CHEMPHYSCHEM ARTICLES DOI: 10.1002/cphc.201402474
Triphenylamine Groups Improve Blocking Behavior of Phenoxazine Dyes in Cobalt-Electrolyte-Based DyeSensitized Solar Cells Yan Hao,[a] Haining Tian,[b] Jiayan Cong,[b] Wenxing Yang,[a] Ilkay Bora,[b, c] Licheng Sun,[b, d] Gerrit Boschloo,*[a] and Anders Hagfeldt[a] Novel phenoxazine dyes are successfully introduced as sensitizers into dye-sensitized solar cells (DSCs) with cobalt-based electrolyte. In sensitizers with triphenylamine (TPA) groups recombination from electrons in the TiO2 conduction band to the cobalt(III) species is suppressed. The effect of the steric properties of the phenoxazine sensitizers on the overall device performance and on recombination and regeneration processes is compared. Optimized DSCs sensitized with IB2 having
two TPA groups in combination with tris(2,2’-bipyridyl) cobalt(II/III) yield efficiencies of 6.3 %, similar to that of IB3, which is equipped with mutiple alkoxy groups. TH310 with only one TPA group gives lower efficiency and open circuit voltage, while IB1 without TPA groups performs even worse. These results demonstrate that both TPA groups on the IB2 are needed for an efficient blocking effect. These results reveal a possible new role for TPA units in DSC sensitizer design.
1. Introduction Redox couples based on cobalt complexes are considered as promising alternative redox mediators for dye-sensitized solar cells (DSCs), since Hagfeldt and co-workers reported that a solar cell sensitized with triphenylamine-based organic dye D35 in combination with tris(2,2’-bipyridyl)cobalt(II/III) ([Co(bpy)3]2 + /3 + ) yielded overall conversion efficiencies of 6.7 %.[1] The best photovoltaic performance to date for DSCs employing cobalt complexes is 13.0 %, obtained using a carefully tailored zinc–porphyrin dye.[2] Cobalt complexes have attracted much interest in DSC research, since they offer the prospect of higher output voltage, show only weak visible light absorption and are less aggressive towards metals than iodine. However, the performance of
[a] Dr. Y. Hao, W. Yang, Dr. G. Boschloo, Prof. A. Hagfeldt Department of Chemistry-ngstrçm Laboratory Center of Molecular Devices Uppsala University Box 523, 751 20 Uppsala (Sweden) E-mail: [email protected] [b] Dr. H. Tian, Dr. J. Cong, I. Bora, Prof. L. Sun School of Chemical Science and Engineering Center of Molecular Devices, Department of Chemistry Royal Institute of Technology (KTH) Teknikringen 30,100 44 Stockholm (Sweden) [c] I. Bora Institute of Organic Chemistry University of Stuttgart 70049 Stuttgart (Germany) [d] Prof. L. Sun State Key Laboratory of Fine Chemicals DUT-KTH Joint Education and Research Center for Molecular Devices Dalian University of Technology (DUT) 2#Linggong Road, 116024 Dalian (China) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201402474.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
cobalt redox mediators in DSCs is limited by the rapid recombination from electrons in the TiO2 conduction band to cobalt(III) species in the electrolyte.[3] A successful strategy to suppress this recombination is to equip the dyes in the adsorbed monolayer with suitable steric groups, so that the monolayer blocks close approach of the CoIII species to the TiO2 surface.[1] In the successful D35 dye two o,p-dibutoxyphenyl groups are introduced to the triphenylamine donor part. Wang and coworkers[4] reported a metal-free d-p-A photosensitizer C218, which not only contains two hexyloxy chains in the donor part, but also has two long carbon chains in the pi-conjugated linker part. The combination of the dye C218 and Co(phen)32 + /3 + gave a power conversion efficiency of 8.3 % determined at 100 mW cm 2 simulated AM 1.5 conditions. A similar dye, C229, with a different pi-conjugated linker, provided an efficiency of 9.4 % together with the same Co-complex redox system.[5] Bach and co-workers used the carbazole dye MK-2 containing four hexyl groups on conjugated linker part for cobalt based DSCs, giving an efficiency of 8.4 % at 100 mW cm 2 simulated AM 1.5 conditions.[6] Grtzel’s group reported the organic dye Y123, containing two o,p-dihexoxyphenyl groups on the triphenylamine donor, which is very similar to the structure of the donor in the sensitizer D35. The combination between Y123 and Co(bpy)32 + /3 + yielded DSCs with an efficiency of 9.6 %.[7] Later, based on a combination of the YD2-o-C8 dye and the Y123, Grtzel’s group achieved a record efficiency of 12.3 %. YD2-o-C8 contains two alkyl chains and four alkoxy chains on the porphyrin ring.[8] Until now, all the successful organic dyes for DSC with cobalt based electrolyte are equipped with multiple long alkoxy chains. Sensitizers with a phenoxazine (POZ) electron-donating group have been applied in DSCs research field, both for iodide/triode electrolyte based liquid and solid state solar cells ChemPhysChem 0000, 00, 1 – 9
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www.chemphyschem.org 2. Results and Discussion
Figure 1. Chemical structure of sensitizers IB1, TH310, IB2 and IB3.
by the Sun group, and gave quite promising efficiencies.[9] The POZ unit displays a strong electron-donating ability with a formal reduction potential of 0.88 V vs. NHE, which can be compared with the well-known triphenylamine donor at 1.04 V vs NHE.[10] This implies that sensitizers with the POZ donor part match better to redox mediators that are currently in use in DSC and may improve red absorption for achieving more efficient solar cells. Another important feature of POZ is the easy structural modification on both sides of the core to tune the properties of the dye. Here, we will report on phenoxazine dyes used in DSCs with cobalt redox couples, and discuss the effect of structural modifications on the photovoltaic properties. A new finding is that triphenylamine (TPA) groups on POZ based sensitizers can provide suitable steric properties necessary for successful use in cobalt-based electrolytes. The TPA moiety is often used in sensitizers for DSCs, because of their electron-donating properties and their potential for low-cost production. Successful TPA-based dyes for use with cobaltbased electrolytes are equipped with alkoxy chains to provide electron-blocking effect.[11] In this paper, four organic sensitizers with phenoxazine donor, thiophene conjugated linker and cyanoacrylic acid as acceptor/binding group (IB1, TH310, IB2 and IB3, see Figure 1) are compared as sensitizer in solar cell devices employing cobalt bipyridine as redox mediator. The IB1 dye is the smallest structure with one octyl group on the phenoxazine part, but no further blocking moieties. IB2 is additionally equipped with two TPA groups, one on the donor part and another one on the linker part, while IB3 has two o,pdibutoxyphenyl group as steric groups instead.[9b] The new dye TH310, which has only one TPA group on the phenoxazine donor part, was specially designed for this paper to investigate in which location the TPA group plays an important role in blocking electron recombination.
The absorption spectra of IB1, TH310, IB2 and IB3 dyes in tertbutanol/acetonitrile (1:1 v/v) solution and on TiO2 films are given in Figure 2. The corresponding photophysical data are collected in Table 1. IB2 dye has a slightly blue-shifted absorption peaks (lmax) and slightly lower extinction coefficient compared with IB1 and TH310 dyes in the tert-butanol/acetonitrile (1:1 v/v) solution, which has been attributed to the relatively weak P-conjugated system.[9b] The absorption spectra of the dyes on TiO2 films showed some broadening compared the solution spectra, as is frequently observed for sensitizers. The IB2sensitized film showed a lower absorbance than the other sensitized films. Dye desorption experiments confirmed that IB2 gives the lowest dye loading on the TiO2 film, followed by IB3. This is directly related to the additional bulky groups in these
Figure 2. UV/Vis absorption spectra of dyes IB1, TH310, IB2 and IB3 in tertbutanol/acetonitrile (1:1 v/v) solution (a) and on TiO2 flim (b).
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Table 1. Spectroscopic data and the amount adsorbed on TiO2 film of dyes IB1, TH310, IB2 and IB3.
lmax [nm]
Absorption E [M 1 cm 1]
lmax on TiO2 [nm]
Dye load ( 10 7) [mol cm 2]
495 492 480 496
17100 18300 15700 16700
470 467 478 468
1.52 1.86 0.88 1.33
Dye IB1 TH310 IB2 IB3
dyes. TH310 dye shows the highest adsorbed amount on the TiO2 film, 1.86 10 7 mol cm 2, about double that of IB2. The photovoltaic performances of phenoxazine-sensitized solar cells with cobalt-based electrolyte are summarized in Figure 3 and Table 2. The current-voltage curves clearly show
Figure 3. Current density versus applied potential curves under 100 mW cm 2 AM1.5G light intensity and in the dark for DSCs sensitized with IB1, TH310, IB2 and IB3. The electrolyte was 0.22 m Co(bpy)3(PF6)2, 0.05 m Co(bpy)3(PF6)3, 0.2 m TBP, and 0.1 m LiClO4 in acetonitrile.
