Alfa Aesar. FTO conductive glasses (sheet resistance of 15 U/sq, Nippon Sheet Glass) were used. 2.2. Characterization techniques 1 H NMR and 13C NMR spectra were recorded on Bruker 400 MHz spectrometer. U-Vvis spectra were obtained on a Varian Cary 300 Conc UV-visible spectrophotometer. HR-MS data were obtained on a Varian 7.0T FTMS. Cyclic voltammetry (CV) experiments and electrochemical impedance spectroscopy (EIS) were recorded using
an electrochemical workstation (Zennium, Zahner Corporation). CV experiments were carried out by a three-electrode system using a glassy carbon electrode as the working electrode, an Ag/AgNO3 electrode as the reference electrode, and a Pt wire as the counter electrode. The redox potentials were measured in dichloromethane (DCM), using 0.1 M n-Bu4NPF6 (TBAPF6) as the supporting electrolyte at a scan rate of 100 mV S1. 2.3. Fabrication and characterization of DSSCs A TiO2 film (~12 mm) for the transparent nanocrystalline layer was prepared by screen-printing method using a commercial 20 nm TiO2 sol (Heptachroma Corporation) onto the treated FTO conductive glass (treated with 0.05 M TiCl4 aqueous solution). The scattering layer (~4 mm) was applied over the transparent layer by screen-printing method using a commercial 200 nm TiO2 sol (Heptachroma Corporation), then the electrode was gradually heated to 500 C and sintered for 1 h. The prepared TiO2 electrodes were treated by 0.04 M TiCl4 aqueous solution at 70 C for 1 h and sintered again at 500 C for 1 h. The platinum electrode was prepared by thermal deposition of 20 mM chloroplatinic acid (isopropanol solution) on the surface of FTO glass at 450 C for 0.5 h. The resulting TiO2 electrodes were immersed in the commercial N719 dye solution (0.3 mM in ethanol) for 12 h. The adsorption of the indolo[3,2-b]carbazole-based dyes on TiO2 electrode was carried out in 0.3 mM dye's THF solution for 12 h. The acetonitrilebased electrolyte composed of 0.3 M DMPII, 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine was used. The DSSCs were fabricated into a sandwich structure and illuminated by a standard solar simulator (XM-500W, Trusttech Corporation) under 100 mW/cm2 irradiation, which was calibrated by a standard silicon solar cell (91150V, Newport Corporation). The current-voltage (J-V) characteristic curves of the DSSC under simulated irradiation were recorded by an electrochemical workstation (Zennium, Zahner Corporation). The incident photon-to-current conversion efficiency (IPCE) was measured using a commercial IPCE system (QTest Station 2000, CrownTech Corporation). 2.4. Synthetic procedures 2.4.1. 6,12-bis(4-ethylphenyl)-5,11-dioctyl-5,11-dihydroindolo[3,2b]carbazole (compound 1a) In a 250 mL round-bottom flask, a solution of aqueous 48% (w/ w) HBF4 (0.42 g, 2.3 mmol) and n-Bu4NI (0.85 g, 2.3 mmol) in acetonitrile (10 mL) was added to a solution of indole (2.71 g, 23 mmol) and 4-ethylbenzaldehyde (3.09 g, 23 mmol) in acetonitrile (80 mL). The resulting dark-red reaction mixture was stirred for 3 h at room temperature. After that, the solid was filtered off and washed with water (200 mL), acetonitrile (50 mL), and methanol (100 mL). The product was dried and tranfered to a 250 mL roundbottom flask. Then, n-Bu4NBr (TBAB) (265 mg, 0.82 mmol), NaOH (1.31 g, 33 mmol), and THF (35 mL) was added and the mixture was heated at 80 C for 0.5 h. Then C8H17Br (6.34 g, 33 mmol) was added dropwise, and the mixture was heated 80 C for 12 h before evaporated. The mixture was extracted with DCM, washed with water and then with brine solution. The organic layer was dried with Na2SO4 and 2,3-dicyano-5,6-dichlorobenzoquinone (DDQ) (2.42 g, 11 mmol) was added. The mixture was heated at 60 C for another 1 h before evaporated and the crude product was purified by silica-gel column chromatography using DCM/PE (PE) (1:4) as the eluent to afford compound 1a as a pale yellow solid (total yield: 68.5%). 1H NMR (400 MHz, CDCl3) d 7.55 (d, J ¼ 14.0 Hz, 4H), 7.42 (d, J ¼ 17.4 Hz, 4H), 7.31 (t, J ¼ 6.8 Hz, 4H), 6.81 (s, 2H), 6.59 (d, J ¼ 7.7 Hz, 2H), 4.21e3.50 (m, 4H), 2.88 (q, J ¼ 7.3 Hz, 4H), 1.59e1.47 (m, 4H), 1.42 (t, J ¼ 7.0 Hz, 6H), 1.35e1.01 (m, 16H), 0.89 (t, J ¼ 6.7 Hz,
X. Qian et al. / Journal of Power Sources 319 (2016) 39e47
41
Fig. 1. Chemical structures of QX01e04.
10H). 13C NMR (100 MHz, CDCl3) d 144.22, 136.01, 132.52, 130.54, 130.43, 128.38, 125.13, 123.09, 122.60, 122.48, 117.76, 117.72, 108.09, 44.49, 31.82, 29.30, 29.27, 28.98, 28.75, 26.79, 22.70, 16.04, 14.14. HR-MS (MALDI): m/z [M]þ calcd for C50H60N2, 688.4756, found, 688.4752. 2.4.2. 4,40 -(5,11-dioctyl-5,11-dihydroindolo[3,2-b]carbazole-6,12diyl)bis(N,N-diethylaniline) (compound 1b) The compound 1b was synthesized according to the procedure described for compound 1a and the crude product was purified by silica-gel column chromatography using DCM/PE (1:2) as the eluent to afford compound 1b as a yellow solid (total yield: 35.1%). 1 H NMR (400 MHz, CDCl3) d 7.48 (d, J ¼ 8.6 Hz, 4H), 7.39e7.32 (m, 2H), 7.29 (d, J ¼ 4.5 Hz, 2H), 6.96 (d, J ¼ 8.6 Hz, 4H), 6.89 (d, J ¼ 3.8 Hz, 4H), 3.99e3.86 (m, 6H), 3.55 (q, J ¼ 7.0 Hz, 8H), 1.66e1.50 (m, 4H), 1.33 (t, J ¼ 7.0 Hz, 12H), 1.31e1.15 (m, 16H), 1.07e0.97 (m, 4H), 0.91 (t, J ¼ 7.0 Hz, 6H). 13C NMR (100 MHz, CDCl3) d 147.54, 142.44, 133.06, 131.17, 125.45, 124.81, 123.49, 123.19, 122.76, 118.10, 117.52, 112.57, 107.93, 44.58, 44.51, 31.83, 30.17, 29.38, 28.87, 26.99, 22.72, 14.13, 12.59. HR-MS (MALDI): m/z [M]þ calcd for C54H70N4, 774.5600, found, 774.5598. 2.4.3. 6,12-bis(4-ethoxyphenyl)-5,11-dioctyl-5,11-dihydroindolo [3,2-b]carbazole (compound 1c) The compound 1c was synthesized according to the procedure described for compound 1a and the crude product was purified by silica-gel column chromatography using DCM/PE (1:4) as the
eluent to afford compound 1c as a pale yellow solid (total yield: 65.2%). 1H NMR (400 MHz, C6D6) d 7.57 (d, J ¼ 8.4 Hz, 4H), 7.36 (t, J ¼ 7.6 Hz, 2H), 7.24 (d, J ¼ 8.3 Hz, 4H), 7.05 (d, J ¼ 7.5 Hz, 2H), 7.01 (d, J ¼ 8.4 Hz, 4H), 4.01e3.79 (m, 4H), 3.71 (q, J ¼ 6.9 Hz, 4H), 1.62e1.44 (m, 4H), 1.30e1.12 (m, 18H), 1.07e0.98 (m, 4H), 0.90 (t, J ¼ 7.1 Hz, 10H). 13C NMR (100 MHz, C6D6) d 159.20, 133.39, 133.23, 131.85, 131.18, 125.39, 124.04, 123.77, 123.61, 122.90, 118.39, 114.88, 108.59, 63.11, 44.39, 31.88, 29.37, 28.87, 26.95, 22.78, 22.73, 14.58, 14.07. HR-MS (MALDI): m/z [M]þ calcd for C50H60N2O2, 720.4655, found, 720.4652. 2.4.4. 5,11-Dioctyl-6,12-bis(4-(octyloxy)phenyl)-5,11-dihydroindolo [3,2-b]carbazole (compound 1d) The compound 1d was synthesized according to the procedure described for compound 1a and the crude product was purified by silica-gel column chromatography using DCM/PE (1:4) as the eluent to afford compound 1d as a pale yellow solid (total yield: 63.3%). 1H NMR (400 MHz, CDCl3) d 7.58 (d, J ¼ 8.4 Hz, 4H), 7.36 (t, J ¼ 7.5 Hz, 2H), 7.29 (d, J ¼ 7.5 Hz, 2H), 7.19 (d, J ¼ 8.5 Hz, 4H), 6.88 (t, J ¼ 7.4 Hz, 2H), 6.70 (d, J ¼ 7.9 Hz, 2H), 4.18 (t, J ¼ 6.6 Hz, 4H), 3.98e3.73 (m, 4H), 2.04e1.85 (m, 4H), 1.73e1.11 (m, 44H), 0.98e0.90 (m, 12H). 13C NMR (100 MHz, CDCl3) d 159.09, 142.44, 132.75, 131.49, 130.74, 125.12, 123.16, 122.97, 122.51, 117.79, 117.58, 114.96, 108.15, 68.34, 44.46, 31.90, 31.82, 29.73, 29.53, 29.45, 29.34, 29.33, 28.78, 26.91, 26.20, 22.73, 22.70, 14.16, 14.13. HR-MS (MALDI): m/z [M]þ calcd for C62H84N2O2, 888.6533, found, 888.6530.
