Dye-Anchoring Functional Groups on the Performance of Dye

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Dye Anchoring Functional Groups on the Performance of Dye Sensitized Solar Cells: Comparison between Alkoxysilyl and Carboxyl Groups Sri Kasi Matta, Kenji Kakiage, Satoshi Makuta, Aisea Veamatahau, Yohei Aoyama, Toru Yano, Minoru Hanaya, and Yasuhiro Tachibana J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5088338 • Publication Date (Web): 15 Nov 2014 Downloaded from http://pubs.acs.org on November 16, 2014

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The Journal of Physical Chemistry

Dye

Anchoring

Performance

of

Functional Dye

Groups

Sensitized

on

Solar

the Cells:

Comparison between Alkoxysilyl and Carboxyl Groups Sri Kasi Matta,† Kenji Kakiage,§ Satoshi Makuta,† Aisea Veamatahau,† Yohei Aoyama,§ Toru Yano,§ Minoru Hanaya*# and Yasuhiro Tachibana*†‡ †

School of Aerospace, Mechanical and Manufacturing Engineering, RMIT University,

Bundoora, VIC 3083, Australia; #Division of Molecular Science, Faculty of Science and Technology, Gunma University; §Environmental & Energy Materials Laboratory, ADEKA CORPORATION, 7-2-35 Higashiogu, Arakawa, Tokyo 116-8554, Japan; ‡Office for UniversityIndustry Collaboration, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan

ACS Paragon Plus Environment

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ABSTRACT We have compared two dye anchoring functional groups, alkoxysilyl and carboxyl groups, to investigate

their

influence

on

the

performance

of

dye

sensitized

solar

cells.

Dimethylaminoazobenzene was selected as a chromophore possessing a donor-accepter transition for the light absorption. Electrochemical and optical measurements were performed for 4-dimethylaminoazobenzene-4'-carboxylic

acid

and

4-dimethylaminoazobenzene-4'-

triethoxysilane attached TiO2 films. Electrochemical measurements and DFT calculation indicated almost identical potential energy levels and electron density of HOMO and LUMO states between these two dyes. Solar cell APCE spectra and charge recombination kinetics at the dye/TiO2 interface revealed almost identical charge transfer rates/yields from and to the dye. The difference was observed on improvement of an open circuit photovoltage (Voc) by 60 mV and on the lifetimes of an electron in the TiO2 conduction band for the dye with the alkoxysilyl functional group compared to the carboxyl group, suggesting that an alkoxysilyl functional group is more attractive to retard the charge recombination reaction between an electron in the TiO2 conduction band and an oxidized form of an electrolyte redox couple. The highest solar energy conversion efficiency of 2.6 % was achieved for dye sensitized solar cells based on an azobenzene dye sensitizer under AM1.5G, one sun condition.

KEY WORDS dimethylaminoazobenzene, charge recombination dynamics, open circuit voltage, highest solar energy conversion efficiency


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1. INTRODUCTION Performance of dye sensitized solar cells is driven by photo-induced charge transfer processes mimicking natural photosynthetic reaction systems.1-4 Following excitation of a dye sensitizer, efficient forward and slow reverse charge transfer processes are required.5-7 These reaction kinetics are controlled at the interfaces of materials and charge transport processes in the cell. Interfacial material design is therefore crucial to improve the performance of dye sensitized solar cells.8-12 Among the material interfaces in dye sensitized solar cells, a titanium dioxide (TiO2) nanocrystal-sensitizer-electrolyte interface is the most critical. To improve charge separation efficiency, sensitizer dye design has been intensively studied, in particular novel organic sensitizer dyes have been designed to replace conventional ruthenium dyes.13-15 In contrast, dye anchoring groups to link a sensitizer chromophore and the TiO2 surface have relatively been less studied, although the linkage is vital for solar cell function. A carboxyl functional group has been commonly employed, since it has shown the best solar cell efficiency and employing other functional groups has shown relatively poor solar cell performance.9, 16-17 Dye sensitized solar cells based on the sensitizer dyes attached through a carboxyl group have, however, indicated relatively poor long term durability.18 Previous studies found that a dye with a carboxyl functional group easily detaches from the TiO2 surface in the presence of water in the electrolyte or in alkaline solution.9, 18-19 The dye desorption occurs from a hydrolysis reaction at the dye/TiO2 interface, resulting in labile solar cell performance.18, 20-21 Thus, it is desirable to design an amphiphilic dye to prevent water from attaching to the TiO2 surface,18 or an alternative dye anchoring group which is durable and strong binding to the TiO2 surface.9, 19


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A functional group to form a tight linkage with the TiO2 surface is extremely attractive. Several alternative groups such as phosphonate, hydroxyl and silanol groups were previously investigated with their tight binding ability to the TiO2 surface.9,

17, 22

Siloxy linkages were

studied by employing dyes with silatrane groups in dye sensitized solar cells.23 These studies identified that siloxy linkages to the TiO2 surface are more stable than the linkages formed by carboxylic or phosphonic acids.22-23 Hanaya et al. employed alkoxysilyl groups as a functional group and concluded their superior surface bonding stability compared to a carboxylic group.19-20, 24