Table 2. J–V characteristics at various simulated sunlight intensities with AM1.5G spectral distribution for cells with different dyes. Dye
Intensity [mW cm 2]
Voc [mV]
Jsc [mA cm 2]
ff
h [%]
IB1
100 46 11.4 100 46 11.4 100 46 11.4 100 46 11.4 100
615 585 525 760 770 735 880 860 810 890 870 815 855
5.64 2.91 0.66 9.01 5.24 1.35 10.25 5.36 1.25 10.36 5.34 1.24 11.55
0.64 0.66 0.65 0.69 0.69 0.70 0.70 0.72 0.73 0.69 0.73 0.75 0.71
2.2 2.4 2.0 4.7 6.1 6.1 6.3 7.3 6.5 6.4 7.4 6.7 7.0
TH310
IB2
IB3
LEG4
Jsc (IPCE)[a] [mA cm 2] 5.50
9.61
10.15
10.29
11.45
[a] Jsc calculated from integration of the IPCE spectrum with respect to the AM1.5G photon flux.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
that IB2 and IB3 give much better performance than IB1 and TH310. The power conversion efficiency at one sun was 6.3 % and 6.4 % for IB2 and IB3, respectively, with a photocurrent density (Jsc) of about 10.3 mA cm 2 for both. Despite very similar optical absorption properties, IB1 had an efficiency of only 2.2 % and a Jsc of 5.6 mA cm 2. Clearly, in DSCs with IB1 severe recombination losses occur. This is also evident from the much lower Voc for IB1, 615 mV, compared to 860 mV and 870 mV for IB2 and IB3 respectively. Although much higher Voc was achieved by TH310 compared with IB1, 760 mV, it is significantly less than that of IB2 and IB3. These results demonstrate a significant blocking effect that can be attributed to TPA groups. More specifically, the TPA groups suppress recombination of electrons in TiO2 to with oxidized cobalt species in the electrolyte by steric hindrance. Furthermore, in IB2 both TPA groups are involved in this blocking. Overall, IB2 and IB3 dyes showed a much more improved Voc in the cobalt electrolyte system compared to the iodide/triiodide system.[9b] The performance of the phenoxazine dyes in DSC with cobalt-based electrolyte is quite impressive due to higher Voc, which is ascribed to more positive redox potential of cobalt redox couple. To assess the solar cell performance further, a comparison was made with a state-of-the-art triphenylamine based sensitizer LEG4, see Table 1 (LEG4 possesses a structure similar to Y123, see supporting information). The device with LEG4 gave a power conversion efficiency of 7.0 %, with a Jsc of 11.55 mA cm 2. The better performance can be ascribed to its broader optical absorption and higher extinction coefficient compared to the phenoxazine dyes.[12] Interestingly, higher Voc values by about 30 mV were obtained for IB2 and IB3 compared to LEG4. The high Voc shows the excellent blocking effect obtained by IB2. Further, DSCs with IB2 and IB3 possess efficiencies close to that of LEG4, which confirms the potential of phenoxazine dyes for cobalt electrolyte based DSCs. By further optimizing the dye structure to increase light harvesting, phenoxazine dyes have the potential to outperform the stateof-the-art dyes LEG4 and Y123 in DSCs with cobalt-based electrolyte. Incident-photon-to-current conversion efficiency (IPCE) spectra were recorded for DSCs sensitized with the organic dyes IB1, TH310, IB2 and IB3, see Figure 4. The DSCs based on IB1 show lowest IPCE values and a relatively narrower IPCE spectrum than the other dyes, in accordance with the lowest photocurrent found. IB2 and IB3-based DSCs gave very similar IPCE spectra, with a maximum IPCE of about 75 %, which was significantly higher than that of TH310 (65 %). The best red IPCE response was found for TH310, which is related its high dye load. In general, the IPCE of a DSC is determined by light harvesting, electron injection efficiency, regeneration efficiency and charge collection efficiency. From optical and electrochemical measurements (see Tables 1 and 3), it was found that all four phenoxazine dyes have similar absorption spectrum and energy levels. The cause of the lower IPCE response for IB1 must be significant electron recombination to the electrolyte during electron transport in the mesoporous TiO2. TH310, possesses a broader IPCE spectrum with higher values than IB1, but maximum values are lower than those of IB2 and IB3. This ChemPhysChem 0000, 00, 1 – 9
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Figure 4. Spectra of incident photon to current efficiency (IPCE) for DSCs sensitized with IB1, TH310, IB2 and IB3.
result shows that the TPA group on the POZ donor part can block electron recombination to some extent, but with an additional TPA group on the linker part (as in IB2) recombination is more efficiently suppressed. Dark current measurements give a direct indication for the recombination process between electrons in TiO2 and cobalt(III) species in the electrolyte. Results in Figure 3 clearly show that DSCs based on IB2 and IB3 give much smaller dark currents than TH310, while IB1 gives the largest dark current. This confirms further that the TPA group on linker part of IB2 plays the same role as o,p-dibutoxyphenyl group in IB3, decreasing recombination rate between electrons in TiO2 and CoIII species in the electrolyte. The electron recombination kinetics were further investigated studying the photovoltage response of the DSCs to a small amplitude light modulation. The electron lifetime was measured under open-circuit conditions for DSCs based on IB1, TH310, IB2 and IB3, and is plotted as function of quasi-Fermi level of the TiO2 in Figure 5. Under conditions of equal internal potential in the TiO2, the electron concentration in the TiO2 will be equal, as no band edge shifts or changes in the trap distri-
Figure 5. Electron lifetime as function of of the quasi-Fermi level of the TiO2 under open circuit conditions for DSCs sensitized with IB1, TH310, IB2 and IB3 employing cobalt electrolyte.
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www.chemphyschem.org bution are expected. In accordance with the current-voltage characteristics, the electron lifetime of DSCs based on IB2 and IB3 is quite similar, much longer than that of LEG4 and TH310, while IB1 has the shortest electron lifetime. Electron lifetimes in DSCs usually only reflect the electron recombination with oxidized form of the redox species in the electrolyte, but in case of slow regeneration of the oxidized dye, also the effect of electron recombination to the oxidized dye will be included in the measured lifetime. As will be shown later, dye regeneration efficiencies of all dyes are rather high, from 70 to 95 %, and recombination of electrons in TiO2 to the oxidized dye is not expected to account for differences in the measured electron lifetimes. The observed trend of increase in recombination for TH310 and IB1 is in agreement with the changes of Voc for different dyes, which further demonstrate that the TPA groups on linker part of IB2 molecules, like the o,p-dibutoxyphenyl groups on IB3, effectively slow down recombination by preventing close contact of the oxidized species in the electrolyte to the TiO2 surface. The dye regeneration reaction in the solar cell, electron transfer between the oxidized dye and the redox couple is expected to depend on the structure of the dyes, since the size of the electron donor and acceptor can affect the reorganization energy and the electronic coupling.[13] The regeneration of the dyes by the cobalt complexes was investigated with nslaser spectroscopy. Results of an initial photoinduced absorption spectroscopy (PIA), performed under conditions similar to the operational conditions of DSCs (white light intensity ca. 100 W m 2) are shown in the Supporting Information. The spectrum of the oxidized dye molecules was determined by using spectroelectrochemistry, see Figure 6 a. A probe wavelength of 880 nm was selected for transient absorption measurements, as all oxidized dye molecules show a significant absorbance there. In contact with inert electrolyte, the phenoxazine dye-sensitized TiO2 electrodes display relatively slow recombination kinetics between electrons in TiO2 and the oxidized dye molecules. IB3 displays significantly longer recombination halftime (t1/2 ca. 1 ms) compared to the other dyes (about 0.5 ms), see Figure 6 a and Table 3. The regeneration kinetics of phenoxazine dye-sensitized TiO2 electrodes in cobalt bipyridine show more significant differences between the different dyes. This is interesting as the driving force for regeneration is nearly constant for all dyes investigated here. IB1 shows the shortest regeneration halftime (17 ms), IB2 and IB3 dyes are 3–4 times slower, while TH310 is a factor of ten times slower (168 ms), see Figure 6 a and Table 3. The differences in regeneration times can be attributed to the differences in chemical structure, which influences the electron coupling between dye and redox couple. Specifically, the isolating steric groups in TH310, IB2 and IB3 may hinder close approach of the cobalt(II) mediator. The more dense packing of TH310 may explain its longest regeneration halftime for the same reason. Regeneration efficiencies were calculated as described in our previous work.[14] As conditions in the ns-laser experiments differ from that under full sun solar cell working conditions, regeneration efficiencies determined here are approximate. ReChemPhysChem 0000, 00, 1 – 9
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Figure 6. a) Oxidized spectra of four different dyes on TiO2 obtained by electrochemical oxidation of the dyes. b) Ns-laser induced transient absorption kinetics of TiO2 sensitized with IB1, TH310, IB2 and IB3 and employing inert electrolyte (black) and cobalt electrolyte (grey).