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2.4.5. 5-(6,12-bis(4-ethylphenyl)-5,11-dioctyl-5,11-dihydroindolo [3,2-b]carbazol- 2-yl)thiophene-2-carbaldehyde (compound 2a) A solution of N-bromosuccinimide (NBS) (260 mg, 1.46 mmol) in DCM (30 mL) was added dropwise to a solution of compound 1a (965 mg, 0.155 mmol) in DCM (80 mL) at 0 C. The mixture was slowly warmed to room temperature and stirred for an additional 1 h before evaporated. The mixture was resolved in DCM and washed with water. Then the organic phase was separated and dried over anhydrous Na2SO4. After the solvent was evaporated, the crude product was transferred to a two-neck round bottomed flask. Then, 5-formylthiophene-2-boronic acid (319 mg, 2.86 mmol), Pd(dppf)Cl2 (119 mg, 0.15 mmol), K2CO3 (2.01 g, 14.6 mmol), toluene (30 mL), methanol (5 mL) were added. The flask was charged with N2. The mixture was heated at 80 C for 4 h before it was poured into water. The organic phase was separated and dried over anhydrous Na2SO4. After the solvent was removed under vacuum, the crude product was purified by silica-gel column chromatography using (DCM/PE ¼ 1:1) as the eluent to afford 2a as an orange solid (total yield: 62.1%). 1H NMR (400 MHz, CDCl3) d 9.81 (s, 1H), 7.68e7.57 (m, 5H), 7.54 (d, J ¼ 8.4 Hz, 3H), 7.46 (d, J ¼ 7.7 Hz, 2H), 7.37e7.31 (m, 1H), 7.27 (d, J ¼ 11.7 Hz, 1H), 7.24 (d, J ¼ 8.8 Hz, 1H), 7.01 (d, J ¼ 3.6 Hz, 1H), 6.84 (t, J ¼ 7.5 Hz, 1H), 6.79 (s, 1H), 6.59 (d, J ¼ 8.0 Hz, 1H), 3.97e3.88 (m, 2H), 3.85e3.64 (m, 2H), 3.04e2.92 (m, 2H), 2.92e2.74 (m, 2H), 1.63e1.47 (m, 9H), 1.42 (t, J ¼ 7.4 Hz, 3H), 1.31e1.10 (m, 18H), 0.89 (t, J ¼ 6.8 Hz, 10H). 13C NMR (100 MHz, CDCl3) d 181.19, 155.92, 143.66, 143.30, 141.89, 141.35, 139.27, 136.61, 134.55, 134.37, 131.69, 131.50, 129.22, 129.12, 127.64, 127.31, 124.40, 122.49, 122.21, 122.14, 121.85, 121.50, 121.46, 121.38, 120.72, 119.64, 117.29, 116.97, 116.90, 107.40, 107.09, 43.48, 43.35, 30.89, 30.71, 29.31, 28.66, 28.33, 28.24, 28.23, 28.15, 28.10, 27.98, 27.87, 27.82, 25.69, 25.64, 21.62, 15.34, 14.92, 13.07. HR-MS (MALDI): m/z [M]þ calcd for C55H62N2OS, 798.4583, found, 798.4580. 2.4.6. 5-(6,12-bis(4-(diethylamino)phenyl)-5,11-dioctyl-5,11dihydroindolo[3,2-b]carbazol-2-yl)thiophene-2-carbaldehyde (compound 2b) The compound 2b was synthesized according to the procedure described for compound 2a and the crude product was purified by silica-gel column chromatography using DCM/PE (1:1) as the eluent to afford compound 2b as a yellow-green solid (total yield: 55.2%). 1H NMR (400 MHz, CDCl3) d 9.83 (s, 1H), 7.65 (dd, J ¼ 8.5, 1.8 Hz, 1H), 7.62 (d, J ¼ 4.0 Hz, 1H), 7.45 (d, J ¼ 8.4 Hz, 4H), 7.40e7.35 (m, 1H), 7.32 (d, J ¼ 8.1 Hz, 1H), 7.23 (d, J ¼ 8.6 Hz, 1H), 7.11 (d, J ¼ 4.0 Hz, 1H), 7.02e6.96 (m, 3H), 6.96e6.89 (m, 4H), 4.24e4.06 (m, 2H), 4.01e3.88 (m, 2H), 3.63 (q, J ¼ 6.9 Hz, 4H), 3.55 (q, J ¼ 7.0 Hz, 4H), 1.65e1.61 (m, 2H), 1.61e1.52 (m, 2H), 1.42e1.30 (m, 16H), 1.27e1.14 (m, 12H), 1.09e0.97 (m, 4H), 0.95e0.88 (m, 6H). 13C NMR (100 MHz, CDCl3) d 182.25, 157.65, 147.75, 147.62, 143.11, 142.43, 140.09, 137.84, 133.33, 133.08, 131.09, 130.91, 125.15, 124.76, 124.42, 124.03, 123.79, 123.37, 123.34, 122.97, 122.88, 122.30, 121.72, 121.13, 118.68, 118.46, 117.74, 112.48, 112.09, 108.36, 108.02, 44.64, 44.56, 44.42, 31.82, 29.41, 29.36, 29.35, 29.31, 29.01, 28.98, 26.98, 26.97, 22.72, 14.14, 12.75, 12.59. HR-MS (MALDI): m/z [M]þ calcd for C59H72N4OS, 884.5427, found, 884.5428. 2.4.7. 5-(6,12-bis(4-ethoxyphenyl)-5,11-dioctyl-5,11-dihydroindolo [3,2-b]carbazol-2-yl)thiophene-2-carbaldehyde (compound 2c) The compound 2c was synthesized according to the procedure described for compound 2a and the crude product was purified by silica-gel column chromatography using DCM/PE (1:1) as the eluent to afford compound 2c as an orange solid (total yield: 56.4%). 1 H NMR (400 MHz, CDCl3) d 9.82 (s, 1H), 7.67e7.60 (m, 2H), 7.55 (t, J ¼ 8.9 Hz, 4H), 7.39e7.32 (m, 1H), 7.29 (d, J ¼ 8.1 Hz, 1H), 7.24 (d, J ¼ 8.5 Hz, 3H), 7.15 (d, J ¼ 8.4 Hz, 2H), 7.07 (d, J ¼ 3.9 Hz, 1H), 6.86 (t, J ¼ 7.4 Hz, 1H), 6.77 (d, J ¼ 1.3 Hz, 1H), 6.65 (d, J ¼ 7.9 Hz, 1H), 4.32 (q,
J ¼ 6.9 Hz, 2H), 4.22 (q, J ¼ 6.9 Hz, 2H), 4.05e3.93 (m, 2H), 3.89e3.73 (m, 2H), 1.63 (t, J ¼ 6.9 Hz, 3H), 1.60e1.44 (m, 9H), 1.33e1.08 (m, 18H), 1.03e0.93 (m, 4H), 0.89 (t, J ¼ 6.7 Hz, 6H). 13C NMR (100 MHz, CDCl3) d 182.35, 159.39, 159.02, 157.13, 143.08, 142.47, 140.50, 137.80, 133.04, 132.79, 131.44, 131.33, 130.32, 130.27, 125.49, 123.72, 123.60, 123.28, 122.99, 122.98, 122.66, 122.61, 121.85, 120.88, 118.13, 118.01, 117.86, 115.36, 114.98, 108.61, 108.26, 64.03, 63.67, 50.00, 44.63, 44.38, 33.72, 31.81, 31.79, 31.08, 29.28, 29.25, 28.91, 28.84, 26.86, 26.83, 22.68, 15.05, 14.95, 14.13. HR-MS (MALDI): m/z [M]þ calcd for C55H62N2O3S, 830.4481, found, 830.4478. 2.4.8. 5-(5,11-dioctyl-6,12-bis(4-(octyloxy)phenyl)-5,11dihydroindolo[3,2-b]carbazol-2-yl)thiophene-2-carbaldehyde (compound 2d) The compound 2d was synthesized according to the procedure described for compound 2a and the crude product was purified by silica-gel column chromatography using DCM/PE (1:1) as the eluent to afford compound 2d as an orange solid (total yield: 55.2%). 1H NMR (400 MHz, CDCl3) d 9.84 (s, 1H), 7.69e7.63 (m, 2H), 7.63e7.54 (m, 4H), 7.42e7.35 (m, 1H), 7.32 (d, J ¼ 8.1 Hz, 1H), 7.28e7.24 (m, 3H), 7.18 (d, J ¼ 8.6 Hz, 2H), 4.24 (t, J ¼ 6.6 Hz, 2H), 4.16 (t, J ¼ 6.7 Hz, 2H), 4.06e3.95 (m, 2H), 3.90e3.78 (m, 2H), 2.07e1.89 (m, 4H), 1.61e1.13 (m, 40H), 1.02e0.88 (m, 16H). 13C NMR (100 MHz, CDCl3) d 182.28, 159.54, 159.19, 157.11, 143.05, 142.44, 140.47, 137.77, 133.03, 132.78, 131.38, 131.31, 130.25, 130.14, 125.47, 123.68, 123.57, 123.40, 122.98, 122.93, 122.62, 121.81, 120.76, 118.14, 118.00, 117.84, 115.31, 114.99, 108.61, 108.24, 68.62, 68.33, 44.63, 44.39, 31.94, 31.90, 31.80, 29.59, 29.52, 29.43, 29.41, 29.35, 29.30, 29.26, 28.92, 28.85, 26.85, 26.27, 26.19, 22.75, 22.74, 22.69, 14.19, 14.14. HR-MS (MALDI): m/z [M]þ calcd for C67H86N2O3S, 998.6359, found, 998.6358. 2.4.9. (E)-3-(5-(6,12-bis(4-ethylphenyl)-5,11-dioctyl-5,11dihydroindolo [3,2-b]carbazol-2-yl)thiophen-2-yl)-2-cyanoacrylic acid (QX01) A mixture of compound 2a (240 mg, 0.30 mmol), cyanoacetic acid (76 mg, 0.90 mmol), piperdine (0.20 mL), CHCl3 (15 mL), and CH3CN (15 mL) was heated at 80 C for 12 h and then acidified with 2 M aqueous hydrochloric acid (10 mL). The crude product was extracted into CHCl3, and the organic layer was washed with water and dried over anhydrous Na2SO4. After removal of the solvent under reduced pressure, the crude product was purified by silicagel column chromatography using DCM/CH3OH (15:1) as the eluent to afford QX01 as a deep red solid (yield: 84.1%). 1H NMR (400 MHz, DMSO-d6) d 8.15 (s, 1H), 7.71 (s, 2H), 7.67 (d, J ¼ 8.8 Hz, 1H), 7.57 (d, J ¼ 10.0 Hz, 4H), 7.52 (d, J ¼ 7.9 Hz, 2H), 7.47 (d, J ¼ 8.7 Hz, 1H), 7.43 (d, J ¼ 7.6 Hz, 1H), 7.35e7.27 (m, 1H), 7.21 (s, 1H), 7.10 (s, 1H), 6.83e6.74 (m, 1H), 6.67 (s, 1H), 6.44 (d, J ¼ 7.8 Hz, 1H), 3.95e3.83 (m, 2H), 3.83e3.71 (m, 2H), 2.97e2.88 (m, 2H), 2.88e2.78 (m, 2H), 2.72e2.62 (m, 2H), 2.07e1.93 (m, 2H), 1.62e0.93 (m, 26H), 0.86 (t, J ¼ 6.8 Hz, 6H). 13C NMR (100 MHz, DMSO-d6) d 156.81, 154.27, 153.71, 146.59, 146.02, 144.39, 144.02, 142.32, 141.96, 140.71, 135.01, 134.91, 134.59, 134.16, 132.65, 132.20, 131.94, 129.89, 129.79, 128.54, 128.22, 125.43, 123.52, 123.49, 122.91, 122.53, 122.28, 121.95, 121.83, 121.46, 119.68, 117.93, 117.75, 109.65, 48.74, 47.36, 44.08, 44.03, 43.85, 31.22, 31.20, 28.78, 28.67, 28.59, 28.57, 28.42, 28.37, 28.33, 26.18, 22.15, 15.95, 15.84, 13.92. HR-MS (MALDI): m/z [M]þ calcd for C58H63N3O2S, 865.4641, found, 865.4638. 2.4.10. (E)-3-(5-(6,12-bis(4-(diethylamino)phenyl)-5,11-dioctyl5,11-dihydro-indolo[3,2-b]carbazol-2-yl)thiophen-2-yl)-2cyanoacrylic acid (QX02) QX02 was synthesized according to the procedure described for
X. Qian et al. / Journal of Power Sources 319 (2016) 39e47
QX01 and the crude product was purified by silica-gel column chromatography using DCM/CH3OH (10:1) as the eluent to afford QX02 as a dark red solid (yield: 75.2%). 1H NMR (400 MHz, DMSOd6) d 8.38 (s, 1H), 7.97 (s, 1H), 7.64 (d, J ¼ 7.5 Hz, 1H), 7.49e7.42 (m, 2H), 7.41e7.36 (m, 3H), 7.37e7.27 (m, 2H), 7.06 (s, 1H), 6.98 (d, J ¼ 7.4 Hz, 2H), 6.94 (d, J ¼ 9.6 Hz, 2H), 6.90e6.79 (m, 2H), 6.72 (d, J ¼ 8.3 Hz, 1H), 4.09e4.02 (m, 2H), 3.93e3.86 (m, 2H), 3.58e3.53 (m, 4H), 3.53e3.49 (m, 4H), 1.56e1.40 (m, 4H), 1.29e1.15 (m, 24H), 1.12e1.05 (m, 4H), 0.97e0.83 (m, 10H). 13C NMR (100 MHz, DMSOd6) d 162.19, 154.37, 147.21, 147.14, 144.11, 144.04, 142.42, 141.97, 133.24, 132.86, 132.55, 130.65, 130.34, 125.03, 124.00, 123.84, 123.45, 123.34, 123.13, 122.97, 122.83, 122.52, 122.01, 121.77, 119.97, 118.34, 118.10, 117.95, 117.58, 111.83, 111.74, 108.99, 108.19, 107.80, 44.22, 44.17, 44.00, 36.21, 31.48, 31.41, 31.11, 29.15, 29.11, 29.03, 28.79, 28.62, 26.64, 26.56, 22.42, 22.39, 18.41, 13.99, 12.56, 12.38. HR-MS (MALDI): m/z [M]þ calcd for C62H73N5O2S, 951.5485, found, 951.5483.
43
3. Results and discussion 3.1. Synthesis of the dyes The synthetic procedure to QX01e04 is depicted in Scheme 1. A concise approach to the synthesis of 6,12-diaryl substituted indolo [3,2-b]carbazoles (compound 1aed) was conducted by reacting the corresponding benzaldehyde with indole under catalysis by acids according to the previous literature [41]. Bromination of compound 1aed with NBS produced the brominated intermediates, and then Suzuki cross-coupling of brominated indolo[3,2-b]carbazoles derivatives and 5-formylthiophene-2-boronic acid produced the pextended indolocarbazole-based aldehydes 2aed. The intermediates 2aed were finally treated with cyanoacetic acid by Knoevenagel reactions to give the four dyes QX01e04.