However, no detailed investigation has been conducted towards influence of an alkoxysilyl

group on interfacial charge transfer dynamics, and thus solar cell performance. Here, we employ alkoxysilyl and carboxyl groups as a functional group for a sensitizer dye to compare the interfacial charge transfer dynamics in addition to the solar cell performance. We have chosen azobenzene dyes as a chromophore owing to low cost and a widely available colorant in industry.17, 25 In particular, dimethylaminoazobenzene has been investigated, as it is one of the simplest structures having a donor-acceptor charge transfer state in a dye. In this study, 4-dimethylaminoazobenzene-4'-carboxylic acid (M0423) and 4-dimethylaminoazobenzene-4'triethoxysilane (SFD-3) were compared. Scheme 1 shows chemical structures of these two sensitizer dyes. Dimethylamino group is attached at one end of azobenzene as an electron donor, and the anchoring functional group is linked to azobenzene at the other end. Bidentate C-O-Ti and tridentate Si-O-Ti linkages to the TiO2 surface are expected to form for M0423 and SFD-3 dyes, respectively. After the linkage formation, the dimethylamino group faces outwards the TiO2 surface. Electrochemical and spectroscopic measurements were performed to correlate potential energy levels and dynamics,


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respectively, with the performance of dye sensitized solar cells to identify a role of a functional group to improve solar cell performance.

M0423

SFD-3

Scheme 1. Chemical structures of dyes M0423 and SFD-3.

2. EXPERIMENTAL METHODS Chemicals. All purchased chemicals were used without any further purification. Ethanol (95 % pure) was purchased from Pacific Laboratory Products Australia Pty Ltd.. Lithium iodide (>97 % pure) and iodine (>99.9 % pure) were purchased from Wako Chemicals. 4-tertbutylpyridine (TBP, >96.0 % pure), acetonitrile (>99.5 % pure) and dimethylaminoazobenzene4'-carboxylic acid (M0423) were purchased from Tokyo Chemical Industry Co., Ltd.. Synthesis of 4-dimethylaminoazobenzene-4'-triethoxysilane (SFD-3) is described in the supporting information. Preparation of dye sensitized TiO2 films for solar cell assembly. TiO2 paste was prepared by mixing P25 TiO2 powder (Nippon Aerosil), water, nitric acid, acetylacetone, Triton X-100, and polyethylene glycol with an ultrasonic system and a homogenizer. TiO2 nanocrystalline films were prepared by screen-printing the TiO2 paste onto a fluorine doped tin oxide (FTO) glass plate (25 × 50 mm2, 9 - 11 Ω/sq., Asahi Glass, type-U) and sintering them at 480 °C for 30 min. The film thickness was estimated to be ~14 µm from a cross-section image using a field emission scanning electron microscope (FE-SEM, JEOL JSM-6330F) with an operating voltage


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of 15.0 kV. The films were then dipped in 40 mM TiCl4 aqueous solution and afterwards sintered at 500 °C for 15 min. Dye sensitized TiO2 films were obtained by immersing the film in 0.3 mM dye dissolved in toluene at 100 °C for 15 hours. We found that this sensitization temperature is more suitable to adsorb a larger amount of the sensitizers on the TiO2 surface compared to room temperature, in order to maximize the solar energy conversion efficiency. The sensitized films were washed in toluene and subsequently methanol to remove excess dyes after the sensitization. Optical characterization of dyes. Absorption spectra for dye in solution phase were measured by a UV-Vis absorption spectrometer (Shimadzu, UV-2450). Absorption spectra of TiO2 films sensitized by M0423 and SFD-3 were measured by a UV-Vis absorption spectrometer with an integrating sphere detector (Hitachi U-3010). Photoluminescence spectra of dye solution were collected in a 1 x 1 cm2 quartz glass cuvette with 90º incident excitation using a PTI UV-Vis fluorometer (Photon Technology International, Inc.) with slit widths of 0.25 mm (1 nm resolution) at room temperature. Photoluminescence quantum yield (QY) was determined by using an integrating sphere emission collector. The dye was excited at an absorption peak, 428 nm for M0423 and 416 nm for SFD-3 (solution absorbance: 0.2~0.3), respectively. Electrochemical measurements. Details of cyclic voltammetry and spectroelectrochemistry measurements for M0423 and SFD-3 dye solutions are described in the supporting information. Photovoltaic measurements. Sandwich type solar cells were assembled by binding an I3-/Iredox electrolyte with the dye-adsorbed TiO2 electrode and a Pt counter electrode through a polyethylene film spacer (30 µm thick). The counter electrode was prepared by sputtering Pt on a FTO plate, and the electrolyte was an acetonitrile solution containing 0.50 M LiI, 0.05 M I2, and 0.50 M 4-tert-butylpyridine (TBP). The photovoltaic performance was assessed by observing incident photon-to-current conversion efficiency (IPCE) spectra and current density – voltage (J