Table 3. Regeneration halftime and regeneration efficiencies for IB1-, TH310-, IB2- and IB3-sensitized DSCs employing cobalt bipyridine redox couple. t1/2 [ms]
Dye IB1 TH310 IB2 IB3
inert
cobalt
385 539 534 1113
17 168 86 54
Freg
HOMO [V] vs. NHE
0.96 0.70 0.84 0.95
0.90 0.93 0.91 0.88
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generation efficiencies of 84–96 % are determined for the IB1, IB2 and IB3 dyes, which indicates that the kinetics for regeneration of these dyes are sufficiently fast. The slower kinetics for regeneration of the oxidized dye found for the TH310 dye, (halflife 168 ms) lead to a much lower regeneration efficiency, 70 %. This is consistent with the IPCE spectrum found for TH310, which is lower than that of IB2 and IB3. Low regeneration efficiency will decrease IPCE independent of wavelength. Shorter electron diffusion length, on the other hand, will lead to a decreased IPCE values and a narrower spectrum IPCE, as was found for IB1. ChemPhysChem 0000, 00, 1 – 9
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3. Conclusions In conclusion, application of suitable bulky groups to the phenoxazine core results in efficient sensitizers for cobalt electrolyte-based dye-sensitized solar cells. The bulky groups suppress the recombination of Scheme 1. electrons in the TiO2 to CoIII species in the electrolyte. As shown before o,p-dibutoxyphenyl groups are well suited for this function, but, interestingly, also bare triphenylamine groups added to the phenoxazine core function well. By careful tuning of the chemical structure to extend the light absorption range, it will be feasible to further develop phenoxazine dyes Scheme 2. for more efficient DSCs. The use of triphenylamine groups as recombination-blocking moieties instead of alkoxy groups may decrease the manufacturing cost of sensitizers, because of the industrialized process of triphenylamine manufacturing.
Experimental Section All chemicals were purchased from Sigma Aldrich unless otherwise noted. IB1, IB2 and IB3 dyes were available from a previous study.[9b] The new sensitizer TH310 was synthesized by following the process shown in Scheme 1. Compound 1 was obtained as a byproduct of synthesis of compound 5 in our previous publication.[8b] A solution of 1 (20 mg, 0.04 mmol), triphenylamine-4-boronic acid (24 mg, 0.08 mmol), 160 mL 0.5 m K2CO3 aqeuous and chloro(2-dicyclohexylphosphino2’,4’,6’-triisopropyl-1,1’-biphenyl)[2-(2’-amino-1,1’-biphenyl)]palladium(II) (X-Phos Pd G2) (1.6 mg, 0.002 mmol) in THF (1 mL) was heated to 40 8C for 2 h under constant stirring. The reaction mixture was allowed to cool and H2O was added. The mixture was extracted with CH2Cl2 twice and the combined organic fractions were concentrated in vacuo. Flash chromatography over silica gel by CH2Cl2/pentane (2/1; v/v) gave crude 2, which is further purified by HPLC using CH2Cl2/pentane (1/1; v/v) with 1 % formic acid as eluent to give 11 mg pure product (yield 42 %) as yellow powder. 1 H NMR (500 MHz, [D6]Acetone): d ppm = 9.88 (s, 1 H), 7.89 (d, J = 3.96 Hz, 1 H), 7.50–7.53 (m, 3 H), 7.31 (t, J1 = 8.18, J2 = 7.61, 4 H), 7.28 (dd, J1 = 2.06, J2 = 8.36, 1 H), 7.16 (dd, J1 = 1.96, J2 = 8.32, 1 H), 7.05– 7.09 (m, 8 H), 7.03 (d, J = 2.08 Hz, 1 H), 6.95 (d, J = 2.02 Hz, 1 H), 6.75–6.79 (m, 2 H), 3.67 (t, 2 H), 1.68–1.75 (m, 2 H), 1.46–1.53 (m, 2 H), 1.30–1.45 (m, 8 H), 0.88 (t, 3 H). A solution of 2 (11 mg, 0.017 mmol), cyanoacetic acid (6 mg, 0.068 mmol) and 2 drops piperidine was refluxed in 5 mL MeCN/ CHCl3 (3/2; v/v) for 4 h under nitrogen protection. The solution was allowed to cool before HCl (0.5 m, 10 mL) was added. The product was extracted with CH2Cl2 (10 mL) twice. The combined organic phase was dried over Na2SO4 and the solvent removed in vacuo. Flash chromatography over silica gel using gradient elution CH2Cl2, MeOH/CH2Cl2 (20/1; v/v) gave product TH310 (10 mg, 83 %) 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
as dark powder (Scheme 2). 1H NMR (500 MHz, [D6]Acetone): d ppm = 8.37 (s, 1 H), 7.89 (d, J = 4.04 Hz, 1 H), 7.50–7.53 (m, 3 H), 7.31 (t, J1 = 8.13, J2 = 7.63, 4 H), 7.26 (dd, J1 = 2.00, J2 = 8.38, 1 H), 7.14 (dd, J1 = 1.87, J2 = 8.34, 1 H), 7.04–7.09 (m, 8 H), 7.02 (d, J = 2.02 Hz, 1 H), 6.94 (d, J = 1.91 Hz, 1 H), 6.75 (m, 2 H), 3.65 (t, 2 H), 1.69–1.72 (m, 2 H), 1.48–1.50 (m, 2 H), 1.30–1.45 (m, 8 H), 0.88 (t, 3 H). 13C NMR (126 MHz, [D6]Acetone): d ppm = 164.2, 154.5, 147.7, 147.9, 147.4, 145.9, 145.7, 141.7, 135.6, 134.8, 134.6. 132.1, 130.4, 127.7, 126.4, 125.3, 124.7, 124.3, 124.1, 123.6, 122.7, 117.1, 114.0, 113.6, 113.4, 113.2, 44.6, 32.7, 27.5, 25.9, 23.4, 14.5 ppm; MS (ESI): Positive ion: m/z = 716.3 [M + H] + , calculated 716.3.
Solar Cell Preparation TiO2 electrodes were prepared on fluorine-doped tin oxide (FTO) glass substrates (Pilkington, TEC15). The glass substrates were cleaned in an ultrasonic bath using (in order) detergent, water, ethanol and acetone, each one hour. The glass substrates were pretreated by immersion in 40 mm aqueous TiCl4 solution at 70 8C for 90 min, washed with water and dried. The photoelectrodes with a size of 0.5 0.5 cm2 were prepared by screen printing, using TiO2 paste (Dyesol DSL 20 NRD-T) as transparent layer and scattering paste (PST-400C, JGC Catalysts and Chemical LTD, gratefully received from JGC Catalysts and Chemical LTD) as scattering layer on top of transparent layer. The thickness of the photoelectrodes was measured with a profilometer (Veeco Dektak 3), about 4 mm and 4 mm was obtained for the transparent layers and scattering layers respectively. The electrodes were sintered in an oven (Nabertherm Controller P320) in an air atmosphere using a temperature gradient program with four levels at 180 8C (10 min), 320 8C (10 min), 390 8C (10 min), and 500 8C (60 min). Prior to use, the electrodes were given a post-treatment by immersing in 40 mm aqueous TiCl4 solution at 70 8C for 30 min, followed by another heating step (500 8C, 60 min). When the temperature cooled to about 90 8C, the electrodes were immersed in a dye bath containing 0.2 mm IB1, TH310, IB2 and IB3 in tert-butanol:acetonitrile = 1:1 and left overnight. The films were then rinsed in ethanol to remove excess dye. ChemPhysChem 0000, 00, 1 – 9
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CHEMPHYSCHEM ARTICLES Solar cells were assembled, using a 30 mm thick thermoplastic Surlyn frame with a platinized counter electrode (TEC8), which was prepared by depositing 10 mL 5 mm H2PtCl6 solution in ethanol to the glass substrate followed by heating in air at 400 8C for 30 min. An electrolyte solution was then injected through one hole predrilled counter electrode and the hole was sealed with thermoplastic Surlyn covers and aluminum foil. The electrolyte consisted of 0.22 m Co(bpy)3(PF6)2, 0.05 m Co(bpy)3(PF6)3, 0.1 m LiClO4, and 0.2 m 4-tert butylpyridine (TBP) in acetonitrile.
Solar Cell Characterization Current–voltage (IV) characteristics were measured using a Keithley 2400 source/meter and a Newport solar simulator (model 91160) giving light with AM 1.5 G spectral distribution, which was calibrated using a certified reference solar cell (Fraunhofer ISE) to an intensity of 1000 W m 2. A black mask with the same area (0.5 0.5 cm2) to the active solar cell area was applied on top of the cell. Photovoltaic measurements at low light intensities were performed using neutral density filters. Incident photon to current conversion efficiency (IPCE) spectra were recorded using a computer-controlled setup consisting of a xenon light source (Spectral Products ASB-XE-175), a monochromator (Spectral Products CM110) and a Keithley 2700 multimeter, calibrated using a certified reference solar cell (Fraunhofer ISE). Electron lifetime was performed using a white LED (Luxeon Star 1W) as the light source. Voltage traces were recorded with a 16-bit resolution digital acquisition board (National Instruments) in combination with a current amplifier (Stanford Research Systems SR570) and a custom-made system using electromagnetic switches. Lifetimes were determined by monitoring photovoltage transients at different light intensities upon applying a small square wave modulation to the base light intensity. The photovoltaic responses were fitted using first-order kinetics to obtain time constants.[15]
Optical Spectroscopy The UV/Vis absorption spectra of the dye-loaded transparent film (2 mm, sensitized 1 h in dye bath) and the dye solution (0.02 mm, in tert-butanol/acetonitrile (1:1, v/v) in 1 cm cuvette) were recorded with an Ocean Optics HR2000 spectrometer and a Micropack DH2000-BAL light-source. The fluorescence spectra of dye solution were recorded on a Fluorolog 3–222 emission spectrometer from Jobin–Yvon by using the same concentration as the UV/Vis measurement.