3.2. UVevis absorption and electrochemical characterization 2.4.11. (E)-3-(5-(6,12-bis(4-ethoxyphenyl)-5,11-dioctyl-5,11dihydroindolo[3,2-b]carbazol-2-yl)thiophen-2-yl)-2-cyanoacrylic acid (QX03) QX03 was synthesized according to the procedure described for QX01 and the crude product was purified by silica-gel column chromatography using DCM/CH3OH (12:1) as the eluent to afford QX03 as a dark red solid (yield: 86.5%). 1H NMR (400 MHz, DMSOd6) d 8.37 (s, 1H), 7.94 (s, 1H), 7.67 (s, 1H), 7.62e7.39 (m, 6H), 7.32 (t, J ¼ 14.3 Hz, 1H), 7.22 (d, J ¼ 14.8 Hz, 4H), 7.14 (s, 1H), 6.83 (t, J ¼ 7.3 Hz, 1H), 6.66 (s, 1H), 6.50 (d, J ¼ 7.9 Hz, 1H), 4.38e4.10 (m, 4H), 3.94 (s, 2H), 3.82 (s, 2H), 1.41 (t, J ¼ 28.0 Hz, 10H), 1.32e0.90 (m, 20H), 0.86 (t, J ¼ 6.8 Hz, 6H). 13C NMR (100 MHz, DMSO-d6) d 163.90, 158.85, 158.59, 155.44, 153.17, 145.48, 142.58, 141.93, 139.44, 139.36, 134.97, 132.98, 132.40, 132.16, 130.95, 129.47, 129.28, 125.36, 123.51, 123.54, 123.01, 122.93, 122.32, 122.28, 121.93, 119.97, 119.90, 117.76, 117.46, 116.57, 115.01, 114.66, 108.97, 108.22, 63.32, 63.12, 44.07, 44.00, 43.75, 31.16, 29.37, 29.06, 28.77, 28.61, 28.55, 28.48, 28.35, 26.60, 26.34, 26.25, 22.09, 14.63, 14.60, 13.85. HR-MS (MALDI): m/z [M]þ calcd for C58H63N3O4S, 897.4539, found, 897.4536.
2.4.12. (E)-2-Cyano-3-(5-(5,11-dioctyl-6,12-bis(4-(octyloxy) phenyl)-5,11-dihydro-indolo[3,2-b]carbazol-2-yl)thiophen-2-yl) acrylic acid (QX04) QX04 was synthesized according to the procedure described for QX01 and the crude product was purified by silica-gel column chromatography using DCM/CH3OH (12:1) as the eluent to afford QX04 as a dark red solid (yield: 88.2%). 1H NMR (400 MHz, DMSOd6) d 8.30 (s, 1H), 7.87 (s, 1H), 7.67 (d, J ¼ 8.2 Hz, 1H), 7.53 (t, J ¼ 7.6 Hz, 3H), 7.50e7.46 (m, 1H), 7.43 (d, J ¼ 7.8 Hz, 1H), 7.33 (t, J ¼ 7.7 Hz, 1H), 7.24 (d, J ¼ 8.3 Hz, 2H), 7.19 (d, J ¼ 8.6 Hz, 2H), 7.13 (s, 1H), 6.82 (t, J ¼ 7.6 Hz, 1H), 6.68 (s, 2H), 6.51 (d, J ¼ 8.2 Hz, 1H), 4.31e4.08 (m, 4H), 4.03e3.88 (m, 2H), 3.89e3.73 (m, 2H), 2.14e1.92 (m, 4H), 1.89e1.68 (m, 4H), 1.54e1.14 (m, 40H), 0.93e0.82 (m, 12H). 13 C NMR (100 MHz, DMSO-d6) d 165.10, 159.41, 159.12, 153.67, 143.45, 142.66, 142.32, 137.22, 134.19, 132.78, 132.49, 131.35, 129.90, 129.69, 129.24, 127.99, 125.76, 123.87, 123.28, 122.79, 122.70, 122.63, 122.35, 122.20, 119.84, 118.32, 118.16, 118.04, 117.80, 115.32, 115.02, 115.02, 109.16, 108.60, 68.33, 68.11, 44.35, 44.18, 31.85, 31.85, 31.81, 31.67, 31.64, 31.64, 29.54, 29.40, 29.40, 29.36, 29.30, 29.30, 29.26, 29.10, 29.10, 28.94, 28.94, 28.78, 26.77, 26.69, 26.17, 26.13, 26.13, 22.64, 22.60, 14.38, 14.37, 14.31. HR-MS (MALDI): m/z [M]þ calcd for C70H87N3O4S, 1065.6417, found, 1065.6417.
The UVevis absorption spectra of QX01e04 in dichloromethane (DCM) and on TiO2 films are depicted in Fig. 2. The data of absorption, electrochemical properties are summarized in Table 1. In DCM solutions, each of these organic dyes exhibited three major absorption bands at 300e350, 350e450, and 450e650 nm, respectively. The high-energy absorption bands ranging from 300 to 450 nm correspond to the pep* transitions of the conjugated aromatic rings. The low-energy bands appearing at 450e650 nm can be attributed to the intramolecular charge transfer (ICT) transitions from donors to acceptors [42e44]. The molar extinction coefficients at lmax of QX01e04 are 3.09 104 (476 nm), 2.50 104 (500 nm), 2.11 104 (505 nm) and 3.13 104 (485 nm) M¡1 cm¡1, respectively. These are more than twice as high as those of conventional polypyridyl Ru(II) complexes dyes, which is conducive to increase the light-harvesting of the cells [45]. As shown in Fig. 2, when anchoring on mesoporous TiO2 films, the four dyes show significantly broadened absorption spectra compared to that measured in DCM and QX02 exhibit the most broad absorption region. However, the absorption peaks of all these dyes showed slight blue-shift as compared to the data recorded in DCM. This blue-shift can be ascribed to the partial deprotonation of carboxylic acid acceptor because of the interaction between the dye molecules and TiO2 [46e48]. Cyclic voltammetry measurements were used to investigate the redox behavior of these dyes in DCM calibrated against ferrocene (0.63 V vs. NHE), using 0.1 M TBAPF6 as the supporting electrolyte. The related cyclic voltammograms recorded in DCM are shown in Fig. 3(a). The HOMO levels of these dyes correspond to the first oxidation potentials (Eox) and the LUMO levels (Ered) could be calculated from Eox E0e0, where E0e0 (zerothezeroth transition energy) was estimated from the onset wavelength in absorption spectra in DCM. The oxidation potentials of these dyes could be determined from the peak potentials in cyclic voltammograms. As shown in Table 1, the oxidation potentials of the four dyes show a first reversible oxidation wave at 0.93 (QX01), 0.83 (QX02), 0.86 (QX03), and 0.85 V (QX04) vs. NHE, respectively. As a result, the HOMO levels of the four dyes are evaluated to be higher than the redox potential of the iodide/triiodide redox couple (0.4 V vs. NHE), which guarantees efficient dye regeneration. On the other hand, the LUMO levels of QX01e04 (1.09, e1.08, e1.12, and 1.12 V vs. NHE, respectively) are all more negative than the conduction band (CB) of TiO2 (0.5 V vs NHE) allowing efficient electron injection from the excited dyes into the TiO2 semiconductor. The energy diagram of HOMO and LUMO energy levels (NHE vs. vacuum level is set to 4.5 V) for the four dyes is shown in Fig. 3(b) [38,49,50].
44
X. Qian et al. / Journal of Power Sources 319 (2016) 39e47
Scheme 1. Synthetic routes to QX01e04.
ε / 104 M-1 cm-1
(a)
5
Table 1 Optical and electrochemical data of the four dyes.a
QX01 QX02 QX03 QX04
4 3 2
0 300
400 500 600 Wavelength / nm
ε/104 M1 cm1
Eox/V
E00/V
Ered/V
QX01 QX02 QX03 QX04
476 500 505 485
3.09 2.50 2.11 3.13
0.93 0.83 0.86 0.85
2.02 1.91 1.98 1.97
1.09 1.08 1.12 1.12
700 3.3. Theoretical calculations
(b) 1.5 QX01 QX02 QX03 QX04
1.2 0.9 Abs
lmax/nm
a First oxidation potentials (Eox) (vs. NHE) in DCM were calibrated with ferrocene (0.63 V vs. NHE); E0e0 values (zeroth-zeroth transition energies) were estimated from the onset of the absorption spectra in dichloromethane; Ered ¼ Eox E0-0.
1
0.6 0.3 0.0
Dye
400
500 600 Wavelength / nm
700
Fig. 2. UVevis absorption spectra of the four dyes (a) in dichloromethane solutions and (b) on TiO2 films.
In order to further understand the geometrical and electronic properties of the studied four dyes, density functional theory (DFT) calculations were conducted using the Gaussian 03 program package at the B3LYP/6-31G* level. Fig. 4 displays the electron distributions of the frontier molecular orbitals of QX01e04. It can be clearly seen that the HOMO orbitals of the four dyes predominantly located on the indolo[3,2-b]carbazole core and the neighboring p-bridge unit, while the LUMOs of the four dyes all delocalized over the cyanoacrylic acid acceptors and their nearby p-bridge unit. As a result, this electron distribution will allow significant charge separation within the dye and hence efficient electron injection from the excited dyes into the acceptor groupconnected TiO2 semiconductor. Meanwhile, the optimized geometries of QX01e04 show the secondary donor groups are perpendicular with indolo[3,2-b]carbazole core, which may suppress the dye aggregation and improve their photovoltaic performances. 3.4. Photovoltaic performance The DSSC performances of the dyes are evaluated under AM
X. Qian et al. / Journal of Power Sources 319 (2016) 39e47
(a)
10 5
QX01 QX02 QX03 QX04
Current density / mA cm-2
Current / uA
15
0 -5 -10 0.0
0.5 1.0 1.5 Potentials vs. NHE (V)
2.0
45
QX01 QX02 QX03 QX04 N719
18 15 12 9 6 3 0 0.0
0.2
0.4 0.6 Voltage / V
0.8
Fig. 5. The photocurrent-voltage (JeV) curves of DSSCs based on QX01e04 and N719.
Fig. 3. (a) Cyclic voltammograms of these dyes recorded in DCM solutions. (b) Energy diagram of HOMO and LUMO energy levels for these dyes.
1.5 G irradiation at 100 mW cm2, employing an iodine electrolyte composed of 0.3 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII), 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tert-butyl pyridine (TBP) in acetonitrile. The DSSC performance parameters of QX01e04 and commercial N719 dye are measured and displayed in Table 2. The corresponding photocurrent density-voltage (JV) curves are plotted in Fig. 5. The solar cell based on dye QX01 with ethylbenzene as the secondary donor group exhibited a short-circuit
Fig. 4. The electron distributions of the frontier orbitals of QX01e04.