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– V) curves under the light irradiation with an irradiation aperture area of 1.20 × 1.20 cm2 using a photo-mask. IPCE spectra were obtained by using a monochromatic light source (Bunkoh-Keiki, SM-25) and a source meter (Advantest, R8240). J – V curves were measured under AM1.5G solar simulated light (one sun condition, 100 mW/cm2 at 25 ± 2 °C) from a solar simulator (Bunkoh-Keiki, OTENTO-SUN III) and using a source meter (Advantest, R6240A). The simulated light power density was calibrated by a reference Si photodiode (Bunkoh-Keiki, BS520). The measurements were performed by sweeping voltage between two electrodes with a step of 5 mV and delay time of 100 ms. Transient open circuit voltage decay measurements. The same solar cells described above were employed to measure transient open circuit voltage decays. The cell was illuminated at an open circuit condition under AM1.5G solar simulated light (one sun condition, 100 mW/cm2 at 25 ± 2 °C) from a solar simulator (Asahi Spectra Co., Ltd., HAL-320). After the open circuit voltage indicates a steady value, the illumination was turned off with a shutter, and an open circuit voltage decay was observed using a potentiostat (Ivium Technologies B.V., Compact Stat) with the measurement step of 100-200 ms. Density Functional Theory (DFT) Calculation. DFT calculation was conducted to estimate potential energy levels and electron densities of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) states of the dye. Dye molecular structures were first optimized, and then the energy levels were calculated using the Gaussian 03 program package.26 A Beck’s three-parameter hybrid functional with the Lee-Yang-Parr gradientcorrected functional (B3LYP) was employed together with 6-31G (d,p) basis set. The calculated molecular orbitals were visualized by the GaussView 3.


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Transient absorption measurements. Transient absorption spectroscopy was employed to monitor charge recombination dynamics between an oxidized dye and an electron in TiO2. Transparent TiO2 nanocrystalline films were prepared by the screen printing method with a TiO2 paste (JGC C&C, NR-18) on a glass slide. After the printing, the film was sintered in an air flow oven at 500 °C for 1 hour, and then immersed in a 30 µM dye ethanol solution for at least 24 hours. The similar absorbance was obtained for both M0423 and SFD-3 sensitized films. The film thickness is approximately 5 µm. Submicrosecond to millisecond transient absorption studies were conducted by a home-built transient absorption spectrometer with a N2 laser (OBB, OL-4300) pumped dye laser (OBB, OL401, 800 ps pulse duration) as a pump source, a 100 W tungsten lamp (Olympus) as a probe source, a photodiode-based detection system (Costronics Electronics), and a TDS-2022 Tektronix oscilloscope. Monochromatic probe light was obtained from the tungsten lamp through two monochromators (JASCO, M10). An excitation wavelength was selected to 428 and 416 nm for M0423 and SFD-3 dye sensitized TiO2 film, respectively (film peak absorbance: 0.2~0.3), while transient decays were monitored at 520 and 670 nm for M0423 and SFD-3 dye, respectively. Experiments were conducted with a low pulse excitation energy density (40~60 µJ/cm2), corresponding to ~1 absorbed photons per TiO2 nanoparticle, with a repetition rate of 2 Hz at 22 °C. The excitation energy adjustment was finally confirmed by obtaining a transient absorption amplitude of ∼ 1.0 m∆O.D. at 800 nm using the TiO2 film sensitized by cisbis(isothiocyanato)bis(2,2'-bipyridyl-4,4'-dicarboxylato)-ruthenium(II), N3 (Solaronix), for each excitation wavelength, following the previous report.5, 27 No change in steady state absorption spectra before and after the transient experiments was observed, suggesting that the samples were stable during the optical experiments.


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3. RESULTS Dye electrochemical data. Cyclic voltammetry and spectroelectrochemical measurements were performed to determine oxidation potentials for M0423 and SFD-3 dyes. The experimental details and results are described in the supporting information. Following cyclic voltammetry data shown in Figure S1, the oxidation potentials obtained for M0423 and SFD-3 dyes are 1.19 V and 1.13 V vs. Ag/AgCl, respectively, from their redox peaks. The oxidation potentials determined from the cyclic voltammetry measurements were corroborated by conducting spectroelectrochemical measurements. Accordingly, the oxidation potentials of M0423 and SFD-3 dyes were determined to be 1.04 and 1.18 V vs. Ag/AgCl, respectively (See Figure S2). These values almost agree with the cyclic voltammetry data. The dye oxidation potentials (Eox) were finally determined by averaging the data obtained by these two different methods, and the results are summarized in Table 1.

Table 1. Optical and electrochemical data for M0423 and SFD-3 dyes. dye

absorptiona λmax / nm (ε / dm3 mole-1 cm-1)

emissiona λmax / nm

E0-0b

emission quantum yield × 105

E / eV

oxidation potential / V vs. Ag/AgClc Eox Eox*

driving force / eVd ∆Ginj

∆Greg

M0423

428 (25,100)

549

5.3±0.8

2.25

1.11 ±0.08

-1.14 ±0.08

-0.49 ±0.08

-0.86 ±0.08

SFD-3

416 (25,800)

537

5.9±0.8

2.30

1.15 ±0.03

-1.15 ±0.03

-0.50 ±0.03

-0.90 ±0.03

a

Steady state absorption and emission spectra were observed using ethanol as a solvent. HOMO-LUMO gap estimated from the absorption onset of the steady state absorption spectrum. c Excited state oxidation potentials (Eox*) estimated from Eox and E0-0 (vs. Ag/AgCl). dDriving forces for electron transfer processes. ∆Ginj: Driving forces for the electron injection from the b


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singlet excited state (Eox*) of the dye to the TiO2 conduction band (-0.65 V vs. Ag/AgCl). ∆Greg: Driving forces for the re-reduction process for the dye radical cation state (Eox) by the I3-/I- redox state (+0.25 V vs. Ag/AgCl).