Electrochemical Measurements Electrochemical experiments were performed with a CH Instruments electrochemical workstation (model 660 A) using a conventional three-electrode electrochemical cell. Measurements were obtained with dye sensitized TiO2 film by using 0.1 m lithium perchlorate (LiClO4) in acetonitrile as supporting electrolyte. Dye sensitized TiO2 electrode was used as the working electrode, a graphite electrode served as a counter electrode. The reference electrode was in all cases aqueous Ag/AgCl/3 m NaCl; a salt bridge electrolyte was interposed between working and reference electrode, containing 0.1 m LiClO4 in acetonitrile, and the scan rate used was 50 mV s 1. All redox potentials were calibrated versus a normal hydrogen electrode (NHE) by the addition of ferrocene as an internal standard taking E0(Fc+/Fc) = 0.63 V vs. NHE.[11b] 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org Dye Coverage Measurements Dye loading were estimated from dye desorption measurements. TiO2 films with two layers of Dyesol DSL 18 NR-T paste, (diluted by 40 wt % polymer mixture, containing 90 wt % terpineol and 10 wt % ethylcellulose) with a thickness of about 4 mm were immersed in dye baths with a concentration of 0.2 mm IB1, TH310, IB2 and IB3 in tert-butanol/acetonitrile (1:1, v/v). After sensitization the samples were put in alkaline solution consisting of 0.05 m TBAOH in CH2Cl2 to desorb the dyes. Next, sulfuric acid was added into the solution in order to reprotonate the dye and the UV/Vis spectrum of the solutions in a 1 cm cuvette was measured, from which the dye coverage was estimated using the Lambert–Beer law.
Spectroelectrochemical Measurements Spectroelectrochemistry was performed on a CH Instruments 660 potentiostat with a three-electrode setup. At the same time as carrying out the electrical measurements, UV/Vis spectra were recorded. All four different dyes-coated mesoporous TiO2 films were used as the working electrodes. A graphite electrode was used as the counter electrode and a Ag/Ag + electrode as a pseudo reference electrode. The electrolyte solution was 0.1 m lithium perchlorate (LiClO4) in acetonitrile. Cyclic voltammetry of a TiO2 film was carried out in the LiClO4 supporting electrolyte with a scan rate of 50 mv s 1.
Nanosecond Transient Absorption Spectroscopy (TAS) Dye regeneration by cobalt-based redox mediator and electron recombination to the oxidized dye were monitored using a Nd:YAG laser (Continuum Surelight II, repetition rate: 10 Hz, pulse width: 10 ns) in combination with an Optical Parametrical Oscillator (Continuum Surelite OPO Plus). Pump pulses of 530 nm for IB1, TH310, IB2 and IB3 was used. The intensity of the laser output was attenuated to 0.3 mJ cm 2 in combination with a 5 % transmittance filter (Thorlabs). Probe light was provided by a near-infrared LED (Osram SHF 484, lmax 880 nm, fwhm 80 nm), and kinetic traces were measured with an amplified Si-photodiode (Thorlabs PDA10AEC). In order to avoid stray light from the laser a cutoff filter (RG 715) was used in front of the detector. PIA spectra were measured for all dyes adsorbed on TiO2 to determine the spectrum of the oxidized dyes. Based on these spectra, 880 nm was chosen as probe light in ns-TAS for measurements. The electrolyte consisted of 0.22 m Co(bpy)3(PF6)2, 0.05 m Co(bpy)3(PF6)3, 0.1 m LiClO4 and 0.2 m TBP in acetonitrile. Recombination dynamics were measured with a solution of 0.2 m TBP and 0.1 m LiClO4 in acetonitrile. Measurements were performed on screen printed TiO2 films of 3 mm thickness which were sensitized according to already mentioned procedure. A sealed sample for recombination and regeneration measurements was applied.
Acknowledgements We gratefully acknowledge the Swedish Energy Agency, the Swedish Research Council (VR), the STandUP for Energy program, the Knut and Alice Wallenberg Foundation and Sony Deutschland GmbH for financial support. Keywords: blocking effect · cobalt redox mediator · dyesensitized solar cells · phenoxazine dyes · triphenylamine
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CHEMPHYSCHEM ARTICLES [1] S. M. Feldt, E. A. Gibson, E. Gabrielsson, L. Sun, G. Boschloo, A. Hagfeldt, J. Am. Chem. Soc. 2010, 132, 16714 – 16724. [2] S. Mathew, A. Yella, P. Gap, R. Humphry-Baker, B. F. E. Curchod, N. Ashari-Astani, I. Tavernelli, U. Rothlisberger, M. K. Nazeeruddin, M. Grtzel, Nat. Chem. 2014, 6, 242 – 247. [3] a) S. Nakade, Y. Makimoto, W. Kubo, T. Kitamura, Y. Wada, S. Yanagida, The Journal of Physical Chemistry B 2005, 109, 3488 – 3493; b) T. W. Hamann, B. S. Brunschwig, N. S. Lewis, The Journal of Physical Chemistry B 2006, 110, 25514 – 25520; c) J. J. Nelson, T. J. Amick, C. M. Elliott, The Journal of Physical Chemistry C 2008, 112, 18255 – 18263; d) M. Liberatore, L. Burtone, T. M. Brown, A. Reale, A. Di Carlo, F. Decker, S. Caramori, C. A. Bignozzi, Applied Physics Letters 2009, 94, 173113. [4] D. Zhou, Q. Yu, N. Cai, Y. Bai, Y. Wang, P. Wang, Energy Environ. Sci. 2011, 4, 2030 – 2034. [5] Y. Bai, J. Zhang, D. Zhou, Y. Wang, M. Zhang, P. Wang, J. Am. Chem. Soc. 2011, 133, 11442 – 11445. [6] M. K. Kashif, J. C. Axelson, N. W. Duffy, C. M. Forsyth, C. J. Chang, J. R. Long, L. Spiccia, U. Bach, J. Am. Chem. Soc. 2012, 134, 16646 – 16653. [7] H. N. Tsao , C. Yi , T. Moehl, J.-H. Yum, S. M. Zakeeruddin, M. K. Nazeeruddin, M. Grtzel, ChemSusChem 2011, 4, 591 – 594. [8] A. Yella, H.-W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Nazeeruddin, E. W.-G. Diau, C.-Y. Yeh, S. M. Zakeeruddin, M. Grtzel, Science 2011, 334, 629 – 634. [9] a) H. Tian, X. Yang, J. Cong, R. Chen, J. Liu, Y. Hao, A. Hagfeldt, L. Sun, Chem. Commun. 2009, 0, 6288 – 6290; b) H. Tian, I. Bora, X. Jiang, E. Ga-
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[10] [11]
[12] [13] [14] [15]
brielsson, K. M. Karlsson, A. Hagfeldt, L. Sun, J. Mater. Chem. 2011, 21, 12462 – 12472; c) H. Tian, X. Yang, R. Chen, A. Hagfeldt, L. Sun, Energy Environ. Sci. 2009, 2, 674 – 677; d) K. M. Karlsson, X. Jiang, S. K. Eriksson, E. Gabrielsson, H. Rensmo, A. Hagfeldt, L. Sun, Chem. Eur. J. 2011, 17, 6415 – 6424. X.-Q. Zhu, Z. Dai, A. Yu, S. Wu, J.-P. Cheng, The Journal of Physical Chemistry B 2008, 112, 11694 – 11707. a) T. Kitamura, M. Ikeda, K. Shigaki, T. Inoue, N. A. Anderson, X. Ai, T. Lian, S. Yanagida, Chem. Mater. 2004, 16, 1806 – 1812; b) D. P. Hagberg, T. Edvinsson, T. Marinado, G. Boschloo, A. Hagfeldt, L. Sun, Chem. Commun. 2006, 0, 2245 – 2247; c) D. P. Hagberg, T. Marinado, K. M. Karlsson, K. Nonomura, P. Qin, G. Boschloo, T. Brinck, A. Hagfeldt, L. Sun, The Journal of Organic Chemistry 2007, 72, 9550 – 9556. E. Gabrielsson, H. Ellis, S. Feldt, H. Tian, G. Boschloo, A. Hagfeldt, L. Sun, Adv. Energy Mater. 2013, 3, 1647 – 1656. S. Cazzanti, S. Caramori, R. Argazzi, C. M. Elliott, C. A. Bignozzi, J. Am. Chem. Soc. 2006, 128, 9996 – 9997. S. M. Feldt, G. Wang, G. Boschloo, A. Hagfeldt, The Journal of Physical Chemistry C 2011, 115, 21500 – 21507. G. Boschloo, L. Hggman, A. Hagfeldt, The Journal of Physical Chemistry B 2006, 110, 13144 – 13150.