Table 2 Photovoltaic performances of QX01e04 with N719 as a reference.
a
Dye
DL/nmol cm2
Voc/mV
Jsc/mA cm2
FF
PCE/%
QX01 QX02 QX03 QX04 N719
180 171 188 160 e
729 745 738 757 740
12.6 15.2 13.9 14.1 15.9
0.68 0.71 0.68 0.71 0.70
6.25 8.09 6.98 7.58 8.26
a The active area of all cells was 0.196 cm2; The electrolyte was composed of 0.3 M DMPII, 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tert-butyl pyridine in acetonitrile.
photocurrent density (Jsc) of 12.6 mA cm2, an open-circuit photovoltage (Voc) of 728 mV, and a fill factor (FF) of 0.68, generating a power conversion efficient (PCE) of 6.25%. When the secondary donor group was replaced by ethyloxylbenzene, the resultant QX03 showed significant improvement of Voc and Jsc (Voc ¼ 738 mV, Jsc ¼ 13.9 mA cm2) and finally gave a higher PCE of 6.98%. With the introduction of the longer octyloxyl group on the secondary donor, QX04-based cell showed a highest Voc of 758 mV and a higher PCE of 7.58%. Notably, benefited from its broader and more intense ICT
X. Qian et al. / Journal of Power Sources 319 (2016) 39e47
band on TiO2 film, QX02 with N,N-diethylaniline as the secondary donor displayed a highest Jsc of 15.2 mA cm2, a moderate Voc of 745 mV, and an increased PCE of 8.09%. The best efficiency of QX02 was very close to that of the commercial N719-based cell (Voc ¼ 740 mV, Jsc ¼ 15.9 mA cm2, FF ¼ 0.70, PCE ¼ 8.26%) that fabricated and measured under the same conditions, indicating that the proper molecular modification of indolo[3,2-b]carbazolebased dyes with strong secondary donor is a promising method to construct effective organic dyes for DSSCs. Based on the abovementioned J-V curve data, the effects of the secondary donors on the photocurrent characteristics were investigated to elucidate the difference between the Jsc values of the four dyes. The incident photon-to-current conversion efficiencies (IPCEs) as a function of incident wavelength for the cells based on QX01e04 are displayed in Fig. 6. As shown in IPCE plots, all of the four dyes could efficiently convert the light to photocurrents in the UVevis region and the onsets of the IPCE spectra for QX01e04 were at 690, 750, 700 and 710 nm, respectively. QX02-based cell gave over 60% IPCE values from 350 to 620 nm with a maximum IPCE value of 85% at 500 nm, which ensured a good light-harvesting ability and a high photocurrent. The broader IPCE response range and high IPCE values of QX02 explained the highest Jsc value of QX02 from the J-V measurement. The relatively highest efficiency observed for QX02 was mainly due to the relatively largest Jsc. Obviously, the four dyes exhibited significantly different DSSC performances, which was caused by the different dye structures. Therefore, the different substituted groups (such as ethylbenzene, N,N-diethylaniline, ethyloxylbenzene, and octyloxylbenzene) introduced into the para position of indolo[3,2-b]carbazole as a couple of secondary donors played an important role on the photophysical, electrochemical properties of the dye molecules and their DSSC performances. The electron-donating ability increases in the order of ethylbenzene < ethyloxylbenzene z octyloxylbenzene < N,Ndiethylaniline. Stronger secondary donors enhance their electrondonating ability of the whole donor part and promote the ICT process (the excited electron is transferred from donor to acceptor). Benefiting from the strongest N,N-diethylaniline donor, QX02 displayed a biggest absorption onset, a smallest E0e0, a broadest IPCE response, and a highest Jsc. The strongest electron-donating ability of QX02 was also reflected by its lowest first oxidation potential. The order of their absorption onsets and first oxidation potentials of QX01e04 are consistent with their electron-donating abilities. As a result, incorporating strong electron-donating group such as N,Ndiethylaniline on indolo[3,2-b]carbazole-based dye structure is an effective route to improve its DSSC performance.
3.5. Electrochemical impedance spectroscopy To elucidate the differences of the photovoltages between these dyes, electrochemical impedance spectroscopy (EIS) was measured to characterize the interfacial charge transfer processes in DSSCs [51,52]. Nyquist plots and Bode phase plots of DSSCs based on QX01e04 were measured in the dark under forward bias (0.70 V) with frequency range of 100 kHz to 0.1 Hz (Fig. 7). In the Nyquist plots, the larger semicircle in the lower frequency region corresponds to the interfacial charge transfer resistances (Rct) at the TiO2/dyes/electrolyte interface and the radius of this semicircle is a measure of Rct [53,54]. A larger radius corresponds to a larger Rct, indicating that the electron recombination in the DSSC are strongly reduced and therefore a smaller dark current and a higher photovoltage well be achieved [55]. The fitted Rct increased in the order of QX01 (52.9 U) < QX03 (61.8 U) < QX02 (84.5 U) < QX04 (130.1 U). This trend appeared to be consistent with the order of Voc of QX01 (729 mV) < QX03 (738 mV) < QX02 (745 mV) < QX04 (757 mV). This clearly indicates that the longer alkyl units in secondary group are helpful for the suppression of dark current and improve the photovoltage. Besides, the peak frequency (f) at lower frequency region is related to the charge recombination rate and the electron lifetime (t) can be calculated using t ¼ 1/(2pf) [56,57]. Generally, longer electron lifetime indicates increased recombination resistance between the injected electrons and the electrolyte, which are consistent with the Voc values [58]. For the four devices, the peak frequency at lower frequency region in Bode phase plots decreased in the order of QX01 > QX03 > QX02 > QX04, and the electron lifetime was enhanced in reverse with the calculated values of QX01 (35.6 ms) < QX03 (40.0 ms) < QX02 (44.9 ms) < QX04 (79.8 ms). Therefore, the order of Voc of the four dyes could be further explained.
(a) 100
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Fig. 7. (a) Nyquist plots and (b) Bode phase plots for DSSCs based on the four dyes measured in the dark under 0.70 V bias.
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4. Conclusions Four indolo[3,2-b]carbazole-based multi-donorepeacceptor type dyes have been synthesized as photosensitizers for DSSCs. The four dyes are composed of an indolo[3,2-b]carbazole skeleton as the main donor with a couple of secondary donors directly attached on its side. Different secondary donors such as ethylbenzene, N,Ndiethylaniline, ethyloxylbenzene, and octyloxylbenzene have been chosen to tune the photoelectric properties of these dyes. With N,N-diethylaniline as the secondary donor, QX02 exhibited a broader absorption region, a broader IPCE response region, and significantly improved IPCE values, which ensured a good lightharvesting ability and a high short-circuit current density of 15.2 mA cm2. Finally, QX02-based cell achieved a high efficiency of 8.09% under AM 1.5G (100 mW cm2) irradiation, demonstrating the proper molecular modification of indolo[3,2-b]carbazole-based dyes with strong secondary donor is a promising method to construct effective organic dyes for highly efficient DSSCs. Acknowledgement We are grateful to the National Natural Science Foundation of China (No: 21376054) for their generous financial support. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.04.043. References [1] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem. Rev. 110 (2010) 6595e6663. [2] G. Calogero, A. Bartolotta, G.D. Marco, A.D. Carlo, F. Bonaccorso, Chem. Soc. Rev. 44 (2015) 3244e3294. [3] F. Bella, C. Gerbaldi, C. Barolo, M. Gr€ atzel, Chem. Soc. Rev. 44 (2015) 3431e3473. [4] M. Liang, J. Chen, Chem. Soc. Rev. 42 (2013) 3453e3488. [5] T. Higashino, H. Imahori, Dalton Trans. 44 (2015) 448e463. [6] M. Zhang, Y. Wang, M. Xu, W. Ma, R. Li, P. Wang, Energy Environ. Sci. 6 (2013) 2944e2949. [7] A. Baheti, K.R.J. Thomas, C.-P. Lee, C.-T. Li, K.-C. Ho, J. Mater. Chem. A 2 (2014) 5766e5779. [8] T. Higashino, Y. Fujimori, K. Sugiura, Y. Tsuji, S. Ito, H. Imahori, Angew. Chem. Int. Ed. 54 (2015) 9052e9056. [9] J. Zhao, X. Yang, M. Cheng, S. Li, L. Sun, ACS Appl. Mater. Interfaces 5 (2013) 5227e5231. [10] X. Yang, J. Zhao, L. Wang, J. Tian, L. Sun, RSC Adv. 4 (2014) 24377e24383. [11] N. Cai, Y. Wang, M. Xu, Y. Fan, R. Li, M. Zhang, P. Wang, Adv. Funct. Mater. 23 (2013) 1846e1854. [12] C.-H. Wu, M.-C. Chen, P.-C. Su, H.-H. Kuo, C.-L. Wang, C.-Y. Lu, C.-H. Tsai, C.C. Wu, C.-Y. Lin, J. Mater. Chem. A 2 (2014) 991e999. [13] C.-L. Wang, J.-Y. Hu, C.-H. Wu, H.-H. Kuo, Y.-C. Chang, Z.-J. Lan, H.-P. Wu, E. Wei-Guang Diau, C.-Y. Lin, Energy Environ. Sci. 7 (2014) 1392e1396. [14] C.-L. Mai, T. Moehl, Y. Kim, F.-Y. Ho, P. Comte, P.-C. Su, C.-W. Hsu, F. Giordano, A. Yella, S.M. Zakeeruddin, C.-Y. Yeh, M. Gratzel, RSC Adv. 4 (2014) 35251e35257. [15] N. Masi Reddy, T.-Y. Pan, Y. Christu Rajan, B.-C. Guo, C.-M. Lan, E. Wei-Guang Diau, C.-Y. Yeh, Phys. Chem. Chem. Phys. 15 (2013) 8409e8415. [16] H. Hayashi, T. Higashino, Y. Kinjo, Y. Fujimori, K. Kurotobi, P. Chabera, V. Sundstrom, S. Isoda, H. Imahori, ACS Appl. Mater. Interfaces 7 (2015) 18689e18696. [17] H. Hayashi, A.S. Touchy, Y. Kinjo, K. Kurotobi, Y. Toude, S. Ito, H. Saarenpaa, N.V. Tkachenko, H. Lemmetyinen, H. Imahori, ChemsusChem 6 (2013) 508e517. [18] Q. Xia, M. Liang, Y. Tan, W. Gao, L. Ouyang, G. Ge, Z. Sun, S. Xue, Org. Electron 17 (2015) 285e294. [19] G. Wu, F. Kong, J. Li, W. Chen, X. Fang, C. Zhang, Q. Chen, X. Zhang, S. Dai, Dyes Pigm. 99 (2013) 653e660.
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Indolo[3,2-b]carbazole-based multi-donorepeacceptor type organic dyes for highly efficient dye-sensitized solar cells Xing Qian, Li Shao, Hongmei Li, Rucai Yan, Xiaoying Wang, Linxi Hou* School of Chemical Engineering, Fuzhou University, Xueyuan Road No. 2, Fuzhou 350116, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Four novel indolo[3,2-b]carbazolebased organic dyes have been synthesized. These dyes are designed as a multidonorepeacceptor type. The secondary donors led to striking changes of the photoelectronic properties. QX02 with diethylaniline as the secondary donor achieved a PCE of 8.09%.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 January 2016 Received in revised form 23 March 2016 Accepted 8 April 2016
Four novel indolo[3,2-b]carbazole-based multi-donorepeacceptor type organic dyes QX01e04 have been designed, synthesized, and applied for dye-sensitized solar cells. These dyes consist of an indolo [3,2-b]carbazole core acting as the main donor group, a couple of groups such as ethylbenzene, N,Ndiethylaniline, ethyloxylbenzene, and octyloxylbenzene acting as the secondary donors. The photophysical, electrochemical, and theoretical studies indicate that the four dyes are all capable as the photosensitizers. When introducing N,N-diethylaniline as the secondary donor, QX02 exhibits a broader absorption region and significantly improved IPCE values, which ensured a good light-harvesting ability and a high Jsc of 15.2 mA cm2. Finally, QX02-based cell achieved a high efficiency of 8.09% which is very close to that of the commercial N719-based cell (8.26%) under 100 mW cm2 irradiation. © 2016 Elsevier B.V. All rights reserved.