Optical characterization of dyes in solution phase. Steady state absorption and emission spectra measurements were performed for dyes in solution phase. Figure 1 shows normalized absorption spectra of M0423 and SFD-3 dyes dissolved in ethanol (3 µM). The main absorption band with a peak around 420 nm for both dyes is assigned to a π – π* transition in the trans form, following the previous reports.20, 28-30 While the extinction coefficients, ε, for these two dyes (summarized in Table 1) indicate essentially the same, the spectrum obtained for M0423 has slightly red-shifted compared to that of SFD-3. This observation is similar to the spectra obtained for the porphyrin dyes with different functional groups, investigated recently by Gust et al.9 A HOMO-LUMO gap of these dyes is estimated from the absorption onset of the spectrum, resulting in 2.25 and 2.30 eV for M0423 and SFD-3 dyes, respectively (Table 1).


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Figure 1. Steady state absorption and emission spectra of the dyes dissolved in ethanol (3 µM). The absorbance was normalized to the absorption maximum of M0423 (428 nm, red dotted line) and SFD-3 (416 nm, blue dotted line) spectra, respectively. The photoluminescence spectra were normalized to the emission maximum for the dyes, M0423 (549 nm, red solid line) and SFD-3 (537 nm, blue solid line), respectively.

Photoluminescence spectra of M0423 and SFD-3 dyes are also shown in Figure 1. Extremely weak signals were observed, however the spectra were obtained after photon accumulation at each wavelength for an extended period, suggesting that the dominant relaxation of the excited state is non-radiative.17 M0423 dye shows an emission peak at 549 nm with a Stokes shift of 5,150 cm-1, while an emission peak at 537 nm with a Stokes shift of 5,420 cm-1 was observed for


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SFD-3 dye. Similar to the absorption spectra, the spectrum obtained for M0423 dye has also redshifted compared to that of SFD-3 dye. The observed photoluminescence quantum yields are 5.3±0.8 × 10-5 and 5.9±0.8 × 10-5 for M0423 and SFD-3 dyes, respectively. Previous ultrafast transient emission and absorption study for an azobenzene derivative shows a similar stokes shift, and a fast emission decay time constant of 100 fs, supporting a low emission quantum yield.31 DFT calculation for sensitizers. DFT calculation was employed to identify electron density profiles of HOMO and LUMO for the two dyes. The calculated results are shown in Figure 2. The HOMO profiles for M0423 and SFD-3 dyes are essentially identical. In contrast, only a slight difference was appeared for the LUMO electron density profile. The LUMO electron density extended to the carboxyl group for M0423 dye while the LUMO state is confined within the chromophore for SFD-3 dye. This extended electron density for M0423 dye may support the red shift of the absorption spectrum shown in Figure 1.

M0423 HOMO

SFD-3 HOMO

M0423 LUMO

SFD-3 LUMO

Figure 2. Electron density profiles of HOMO and LUMO states of M0423 and SFD-3 dyes, resulting from DFT calculation.


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Comparing the electron densities of HOMO and LUMO states, both dyes indicate that the HOMO state is localized in the donor region, while the electron density increases closer to the anchoring functional group for the LUMO state. Cole et al. recently reported similar DFT calculation results for M0423 dye, however in their case, dye attached TiO2 configurations were employed.17, 29 This electron density shift favors the electron injection process from the sensitizer to the TiO2 and the retardation of the charge recombination process between an oxidized dye and an electron in the TiO2. It is worth noting that the dyes rotate at any angle against the TiO2 surface after adsorbed to TiO2 surface, altering the distance between the LUMO state and the TiO2 surface. Nevertheless, the molecular density profiles obtained for the HOMO and the LUMO are still attractive for efficient electron injection and retarded charge recombination processes. Photovoltaic performance. IPCE spectra of dye sensitized solar cells are shown in Figure 3a. The M0432 dye sensitized solar cell indicates slightly higher IPCEs around 400~500 nm, while SFD-3 dye shows slightly higher IPCEs at longer wavelengths. Interestingly, both spectra indicate photocurrent response onset of approximately 700 nm, significantly red-shifted compared to the absorption onset of the dyes in solution phase. In order to assess forward charge transfer processes at short circuit condition, APCE (absorbed photon-to-current conversion efficiency) spectra are evaluated by the following equation (1).

= 1 − × ×

(1)


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where λ is monochromatic light wavelength, r(λ) is incident light loss before reaching the TiO2 film in the cell, e.g. the light reflection loss at the interfaces in the cell,7 and LHE(λ) is light harvesting efficiency.

(a)

(b)

LHE

IPCE

Figure 3. (a) IPCE (solid line) and LHE (dotted line) spectra, and (b) APCE spectra for TiO2 films sensitized by M0432 (red line) or SFD-3 (blue line) dye.