Received: July 2, 2014 Published online on && &&, 2014
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ARTICLES Anything but sunblock: Triphenylamine groups give a good blocking effect in phenoxazine dyes for cobalt electrolyte-based dye-sensitized solar cells.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Y. Hao, H. Tian, J. Cong, W. Yang, I. Bora, L. Sun, G. Boschloo,* A. Hagfeldt && – && Triphenylamine Groups Improve Blocking Behavior of Phenoxazine Dyes in Cobalt-Electrolyte-Based DyeSensitized Solar Cells
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Triphenylamine Groups Improve Blocking Behavior of Phenoxazine Dyes in Cobalt-Electrolyte-Based DyeSensitized Solar Cells Yan Hao,[a] Haining Tian,[b] Jiayan Cong,[b] Wenxing Yang,[a] Ilkay Bora,[b, c] Licheng Sun,[b, d] Gerrit Boschloo,*[a] and Anders Hagfeldt[a] Novel phenoxazine dyes are successfully introduced as sensitizers into dye-sensitized solar cells (DSCs) with cobalt-based electrolyte. In sensitizers with triphenylamine (TPA) groups recombination from electrons in the TiO2 conduction band to the cobalt(III) species is suppressed. The effect of the steric properties of the phenoxazine sensitizers on the overall device performance and on recombination and regeneration processes is compared. Optimized DSCs sensitized with IB2 having
two TPA groups in combination with tris(2,2’-bipyridyl) cobalt(II/III) yield efficiencies of 6.3 %, similar to that of IB3, which is equipped with mutiple alkoxy groups. TH310 with only one TPA group gives lower efficiency and open circuit voltage, while IB1 without TPA groups performs even worse. These results demonstrate that both TPA groups on the IB2 are needed for an efficient blocking effect. These results reveal a possible new role for TPA units in DSC sensitizer design.
1. Introduction Redox couples based on cobalt complexes are considered as promising alternative redox mediators for dye-sensitized solar cells (DSCs), since Hagfeldt and co-workers reported that a solar cell sensitized with triphenylamine-based organic dye D35 in combination with tris(2,2’-bipyridyl)cobalt(II/III) ([Co(bpy)3]2 + /3 + ) yielded overall conversion efficiencies of 6.7 %.[1] The best photovoltaic performance to date for DSCs employing cobalt complexes is 13.0 %, obtained using a carefully tailored zinc–porphyrin dye.[2] Cobalt complexes have attracted much interest in DSC research, since they offer the prospect of higher output voltage, show only weak visible light absorption and are less aggressive towards metals than iodine. However, the performance of
[a] Dr. Y. Hao, W. Yang, Dr. G. Boschloo, Prof. A. Hagfeldt Department of Chemistry-ngstrçm Laboratory Center of Molecular Devices Uppsala University Box 523, 751 20 Uppsala (Sweden) E-mail: [email protected] [b] Dr. H. Tian, Dr. J. Cong, I. Bora, Prof. L. Sun School of Chemical Science and Engineering Center of Molecular Devices, Department of Chemistry Royal Institute of Technology (KTH) Teknikringen 30,100 44 Stockholm (Sweden) [c] I. Bora Institute of Organic Chemistry University of Stuttgart 70049 Stuttgart (Germany) [d] Prof. L. Sun State Key Laboratory of Fine Chemicals DUT-KTH Joint Education and Research Center for Molecular Devices Dalian University of Technology (DUT) 2#Linggong Road, 116024 Dalian (China) Supporting Information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201402474.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
cobalt redox mediators in DSCs is limited by the rapid recombination from electrons in the TiO2 conduction band to cobalt(III) species in the electrolyte.[3] A successful strategy to suppress this recombination is to equip the dyes in the adsorbed monolayer with suitable steric groups, so that the monolayer blocks close approach of the CoIII species to the TiO2 surface.[1] In the successful D35 dye two o,p-dibutoxyphenyl groups are introduced to the triphenylamine donor part. Wang and coworkers[4] reported a metal-free d-p-A photosensitizer C218, which not only contains two hexyloxy chains in the donor part, but also has two long carbon chains in the pi-conjugated linker part. The combination of the dye C218 and Co(phen)32 + /3 + gave a power conversion efficiency of 8.3 % determined at 100 mW cm 2 simulated AM 1.5 conditions. A similar dye, C229, with a different pi-conjugated linker, provided an efficiency of 9.4 % together with the same Co-complex redox system.[5] Bach and co-workers used the carbazole dye MK-2 containing four hexyl groups on conjugated linker part for cobalt based DSCs, giving an efficiency of 8.4 % at 100 mW cm 2 simulated AM 1.5 conditions.[6] Grtzel’s group reported the organic dye Y123, containing two o,p-dihexoxyphenyl groups on the triphenylamine donor, which is very similar to the structure of the donor in the sensitizer D35. The combination between Y123 and Co(bpy)32 + /3 + yielded DSCs with an efficiency of 9.6 %.[7] Later, based on a combination of the YD2-o-C8 dye and the Y123, Grtzel’s group achieved a record efficiency of 12.3 %. YD2-o-C8 contains two alkyl chains and four alkoxy chains on the porphyrin ring.[8] Until now, all the successful organic dyes for DSC with cobalt based electrolyte are equipped with multiple long alkoxy chains. Sensitizers with a phenoxazine (POZ) electron-donating group have been applied in DSCs research field, both for iodide/triode electrolyte based liquid and solid state solar cells ChemPhysChem 0000, 00, 1 – 9
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www.chemphyschem.org 2. Results and Discussion
Figure 1. Chemical structure of sensitizers IB1, TH310, IB2 and IB3.
by the Sun group, and gave quite promising efficiencies.[9] The POZ unit displays a strong electron-donating ability with a formal reduction potential of 0.88 V vs. NHE, which can be compared with the well-known triphenylamine donor at 1.04 V vs NHE.[10] This implies that sensitizers with the POZ donor part match better to redox mediators that are currently in use in DSC and may improve red absorption for achieving more efficient solar cells. Another important feature of POZ is the easy structural modification on both sides of the core to tune the properties of the dye. Here, we will report on phenoxazine dyes used in DSCs with cobalt redox couples, and discuss the effect of structural modifications on the photovoltaic properties. A new finding is that triphenylamine (TPA) groups on POZ based sensitizers can provide suitable steric properties necessary for successful use in cobalt-based electrolytes. The TPA moiety is often used in sensitizers for DSCs, because of their electron-donating properties and their potential for low-cost production. Successful TPA-based dyes for use with cobaltbased electrolytes are equipped with alkoxy chains to provide electron-blocking effect.[11] In this paper, four organic sensitizers with phenoxazine donor, thiophene conjugated linker and cyanoacrylic acid as acceptor/binding group (IB1, TH310, IB2 and IB3, see Figure 1) are compared as sensitizer in solar cell devices employing cobalt bipyridine as redox mediator. The IB1 dye is the smallest structure with one octyl group on the phenoxazine part, but no further blocking moieties. IB2 is additionally equipped with two TPA groups, one on the donor part and another one on the linker part, while IB3 has two o,pdibutoxyphenyl group as steric groups instead.[9b] The new dye TH310, which has only one TPA group on the phenoxazine donor part, was specially designed for this paper to investigate in which location the TPA group plays an important role in blocking electron recombination.
The absorption spectra of IB1, TH310, IB2 and IB3 dyes in tertbutanol/acetonitrile (1:1 v/v) solution and on TiO2 films are given in Figure 2. The corresponding photophysical data are collected in Table 1. IB2 dye has a slightly blue-shifted absorption peaks (lmax) and slightly lower extinction coefficient compared with IB1 and TH310 dyes in the tert-butanol/acetonitrile (1:1 v/v) solution, which has been attributed to the relatively weak P-conjugated system.[9b] The absorption spectra of the dyes on TiO2 films showed some broadening compared the solution spectra, as is frequently observed for sensitizers. The IB2sensitized film showed a lower absorbance than the other sensitized films. Dye desorption experiments confirmed that IB2 gives the lowest dye loading on the TiO2 film, followed by IB3. This is directly related to the additional bulky groups in these
Figure 2. UV/Vis absorption spectra of dyes IB1, TH310, IB2 and IB3 in tertbutanol/acetonitrile (1:1 v/v) solution (a) and on TiO2 flim (b).
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Table 1. Spectroscopic data and the amount adsorbed on TiO2 film of dyes IB1, TH310, IB2 and IB3.
lmax [nm]
Absorption E [M 1 cm 1]
lmax on TiO2 [nm]
Dye load ( 10 7) [mol cm 2]
495 492 480 496
17100 18300 15700 16700
470 467 478 468
1.52 1.86 0.88 1.33
Dye IB1 TH310 IB2 IB3
dyes. TH310 dye shows the highest adsorbed amount on the TiO2 film, 1.86 10 7 mol cm 2, about double that of IB2. The photovoltaic performances of phenoxazine-sensitized solar cells with cobalt-based electrolyte are summarized in Figure 3 and Table 2. The current-voltage curves clearly show
Figure 3. Current density versus applied potential curves under 100 mW cm 2 AM1.5G light intensity and in the dark for DSCs sensitized with IB1, TH310, IB2 and IB3. The electrolyte was 0.22 m Co(bpy)3(PF6)2, 0.05 m Co(bpy)3(PF6)3, 0.2 m TBP, and 0.1 m LiClO4 in acetonitrile.
