oxide interface via

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Cite this: DOI: 10.1039/c5cp01816a

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Understanding the role of the dye/oxide interface via SnO2-based MK-2 dye-sensitized solar cells Dae-Yong Son,a Chang-Ryul Lee,a Hee-Won Shin,a In-Hyuk Jang,b Hyun Suk Jung,c Tae Kyu Ahn*a and Nam-Gyu Park*ab To understand the role of the dye/oxide interface, a model system using a nanocrystalline SnO2 and 3-hexyl thiophene based MK-2 dye is proposed. A thin interfacial TiO2 blocking layer (IBL) is introduced in between SnO2 and MK-2 and its effects on photocurrent–voltage, electron transport-recombination, and density of states (DOS) are systematically investigated. Compared to the bare SnO2 film, the insertion of IBL leads to a 14-fold improvement in the power conversion efficiency (PCE) despite little change in the dye adsorption amount, which is due to the 7-fold and 2-fold increase in the photocurrent density and voltage, respectively. The charge collection efficiency is substantially improved from 38% to 96% mainly due to the increase in the electron lifetime. The IBL is also found to enhance the dye regeneration

Received 28th March 2015, Accepted 29th April 2015

efficiency as confirmed by the 15-fold faster dye bleaching recovery dynamics. The recombination resistance increases and the DOS decreases after surface modification of SnO2, which is responsible for

DOI: 10.1039/c5cp01816a

the doubly increased voltage. This study suggests that the interfacial layer between the oxide and the dye plays a crucial role in retarding recombination, improving charge collection efficiency, increasing diffusion

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length, accelerating dye regeneration and narrowing the density of states.

1. Introduction Dye-sensitized solar cells (DSSCs) based on nanocrystalline wide band gap oxide semiconductors have been intensively studied for the last two decades. Although the key breakthrough for DSSCs was the use of TiO2 in the form of mesoporous films,1 many other oxide semiconductors have also been tested, such as ZnO2 and SnO2.3–13 SnO2 is superior to TiO2 in terms of the orders of magnitude faster electron conductivity and higher stability under long-term UV exposure.14,15 Nevertheless, the photovoltaic performance of SnO2-based DSSCs is inferior to that of the TiO2-based ones due to faster electron recombination16 and the low-lying conduction band edge.17 Moreover, adsorption of the conventional ruthenium-based dyes on the SnO2 surface is rather restricted due to the similarity between the pH of the dye solution and the isoelectric point (IEP) of SnO2 (pH = 4–5), which is one of the reasons for a low photocurrent.18 To overcome such a low dye coverage, a modification of the SnO2 surface has been proposed by the introduction of thin layers of metal oxides19–29 with high IEPs such as TiO2 (IEP at pH 4.7 for rutile

a

Department of Energy Science, Sungkyunkwan University, Suwon 440-746, Korea. E-mail: [email protected], [email protected]; Fax: +82-31-290-7272; Tel: +82-31-290-7241 b School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Korea c School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Korea

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and pH 6.2 for anatase), Cr2O3 (IEP at pH 7.0), Y2O3 (IEP at pH 9), ZnO (IEP at pH B 9), Al2O3 (IEP at pH 9) and MgO (IEP at pH 12). Surface modification with high IEP oxides is anticipated to increase the amount of dye adsorption due to the fact that the carboxylate anions are electrostatically attracted to the positively charged oxide surface at pH below the IEP. It was observed that amount of the adsorbed N719 dye was increased by about 75% after surface modification of SnO2 with ZnO, which was responsible for the 67% increase in the photocurrent density.22 Besides the role of increasing the dye adsorption amount, the thin oxide layer on the SnO2 surface plays another important role in the protection of the electron back reaction from SnO2 to the electrolyte,27 which is in part responsible for the increased voltage after surface modification. Since the N719 dye cannot be fully covered on the bare SnO2, the empty site without the dye can act as a recombination center because it is directly exposed to the electrolyte. The fast electron recombination is therefore closely related to insufficient dye coverage. In the case of N719, the role of the thin oxide layer coated on the SnO2 surface is to increase the amount of dye adsorption. We have been interested in the effect of surface treatment on photovoltaic properties without any change in the amount of the adsorbed dye before and after surface treatment of SnO2 in order to understand clearly the role of the interfacial layer formed by surface treatment. For this purpose, an organic dye with a monodentate ligand will be better than the bidentateligand bearing N719 since the adsorbed N719 was found to be