Absorption spectra of TiO2 films sensitized by M0423 and SFD-3 were measured, and then the LHE(λ) was calculated. The LHE spectra are shown in Figure 3a. For both dye sensitized films, LHE(λ) reached the maximum in a wavelength range of 400~500 nm. Interestingly, in contrast to the dye in solution phase, the spectrum obtained for SFD-3 sensitized film is red-shifted compared to M0423 sensitized film. Moreover, for both dyes, the absorption onset of the sensitized films is largely red-shifted by approximately 150 and 200 nm, i.e. approximately 700 and 750 nm onset for M0423 and SFD-3 dyes, respectively. Recent time-dependent density function theory calculation for M0423 dye with the comparison of the energy levels between a free dye and a dye sensitized TiO2 form suggested that the energy gap between the HOMO and


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LUMO levels decreases when the dye is attached to the TiO2 surface.29 Further, the calculated absorption spectrum of the dye sensitized TiO2 film has been red-shifted compared to the free dye, in agreement with our observed results (Figure 3a). We also postulate that this spectral shift may result from the interaction between the dye molecules attached on the TiO2 surface. Such interaction was typically observed as a result of J-aggregate formation for cyanine dyes.32 We have previously evaluated incident light loss, r(λ), by conducting optical simulation.33 Using this method, incident light loss was calculated for the film with the porosity of 0.73, typically obtained for the TiO2 film prepared from P25, in the supporting information. Figure S3 shows [1 - r(λ)] spectrum, i.e. transmittance spectrum, for the light entering to the TiO2 layer after reflection at the interfaces of a fluorine doped tin oxide (FTO) glass and absorption by a FTO layer. Using the LHE spectrum shown in Figure 3a and the [1 - r(λ)] spectrum shown in Figure S3, APCE spectra for M0423 and SFD-3 dyes were calculated, and the results are shown in Figure 3b. The data clearly indicate that APCE spectra for M0423 and SFD-3 dyes are indistinguishable except for a 400~500 nm wavelength region. Moreover, in both cases, wavelength dependent APCE was observed, implying that the electron injection efficiency from the dye to TiO2 is wavelength dependent. The details of this wavelength dependence are discussed later. Figure 4 shows J – V curves for M0423 and SFD-3 dye sensitized TiO2 solar cells under the light irradiation (AM1.5G, 1 sun), and the performance data are summarized in Table 2. The difference in the short circuit photocurrent is indistinguishable. The integration of the IPCE spectra shown in Figure 3a with the AM1.5G solar spectrum7 results in 5.8 mA cm-2 for both M0423 and SFD-3 dyes, in close agreement with the observed Jsc values in Table 2. In contrast, increase in Voc was clearly observed for SFD-3 dye by 60 mV, compared to M0423. Owing to


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this increased Voc, the SFD-3 dye sensitized solar cell has indicated the highest solar energy conversion efficiency of 2.6 % for azobenzene dye sensitizer based solar cells.

SFD-3

M0423

Figure 4. J – V curves for M0423 (red line) and SFD-3 (blue line) dye sensitized TiO2 solar cells. The measurements were performed under AM1.5G solar simulated light, 100 mW cm-2 at 25 ± 2 °C (solid line) and in dark (dotted line).

Table 2. Comparison of solar cell performance for M0423 and SFD-3 dyes.


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dye

Jsc / mA cm-2

Voc / V

FF

efficiency / %

M0423

5.9 ± 0.1

0.61 ± 0.02

0.67 ± 0.02

2.4 ± 0.1

SFD-3

5.8 ± 0.1

0.67 ± 0.01

0.66 ± 0.02

2.6 ± 0.1

Comparing with the J – V curves observed in dark suggests that this Voc increase for the SFD3 dye sensitized TiO2 solar cells originates from slower charge recombination between an oxidized electrolyte and an electron in the TiO2. To clarify this slower charge recombination mechanism, transient open circuit voltage decays were observed for both M0423 and SFD-3 dye sensitized solar cells at an open circuit condition after turning off the solar simulated light irradiation, and the results are shown in Figure 5a. The data clearly indicate a slower Voc decay for the SFD-3 dye sensitized solar cell. Following these data, we have calculated the lifetimes of the electrons in the TiO2 conduction band based on the following equation (2).34

= −

(2)

where τn is lifetime of an electron in the TiO2 conduction band, k is Boltzmann constant, T is temperature, e is an elementary charge, and t is time after turning off the solar simulated light. The calculated lifetimes as a function of open circuit voltage are shown in Figure 5b. The results clearly indicate that the electron lifetime is significantly larger for the SFD-3 dye sensitized solar cell at higher Voc region in agreement with the increase of Voc in the solar cell performance, compared to that of the M0423 dye sensitized solar cell.


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(a)

Page 18 of 36

(b)

Figure 5. (a) Open circuit voltage decays for M0423 (red line) and SFD-3 (blue line) dye sensitized TiO2 solar cells obtained after turning off the solar simulated light irradiation. (b) Lifetimes of the electrons in the TiO2 conduction band plotted as a function of the open circuit voltage.

