Structural Effect of Donor in Organic Dye on Recombination in

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Structural Effect of Donor in Organic Dye on Recombination in DyeSensitized Solar Cells with Cobalt Complex Electrolyte Takurou N. Murakami,*,† Nagatoshi Koumura,*,† Mutsumi Kimura,‡ and Shogo Mori‡ †

Research Institute for Innovation in Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan ‡ Division of Chemistry and Materials, Faculty of Textile Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan S Supporting Information *

ABSTRACT: The effect of the donor in an organic dye on the electron lifetime of dye-sensitized solar cells (DSSCs) employing a cobalt redox electrolyte was investigated. We synthesized organic dyes with donor moieties of carbazole, coumarin, triphenylamine, and N-phenyl-carbazole and measured the current−voltage characteristics and electron lifetimes of the DSSCs with these dyes. The cell with the triphenylamine donor dye produced the highest open circuit voltage and longest electron lifetime. On the other hand, the lowest open circuit voltage and shortest electron lifetime was obtained with coumarin donor dye, suggesting that the coumarin attracted the cobalt redox couples to the surface of the TiO2 layer, thus increasing the concentration of cobalt complex. On the other hand, the longest electron lifetime with triphenylamine was attributed to the blocking effect by steric hindrance of the nonplanar structure of the donor.

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INTRODUCTION Dye-sensitized solar cells (DSSCs) with cobalt complex redox electrolytes have achieved high photoconversion efficiencies of over 12% and high open-circuit voltages (Voc) because of the higher redox potential in cobalt complex redox systems as compared to that of conventional iodide/tri-iodide redox systems.1−9 However, low Voc and low efficiency in cobalt redox electrolyte due to the fast recombination reaction at the interface between the TiO2 layer and the electrolyte have been reported.10−19 The ideal performance of DSSCs with cobalt redox electrolytes, which includes attaining a high Voc value by the high redox potential of the cobalt complex, depends on the ability of the dye to inhibit recombination.20−28 In our previous report, we suggested that using hexyloxyphenyl groups to provide steric hindrance in the donor groups and short alkyl side chains, such as propyl groups in π-linker groups in organic dyes consisting of donor−π-linker−acceptors (D-π-As), was effective for inhibiting the recombination reaction.24 Sustaining the cobalt redox couple away from TiO2 surfaces is one of the key requirements for realizing a high photovoltaic performance with cobalt redox electrolytes. Further, in iodide redox systems, the type of donor used is also quite sensitive influencing the electron lifetime. For example, short electron lifetime and low Voc for coumarin donor dyes such as NKX-2587 have been reported.29−33 These results are attributed to the partial charge effect of the coumarin donor for attracting lithium cations and © 2014 American Chemical Society

iodide/tri-iodide anions, hence increasing the tri-iodide concentration at the TiO2 surface.32 Also, the densely adsorbed dye of triphenylamine directly connecting to cyanoacrylic acid lead to a longer electron lifetime because of the blocking effect of tri-iodide approaching to TiO2 surface.34,35 While similar charge dynamics are expected to apply in cobalt complex systems; however, the relationship between the type of donor and DSSC performance has not been studied systematically. The difference in charge, polarity, and size of the cobalt complexes compared to iodide/tri-iodide system may lead to new insights in charge dynamics. Well performing organic dyes for DSSCs with a cobalt redox electrolyte primarily employ triphenylamine-based donors such as D35, Y-123, and C229.2,4,5,9,20,27,36−38 On the other hand, carbazole donor dyes have also achieved high performance of over 9% efficiency.7,8,39 It is thus important to understand the blocking and attracting features of donors to cobalt complex electrolytes in order to develop high-performance dyes. In this study, we prepared organic dyes using carbazole (MK-1), coumarin (MK31), triphenylamine (MK-88), and N-phenyl-carbazole (MK90) in Figure 1, and we investigated the relationship between the donor group and the electron lifetime in the DSSCs. Received: December 16, 2013 Revised: February 5, 2014 Published: February 6, 2014 2274