Table 2. J–V characteristics at various simulated sunlight intensities with AM1.5G spectral distribution for cells with different dyes. Dye
Intensity [mW cm 2]
Voc [mV]
Jsc [mA cm 2]
ff
h [%]
IB1
100 46 11.4 100 46 11.4 100 46 11.4 100 46 11.4 100
615 585 525 760 770 735 880 860 810 890 870 815 855
5.64 2.91 0.66 9.01 5.24 1.35 10.25 5.36 1.25 10.36 5.34 1.24 11.55
0.64 0.66 0.65 0.69 0.69 0.70 0.70 0.72 0.73 0.69 0.73 0.75 0.71
2.2 2.4 2.0 4.7 6.1 6.1 6.3 7.3 6.5 6.4 7.4 6.7 7.0
TH310
IB2
IB3
LEG4
Jsc (IPCE)[a] [mA cm 2] 5.50
9.61
10.15
10.29
11.45
[a] Jsc calculated from integration of the IPCE spectrum with respect to the AM1.5G photon flux.
2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
that IB2 and IB3 give much better performance than IB1 and TH310. The power conversion efficiency at one sun was 6.3 % and 6.4 % for IB2 and IB3, respectively, with a photocurrent density (Jsc) of about 10.3 mA cm 2 for both. Despite very similar optical absorption properties, IB1 had an efficiency of only 2.2 % and a Jsc of 5.6 mA cm 2. Clearly, in DSCs with IB1 severe recombination losses occur. This is also evident from the much lower Voc for IB1, 615 mV, compared to 860 mV and 870 mV for IB2 and IB3 respectively. Although much higher Voc was achieved by TH310 compared with IB1, 760 mV, it is significantly less than that of IB2 and IB3. These results demonstrate a significant blocking effect that can be attributed to TPA groups. More specifically, the TPA groups suppress recombination of electrons in TiO2 to with oxidized cobalt species in the electrolyte by steric hindrance. Furthermore, in IB2 both TPA groups are involved in this blocking. Overall, IB2 and IB3 dyes showed a much more improved Voc in the cobalt electrolyte system compared to the iodide/triiodide system.[9b] The performance of the phenoxazine dyes in DSC with cobalt-based electrolyte is quite impressive due to higher Voc, which is ascribed to more positive redox potential of cobalt redox couple. To assess the solar cell performance further, a comparison was made with a state-of-the-art triphenylamine based sensitizer LEG4, see Table 1 (LEG4 possesses a structure similar to Y123, see supporting information). The device with LEG4 gave a power conversion efficiency of 7.0 %, with a Jsc of 11.55 mA cm 2. The better performance can be ascribed to its broader optical absorption and higher extinction coefficient compared to the phenoxazine dyes.[12] Interestingly, higher Voc values by about 30 mV were obtained for IB2 and IB3 compared to LEG4. The high Voc shows the excellent blocking effect obtained by IB2. Further, DSCs with IB2 and IB3 possess efficiencies close to that of LEG4, which confirms the potential of phenoxazine dyes for cobalt electrolyte based DSCs. By further optimizing the dye structure to increase light harvesting, phenoxazine dyes have the potential to outperform the stateof-the-art dyes LEG4 and Y123 in DSCs with cobalt-based electrolyte. Incident-photon-to-current conversion efficiency (IPCE) spectra were recorded for DSCs sensitized with the organic dyes IB1, TH310, IB2 and IB3, see Figure 4. The DSCs based on IB1 show lowest IPCE values and a relatively narrower IPCE spectrum than the other dyes, in accordance with the lowest photocurrent found. IB2 and IB3-based DSCs gave very similar IPCE spectra, with a maximum IPCE of about 75 %, which was significantly higher than that of TH310 (65 %). The best red IPCE response was found for TH310, which is related its high dye load. In general, the IPCE of a DSC is determined by light harvesting, electron injection efficiency, regeneration efficiency and charge collection efficiency. From optical and electrochemical measurements (see Tables 1 and 3), it was found that all four phenoxazine dyes have similar absorption spectrum and energy levels. The cause of the lower IPCE response for IB1 must be significant electron recombination to the electrolyte during electron transport in the mesoporous TiO2. TH310, possesses a broader IPCE spectrum with higher values than IB1, but maximum values are lower than those of IB2 and IB3. This ChemPhysChem 0000, 00, 1 – 9
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Figure 4. Spectra of incident photon to current efficiency (IPCE) for DSCs sensitized with IB1, TH310, IB2 and IB3.
result shows that the TPA group on the POZ donor part can block electron recombination to some extent, but with an additional TPA group on the linker part (as in IB2) recombination is more efficiently suppressed. Dark current measurements give a direct indication for the recombination process between electrons in TiO2 and cobalt(III) species in the electrolyte. Results in Figure 3 clearly show that DSCs based on IB2 and IB3 give much smaller dark currents than TH310, while IB1 gives the largest dark current. This confirms further that the TPA group on linker part of IB2 plays the same role as o,p-dibutoxyphenyl group in IB3, decreasing recombination rate between electrons in TiO2 and CoIII species in the electrolyte. The electron recombination kinetics were further investigated studying the photovoltage response of the DSCs to a small amplitude light modulation. The electron lifetime was measured under open-circuit conditions for DSCs based on IB1, TH310, IB2 and IB3, and is plotted as function of quasi-Fermi level of the TiO2 in Figure 5. Under conditions of equal internal potential in the TiO2, the electron concentration in the TiO2 will be equal, as no band edge shifts or changes in the trap distri-
Figure 5. Electron lifetime as function of of the quasi-Fermi level of the TiO2 under open circuit conditions for DSCs sensitized with IB1, TH310, IB2 and IB3 employing cobalt electrolyte.
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www.chemphyschem.org bution are expected. In accordance with the current-voltage characteristics, the electron lifetime of DSCs based on IB2 and IB3 is quite similar, much longer than that of LEG4 and TH310, while IB1 has the shortest electron lifetime. Electron lifetimes in DSCs usually only reflect the electron recombination with oxidized form of the redox species in the electrolyte, but in case of slow regeneration of the oxidized dye, also the effect of electron recombination to the oxidized dye will be included in the measured lifetime. As will be shown later, dye regeneration efficiencies of all dyes are rather high, from 70 to 95 %, and recombination of electrons in TiO2 to the oxidized dye is not expected to account for differences in the measured electron lifetimes. The observed trend of increase in recombination for TH310 and IB1 is in agreement with the changes of Voc for different dyes, which further demonstrate that the TPA groups on linker part of IB2 molecules, like the o,p-dibutoxyphenyl groups on IB3, effectively slow down recombination by preventing close contact of the oxidized species in the electrolyte to the TiO2 surface. The dye regeneration reaction in the solar cell, electron transfer between the oxidized dye and the redox couple is expected to depend on the structure of the dyes, since the size of the electron donor and acceptor can affect the reorganization energy and the electronic coupling.[13] The regeneration of the dyes by the cobalt complexes was investigated with nslaser spectroscopy. Results of an initial photoinduced absorption spectroscopy (PIA), performed under conditions similar to the operational conditions of DSCs (white light intensity ca. 100 W m 2) are shown in the Supporting Information. The spectrum of the oxidized dye molecules was determined by using spectroelectrochemistry, see Figure 6 a. A probe wavelength of 880 nm was selected for transient absorption measurements, as all oxidized dye molecules show a significant absorbance there. In contact with inert electrolyte, the phenoxazine dye-sensitized TiO2 electrodes display relatively slow recombination kinetics between electrons in TiO2 and the oxidized dye molecules. IB3 displays significantly longer recombination halftime (t1/2 ca. 1 ms) compared to the other dyes (about 0.5 ms), see Figure 6 a and Table 3. The regeneration kinetics of phenoxazine dye-sensitized TiO2 electrodes in cobalt bipyridine show more significant differences between the different dyes. This is interesting as the driving force for regeneration is nearly constant for all dyes investigated here. IB1 shows the shortest regeneration halftime (17 ms), IB2 and IB3 dyes are 3–4 times slower, while TH310 is a factor of ten times slower (168 ms), see Figure 6 a and Table 3. The differences in regeneration times can be attributed to the differences in chemical structure, which influences the electron coupling between dye and redox couple. Specifically, the isolating steric groups in TH310, IB2 and IB3 may hinder close approach of the cobalt(II) mediator. The more dense packing of TH310 may explain its longest regeneration halftime for the same reason. Regeneration efficiencies were calculated as described in our previous work.[14] As conditions in the ns-laser experiments differ from that under full sun solar cell working conditions, regeneration efficiencies determined here are approximate. ReChemPhysChem 0000, 00, 1 – 9
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Figure 6. a) Oxidized spectra of four different dyes on TiO2 obtained by electrochemical oxidation of the dyes. b) Ns-laser induced transient absorption kinetics of TiO2 sensitized with IB1, TH310, IB2 and IB3 and employing inert electrolyte (black) and cobalt electrolyte (grey).