Note that the solar cells in the present study appear to be stable over several hours without any sign of decrease in performance. We did not observe any photoisomerization process of our dyes attached to the TiO2 surface during the simulated light irradiation. Cole et al. recently reported TiO2 catalyzed photoisomerization from the trans to cis form for M0423 sensitized TiO2 films.29 We have considered several cases to explain this difference, i.e. our stable solar cell behavior. Firstly, our sensitization method may form a close packed dye monolayer on the TiO2 surface, and thus the isomerization from the trans to cis form is sterically hindered. Secondly, the magnitude of the isomerization is negligible. Joo et al. recently reported that negligible trans to cis photoisomerization process was observed for M0423 dyes in water or ethanol probably owing


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to fast thermal recovery to the trans form with low activation energy.28 The similar fast thermal recovery possibly occurs for M0423 dye surrounded by highly concentrated ions in the solar cell. Finally, the electron injection process from the excited trans dye to the TiO2 is faster than the dye isomerization process. As discussed above, we have observed relatively high APCE (see Figure 3b), and thus the last case, i.e. the faster electron injection than the photoisomerization process, is the most plausible. Transient absorption data. Transient absorption studies were performed for a TiO2 film sensitized by M0423 or SFD-3 dye. Figures 6a and 6c show transient absorption spectra at 2 µs after pulse excitation, obtained for M0423 and SFD-3 dyes, respectively. Comparing with the steady state absorption spectra (inverted spectra are shown in the same figure), the negative amplitudes can be assigned to the ground state bleach. In contrast, the transient absorption spectra between M0423 and SFD-3 dye sensitized TiO2 films are slightly different, i.e. a larger absorption amplitude for M0423. The difference in these positive amplitudes can be attributed to the difference in the spectra of the dye cation states by comparing the spectroelectrochemically generated oxidized dye spectra (see Figures S2a and S2b in the supporting information). The results therefore suggest that the transient spectra indicate charge separated spectra following the electron injection process from the sensitizer dye to the TiO2. Note that the injected electron in the TiO2 exhibits relatively small amplitude compared to the dye cation state.5, 35


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(a)

(b)

(c)

(d)

Figure 6. (a, c) Transient absorption spectra for the TiO2 film sensitized by M0423 dye (a, brown circle and red solid line) and SFD-3 dye (c, dark blue circle and blue solid line) at 2 µs after 428 and 416 nm excitation, respectively. The normalized inverted steady state absorption spectrum is also shown as a broken line. (b, d) Transient absorption decays for M0423 dye (b, red dots), SFD-3 dye (d, blue dots) and N3 dye (green dots) sensitized TiO2 films monitored at 520, 670 and 800 nm, respectively. The results of the fit using a stretched exponential function


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are shown as black solid lines. The decay amplitudes were normalized to 1.0 mΔO.D. at the initial fitted time (i.e. 0 s).

Figures 6b and 6d show transient absorption decays monitored at the cation absorption peak for M0423 and SFD-3 dyes, respectively. As comparison, transient absorption decay for N3 sensitized TiO2 film is also shown in the same figure. The data clearly indicate that the decays for both dyes are indistinguishable with that for N3 sensitized film. These decay profiles were analyzed by a stretched exponential function shown as the following equation (3).

-

∆"#$ = ∆"#% exp )− *$+ , .

(3)

where ∆OD(t) is the differential optical density at time t, ∆OD0 is the initial differential optical density when t = 0, t is time after pulse excitation, τ is lifetime, β is a stretched parameter. The analysis indicates that the decay lifetime is 6.9 ± 1.7 ms for SFD-3 and 5.2 ± 2.0 ms for M0423, whereas the reference decay lifetime of N3 sensitized film is 7.7 ± 3.0 ms, in agreement with the previous observation.5,

35

As mentioned above, since no electrolyte is involved in these

measurements, no dye regeneration reaction occurs in this kinetic experiment. Therefore, these lifetimes directly relate to geminate charge recombination reaction between an oxidized dye and an electron in the TiO2. Moreover, comparison of the decay lifetimes for these two dyes has revealed that the dye anchoring group type does not influence the charge recombination reactions at the dye/TiO2 interface.


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4. DISCUSSION Influence of an anchoring functional group type on dye adsorption. The adsorption of the dyes to TiO2 would occur through the carboxyl or alkoxysilyl group. Since the similar LHE spectra (Figure 3a) and APCE spectra (Figure 3b) were obtained, the dye adsorption amount must be similar for both dyes. These results are in contrast to the results reported by Gust et al. who demonstrated that a larger amount of porphyrin dyes with a carboxyl group were attached to the TiO2 surface, compared to the dyes with a phosphonate group or a silatrane group.9 For a carboxyl functional group, it has been speculated that a unidentate, ester-like, linkage is formed on the surface of TiO2,36 however previous IR studies support bidentate bridging or chelating linkage.9,

16, 37-40

Theoretical studies using a DFT method also suggested that the

bidentate mode is more stable for the dye attachment through the carboxyl group.17, 41 We have conducted FT-IR measurements of M0423 dye sensitized TiO2 films. The result in Figure S4 indeed shows a typical strong absorption band at 1,410 cm-1, attributable to a carboxylate symmetric (-COO-s) band9,

16, 37-40

, while a weak (C=O) stretching mode from the carboxyl

anchoring group was observed at 1,685 cm-1.16, 40 Based on the comparison with the previous studies,9, 16, 37-40 we conclude that M0423 dye mainly forms a bidentate linkage. With respect to alkoxysilyl functional group, the adsorption is achieved by a covalent bond through Si-O-Ti bonding.20,