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compounds were fully confirmed using 1H and 13C NMR spectroscopy. To eliminate the recombination reaction at the FTO/electrolyte interface, the surface of the FTO was covered with a thin compact TiO2 layer that was around 100 nm thick. A 0.2 M solution of titanium isopropoxide−acetylacetonate and 75% v/v isopropyl alcohol was diluted to 20% v/v with anhydrous ethanol, and the compact TiO2 layer was formed from this solution by spray pyrolysis at 500 °C under air.42,43 The TiO2 mesoporous layers were prepared on the TiO2coated FTO-glass substrates by the screen-printing method, that is, the TiO2 paste was coated onto the substrate over a 5 mm × 5 mm square area and sintered at 500 °C for 30 min. The thickness of the transparent TiO2 layer was 4 μm. Immediately after reheating the TiO2 layers at 450 °C for 30 min, the layers were immersed in the dye solution for 18 h at 30 °C. The counter electrodes were prepared by drilling two holes into the FTO-glass substrates for electrolyte injection, followed by heat deposition of hexachloroplatinic acid, which was used as a catalyst, onto the substrate. The photoelectrode and counter electrode were sandwiched by the insertion of a 30 μm thick layer of thermobonding polymer (Surlyn, DuPont), and these electrodes were heated for sealing. After sealing, the electrolyte was injected between the two electrodes by means of the hole in the counter electrode; the hole was then also sealed with Surlyn and a cover glass. The UV−visible spectra were obtained by SHIMADZU UV3101PC spectrometer. The photocurrent−voltage (I−V) characteristics were measured using a Advantest R6246 DC source/meter in combination with a solar simulator (YSS-150A, Yamashita Denso Co.) with AM 1.5G filters. The light intensity of 100 mW cm−2 was calibrated with a standard amorphous silicon PV. The aperture cell area was fixed at 0.16 cm2 by using a photomask on top of the cells. Incident photon-to-electron conversion efficiency (IPCE) spectra were measured with a unit composed of source meter, light intensity controllable monochromator, and xenon arc lamp (Bunkokeiki, CEP99W). The electron lifetimes and densities at open circuit were measured by using the stepped light-induced photocurrent and voltage transients (SLIM-PCV)32,44 and charge extraction methods,45 respectively.

Figure 1. Chemical structure of the organic dyes with different donor groups of carbazole (MK-1), coumarin (MK-31), triphenylamine (MK-88), and N-phenyl carbazole (MK-90).

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RESULTS AND DISCUSSION The absorption spectra of the dyes on the porous TiO2 layer are shown in Figure 2. The order of the absorption peak

EXPERIMENTAL SECTION

Fluorine-doped SnO2-coated glass (FTO-glass), used as a transparent conductive glass for photoelectrodes and counter electrodes, was purchased from Nippon Sheet Glass Co., Ltd. (TEC A9X, sheet resistance ≤10 Ω/sq). TiO2 paste (DSL18NRT) consisting of 20 nm nanocrystalline particles was purchased from Dyesol Inc. An organic dye that we synthesized (see the Supporting Information for more detail) consisting of a 0.3 mM solution in toluene was used for carbazole and triphenylamine dye adsorption, and the same dye concentration in a mixed solvent solution of acetonitrile, tert-butyl alcohol, and ethanol (2:2:1 volume ratio) was used for coumarin dye adsorption. The standard electrolyte consisted of 0.22 M cobalt(II) tris-bipyridyl BCN4, 0.02 M cobalt(III) tris-bipyridyl BCN4, 0.1 M LiClO4, and 0.2 M 4-tert-butylpyridine (Sigma−Aldrich Co. LLC) in acetonitrile (Wako Pure Chemical Industries, Ltd.). Cobalt(II)/(III) bipyridyl BCN4 was synthesized by the methods reported in an earlier study.20,36 To attain low redox concentrations of 10%, electrolytes consisting of 0.022 M cobalt(II) tris-bipyridyl BCN4 and 0.002 M cobalt(III) tris-bipyridyl BCN4 were prepared. The other solute concentrations in this modified electrolyte are the same as those in the standard electrolyte. MK-1 dyes were synthesized using methods reported in the literature.40,41 The synthesis of an oligothiophene linker was performed by repeating the Suzuki coupling reaction with the corresponding n-hexylthiophene or n-propylthiophene boronic acid ester and bromination with N-bromosuccinimide. These dyes were obtained by using the Vilsmeier reaction and Knoevenagel condensation with cyanoacetic acid. The detailed procedures for the syntheses of the new dyes, MK-31, MK-88, and MK-90, are described in the Supporting Information. The molecular structures of the

Figure 2. Absorption spectra of dyes adsorbed on the TiO2 layer.

wavelength and the onset wavelength from long to short was MK-31 (coumarin) > MK-1 (carbazole) > MK-88 (triphenylamine) > MK-90 (N-phenyl-carbazole). The absorption differences can be attributed to the donor ability of the donor moiety in the organic dye; the absorption spectra (Figure 2) suggested that coumarin had the best donor ability. The I−V curves of the DSSCs employing dyes MK-1 (MK-1cell), MK-31 (MK-31-cell), and MK-88 (MK-88-cell) are shown in Figure 3, and their photovoltaic performance is 2275

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Figure 4. IPCE spectra of the DSSCs shown in Figure 3.