Kyewords: Dye-sensitized solar cells Organic dyes Indolo[3,2-b]carbazole Secondary donor Intramolecular charge transfer Photovoltaic performances
1. Introduction Dye-sensitized solar cells (DSSCs) are promising alternatives to traditional inorganic semiconductor-based solar cells because of their easy manufacturing processes, low cost, less environmental issues, and high power conversion efficiency (PCE) [1e3]. A typical DSSC has four main components: a semiconductor photoanode, an electrolyte, a counter electrode, and a dye sensitizer [4]. The
* Corresponding author. E-mail address: [email protected] (L. Hou). http://dx.doi.org/10.1016/j.jpowsour.2016.04.043 0378-7753/© 2016 Elsevier B.V. All rights reserved.
sensitizer, as a critical component in a DSSC, plays a crucial role in the light-harvesting, electron generation, electron transfer/injection [5e10]. Recently, numerous research efforts have been devoted to explore new photosensitizers in the development of DSSCs [11e17]. More and more attention have been paid to metal-free organic dye sensitizers because of their low cost, easy synthesis, high molar extinction coefficients, as well as low toxicity [18e21]. Indole and carbazole are both well-known heterocyclic compounds with strong electron-rich properties. Thus, indole and carbazole derivatives are widely used as the electron donor units for organic dyes which are applied in DSSCs due to their significant
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electronic and optical properties [22e27]. Indolo[3,2-b]carbazole contains an electron-rich indole unit fused with another electronrich carbazole unit and presents a planar structure similar to pentacene. Moreover, the solubility of the indolo[3,2-b]carbazole derivatives can be enhanced with the addition of long alkyl chains on the nitrogen atoms [28,29]. This unique structure endows the resultant indolo[3,2-b]carbazole with excellent potentials of strong electron donating ability and excellent hole-transporting ability, so indolo[3,2-b]carbazole derivatives have been applied in fields of organic light-emitting diodes (OLEDs) [30,31], organic field-effect transistors (OFETs) [32,33], hole transporting materials (HTM) [34], and photovoltaic cells [28,35,36]. Donorepeacceptor (DepeA) type have proven to be a nice design pattern for efficient metal-free organic dyes. In these dyes, cyanoacrylic acid is commonly applied as the acceptor/anchoring group because of the increased spectral response based on a strong intramolecular charge transfer (ICT) and its good electron injection property [37e39]. Generally, organic dyes used for efficient DSSCs should have a broad and intense spectral absorption in the UVevisible region. One of the most used method is to design a long rod-like structure by introducing more conjugated p-bridges. However, organic dyes with this structure may facilitate the recombination of electrons with the redox couple in the electrolyte and the intermolecular aggregations of dye molecules. Incorporating a nonplanar secondary electron donor unit into the organic donor framework has been proved to be an effective method to suppress intermolecular aggregation, retard charge recombination rate, and enhance the photovoltage [21,40]. On the other hand, incorporating extra electron donor units into the organic donor framework may increase the electron donating ability of the DepeA type dye and therefore enhance the ICT progress. Because of the highly p-conjugated planar structure, indolo[3,2-b]carbazole-based dyes may easily tend to aggregate when anchored on TiO2 films. So the suppression of dye aggregation is very important for designing efficient indolo[3,2-b]carbazole-based dyes. In this report, we present a new extension of multi-donorepeacceptor dyes by introducing secondary donor groups into the side of indolo [3,2-b]carbazole skeleton, with the aims of further expanding the possibility of improving the optical and ICT properties of the sensitizers as well as the suppression of dye aggregations. Therefore, four novel multi-donorepeacceptor type organic dyes QX01e04 have been designed and synthesized. These dyes consist of an indolo[3,2-b]carbazole core acting as the main donor group, a couple of groups such as ethylbenzene, N,N-diethylaniline, ethyloxylbenzene, and octyloxylbenzene acting as the secondary donor groups, a thiophene unit acting as the p-conjugated linker, and a 2cyanoacylic acid unit acting as the electron acceptor/anchoring group. The molecular structures of these dyes are shown in Fig. 1. 2. Experimental section 2.1. Materials and reagents All solvents were purified by common standard purification methods, and all NMR reagents were used as received. All reagents were purchased from Aldrich, TCI, and Alfa Aesar. FTO conductive glasses (sheet resistance of 15 U/sq, Nippon Sheet Glass) were used. 2.2. Characterization techniques 1 H NMR and 13C NMR spectra were recorded on Bruker 400 MHz spectrometer. U-Vvis spectra were obtained on a Varian Cary 300 Conc UV-visible spectrophotometer. HR-MS data were obtained on a Varian 7.0T FTMS. Cyclic voltammetry (CV) experiments and electrochemical impedance spectroscopy (EIS) were recorded using
an electrochemical workstation (Zennium, Zahner Corporation). CV experiments were carried out by a three-electrode system using a glassy carbon electrode as the working electrode, an Ag/AgNO3 electrode as the reference electrode, and a Pt wire as the counter electrode. The redox potentials were measured in dichloromethane (DCM), using 0.1 M n-Bu4NPF6 (TBAPF6) as the supporting electrolyte at a scan rate of 100 mV S1. 2.3. Fabrication and characterization of DSSCs A TiO2 film (~12 mm) for the transparent nanocrystalline layer was prepared by screen-printing method using a commercial 20 nm TiO2 sol (Heptachroma Corporation) onto the treated FTO conductive glass (treated with 0.05 M TiCl4 aqueous solution). The scattering layer (~4 mm) was applied over the transparent layer by screen-printing method using a commercial 200 nm TiO2 sol (Heptachroma Corporation), then the electrode was gradually heated to 500 C and sintered for 1 h. The prepared TiO2 electrodes were treated by 0.04 M TiCl4 aqueous solution at 70 C for 1 h and sintered again at 500 C for 1 h. The platinum electrode was prepared by thermal deposition of 20 mM chloroplatinic acid (isopropanol solution) on the surface of FTO glass at 450 C for 0.5 h. The resulting TiO2 electrodes were immersed in the commercial N719 dye solution (0.3 mM in ethanol) for 12 h. The adsorption of the indolo[3,2-b]carbazole-based dyes on TiO2 electrode was carried out in 0.3 mM dye's THF solution for 12 h. The acetonitrilebased electrolyte composed of 0.3 M DMPII, 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tert-butylpyridine was used. The DSSCs were fabricated into a sandwich structure and illuminated by a standard solar simulator (XM-500W, Trusttech Corporation) under 100 mW/cm2 irradiation, which was calibrated by a standard silicon solar cell (91150V, Newport Corporation). The current-voltage (J-V) characteristic curves of the DSSC under simulated irradiation were recorded by an electrochemical workstation (Zennium, Zahner Corporation). The incident photon-to-current conversion efficiency (IPCE) was measured using a commercial IPCE system (QTest Station 2000, CrownTech Corporation). 2.4. Synthetic procedures 2.4.1. 6,12-bis(4-ethylphenyl)-5,11-dioctyl-5,11-dihydroindolo[3,2b]carbazole (compound 1a) In a 250 mL round-bottom flask, a solution of aqueous 48% (w/ w) HBF4 (0.42 g, 2.3 mmol) and n-Bu4NI (0.85 g, 2.3 mmol) in acetonitrile (10 mL) was added to a solution of indole (2.71 g, 23 mmol) and 4-ethylbenzaldehyde (3.09 g, 23 mmol) in acetonitrile (80 mL). The resulting dark-red reaction mixture was stirred for 3 h at room temperature. After that, the solid was filtered off and washed with water (200 mL), acetonitrile (50 mL), and methanol (100 mL). The product was dried and tranfered to a 250 mL roundbottom flask. Then, n-Bu4NBr (TBAB) (265 mg, 0.82 mmol), NaOH (1.31 g, 33 mmol), and THF (35 mL) was added and the mixture was heated at 80 C for 0.5 h. Then C8H17Br (6.34 g, 33 mmol) was added dropwise, and the mixture was heated 80 C for 12 h before evaporated. The mixture was extracted with DCM, washed with water and then with brine solution. The organic layer was dried with Na2SO4 and 2,3-dicyano-5,6-dichlorobenzoquinone (DDQ) (2.42 g, 11 mmol) was added. The mixture was heated at 60 C for another 1 h before evaporated and the crude product was purified by silica-gel column chromatography using DCM/PE (PE) (1:4) as the eluent to afford compound 1a as a pale yellow solid (total yield: 68.5%). 1H NMR (400 MHz, CDCl3) d 7.55 (d, J ¼ 14.0 Hz, 4H), 7.42 (d, J ¼ 17.4 Hz, 4H), 7.31 (t, J ¼ 6.8 Hz, 4H), 6.81 (s, 2H), 6.59 (d, J ¼ 7.7 Hz, 2H), 4.21e3.50 (m, 4H), 2.88 (q, J ¼ 7.3 Hz, 4H), 1.59e1.47 (m, 4H), 1.42 (t, J ¼ 7.0 Hz, 6H), 1.35e1.01 (m, 16H), 0.89 (t, J ¼ 6.7 Hz,
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41
Fig. 1. Chemical structures of QX01e04.
10H). 13C NMR (100 MHz, CDCl3) d 144.22, 136.01, 132.52, 130.54, 130.43, 128.38, 125.13, 123.09, 122.60, 122.48, 117.76, 117.72, 108.09, 44.49, 31.82, 29.30, 29.27, 28.98, 28.75, 26.79, 22.70, 16.04, 14.14. HR-MS (MALDI): m/z [M]þ calcd for C50H60N2, 688.4756, found, 688.4752. 2.4.2. 4,40 -(5,11-dioctyl-5,11-dihydroindolo[3,2-b]carbazole-6,12diyl)bis(N,N-diethylaniline) (compound 1b) The compound 1b was synthesized according to the procedure described for compound 1a and the crude product was purified by silica-gel column chromatography using DCM/PE (1:2) as the eluent to afford compound 1b as a yellow solid (total yield: 35.1%). 1 H NMR (400 MHz, CDCl3) d 7.48 (d, J ¼ 8.6 Hz, 4H), 7.39e7.32 (m, 2H), 7.29 (d, J ¼ 4.5 Hz, 2H), 6.96 (d, J ¼ 8.6 Hz, 4H), 6.89 (d, J ¼ 3.8 Hz, 4H), 3.99e3.86 (m, 6H), 3.55 (q, J ¼ 7.0 Hz, 8H), 1.66e1.50 (m, 4H), 1.33 (t, J ¼ 7.0 Hz, 12H), 1.31e1.15 (m, 16H), 1.07e0.97 (m, 4H), 0.91 (t, J ¼ 7.0 Hz, 6H). 13C NMR (100 MHz, CDCl3) d 147.54, 142.44, 133.06, 131.17, 125.45, 124.81, 123.49, 123.19, 122.76, 118.10, 117.52, 112.57, 107.93, 44.58, 44.51, 31.83, 30.17, 29.38, 28.87, 26.99, 22.72, 14.13, 12.59. HR-MS (MALDI): m/z [M]þ calcd for C54H70N4, 774.5600, found, 774.5598. 2.4.3. 6,12-bis(4-ethoxyphenyl)-5,11-dioctyl-5,11-dihydroindolo [3,2-b]carbazole (compound 1c) The compound 1c was synthesized according to the procedure described for compound 1a and the crude product was purified by silica-gel column chromatography using DCM/PE (1:4) as the
eluent to afford compound 1c as a pale yellow solid (total yield: 65.2%). 1H NMR (400 MHz, C6D6) d 7.57 (d, J ¼ 8.4 Hz, 4H), 7.36 (t, J ¼ 7.6 Hz, 2H), 7.24 (d, J ¼ 8.3 Hz, 4H), 7.05 (d, J ¼ 7.5 Hz, 2H), 7.01 (d, J ¼ 8.4 Hz, 4H), 4.01e3.79 (m, 4H), 3.71 (q, J ¼ 6.9 Hz, 4H), 1.62e1.44 (m, 4H), 1.30e1.12 (m, 18H), 1.07e0.98 (m, 4H), 0.90 (t, J ¼ 7.1 Hz, 10H). 13C NMR (100 MHz, C6D6) d 159.20, 133.39, 133.23, 131.85, 131.18, 125.39, 124.04, 123.77, 123.61, 122.90, 118.39, 114.88, 108.59, 63.11, 44.39, 31.88, 29.37, 28.87, 26.95, 22.78, 22.73, 14.58, 14.07. HR-MS (MALDI): m/z [M]þ calcd for C50H60N2O2, 720.4655, found, 720.4652. 2.4.4. 5,11-Dioctyl-6,12-bis(4-(octyloxy)phenyl)-5,11-dihydroindolo [3,2-b]carbazole (compound 1d) The compound 1d was synthesized according to the procedure described for compound 1a and the crude product was purified by silica-gel column chromatography using DCM/PE (1:4) as the eluent to afford compound 1d as a pale yellow solid (total yield: 63.3%). 1H NMR (400 MHz, CDCl3) d 7.58 (d, J ¼ 8.4 Hz, 4H), 7.36 (t, J ¼ 7.5 Hz, 2H), 7.29 (d, J ¼ 7.5 Hz, 2H), 7.19 (d, J ¼ 8.5 Hz, 4H), 6.88 (t, J ¼ 7.4 Hz, 2H), 6.70 (d, J ¼ 7.9 Hz, 2H), 4.18 (t, J ¼ 6.6 Hz, 4H), 3.98e3.73 (m, 4H), 2.04e1.85 (m, 4H), 1.73e1.11 (m, 44H), 0.98e0.90 (m, 12H). 13C NMR (100 MHz, CDCl3) d 159.09, 142.44, 132.75, 131.49, 130.74, 125.12, 123.16, 122.97, 122.51, 117.79, 117.58, 114.96, 108.15, 68.34, 44.46, 31.90, 31.82, 29.73, 29.53, 29.45, 29.34, 29.33, 28.78, 26.91, 26.20, 22.73, 22.70, 14.16, 14.13. HR-MS (MALDI): m/z [M]þ calcd for C62H84N2O2, 888.6533, found, 888.6530.