42-43

Previous studies reported that trialkoxysilyl groups possibly

form a bidentate or tridentate linkage on a metal oxide surface.42-43 Such linkage is formed by direct condensation reaction between the alkoxysilyl group and the hydroxyl group on a metal oxide surface. FT-IR spectrum obtained for SFD-3 dye sensitized TiO2 films is also shown in Figure S4. A stretching Ti-O-Si mode was clearly observed around 930~960 cm-1 in agreement


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with the previous study.44 Also strong bands were observed at 1,120 and around 1,000 cm-1, attributable to asymmetric (Si-O-Si) stretching modes.43-45 Thus, we conclude that SFD-3 dye can form a bond with Ti on the TiO2 surface through a Si-O-Ti bond, and the attached dye molecules interact to each other with the formation of Si-O-Si bonds. Comparison of potential energy levels. The estimated HOMO-LUMO gaps obtained from the onset of the steady state absorption spectra and the electrochemically determined HOMO level are summarized in Table 1. These potential energy levels with N3 dye, TiO2 conduction band edge, and I3-/I- redox potential data from the previous reports2 are described in the potential energy diagram in Scheme 2. The driving forces for the electron injection and dye regeneration processes are almost identical for M0423 and SFD-3.

Sensitizer dye (1)

-1.14

-1.15

-1

-0.75

-0.65 (4)

M0423

0

TiO2

N3 Dye SFD-3

I3-/I+0.25

(3) (2)

1

+0.85 +1.11

+1.15

V vs. Ag/AgCl ACS Paragon Plus Environment

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Scheme 2. Potential energy diagram of M0423 and SFD-3 dyes. TiO2 conduction band edge, I3/I- redox and N3 dye potentials are also presented for the comparison. The arrow indicates an electron transfer process; (1) an electron injection from a dye excited state to the TiO2 conduction band, (2) a dye regeneration reaction from a redox electrolyte, (3) an electron transfer from the TiO2 conduction band to an oxidized dye (recombination at the TiO2/dye interface), and (4) an electron transfer from the TiO2 conduction band to an oxidized form in the electrolyte (recombination at the TiO2/electrolyte interface).

These forward reaction processes may be thermodynamically more favorable to occur compared to N3. Previous reports suggest that driving forces of 20 kJ mol−1 (∆E ≈ 0.20 eV) and 20~25 kJ mol−1 (∆E ≈ 0.20~0.25 eV) would be sufficient to achieve 99.9% electron injection and dye regeneration processes under the typical solar cell operating conditions.3, 5 Based on this analysis, we could expect that these two forward processes occur efficiently for M0423 and SFD-3 dyes. Following the results shown in Figure 3b, the wavelength dependent APCEs were clearly observed for both dyes. Recently, Cole and co-workers conducted theoretical studies of HOMOLUMO level of M0423 dye using a time-dependent density functional theory.29 Their study predicted that the energy gap between the LUMO and the TiO2 conduction band edge decreases for the dye sensitized TiO2 film compared to the free dye form. Previous studies have shown that an electron injection process occurs with multi-exponential kinetics.5, 35 The multi-exponential dynamics may originate from the exponential distribution of the electron acceptor states in the TiO2 conduction band. This implies that an electron injection process may occur with a lower


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efficiency by excitation of the lower lying absorption band of a M0423 or SFD-3 dye sensitized TiO2 film. To clarify the influence of the exponential distribution of the electron acceptor states on the electron injection processes, we have conducted IPCE measurements for SFD-3 dye sensitized TiO2 films by shifting the potential energy levels of the TiO2 conduction band with modification of electrolyte composition. The measurements were performed in the presence and absence of TBP in the electrolyte, since the presence of TBP is known to shift the TiO2 conduction band edge negatively.46 As shown in Figure S5, the IPCEs of the solar cell in the absence of TBP in the electrolyte were increased by approximately 5 % compared to those in the presence of TBP over the entire wavelengths. Previous studies suggest that the negative shift of the conduction band edge results in retardation of the electron injection reactions competing with the excited state decay kinetics.47-48 These studies suggest that the excitation of the lower lying absorption band of the dyes decreases the electron injection efficiency owing to the decreased number of the electron acceptor states, and therefore, the wavelength dependent APCEs observed in Figure 3b originate from the wavelength dependent electron injection efficiency. Influence of an anchoring functional group type on solar cell performance. Following the potential energy diagram shown in Scheme 2, four different charge transfer processes can be considered to control solar cell performance. Two are forward electron transfer processes, i.e. (1) an electron injection from the dye to the TiO2 conduction band, and (2) a dye regeneration reaction from the redox electrolyte. The other two are charge recombination processes between (3) an oxidized dye and an electron in the TiO2 conduction band, and (4) an oxidized form in the electrolyte and an electron in the TiO2 conduction band, as shown in Scheme 2.