Figure 3. I−V curves of the DSSCs employing the dyes with donor moieties of carbazole (MK-1), coumarin (MK-31), and triphenylamine (MK-88). Bare is the cell without dyes.

complex is less likely to access the TiO2 surface. For TiO2 films with low dye loading, the electron lifetime will decrease due to the increased interaction of the cobalt complex with the TiO2 surface leading to increased recombination. In addition, the electrolyte concentration was decreased to enhance the effect of interaction between the dyes and cobalt redox couple on the electron lifetime.34 Figure 6a,b shows the Voc and the electron lifetime, respectively, of the DSSCs with approximately 20% dye loadings and with the 10% redox concentration electrolyte versus the standard electrolyte. The amount of dye loading and the ratio of the dye adsorption amount versus the dye amount at full adsorption on TiO2 were 0.54 mol cm−3 (22%) for MK1, 0.42 mol cm−3 (17%) for MK-31, and 0.51 mol cm−3 (24%) for MK-88. There is no significant difference in the relationship between Voc and electron density in any of the dyes under the condition of low redox concentration with low dye loading. The electron lifetimes of the MK-1-cell and the MK-88-cell were almost the same and that of the MK-31-cell was shorter than the others. This suggests that the longer electron lifetime of the MK-88-cell than the MK-1-cell with full dye loading conditions with the standard electrolyte can be attributed to the difference of the blocking effect by the donor moiety not due to the surface coverage. The shorter electron lifetime in the MK-31cell under the conditions of low dye loading suggests that the coumarin donor in MK-31 attracted the cobalt complex to the TiO2 surface. In our previous report, the acceleration of the recombination with coumarin dye was explained as follows: The partial charge of the oxygen in the carbonyl part of coumarin attracts cations such as lithium cations, and then, the lithium cations attract tri-iodide anions, thus increasing the surface concentration of tri-iodide on the TiO2 layer. In the case of the cobalt complex electrolyte, it can be considered that the partial charge in coumarin attracted the cobalt complex cation directly. Here, the electron lifetimes of the cells with all dyes in Figure 6b were longer than those in Figure 5b. Lower dye coverage may increase the rate of recombination because redox species could access the TiO2 surface more easily. On the other hand, the low redox concentration electrolytes decreases the recombination rate because the reaction rate between electron in TiO2 and the triad cobalt complex in electrolyte is reduced. The increased lifetime suggests that the effect of the

described in Table 1. The MK-31-cell showed the highest short-circuit current (Jsc) and the lowest open circuit voltage (Voc), and the MK-88-cell showed the lowest Jsc and the highest Voc. In the IPCE spectra of the DSSCs, shown in Figure 4, the order of the onset wavelength was the same as that of the absorption spectra in Figure 2. On the other hand, the MK-31cell gave the lowest value for the IPCE. Among the three dyes, MK-31 has the largest energy difference between the conduction band edge of TiO2 and the lowest unoccupied molecular orbital (LUMO) level of the dyes (Table S1, Supporting Information). Thus, the lower IPCE must be attributed to other factors than thermodynamic driving force. For DSSCs, Voc is determined by the difference between the Fermi level of TiO2 and the redox potential of the electrolyte. The Fermi level can be shifted by changing the electron density in TiO2. Since a higher electron density is obtained by longer electron lifetime, the higher Voc of the MK-88-cell could be related with longer electron lifetime if the conduction band edge potential is the same among the cells. To examine the relationship between the recombination rate and the type of donor, the electron lifetime of the DSSCs was measured, and the conduction band edge potential was evaluated from the open circuit voltage and the electron density plot. Figure 5a,b displays Voc and electron lifetime, respectively, as a function of electron density in the TiO2 electrodes. Figure 5 represents the results from the DSSCs with the standard electrolyte. There are differences up to 20 mV in the Voc and electron density plot, showing the dyes have little effect on the potential of the conduction band edge of TiO2. Figure 5b shows a clear trend in the order of the electron lifetime at the same electron density: MK-88-cell > MK-1-cell > MK-31-cell. Hence, we can attribute the higher Voc (from I−V measurement as seen in Figure 3) to the increased electron lifetime but not to the conduction band edge shift. To elucidate the mechanism of the differences in the recombination rate with the donor type, the electron lifetimes of the DSSCs with a low-dye-loading photoelectrode and a lowredox-couple-concentration electrolyte were measured. If the dense dye layer has a blocking function, then the cobalt Table 1. Performances of the DSSCs Shown in Figure 3 dye