Table 3. Regeneration halftime and regeneration efficiencies for IB1-, TH310-, IB2- and IB3-sensitized DSCs employing cobalt bipyridine redox couple. t1/2 [ms]
Dye IB1 TH310 IB2 IB3
inert
cobalt
385 539 534 1113
17 168 86 54
Freg
HOMO [V] vs. NHE
0.96 0.70 0.84 0.95
0.90 0.93 0.91 0.88
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generation efficiencies of 84–96 % are determined for the IB1, IB2 and IB3 dyes, which indicates that the kinetics for regeneration of these dyes are sufficiently fast. The slower kinetics for regeneration of the oxidized dye found for the TH310 dye, (halflife 168 ms) lead to a much lower regeneration efficiency, 70 %. This is consistent with the IPCE spectrum found for TH310, which is lower than that of IB2 and IB3. Low regeneration efficiency will decrease IPCE independent of wavelength. Shorter electron diffusion length, on the other hand, will lead to a decreased IPCE values and a narrower spectrum IPCE, as was found for IB1. ChemPhysChem 0000, 00, 1 – 9
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3. Conclusions In conclusion, application of suitable bulky groups to the phenoxazine core results in efficient sensitizers for cobalt electrolyte-based dye-sensitized solar cells. The bulky groups suppress the recombination of Scheme 1. electrons in the TiO2 to CoIII species in the electrolyte. As shown before o,p-dibutoxyphenyl groups are well suited for this function, but, interestingly, also bare triphenylamine groups added to the phenoxazine core function well. By careful tuning of the chemical structure to extend the light absorption range, it will be feasible to further develop phenoxazine dyes Scheme 2. for more efficient DSCs. The use of triphenylamine groups as recombination-blocking moieties instead of alkoxy groups may decrease the manufacturing cost of sensitizers, because of the industrialized process of triphenylamine manufacturing.
Experimental Section All chemicals were purchased from Sigma Aldrich unless otherwise noted. IB1, IB2 and IB3 dyes were available from a previous study.[9b] The new sensitizer TH310 was synthesized by following the process shown in Scheme 1. Compound 1 was obtained as a byproduct of synthesis of compound 5 in our previous publication.[8b] A solution of 1 (20 mg, 0.04 mmol), triphenylamine-4-boronic acid (24 mg, 0.08 mmol), 160 mL 0.5 m K2CO3 aqeuous and chloro(2-dicyclohexylphosphino2’,4’,6’-triisopropyl-1,1’-biphenyl)[2-(2’-amino-1,1’-biphenyl)]palladium(II) (X-Phos Pd G2) (1.6 mg, 0.002 mmol) in THF (1 mL) was heated to 40 8C for 2 h under constant stirring. The reaction mixture was allowed to cool and H2O was added. The mixture was extracted with CH2Cl2 twice and the combined organic fractions were concentrated in vacuo. Flash chromatography over silica gel by CH2Cl2/pentane (2/1; v/v) gave crude 2, which is further purified by HPLC using CH2Cl2/pentane (1/1; v/v) with 1 % formic acid as eluent to give 11 mg pure product (yield 42 %) as yellow powder. 1 H NMR (500 MHz, [D6]Acetone): d ppm = 9.88 (s, 1 H), 7.89 (d, J = 3.96 Hz, 1 H), 7.50–7.53 (m, 3 H), 7.31 (t, J1 = 8.18, J2 = 7.61, 4 H), 7.28 (dd, J1 = 2.06, J2 = 8.36, 1 H), 7.16 (dd, J1 = 1.96, J2 = 8.32, 1 H), 7.05– 7.09 (m, 8 H), 7.03 (d, J = 2.08 Hz, 1 H), 6.95 (d, J = 2.02 Hz, 1 H), 6.75–6.79 (m, 2 H), 3.67 (t, 2 H), 1.68–1.75 (m, 2 H), 1.46–1.53 (m, 2 H), 1.30–1.45 (m, 8 H), 0.88 (t, 3 H). A solution of 2 (11 mg, 0.017 mmol), cyanoacetic acid (6 mg, 0.068 mmol) and 2 drops piperidine was refluxed in 5 mL MeCN/ CHCl3 (3/2; v/v) for 4 h under nitrogen protection. The solution was allowed to cool before HCl (0.5 m, 10 mL) was added. The product was extracted with CH2Cl2 (10 mL) twice. The combined organic phase was dried over Na2SO4 and the solvent removed in vacuo. Flash chromatography over silica gel using gradient elution CH2Cl2, MeOH/CH2Cl2 (20/1; v/v) gave product TH310 (10 mg, 83 %) 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
as dark powder (Scheme 2). 1H NMR (500 MHz, [D6]Acetone): d ppm = 8.37 (s, 1 H), 7.89 (d, J = 4.04 Hz, 1 H), 7.50–7.53 (m, 3 H), 7.31 (t, J1 = 8.13, J2 = 7.63, 4 H), 7.26 (dd, J1 = 2.00, J2 = 8.38, 1 H), 7.14 (dd, J1 = 1.87, J2 = 8.34, 1 H), 7.04–7.09 (m, 8 H), 7.02 (d, J = 2.02 Hz, 1 H), 6.94 (d, J = 1.91 Hz, 1 H), 6.75 (m, 2 H), 3.65 (t, 2 H), 1.69–1.72 (m, 2 H), 1.48–1.50 (m, 2 H), 1.30–1.45 (m, 8 H), 0.88 (t, 3 H). 13C NMR (126 MHz, [D6]Acetone): d ppm = 164.2, 154.5, 147.7, 147.9, 147.4, 145.9, 145.7, 141.7, 135.6, 134.8, 134.6. 132.1, 130.4, 127.7, 126.4, 125.3, 124.7, 124.3, 124.1, 123.6, 122.7, 117.1, 114.0, 113.6, 113.4, 113.2, 44.6, 32.7, 27.5, 25.9, 23.4, 14.5 ppm; MS (ESI): Positive ion: m/z = 716.3 [M + H] + , calculated 716.3.
Solar Cell Preparation TiO2 electrodes were prepared on fluorine-doped tin oxide (FTO) glass substrates (Pilkington, TEC15). The glass substrates were cleaned in an ultrasonic bath using (in order) detergent, water, ethanol and acetone, each one hour. The glass substrates were pretreated by immersion in 40 mm aqueous TiCl4 solution at 70 8C for 90 min, washed with water and dried. The photoelectrodes with a size of 0.5 0.5 cm2 were prepared by screen printing, using TiO2 paste (Dyesol DSL 20 NRD-T) as transparent layer and scattering paste (PST-400C, JGC Catalysts and Chemical LTD, gratefully received from JGC Catalysts and Chemical LTD) as scattering layer on top of transparent layer. The thickness of the photoelectrodes was measured with a profilometer (Veeco Dektak 3), about 4 mm and 4 mm was obtained for the transparent layers and scattering layers respectively. The electrodes were sintered in an oven (Nabertherm Controller P320) in an air atmosphere using a temperature gradient program with four levels at 180 8C (10 min), 320 8C (10 min), 390 8C (10 min), and 500 8C (60 min). Prior to use, the electrodes were given a post-treatment by immersing in 40 mm aqueous TiCl4 solution at 70 8C for 30 min, followed by another heating step (500 8C, 60 min). When the temperature cooled to about 90 8C, the electrodes were immersed in a dye bath containing 0.2 mm IB1, TH310, IB2 and IB3 in tert-butanol:acetonitrile = 1:1 and left overnight. The films were then rinsed in ethanol to remove excess dye. ChemPhysChem 0000, 00, 1 – 9
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CHEMPHYSCHEM ARTICLES Solar cells were assembled, using a 30 mm thick thermoplastic Surlyn frame with a platinized counter electrode (TEC8), which was prepared by depositing 10 mL 5 mm H2PtCl6 solution in ethanol to the glass substrate followed by heating in air at 400 8C for 30 min. An electrolyte solution was then injected through one hole predrilled counter electrode and the hole was sealed with thermoplastic Surlyn covers and aluminum foil. The electrolyte consisted of 0.22 m Co(bpy)3(PF6)2, 0.05 m Co(bpy)3(PF6)3, 0.1 m LiClO4, and 0.2 m 4-tert butylpyridine (TBP) in acetonitrile.
Solar Cell Characterization Current–voltage (IV) characteristics were measured using a Keithley 2400 source/meter and a Newport solar simulator (model 91160) giving light with AM 1.5 G spectral distribution, which was calibrated using a certified reference solar cell (Fraunhofer ISE) to an intensity of 1000 W m 2. A black mask with the same area (0.5 0.5 cm2) to the active solar cell area was applied on top of the cell. Photovoltaic measurements at low light intensities were performed using neutral density filters. Incident photon to current conversion efficiency (IPCE) spectra were recorded using a computer-controlled setup consisting of a xenon light source (Spectral Products ASB-XE-175), a monochromator (Spectral Products CM110) and a Keithley 2700 multimeter, calibrated using a certified reference solar cell (Fraunhofer ISE). Electron lifetime was performed using a white LED (Luxeon Star 1W) as the light source. Voltage traces were recorded with a 16-bit resolution digital acquisition board (National Instruments) in combination with a current amplifier (Stanford Research Systems SR570) and a custom-made system using electromagnetic switches. Lifetimes were determined by monitoring photovoltage transients at different light intensities upon applying a small square wave modulation to the base light intensity. The photovoltaic responses were fitted using first-order kinetics to obtain time constants.[15]
Optical Spectroscopy The UV/Vis absorption spectra of the dye-loaded transparent film (2 mm, sensitized 1 h in dye bath) and the dye solution (0.02 mm, in tert-butanol/acetonitrile (1:1, v/v) in 1 cm cuvette) were recorded with an Ocean Optics HR2000 spectrometer and a Micropack DH2000-BAL light-source. The fluorescence spectra of dye solution were recorded on a Fluorolog 3–222 emission spectrometer from Jobin–Yvon by using the same concentration as the UV/Vis measurement.