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2.4.5. 5-(6,12-bis(4-ethylphenyl)-5,11-dioctyl-5,11-dihydroindolo [3,2-b]carbazol- 2-yl)thiophene-2-carbaldehyde (compound 2a) A solution of N-bromosuccinimide (NBS) (260 mg, 1.46 mmol) in DCM (30 mL) was added dropwise to a solution of compound 1a (965 mg, 0.155 mmol) in DCM (80 mL) at 0 C. The mixture was slowly warmed to room temperature and stirred for an additional 1 h before evaporated. The mixture was resolved in DCM and washed with water. Then the organic phase was separated and dried over anhydrous Na2SO4. After the solvent was evaporated, the crude product was transferred to a two-neck round bottomed flask. Then, 5-formylthiophene-2-boronic acid (319 mg, 2.86 mmol), Pd(dppf)Cl2 (119 mg, 0.15 mmol), K2CO3 (2.01 g, 14.6 mmol), toluene (30 mL), methanol (5 mL) were added. The flask was charged with N2. The mixture was heated at 80 C for 4 h before it was poured into water. The organic phase was separated and dried over anhydrous Na2SO4. After the solvent was removed under vacuum, the crude product was purified by silica-gel column chromatography using (DCM/PE ¼ 1:1) as the eluent to afford 2a as an orange solid (total yield: 62.1%). 1H NMR (400 MHz, CDCl3) d 9.81 (s, 1H), 7.68e7.57 (m, 5H), 7.54 (d, J ¼ 8.4 Hz, 3H), 7.46 (d, J ¼ 7.7 Hz, 2H), 7.37e7.31 (m, 1H), 7.27 (d, J ¼ 11.7 Hz, 1H), 7.24 (d, J ¼ 8.8 Hz, 1H), 7.01 (d, J ¼ 3.6 Hz, 1H), 6.84 (t, J ¼ 7.5 Hz, 1H), 6.79 (s, 1H), 6.59 (d, J ¼ 8.0 Hz, 1H), 3.97e3.88 (m, 2H), 3.85e3.64 (m, 2H), 3.04e2.92 (m, 2H), 2.92e2.74 (m, 2H), 1.63e1.47 (m, 9H), 1.42 (t, J ¼ 7.4 Hz, 3H), 1.31e1.10 (m, 18H), 0.89 (t, J ¼ 6.8 Hz, 10H). 13C NMR (100 MHz, CDCl3) d 181.19, 155.92, 143.66, 143.30, 141.89, 141.35, 139.27, 136.61, 134.55, 134.37, 131.69, 131.50, 129.22, 129.12, 127.64, 127.31, 124.40, 122.49, 122.21, 122.14, 121.85, 121.50, 121.46, 121.38, 120.72, 119.64, 117.29, 116.97, 116.90, 107.40, 107.09, 43.48, 43.35, 30.89, 30.71, 29.31, 28.66, 28.33, 28.24, 28.23, 28.15, 28.10, 27.98, 27.87, 27.82, 25.69, 25.64, 21.62, 15.34, 14.92, 13.07. HR-MS (MALDI): m/z [M]þ calcd for C55H62N2OS, 798.4583, found, 798.4580. 2.4.6. 5-(6,12-bis(4-(diethylamino)phenyl)-5,11-dioctyl-5,11dihydroindolo[3,2-b]carbazol-2-yl)thiophene-2-carbaldehyde (compound 2b) The compound 2b was synthesized according to the procedure described for compound 2a and the crude product was purified by silica-gel column chromatography using DCM/PE (1:1) as the eluent to afford compound 2b as a yellow-green solid (total yield: 55.2%). 1H NMR (400 MHz, CDCl3) d 9.83 (s, 1H), 7.65 (dd, J ¼ 8.5, 1.8 Hz, 1H), 7.62 (d, J ¼ 4.0 Hz, 1H), 7.45 (d, J ¼ 8.4 Hz, 4H), 7.40e7.35 (m, 1H), 7.32 (d, J ¼ 8.1 Hz, 1H), 7.23 (d, J ¼ 8.6 Hz, 1H), 7.11 (d, J ¼ 4.0 Hz, 1H), 7.02e6.96 (m, 3H), 6.96e6.89 (m, 4H), 4.24e4.06 (m, 2H), 4.01e3.88 (m, 2H), 3.63 (q, J ¼ 6.9 Hz, 4H), 3.55 (q, J ¼ 7.0 Hz, 4H), 1.65e1.61 (m, 2H), 1.61e1.52 (m, 2H), 1.42e1.30 (m, 16H), 1.27e1.14 (m, 12H), 1.09e0.97 (m, 4H), 0.95e0.88 (m, 6H). 13C NMR (100 MHz, CDCl3) d 182.25, 157.65, 147.75, 147.62, 143.11, 142.43, 140.09, 137.84, 133.33, 133.08, 131.09, 130.91, 125.15, 124.76, 124.42, 124.03, 123.79, 123.37, 123.34, 122.97, 122.88, 122.30, 121.72, 121.13, 118.68, 118.46, 117.74, 112.48, 112.09, 108.36, 108.02, 44.64, 44.56, 44.42, 31.82, 29.41, 29.36, 29.35, 29.31, 29.01, 28.98, 26.98, 26.97, 22.72, 14.14, 12.75, 12.59. HR-MS (MALDI): m/z [M]þ calcd for C59H72N4OS, 884.5427, found, 884.5428. 2.4.7. 5-(6,12-bis(4-ethoxyphenyl)-5,11-dioctyl-5,11-dihydroindolo [3,2-b]carbazol-2-yl)thiophene-2-carbaldehyde (compound 2c) The compound 2c was synthesized according to the procedure described for compound 2a and the crude product was purified by silica-gel column chromatography using DCM/PE (1:1) as the eluent to afford compound 2c as an orange solid (total yield: 56.4%). 1 H NMR (400 MHz, CDCl3) d 9.82 (s, 1H), 7.67e7.60 (m, 2H), 7.55 (t, J ¼ 8.9 Hz, 4H), 7.39e7.32 (m, 1H), 7.29 (d, J ¼ 8.1 Hz, 1H), 7.24 (d, J ¼ 8.5 Hz, 3H), 7.15 (d, J ¼ 8.4 Hz, 2H), 7.07 (d, J ¼ 3.9 Hz, 1H), 6.86 (t, J ¼ 7.4 Hz, 1H), 6.77 (d, J ¼ 1.3 Hz, 1H), 6.65 (d, J ¼ 7.9 Hz, 1H), 4.32 (q,
J ¼ 6.9 Hz, 2H), 4.22 (q, J ¼ 6.9 Hz, 2H), 4.05e3.93 (m, 2H), 3.89e3.73 (m, 2H), 1.63 (t, J ¼ 6.9 Hz, 3H), 1.60e1.44 (m, 9H), 1.33e1.08 (m, 18H), 1.03e0.93 (m, 4H), 0.89 (t, J ¼ 6.7 Hz, 6H). 13C NMR (100 MHz, CDCl3) d 182.35, 159.39, 159.02, 157.13, 143.08, 142.47, 140.50, 137.80, 133.04, 132.79, 131.44, 131.33, 130.32, 130.27, 125.49, 123.72, 123.60, 123.28, 122.99, 122.98, 122.66, 122.61, 121.85, 120.88, 118.13, 118.01, 117.86, 115.36, 114.98, 108.61, 108.26, 64.03, 63.67, 50.00, 44.63, 44.38, 33.72, 31.81, 31.79, 31.08, 29.28, 29.25, 28.91, 28.84, 26.86, 26.83, 22.68, 15.05, 14.95, 14.13. HR-MS (MALDI): m/z [M]þ calcd for C55H62N2O3S, 830.4481, found, 830.4478. 2.4.8. 5-(5,11-dioctyl-6,12-bis(4-(octyloxy)phenyl)-5,11dihydroindolo[3,2-b]carbazol-2-yl)thiophene-2-carbaldehyde (compound 2d) The compound 2d was synthesized according to the procedure described for compound 2a and the crude product was purified by silica-gel column chromatography using DCM/PE (1:1) as the eluent to afford compound 2d as an orange solid (total yield: 55.2%). 1H NMR (400 MHz, CDCl3) d 9.84 (s, 1H), 7.69e7.63 (m, 2H), 7.63e7.54 (m, 4H), 7.42e7.35 (m, 1H), 7.32 (d, J ¼ 8.1 Hz, 1H), 7.28e7.24 (m, 3H), 7.18 (d, J ¼ 8.6 Hz, 2H), 4.24 (t, J ¼ 6.6 Hz, 2H), 4.16 (t, J ¼ 6.7 Hz, 2H), 4.06e3.95 (m, 2H), 3.90e3.78 (m, 2H), 2.07e1.89 (m, 4H), 1.61e1.13 (m, 40H), 1.02e0.88 (m, 16H). 13C NMR (100 MHz, CDCl3) d 182.28, 159.54, 159.19, 157.11, 143.05, 142.44, 140.47, 137.77, 133.03, 132.78, 131.38, 131.31, 130.25, 130.14, 125.47, 123.68, 123.57, 123.40, 122.98, 122.93, 122.62, 121.81, 120.76, 118.14, 118.00, 117.84, 115.31, 114.99, 108.61, 108.24, 68.62, 68.33, 44.63, 44.39, 31.94, 31.90, 31.80, 29.59, 29.52, 29.43, 29.41, 29.35, 29.30, 29.26, 28.92, 28.85, 26.85, 26.27, 26.19, 22.75, 22.74, 22.69, 14.19, 14.14. HR-MS (MALDI): m/z [M]þ calcd for C67H86N2O3S, 998.6359, found, 998.6358. 2.4.9. (E)-3-(5-(6,12-bis(4-ethylphenyl)-5,11-dioctyl-5,11dihydroindolo [3,2-b]carbazol-2-yl)thiophen-2-yl)-2-cyanoacrylic acid (QX01) A mixture of compound 2a (240 mg, 0.30 mmol), cyanoacetic acid (76 mg, 0.90 mmol), piperdine (0.20 mL), CHCl3 (15 mL), and CH3CN (15 mL) was heated at 80 C for 12 h and then acidified with 2 M aqueous hydrochloric acid (10 mL). The crude product was extracted into CHCl3, and the organic layer was washed with water and dried over anhydrous Na2SO4. After removal of the solvent under reduced pressure, the crude product was purified by silicagel column chromatography using DCM/CH3OH (15:1) as the eluent to afford QX01 as a deep red solid (yield: 84.1%). 1H NMR (400 MHz, DMSO-d6) d 8.15 (s, 1H), 7.71 (s, 2H), 7.67 (d, J ¼ 8.8 Hz, 1H), 7.57 (d, J ¼ 10.0 Hz, 4H), 7.52 (d, J ¼ 7.9 Hz, 2H), 7.47 (d, J ¼ 8.7 Hz, 1H), 7.43 (d, J ¼ 7.6 Hz, 1H), 7.35e7.27 (m, 1H), 7.21 (s, 1H), 7.10 (s, 1H), 6.83e6.74 (m, 1H), 6.67 (s, 1H), 6.44 (d, J ¼ 7.8 Hz, 1H), 3.95e3.83 (m, 2H), 3.83e3.71 (m, 2H), 2.97e2.88 (m, 2H), 2.88e2.78 (m, 2H), 2.72e2.62 (m, 2H), 2.07e1.93 (m, 2H), 1.62e0.93 (m, 26H), 0.86 (t, J ¼ 6.8 Hz, 6H). 13C NMR (100 MHz, DMSO-d6) d 156.81, 154.27, 153.71, 146.59, 146.02, 144.39, 144.02, 142.32, 141.96, 140.71, 135.01, 134.91, 134.59, 134.16, 132.65, 132.20, 131.94, 129.89, 129.79, 128.54, 128.22, 125.43, 123.52, 123.49, 122.91, 122.53, 122.28, 121.95, 121.83, 121.46, 119.68, 117.93, 117.75, 109.65, 48.74, 47.36, 44.08, 44.03, 43.85, 31.22, 31.20, 28.78, 28.67, 28.59, 28.57, 28.42, 28.37, 28.33, 26.18, 22.15, 15.95, 15.84, 13.92. HR-MS (MALDI): m/z [M]þ calcd for C58H63N3O2S, 865.4641, found, 865.4638. 2.4.10. (E)-3-(5-(6,12-bis(4-(diethylamino)phenyl)-5,11-dioctyl5,11-dihydro-indolo[3,2-b]carbazol-2-yl)thiophen-2-yl)-2cyanoacrylic acid (QX02) QX02 was synthesized according to the procedure described for
X. Qian et al. / Journal of Power Sources 319 (2016) 39e47
QX01 and the crude product was purified by silica-gel column chromatography using DCM/CH3OH (10:1) as the eluent to afford QX02 as a dark red solid (yield: 75.2%). 1H NMR (400 MHz, DMSOd6) d 8.38 (s, 1H), 7.97 (s, 1H), 7.64 (d, J ¼ 7.5 Hz, 1H), 7.49e7.42 (m, 2H), 7.41e7.36 (m, 3H), 7.37e7.27 (m, 2H), 7.06 (s, 1H), 6.98 (d, J ¼ 7.4 Hz, 2H), 6.94 (d, J ¼ 9.6 Hz, 2H), 6.90e6.79 (m, 2H), 6.72 (d, J ¼ 8.3 Hz, 1H), 4.09e4.02 (m, 2H), 3.93e3.86 (m, 2H), 3.58e3.53 (m, 4H), 3.53e3.49 (m, 4H), 1.56e1.40 (m, 4H), 1.29e1.15 (m, 24H), 1.12e1.05 (m, 4H), 0.97e0.83 (m, 10H). 13C NMR (100 MHz, DMSOd6) d 162.19, 154.37, 147.21, 147.14, 144.11, 144.04, 142.42, 141.97, 133.24, 132.86, 132.55, 130.65, 130.34, 125.03, 124.00, 123.84, 123.45, 123.34, 123.13, 122.97, 122.83, 122.52, 122.01, 121.77, 119.97, 118.34, 118.10, 117.95, 117.58, 111.83, 111.74, 108.99, 108.19, 107.80, 44.22, 44.17, 44.00, 36.21, 31.48, 31.41, 31.11, 29.15, 29.11, 29.03, 28.79, 28.62, 26.64, 26.56, 22.42, 22.39, 18.41, 13.99, 12.56, 12.38. HR-MS (MALDI): m/z [M]þ calcd for C62H73N5O2S, 951.5485, found, 951.5483.