donor type

dye loading/10−4 mol cm−3

Jsc/mA cm−2

Voc/V

FF

efficiency/%

MK-1 MK-31 MK-88

carbazole coumarin triphenylamine

2.4 ± 0.1 2.5 ± 0.1 2.1 ± 0.2

11 12 10

0.779 0.758 0.806

0.72 0.69 0.70

6.0 6.1 5.8

2276

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Figure 5. (a) Voc and (b) electron lifetime as a function of electron density for the DSSCs with various organic dyes with different donor moieties. The electrolytes consist of 0.22 M cobalt(II) tris-bipyridyl BCN4 and 0.02 M cobalt(III) tris-bipyridyl BCN4.

Figure 6. (a) Voc and (b) electron lifetime as a function of electron density for the DSSCs with 20% adsorbed dye. The electrolytes consist of 0.022 M cobalt(II) tris-bipyridyl BCN4 and 0.002 M cobalt(III) tris-bipyridyl BCN4.

low redox concentration was probably dominant to the results in Figure 6b. The blocking effect of triphenylamine is possibly due to steric hindrance by the nonplanar structure of the triphenylamine. To examine this hypothesis, MK-90 dye was synthesized, and the electron lifetimes of the MK-90-cell (2.3 ± 0.0 mol cm−3 dye loading) and the MK-88-cell (2.1 ± 0.2 mol cm−3 dye loading) were compared. The MK-90 donor has two phenyl groups in donor moiety with planar structure. Figure 7, Table 2, and Figure 8 show the I−V characteristics, photovoltaic performances, and electron lifetimes of the MK-90-cell and the MK-88cell, respectively, measured at the same time. The MK-90-cell clearly shows a lower electron lifetime than the MK-88-cell; this result suggests that the two phenyl groups at the tip of

Table 2. Performances of the DSSCs Shown in Figure 7 dye

donor type

Jsc/ mA cm−2

Voc/V

FF

efficiency/%

MK-88 MK-90

triphenylamine N-phenyl carbazole

11 8.1

0.792 0.734

0.69 0.69

5.8 4.1

triphenylamine donor group play a significant role in blocking the recombination reaction.

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CONCLUSIONS

Dyes having donor moieties of carbazole, coumarin, triphenylamine, and N-phenyl-carbazole were compared to examine the relationship between the type of donor in an organic dye and the recombination in DSSCs. Triphenylamine dye showed the highest Voc and the longest electron lifetime. The structure of triphenylamine resulted in steric hindrance that prevented cobalt complexes from reaching the TiO2 surface, thus impeding the recombination reaction. On the other hand, coumarin attracted the cobalt complex to the TiO2 surface, increasing the recombination rate. However, the coumarin dye exhibited wide-range visible light absorption and showed the highest Jsc. Our results outline important design rules for highperformance organic dyes in DSSCs employing cobalt redox electrolytes. We show that organic dyes should possess not only high donor ability and a large donor size for steric hindrance, but the partial charge of donor must also be carefully engineered to inhibit recombination reaction in redox couples.

Figure 7. I−V curves of the DSSCs employing the dyes with triphenylamine (MK-88) and N-phenyl carbazole (MK-90). 2277

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Figure 8. (left) Voc and (right) electron lifetime as a function of electron density for the DSSCs with triphenylamine dye (MK-88) and N-phenylcarbazole dye (MK-90). The electrolytes consist of 0.22 M cobalt(II) tris-bipyridyl BCN4 and 0.02 M cobalt(III) tris-bipyridyl BCN4.

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ASSOCIATED CONTENT

S Supporting Information *

General methods; synthetic procedures and schemes; NMR spectra; HOMO and LUMO characteristics; and I−V curves, IPCE spectra, and solar cell performance of the DSSCs. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Dr. N. Masaki and Dr. M. M. Lee for valuable discussions and H. Kodama for her technical assistance. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

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dx.doi.org/10.1021/la4047808 | Langmuir 2014, 30, 2274−2279