Electrochemical Measurements Electrochemical experiments were performed with a CH Instruments electrochemical workstation (model 660 A) using a conventional three-electrode electrochemical cell. Measurements were obtained with dye sensitized TiO2 film by using 0.1 m lithium perchlorate (LiClO4) in acetonitrile as supporting electrolyte. Dye sensitized TiO2 electrode was used as the working electrode, a graphite electrode served as a counter electrode. The reference electrode was in all cases aqueous Ag/AgCl/3 m NaCl; a salt bridge electrolyte was interposed between working and reference electrode, containing 0.1 m LiClO4 in acetonitrile, and the scan rate used was 50 mV s 1. All redox potentials were calibrated versus a normal hydrogen electrode (NHE) by the addition of ferrocene as an internal standard taking E0(Fc+/Fc) = 0.63 V vs. NHE.[11b] 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemphyschem.org Dye Coverage Measurements Dye loading were estimated from dye desorption measurements. TiO2 films with two layers of Dyesol DSL 18 NR-T paste, (diluted by 40 wt % polymer mixture, containing 90 wt % terpineol and 10 wt % ethylcellulose) with a thickness of about 4 mm were immersed in dye baths with a concentration of 0.2 mm IB1, TH310, IB2 and IB3 in tert-butanol/acetonitrile (1:1, v/v). After sensitization the samples were put in alkaline solution consisting of 0.05 m TBAOH in CH2Cl2 to desorb the dyes. Next, sulfuric acid was added into the solution in order to reprotonate the dye and the UV/Vis spectrum of the solutions in a 1 cm cuvette was measured, from which the dye coverage was estimated using the Lambert–Beer law.
Spectroelectrochemical Measurements Spectroelectrochemistry was performed on a CH Instruments 660 potentiostat with a three-electrode setup. At the same time as carrying out the electrical measurements, UV/Vis spectra were recorded. All four different dyes-coated mesoporous TiO2 films were used as the working electrodes. A graphite electrode was used as the counter electrode and a Ag/Ag + electrode as a pseudo reference electrode. The electrolyte solution was 0.1 m lithium perchlorate (LiClO4) in acetonitrile. Cyclic voltammetry of a TiO2 film was carried out in the LiClO4 supporting electrolyte with a scan rate of 50 mv s 1.
Nanosecond Transient Absorption Spectroscopy (TAS) Dye regeneration by cobalt-based redox mediator and electron recombination to the oxidized dye were monitored using a Nd:YAG laser (Continuum Surelight II, repetition rate: 10 Hz, pulse width: 10 ns) in combination with an Optical Parametrical Oscillator (Continuum Surelite OPO Plus). Pump pulses of 530 nm for IB1, TH310, IB2 and IB3 was used. The intensity of the laser output was attenuated to 0.3 mJ cm 2 in combination with a 5 % transmittance filter (Thorlabs). Probe light was provided by a near-infrared LED (Osram SHF 484, lmax 880 nm, fwhm 80 nm), and kinetic traces were measured with an amplified Si-photodiode (Thorlabs PDA10AEC). In order to avoid stray light from the laser a cutoff filter (RG 715) was used in front of the detector. PIA spectra were measured for all dyes adsorbed on TiO2 to determine the spectrum of the oxidized dyes. Based on these spectra, 880 nm was chosen as probe light in ns-TAS for measurements. The electrolyte consisted of 0.22 m Co(bpy)3(PF6)2, 0.05 m Co(bpy)3(PF6)3, 0.1 m LiClO4 and 0.2 m TBP in acetonitrile. Recombination dynamics were measured with a solution of 0.2 m TBP and 0.1 m LiClO4 in acetonitrile. Measurements were performed on screen printed TiO2 films of 3 mm thickness which were sensitized according to already mentioned procedure. A sealed sample for recombination and regeneration measurements was applied.
Acknowledgements We gratefully acknowledge the Swedish Energy Agency, the Swedish Research Council (VR), the STandUP for Energy program, the Knut and Alice Wallenberg Foundation and Sony Deutschland GmbH for financial support. Keywords: blocking effect · cobalt redox mediator · dyesensitized solar cells · phenoxazine dyes · triphenylamine
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CHEMPHYSCHEM ARTICLES [1] S. M. Feldt, E. A. Gibson, E. Gabrielsson, L. Sun, G. Boschloo, A. Hagfeldt, J. Am. Chem. Soc. 2010, 132, 16714 – 16724. [2] S. Mathew, A. Yella, P. Gap, R. Humphry-Baker, B. F. E. Curchod, N. Ashari-Astani, I. Tavernelli, U. Rothlisberger, M. K. Nazeeruddin, M. Grtzel, Nat. Chem. 2014, 6, 242 – 247. [3] a) S. Nakade, Y. Makimoto, W. Kubo, T. Kitamura, Y. Wada, S. Yanagida, The Journal of Physical Chemistry B 2005, 109, 3488 – 3493; b) T. W. Hamann, B. S. Brunschwig, N. S. Lewis, The Journal of Physical Chemistry B 2006, 110, 25514 – 25520; c) J. J. Nelson, T. J. Amick, C. M. Elliott, The Journal of Physical Chemistry C 2008, 112, 18255 – 18263; d) M. Liberatore, L. Burtone, T. M. Brown, A. Reale, A. Di Carlo, F. Decker, S. Caramori, C. A. Bignozzi, Applied Physics Letters 2009, 94, 173113. [4] D. Zhou, Q. Yu, N. Cai, Y. Bai, Y. Wang, P. Wang, Energy Environ. Sci. 2011, 4, 2030 – 2034. [5] Y. Bai, J. Zhang, D. Zhou, Y. Wang, M. Zhang, P. Wang, J. Am. Chem. Soc. 2011, 133, 11442 – 11445. [6] M. K. Kashif, J. C. Axelson, N. W. Duffy, C. M. Forsyth, C. J. Chang, J. R. Long, L. Spiccia, U. Bach, J. Am. Chem. Soc. 2012, 134, 16646 – 16653. [7] H. N. Tsao , C. Yi , T. Moehl, J.-H. Yum, S. M. Zakeeruddin, M. K. Nazeeruddin, M. Grtzel, ChemSusChem 2011, 4, 591 – 594. [8] A. Yella, H.-W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Nazeeruddin, E. W.-G. Diau, C.-Y. Yeh, S. M. Zakeeruddin, M. Grtzel, Science 2011, 334, 629 – 634. [9] a) H. Tian, X. Yang, J. Cong, R. Chen, J. Liu, Y. Hao, A. Hagfeldt, L. Sun, Chem. Commun. 2009, 0, 6288 – 6290; b) H. Tian, I. Bora, X. Jiang, E. Ga-
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brielsson, K. M. Karlsson, A. Hagfeldt, L. Sun, J. Mater. Chem. 2011, 21, 12462 – 12472; c) H. Tian, X. Yang, R. Chen, A. Hagfeldt, L. Sun, Energy Environ. Sci. 2009, 2, 674 – 677; d) K. M. Karlsson, X. Jiang, S. K. Eriksson, E. Gabrielsson, H. Rensmo, A. Hagfeldt, L. Sun, Chem. Eur. J. 2011, 17, 6415 – 6424. X.-Q. Zhu, Z. Dai, A. Yu, S. Wu, J.-P. Cheng, The Journal of Physical Chemistry B 2008, 112, 11694 – 11707. a) T. Kitamura, M. Ikeda, K. Shigaki, T. Inoue, N. A. Anderson, X. Ai, T. Lian, S. Yanagida, Chem. Mater. 2004, 16, 1806 – 1812; b) D. P. Hagberg, T. Edvinsson, T. Marinado, G. Boschloo, A. Hagfeldt, L. Sun, Chem. Commun. 2006, 0, 2245 – 2247; c) D. P. Hagberg, T. Marinado, K. M. Karlsson, K. Nonomura, P. Qin, G. Boschloo, T. Brinck, A. Hagfeldt, L. Sun, The Journal of Organic Chemistry 2007, 72, 9550 – 9556. E. Gabrielsson, H. Ellis, S. Feldt, H. Tian, G. Boschloo, A. Hagfeldt, L. Sun, Adv. Energy Mater. 2013, 3, 1647 – 1656. S. Cazzanti, S. Caramori, R. Argazzi, C. M. Elliott, C. A. Bignozzi, J. Am. Chem. Soc. 2006, 128, 9996 – 9997. S. M. Feldt, G. Wang, G. Boschloo, A. Hagfeldt, The Journal of Physical Chemistry C 2011, 115, 21500 – 21507. G. Boschloo, L. Hggman, A. Hagfeldt, The Journal of Physical Chemistry B 2006, 110, 13144 – 13150.
Received: July 2, 2014 Published online on && &&, 2014
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ARTICLES Anything but sunblock: Triphenylamine groups give a good blocking effect in phenoxazine dyes for cobalt electrolyte-based dye-sensitized solar cells.
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Y. Hao, H. Tian, J. Cong, W. Yang, I. Bora, L. Sun, G. Boschloo,* A. Hagfeldt && – && Triphenylamine Groups Improve Blocking Behavior of Phenoxazine Dyes in Cobalt-Electrolyte-Based DyeSensitized Solar Cells
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