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increased in the surface treated SnO2 system. We select a commercially available organic dye, coded MK-2.30 Here we report on the effects of the interfacial blocking layer between nanocrystalline SnO2 and the organic MK-2 dye on the photocurrent, voltage, electron transport, electron recombination, transient absorption and density of states (DOS). As expected, the amount of the adsorbed dye is almost identical before and after surface modification. However, a remarkable enhancement in photocurrent and voltage is observed, resulting in a 14-fold enhancement in the power conversion efficiency.

2. Experimental 2.1.

Materials and device fabrication

Nanocrystalline SnO2 particles were hydrothermally synthesized as follows. Tin chloride solution was prepared by dissolving 20.072 g of SnCl45H2O (98%, Aldrich) in 100 mL of deionized water and ammonium hydroxide solution was separately prepared by mixing 77.9 mL of NH4OH (ACS reagent, 28.0–30.0% NH3 basis) with 120 mL of deionized water. The tin chloride solution was slowly added to the ammonium hydroxide solution by using a dropping funnel and the mixture was stirred for 2 h, where the solution pH reached 12 and the final concentration of SnCl45H2O and ammonium hydroxide was 0.2 M and 2 M, respectively. The solution was transferred to a non-stirred titanium vessel and heated at 250 1C for 12 h using a high-pressure reactor (Parr Instrument). The white precipitate was washed with water until no chloride ions were detected using 0.1 M aqueous solution of AgNO3. For preparing a screen-printable paste, water in the SnO2 colloid solution was replaced by ethanol. Terpineol (Aldrich) and ethyl cellulose (Aldrich) were added to the ethanolic SnO2 solution, followed by evaporation of ethanol using a rotary evaporator. The viscous paste was treated with a three-roll-mill. The nominal composition of SnO2/terpineol/ethyl cellulose was 1/4/0.5 in weight percent. Fluorine-doped tin oxide (FTO) conductive glass (Pilkington, TEC-8, 8 O sq1) was ultrasonically cleaned in ethanol for 10 min. A blocking layer was covered on the FTO layer by spin-coating of 0.1 M 1-butanol solution of titanium(IV) bis(ethyl acetoacetato)diisopropoxide (Aldrich) and heated at 500 1C for 15 min. The nanocrystalline SnO2 paste was deposited on the blocking-layercoated FTO substrate and heated at 550 1C for 1 h in air (bare SnO2 film). For surface treatment of SnO2, the bare SnO2 film was dipped in 20 mM aqueous TiCl4 (Aldrich, 498%) solution for 30 min at 75 1C, which was washed with water and heated at 500 1C for 30 min. The thicknesses of the annealed SnO2 films were determined using an alpha-step IQ surface profiler (KAL Tencor). The bare and the TiCl4-treated SnO2 films were immersed in 0.3 mM of toluene solution of the MK-2 (2-cyano-3-[5 0 0 0 -(9-ethyl9H-carbazol-3-yl)-30 ,300 ,3 0 0 0 ,4-tetra-n-hexyl-[2,20 ,5 0 ,200 ,500 ,2 0 0 0 ]-quater thiophen-5-yl] acrylic acid) dye (Aldrich, 95%) for 12 h at 40 1C. The counter electrode was prepared by spin coating of 0.7 mM 2-propanol solution of H2PtCl6 on top of a FTO glass and then heating at 400 1C for 20 min. Two electrodes were sandwiched

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using a 60 mm-thick Surlyn (Meltonix 1170-60, Solaronix). The electrolyte was composed of 0.5 M 1-methyl-3-propyl imidazolium iodide (MPII), 0.1 M LiI, 0.05 M I2 and 0.6 M 4-tert-butylpyridine in acetonitrile. 2.2.