43
3. Results and discussion 3.1. Synthesis of the dyes The synthetic procedure to QX01e04 is depicted in Scheme 1. A concise approach to the synthesis of 6,12-diaryl substituted indolo [3,2-b]carbazoles (compound 1aed) was conducted by reacting the corresponding benzaldehyde with indole under catalysis by acids according to the previous literature [41]. Bromination of compound 1aed with NBS produced the brominated intermediates, and then Suzuki cross-coupling of brominated indolo[3,2-b]carbazoles derivatives and 5-formylthiophene-2-boronic acid produced the pextended indolocarbazole-based aldehydes 2aed. The intermediates 2aed were finally treated with cyanoacetic acid by Knoevenagel reactions to give the four dyes QX01e04.
3.2. UVevis absorption and electrochemical characterization 2.4.11. (E)-3-(5-(6,12-bis(4-ethoxyphenyl)-5,11-dioctyl-5,11dihydroindolo[3,2-b]carbazol-2-yl)thiophen-2-yl)-2-cyanoacrylic acid (QX03) QX03 was synthesized according to the procedure described for QX01 and the crude product was purified by silica-gel column chromatography using DCM/CH3OH (12:1) as the eluent to afford QX03 as a dark red solid (yield: 86.5%). 1H NMR (400 MHz, DMSOd6) d 8.37 (s, 1H), 7.94 (s, 1H), 7.67 (s, 1H), 7.62e7.39 (m, 6H), 7.32 (t, J ¼ 14.3 Hz, 1H), 7.22 (d, J ¼ 14.8 Hz, 4H), 7.14 (s, 1H), 6.83 (t, J ¼ 7.3 Hz, 1H), 6.66 (s, 1H), 6.50 (d, J ¼ 7.9 Hz, 1H), 4.38e4.10 (m, 4H), 3.94 (s, 2H), 3.82 (s, 2H), 1.41 (t, J ¼ 28.0 Hz, 10H), 1.32e0.90 (m, 20H), 0.86 (t, J ¼ 6.8 Hz, 6H). 13C NMR (100 MHz, DMSO-d6) d 163.90, 158.85, 158.59, 155.44, 153.17, 145.48, 142.58, 141.93, 139.44, 139.36, 134.97, 132.98, 132.40, 132.16, 130.95, 129.47, 129.28, 125.36, 123.51, 123.54, 123.01, 122.93, 122.32, 122.28, 121.93, 119.97, 119.90, 117.76, 117.46, 116.57, 115.01, 114.66, 108.97, 108.22, 63.32, 63.12, 44.07, 44.00, 43.75, 31.16, 29.37, 29.06, 28.77, 28.61, 28.55, 28.48, 28.35, 26.60, 26.34, 26.25, 22.09, 14.63, 14.60, 13.85. HR-MS (MALDI): m/z [M]þ calcd for C58H63N3O4S, 897.4539, found, 897.4536.
2.4.12. (E)-2-Cyano-3-(5-(5,11-dioctyl-6,12-bis(4-(octyloxy) phenyl)-5,11-dihydro-indolo[3,2-b]carbazol-2-yl)thiophen-2-yl) acrylic acid (QX04) QX04 was synthesized according to the procedure described for QX01 and the crude product was purified by silica-gel column chromatography using DCM/CH3OH (12:1) as the eluent to afford QX04 as a dark red solid (yield: 88.2%). 1H NMR (400 MHz, DMSOd6) d 8.30 (s, 1H), 7.87 (s, 1H), 7.67 (d, J ¼ 8.2 Hz, 1H), 7.53 (t, J ¼ 7.6 Hz, 3H), 7.50e7.46 (m, 1H), 7.43 (d, J ¼ 7.8 Hz, 1H), 7.33 (t, J ¼ 7.7 Hz, 1H), 7.24 (d, J ¼ 8.3 Hz, 2H), 7.19 (d, J ¼ 8.6 Hz, 2H), 7.13 (s, 1H), 6.82 (t, J ¼ 7.6 Hz, 1H), 6.68 (s, 2H), 6.51 (d, J ¼ 8.2 Hz, 1H), 4.31e4.08 (m, 4H), 4.03e3.88 (m, 2H), 3.89e3.73 (m, 2H), 2.14e1.92 (m, 4H), 1.89e1.68 (m, 4H), 1.54e1.14 (m, 40H), 0.93e0.82 (m, 12H). 13 C NMR (100 MHz, DMSO-d6) d 165.10, 159.41, 159.12, 153.67, 143.45, 142.66, 142.32, 137.22, 134.19, 132.78, 132.49, 131.35, 129.90, 129.69, 129.24, 127.99, 125.76, 123.87, 123.28, 122.79, 122.70, 122.63, 122.35, 122.20, 119.84, 118.32, 118.16, 118.04, 117.80, 115.32, 115.02, 115.02, 109.16, 108.60, 68.33, 68.11, 44.35, 44.18, 31.85, 31.85, 31.81, 31.67, 31.64, 31.64, 29.54, 29.40, 29.40, 29.36, 29.30, 29.30, 29.26, 29.10, 29.10, 28.94, 28.94, 28.78, 26.77, 26.69, 26.17, 26.13, 26.13, 22.64, 22.60, 14.38, 14.37, 14.31. HR-MS (MALDI): m/z [M]þ calcd for C70H87N3O4S, 1065.6417, found, 1065.6417.
The UVevis absorption spectra of QX01e04 in dichloromethane (DCM) and on TiO2 films are depicted in Fig. 2. The data of absorption, electrochemical properties are summarized in Table 1. In DCM solutions, each of these organic dyes exhibited three major absorption bands at 300e350, 350e450, and 450e650 nm, respectively. The high-energy absorption bands ranging from 300 to 450 nm correspond to the pep* transitions of the conjugated aromatic rings. The low-energy bands appearing at 450e650 nm can be attributed to the intramolecular charge transfer (ICT) transitions from donors to acceptors [42e44]. The molar extinction coefficients at lmax of QX01e04 are 3.09 104 (476 nm), 2.50 104 (500 nm), 2.11 104 (505 nm) and 3.13 104 (485 nm) M¡1 cm¡1, respectively. These are more than twice as high as those of conventional polypyridyl Ru(II) complexes dyes, which is conducive to increase the light-harvesting of the cells [45]. As shown in Fig. 2, when anchoring on mesoporous TiO2 films, the four dyes show significantly broadened absorption spectra compared to that measured in DCM and QX02 exhibit the most broad absorption region. However, the absorption peaks of all these dyes showed slight blue-shift as compared to the data recorded in DCM. This blue-shift can be ascribed to the partial deprotonation of carboxylic acid acceptor because of the interaction between the dye molecules and TiO2 [46e48]. Cyclic voltammetry measurements were used to investigate the redox behavior of these dyes in DCM calibrated against ferrocene (0.63 V vs. NHE), using 0.1 M TBAPF6 as the supporting electrolyte. The related cyclic voltammograms recorded in DCM are shown in Fig. 3(a). The HOMO levels of these dyes correspond to the first oxidation potentials (Eox) and the LUMO levels (Ered) could be calculated from Eox E0e0, where E0e0 (zerothezeroth transition energy) was estimated from the onset wavelength in absorption spectra in DCM. The oxidation potentials of these dyes could be determined from the peak potentials in cyclic voltammograms. As shown in Table 1, the oxidation potentials of the four dyes show a first reversible oxidation wave at 0.93 (QX01), 0.83 (QX02), 0.86 (QX03), and 0.85 V (QX04) vs. NHE, respectively. As a result, the HOMO levels of the four dyes are evaluated to be higher than the redox potential of the iodide/triiodide redox couple (0.4 V vs. NHE), which guarantees efficient dye regeneration. On the other hand, the LUMO levels of QX01e04 (1.09, e1.08, e1.12, and 1.12 V vs. NHE, respectively) are all more negative than the conduction band (CB) of TiO2 (0.5 V vs NHE) allowing efficient electron injection from the excited dyes into the TiO2 semiconductor. The energy diagram of HOMO and LUMO energy levels (NHE vs. vacuum level is set to 4.5 V) for the four dyes is shown in Fig. 3(b) [38,49,50].
44
X. Qian et al. / Journal of Power Sources 319 (2016) 39e47
Scheme 1. Synthetic routes to QX01e04.
ε / 104 M-1 cm-1
(a)
5
Table 1 Optical and electrochemical data of the four dyes.a
QX01 QX02 QX03 QX04
4 3 2
0 300
400 500 600 Wavelength / nm
ε/104 M1 cm1
Eox/V
E00/V
Ered/V
QX01 QX02 QX03 QX04
476 500 505 485
3.09 2.50 2.11 3.13
0.93 0.83 0.86 0.85
2.02 1.91 1.98 1.97
1.09 1.08 1.12 1.12
700 3.3. Theoretical calculations
(b) 1.5 QX01 QX02 QX03 QX04
1.2 0.9 Abs
lmax/nm
a First oxidation potentials (Eox) (vs. NHE) in DCM were calibrated with ferrocene (0.63 V vs. NHE); E0e0 values (zeroth-zeroth transition energies) were estimated from the onset of the absorption spectra in dichloromethane; Ered ¼ Eox E0-0.
1
0.6 0.3 0.0
Dye
400
500 600 Wavelength / nm
700
Fig. 2. UVevis absorption spectra of the four dyes (a) in dichloromethane solutions and (b) on TiO2 films.
In order to further understand the geometrical and electronic properties of the studied four dyes, density functional theory (DFT) calculations were conducted using the Gaussian 03 program package at the B3LYP/6-31G* level. Fig. 4 displays the electron distributions of the frontier molecular orbitals of QX01e04. It can be clearly seen that the HOMO orbitals of the four dyes predominantly located on the indolo[3,2-b]carbazole core and the neighboring p-bridge unit, while the LUMOs of the four dyes all delocalized over the cyanoacrylic acid acceptors and their nearby p-bridge unit. As a result, this electron distribution will allow significant charge separation within the dye and hence efficient electron injection from the excited dyes into the acceptor groupconnected TiO2 semiconductor. Meanwhile, the optimized geometries of QX01e04 show the secondary donor groups are perpendicular with indolo[3,2-b]carbazole core, which may suppress the dye aggregation and improve their photovoltaic performances. 3.4. Photovoltaic performance The DSSC performances of the dyes are evaluated under AM
X. Qian et al. / Journal of Power Sources 319 (2016) 39e47
(a)
10 5
QX01 QX02 QX03 QX04
Current density / mA cm-2
Current / uA
15
0 -5 -10 0.0
0.5 1.0 1.5 Potentials vs. NHE (V)
2.0
45
QX01 QX02 QX03 QX04 N719
18 15 12 9 6 3 0 0.0
0.2
0.4 0.6 Voltage / V
0.8
Fig. 5. The photocurrent-voltage (JeV) curves of DSSCs based on QX01e04 and N719.