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From the electron density profiles (Figure 2), the LUMO electron density extended to the carboxyl group for M0423 dye while the LUMO state is confined within the chromophore for SFD-3 dye. These results suggest that the electronic coupling between the M0423 dye LUMO and the TiO2 conduction band may be larger than that for SFD-3 dye, implying that the electron injection rate from the M0423 dye excited state may be faster. Since the similar APCE spectra were obtained for both dyes (Figure 3b), most of the electron injection processes must be completed prior to the excited state internal conversion process and the decay to the ground state. Reid et al. reported ultrafast studies of trans-dimethylaminoazobenzene in polar solvent that the excited state internal conversion occurs with time constants of 0.8~1.0 ps and the excited state decay to the ground state with time constants of 3~5 ps.30 Thus, the majority of the electron injection processes at 400~500 nm excitations occur at least earlier than 5 ps. Thus, we conclude that the electron injection efficiency is not influenced by the difference in the electronic couplings. Following the driving forces (Table 1) and the APCE spectra (Figure 3b), it is plausible to consider that (2) a dye regeneration reaction from the redox electrolyte occur on the similar timescales for M0423 and SFD-3 dyes. In addition, the charge recombination kinetics (3) between an oxidized dye and an electron in the TiO2 (Figures 6b and 6d) are almost identical for these two dyes. Based on these similar results, we conclude that the difference between the alkoxysilyl and carboxyl groups does not influence charge transfer dynamics in processes (1)-(3). Recently Gust et al. reported the influence of a dye anchoring functional group on the solar cell performance, and concluded that the porphyrin dyes with a carboxyl, phosphonate or silatrane group show indistinguishable photocurrent production performance, as long as the dye loading amount on the TiO2 surface is similar.9 Our present studies essentially agree with their conclusion.


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In contrast, the clear difference in Voc was observed in Figure 4. The SFD-3 dye sensitized solar cell indicates higher Voc by 60 mV. As discussed above, based on the J-V curves observed in dark (Figure 4) and transient Voc decays (Figure 5), this Voc difference originates from the difference in charge recombination kinetics between an electron in the TiO2 and an oxidized form in the electrolyte (4). The FT-IR spectra (Figure S4) suggested that the siloxy functional group derived from hydrolysis of an alkoxysilyl group form Si-O-Si bonds resulting from intermolecular condensations.9,

42-43

In such a case, approach of an oxidized form of the

electrolyte to the TiO2 surface must be hindered. The result of the Voc increase therefore originates from the multiple bonds formed through alkoxysilyl functional groups on the TiO2 surface, reducing the TiO2 recombination sites, and/or the intermolecular Si-O-Si bonds between the adsorbed dyes, decreasing the rate of the charge recombination between an electron in TiO2 and an oxidized form of the electrolyte. These simultaneous linkages to the electrode also support a long term stability of the dye attachment during the solar cell operation, compared to the linkage through the carboxyl group.

5. CONCLUSION In this study, we have investigated influence of the difference between dye carboxyl and alkoxysilyl functional groups on performance of dye sensitized solar cells. The similar potential energy levels and electron density profiles were obtained from electrochemical measurements and DFT calculation for the two dyes. The forward and backward charge transfer kinetics at the dye/TiO2 interface appear to be almost identical for the two dyes. In contrast, the dye with the alkoxysilyl functional group indicates increases in the open circuit photovoltage (Voc) by 60 mV and in the lifetimes of an electron in the TiO2 conduction band, compared to the dye with the


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Page 28 of 36

carboxyl group. The multiple bonds through the alkoxysilyl group on the TiO2 surface and/or the intermolecular Si-O-Si bonds between the adsorbed dyes retard the charge recombination reaction between an electron in the TiO2 and an oxidized form of the electrolyte. We therefore conclude that alkoxysilyl functional group is more attractive, compared to a carboxyl group, to increase Voc, and thus to improve solar cell performance.

AUTHOR INFORMATION Corresponding Author Tel: +61 (0)3 9925 6127; Fax: +61 (0)3 9925 6139 *E-mail: [email protected]; [email protected]; [email protected]

ACKNOWLEDGMENT This work was supported by the “Element Innovation” Project by the Ministry of Education, Culture, Sports, Science & Technology in Japan, and the Office for University-Industry Collaboration, Osaka University, Japan. We thank Mr. Frank Antolasic, School of Applied Sciences, RMIT University, for FT-IR measurements.

ASSOCIATED CONTENT Supporting Information. Experimental details and results of dye electrochemical and spectroelectrochemical measurements, FT-IR measurements of dye sensitized TiO2 films, and IPCE measurements of


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Kinetics

for

the

Nanocrystalline

TiO2

Films

Sensitized

with

the

Dye

(Bu4N)2Ru(dcbpyH)2(NCS)2. Chem. Phys. 2002, 285, 127-132. (48) Haque, S. A.; Palomares, E.; Cho, B. M.; Green, A. N. M.; Hirata, N.; Klug, D. R.; Durrant, J. R., Charge Separation versus Recombination in Dye-Sensitized Nanocrystalline Solar Cells: the Minimization of Kinetic Redundancy. J. Am Chem. Soc. 2005, 127, 3456-3462.


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Table of Contents (TOC) Image

C-O-Ti linkage I3 Faster

N

N

recombination

N

-

e O N

N

TiO2

O

Si-O-Ti linkage I3 Slower

recombination

N

-

e Si O O O

TiO2


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Dye-Anchoring Functional Groups on the Performance of Dye

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