Characterization

X-ray diffraction (XRD) patterns were recorded on a Philips PW 1050 diffractometer using graphite-monochromated CuKa radiation (l = 1.54184 Å). The 550 1C-annealed SnO2 powder was detached from the substrate and used for XRD measurements. Elemental analysis was performed using energy dispersive X-ray spectroscopy (EDXS) combined with field-emission scanning electron microscopy (JEOL JSM 6700F). Transmission electron microscopy (TEM) images were obtained using high-resolution TEM (JEOL JEM-2100F) at an acceleration voltage of 200 kV. The Brunauer–Emmett–Teller (BET) surface area was determined by the nitrogen adsorption using a Micromeritics ASAP 2020. The amount of the adsorbed dye was estimated using UV-vis spectrophotometry (Agilent 8453), where the adsorbed dye was desorbed using 0.05 M NaOH in toluene and ethanol (v/v 1 : 1). Fourier transformed infrared (FT-IR) spectra were measured using a Bruker IFS-66/S. The number of scans was 32 times and the resolution was better than 0.1 cm1 at the measured wavenumber from 4000 cm1 to 500 cm1. For FT-IR measurements, a 0.3 mM solution of MK-2 in anhydrous toluene was prepared as a reference sample and the solid samples of MK-2 adsorbed SnO2 and SnO2@TiO2 films were prepared by immersing the oxide films in MK-2 solutions at different concentrations (0.3 mM and 0.03 mM). Pure anhydrous toluene solvent was used for baseline correction. Photocurrent and voltage were measured using a solar simulator equipped with a 450 W xenon lamp (Newport 6279NS) and a Keithley 2400 source meter. The light intensity was adjusted using a NREL-calibrated Si solar cell having a KG-2 filter for approximating AM 1.5G one sun light intensity (100 mW cm2). While measuring the current and voltage, the cell was covered with a black mask having an aperture. The incident photon-to-electron conversion efficiency (IPCE) was measured using an IPCE system (PV measurement Inc.) in DC mode,31 where a 75 W xenon lamp was used as a light source for generating a monochromatic beam. Time constants for electron transport and recombination were obtained using photocurrent and photovoltage transient spectroscopy. The details of the setup and measurements are described elsewhere.32 Nanosecond transient absorption data were measured using the flash photolysis technique. An excitation pulse of 500 nm was generated using an OPO (Spectra-Physics, basiScan), which was pumped by an Nd-YAG (Spectra-Physics, INDI-40-10) laser. The time duration (FWHM) of the excitation pulse was ca. 6 ns, and the pulse energy was B1.0 mJ per pulse. A CW Xe lamp (ABET technologies, LS-150-XE) was used as a probe light. The sample films were oriented at an angle of 451 with respect to the pump and the probe beam. The probe light transmitted through the sample was spectrally resolved using a monochromator (Princeton Instruments, SP2150) and then detected using a PMT (Hamamatsu, H10721-20). The output signal from the PMT was

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recorded using a 500 MHz digital oscilloscope (Agilent, DSO-X 3054A). A cut off filter was used to remove the scattered excitation light. All measurements were performed at room temperature. Electrochemical impedance spectroscopy (EIS) measurements were conducted using a potentiostat/galvanostat (PGSTAT 128N, Autolab, Eco-Chemie, the Netherlands). EIS data were recorded under one-sun (100 mW cm2) illumination conditions and analyzed using the Z-wave software.