Fig. 3. (a) Cyclic voltammograms of these dyes recorded in DCM solutions. (b) Energy diagram of HOMO and LUMO energy levels for these dyes.
1.5 G irradiation at 100 mW cm2, employing an iodine electrolyte composed of 0.3 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII), 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tert-butyl pyridine (TBP) in acetonitrile. The DSSC performance parameters of QX01e04 and commercial N719 dye are measured and displayed in Table 2. The corresponding photocurrent density-voltage (JV) curves are plotted in Fig. 5. The solar cell based on dye QX01 with ethylbenzene as the secondary donor group exhibited a short-circuit
Fig. 4. The electron distributions of the frontier orbitals of QX01e04.
Table 2 Photovoltaic performances of QX01e04 with N719 as a reference.
a
Dye
DL/nmol cm2
Voc/mV
Jsc/mA cm2
FF
PCE/%
QX01 QX02 QX03 QX04 N719
180 171 188 160 e
729 745 738 757 740
12.6 15.2 13.9 14.1 15.9
0.68 0.71 0.68 0.71 0.70
6.25 8.09 6.98 7.58 8.26
a The active area of all cells was 0.196 cm2; The electrolyte was composed of 0.3 M DMPII, 0.1 M LiI, 0.05 M I2, and 0.5 M 4-tert-butyl pyridine in acetonitrile.
photocurrent density (Jsc) of 12.6 mA cm2, an open-circuit photovoltage (Voc) of 728 mV, and a fill factor (FF) of 0.68, generating a power conversion efficient (PCE) of 6.25%. When the secondary donor group was replaced by ethyloxylbenzene, the resultant QX03 showed significant improvement of Voc and Jsc (Voc ¼ 738 mV, Jsc ¼ 13.9 mA cm2) and finally gave a higher PCE of 6.98%. With the introduction of the longer octyloxyl group on the secondary donor, QX04-based cell showed a highest Voc of 758 mV and a higher PCE of 7.58%. Notably, benefited from its broader and more intense ICT
X. Qian et al. / Journal of Power Sources 319 (2016) 39e47
band on TiO2 film, QX02 with N,N-diethylaniline as the secondary donor displayed a highest Jsc of 15.2 mA cm2, a moderate Voc of 745 mV, and an increased PCE of 8.09%. The best efficiency of QX02 was very close to that of the commercial N719-based cell (Voc ¼ 740 mV, Jsc ¼ 15.9 mA cm2, FF ¼ 0.70, PCE ¼ 8.26%) that fabricated and measured under the same conditions, indicating that the proper molecular modification of indolo[3,2-b]carbazolebased dyes with strong secondary donor is a promising method to construct effective organic dyes for DSSCs. Based on the abovementioned J-V curve data, the effects of the secondary donors on the photocurrent characteristics were investigated to elucidate the difference between the Jsc values of the four dyes. The incident photon-to-current conversion efficiencies (IPCEs) as a function of incident wavelength for the cells based on QX01e04 are displayed in Fig. 6. As shown in IPCE plots, all of the four dyes could efficiently convert the light to photocurrents in the UVevis region and the onsets of the IPCE spectra for QX01e04 were at 690, 750, 700 and 710 nm, respectively. QX02-based cell gave over 60% IPCE values from 350 to 620 nm with a maximum IPCE value of 85% at 500 nm, which ensured a good light-harvesting ability and a high photocurrent. The broader IPCE response range and high IPCE values of QX02 explained the highest Jsc value of QX02 from the J-V measurement. The relatively highest efficiency observed for QX02 was mainly due to the relatively largest Jsc. Obviously, the four dyes exhibited significantly different DSSC performances, which was caused by the different dye structures. Therefore, the different substituted groups (such as ethylbenzene, N,N-diethylaniline, ethyloxylbenzene, and octyloxylbenzene) introduced into the para position of indolo[3,2-b]carbazole as a couple of secondary donors played an important role on the photophysical, electrochemical properties of the dye molecules and their DSSC performances. The electron-donating ability increases in the order of ethylbenzene < ethyloxylbenzene z octyloxylbenzene < N,Ndiethylaniline. Stronger secondary donors enhance their electrondonating ability of the whole donor part and promote the ICT process (the excited electron is transferred from donor to acceptor). Benefiting from the strongest N,N-diethylaniline donor, QX02 displayed a biggest absorption onset, a smallest E0e0, a broadest IPCE response, and a highest Jsc. The strongest electron-donating ability of QX02 was also reflected by its lowest first oxidation potential. The order of their absorption onsets and first oxidation potentials of QX01e04 are consistent with their electron-donating abilities. As a result, incorporating strong electron-donating group such as N,Ndiethylaniline on indolo[3,2-b]carbazole-based dye structure is an effective route to improve its DSSC performance.
3.5. Electrochemical impedance spectroscopy To elucidate the differences of the photovoltages between these dyes, electrochemical impedance spectroscopy (EIS) was measured to characterize the interfacial charge transfer processes in DSSCs [51,52]. Nyquist plots and Bode phase plots of DSSCs based on QX01e04 were measured in the dark under forward bias (0.70 V) with frequency range of 100 kHz to 0.1 Hz (Fig. 7). In the Nyquist plots, the larger semicircle in the lower frequency region corresponds to the interfacial charge transfer resistances (Rct) at the TiO2/dyes/electrolyte interface and the radius of this semicircle is a measure of Rct [53,54]. A larger radius corresponds to a larger Rct, indicating that the electron recombination in the DSSC are strongly reduced and therefore a smaller dark current and a higher photovoltage well be achieved [55]. The fitted Rct increased in the order of QX01 (52.9 U) < QX03 (61.8 U) < QX02 (84.5 U) < QX04 (130.1 U). This trend appeared to be consistent with the order of Voc of QX01 (729 mV) < QX03 (738 mV) < QX02 (745 mV) < QX04 (757 mV). This clearly indicates that the longer alkyl units in secondary group are helpful for the suppression of dark current and improve the photovoltage. Besides, the peak frequency (f) at lower frequency region is related to the charge recombination rate and the electron lifetime (t) can be calculated using t ¼ 1/(2pf) [56,57]. Generally, longer electron lifetime indicates increased recombination resistance between the injected electrons and the electrolyte, which are consistent with the Voc values [58]. For the four devices, the peak frequency at lower frequency region in Bode phase plots decreased in the order of QX01 > QX03 > QX02 > QX04, and the electron lifetime was enhanced in reverse with the calculated values of QX01 (35.6 ms) < QX03 (40.0 ms) < QX02 (44.9 ms) < QX04 (79.8 ms). Therefore, the order of Voc of the four dyes could be further explained.
(a) 100
60 40 20 0
QX01 QX02 QX03 QX04
60
90
120 Z' / Ω
150
180
60
QX01 QX02 QX03 QX04
40 Phase / o
80 IPCE (%)
30
(b) 50
100
40
30 20 10
20 0
QX01 QX02 QX03 QX04
80
Z'' / Ω
46
400
500
600
700
800
Wavelength / nm Fig. 6. IPCE spectra of the DSSCs based on the four dyes.
0 -1 10
0
10
1
2
3
10 10 10 Frequence / Hz
4
10
5
10
Fig. 7. (a) Nyquist plots and (b) Bode phase plots for DSSCs based on the four dyes measured in the dark under 0.70 V bias.
X. Qian et al. / Journal of Power Sources 319 (2016) 39e47
4. Conclusions Four indolo[3,2-b]carbazole-based multi-donorepeacceptor type dyes have been synthesized as photosensitizers for DSSCs. The four dyes are composed of an indolo[3,2-b]carbazole skeleton as the main donor with a couple of secondary donors directly attached on its side. Different secondary donors such as ethylbenzene, N,Ndiethylaniline, ethyloxylbenzene, and octyloxylbenzene have been chosen to tune the photoelectric properties of these dyes. With N,N-diethylaniline as the secondary donor, QX02 exhibited a broader absorption region, a broader IPCE response region, and significantly improved IPCE values, which ensured a good lightharvesting ability and a high short-circuit current density of 15.2 mA cm2. Finally, QX02-based cell achieved a high efficiency of 8.09% under AM 1.5G (100 mW cm2) irradiation, demonstrating the proper molecular modification of indolo[3,2-b]carbazole-based dyes with strong secondary donor is a promising method to construct effective organic dyes for highly efficient DSSCs. Acknowledgement We are grateful to the National Natural Science Foundation of China (No: 21376054) for their generous financial support. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.04.043. References [1] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem. Rev. 110 (2010) 6595e6663. [2] G. Calogero, A. Bartolotta, G.D. Marco, A.D. Carlo, F. Bonaccorso, Chem. Soc. Rev. 44 (2015) 3244e3294. [3] F. Bella, C. Gerbaldi, C. Barolo, M. Gr€ atzel, Chem. Soc. Rev. 44 (2015) 3431e3473. [4] M. Liang, J. Chen, Chem. Soc. Rev. 42 (2013) 3453e3488. [5] T. Higashino, H. Imahori, Dalton Trans. 44 (2015) 448e463. [6] M. Zhang, Y. Wang, M. Xu, W. Ma, R. Li, P. Wang, Energy Environ. Sci. 6 (2013) 2944e2949. [7] A. Baheti, K.R.J. Thomas, C.-P. Lee, C.-T. Li, K.-C. Ho, J. Mater. Chem. A 2 (2014) 5766e5779. [8] T. Higashino, Y. Fujimori, K. Sugiura, Y. Tsuji, S. Ito, H. Imahori, Angew. Chem. Int. Ed. 54 (2015) 9052e9056. [9] J. Zhao, X. Yang, M. Cheng, S. Li, L. Sun, ACS Appl. Mater. Interfaces 5 (2013) 5227e5231. [10] X. Yang, J. Zhao, L. Wang, J. Tian, L. Sun, RSC Adv. 4 (2014) 24377e24383. [11] N. Cai, Y. Wang, M. Xu, Y. Fan, R. Li, M. Zhang, P. Wang, Adv. Funct. Mater. 23 (2013) 1846e1854. [12] C.-H. Wu, M.-C. Chen, P.-C. Su, H.-H. Kuo, C.-L. Wang, C.-Y. Lu, C.-H. Tsai, C.C. Wu, C.-Y. Lin, J. Mater. Chem. A 2 (2014) 991e999. [13] C.-L. Wang, J.-Y. Hu, C.-H. Wu, H.-H. Kuo, Y.-C. Chang, Z.-J. Lan, H.-P. Wu, E. Wei-Guang Diau, C.-Y. Lin, Energy Environ. Sci. 7 (2014) 1392e1396. [14] C.-L. Mai, T. Moehl, Y. Kim, F.-Y. Ho, P. Comte, P.-C. Su, C.-W. Hsu, F. Giordano, A. Yella, S.M. Zakeeruddin, C.-Y. Yeh, M. Gratzel, RSC Adv. 4 (2014) 35251e35257. [15] N. Masi Reddy, T.-Y. Pan, Y. Christu Rajan, B.-C. Guo, C.-M. Lan, E. Wei-Guang Diau, C.-Y. Yeh, Phys. Chem. Chem. Phys. 15 (2013) 8409e8415. [16] H. Hayashi, T. Higashino, Y. Kinjo, Y. Fujimori, K. Kurotobi, P. Chabera, V. Sundstrom, S. Isoda, H. Imahori, ACS Appl. Mater. Interfaces 7 (2015) 18689e18696. [17] H. Hayashi, A.S. Touchy, Y. Kinjo, K. Kurotobi, Y. Toude, S. Ito, H. Saarenpaa, N.V. Tkachenko, H. Lemmetyinen, H. Imahori, ChemsusChem 6 (2013) 508e517. [18] Q. Xia, M. Liang, Y. Tan, W. Gao, L. Ouyang, G. Ge, Z. Sun, S. Xue, Org. Electron 17 (2015) 285e294. [19] G. Wu, F. Kong, J. Li, W. Chen, X. Fang, C. Zhang, Q. Chen, X. Zhang, S. Dai, Dyes Pigm. 99 (2013) 653e660.
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