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SnO2 can be designated as SnO2@TiO2 because of the core (SnO2)–shell (TiO2) formation. Fig. 2 compares I–V and IPCE curves of the MK-2 sensitized solar cells based on the SnO2 electrodes before and after TiCl4 treatment. A preponderant enhancement in photocurrent and voltage is observed after surface treatment with TiCl4 (Fig. 2a). Compared to the bare SnO2 electrode, the photocurrent density increases from 1.33 mA cm2 to 9.46 mA cm2, corresponding to more than 7-fold increment, and the voltage is doubled from

3. Results and discussion Fig. 1 shows the TEM images of SnO2 nanoparticles before and after TiCl4 treatment. Bare SnO2 particles that are sintered at 550 1C in air show a spherical shape with an average diameter of about 10.8 nm as estimated from 60 particles (Fig. 1a). Aqueous TiCl4 treatment of the sintered SnO2 film results in a core–shell structure, as can be seen in Fig. 1b. Compared to the clear lattice fringe in SnO2, the TiO2 shell shows no distinct lattice fringe, which indicates that amorphous TiO2 forms on the surface of SnO2. The thickness of the TiO2 shell is estimated to be about 1 nm. EDXS confirms that Ti is not detected in bare SnO2 (Fig. 1c), whereas the material wrapping SnO2 is TiO2 and the elemental ratio of Sn4+ : Ti4+ is found to be 1 : 0.41 (Fig. 1d). According to XRD diffraction patterns (not shown), SnO2 can be indexed to a tetragonal rutile structure, which is well matched with the reference pattern (JCPDS card no. 41-1445). Using Scherrer’s formula,23 d = [0.9l]/[b cos y], the average diameter (d) calculated based on the (110), (101) and (200) peaks is about 9 nm which is consistent with the size observed by TEM. The BET surface area of the 550 1C-annealed SnO2 film is estimated to be 68 cm3 g1. Based on the TEM and EDXS studies, the TiCl4-treated

Fig. 2 (a) Current–voltage curves and (b) IPCE of MK-2 dye-sensitized bare SnO2 and TiCl4-treated SnO2 (SnO2@TiO2) electrodes. The oxide film thickness was ca. 4 mm.

Table 1 Photovoltaic parameters of short-circuit photocurrent density JSC, open-circuit voltage VOC, fill factor FF and efficiency Z, and amount of the adsorbed MK-2 dye for MK-2 dye-sensitized bare SnO2 and TiCl4treated SnO2 (SnO2@TiO2) electrodes. Oxide film thickness was ca. 4 mm

VOC MK-2 JSC sensitized SnO2 (mA cm2) (V) w/o TiCl4 w/TiCl4

1.33 9.46

FF

Z (%)

Amount of adsorbed dye (mmol cm2)

0.315 0.53 0.22 1.213 0.634 0.52 3.13 1.282

Fig. 1 TEM images of (a) bare SnO2 and (b) TiCl4-treated SnO2, showing a 1 nm-thick amorphous TiO2 layer. EDX spectra of (c) bare SnO2 and (d) TiCl4-treated SnO2.

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Fig. 3 (a) Current–voltage curves and (b) IPCE of N719 dye-sensitized bare SnO2 and TiCl4-treated SnO2 (SnO2@TiO2) electrodes. The oxide film thickness was ca. 4 mm.

Fig. 4 Changes in JSC and dye loading before (SnO2) and after (SnO2@TiO2) TiCl4 treatment for (a) the MK-2 dye and (b) the N719 dye.

Table 2 Photovoltaic parameters of short-circuit photocurrent density JSC, open-circuit voltage VOC, fill factor FF and efficiency Z, and amount of the adsorbed N719 dye for N719 dye-sensitized bare SnO2 and TiCl4treated SnO2 (SnO2@TiO2) electrodes. Oxide film thickness was ca. 4 mm

N719 sensitized SnO2

JSC (mA cm2)

VOC (V)

w/o TiCl4 wTiCl4

5.07 10.97

0.379 0.622

FF

Z (%)

Amount of adsorbed dye (mmol cm2)

58.00 63.31

1.11 4.32

0.0627 0.1226

0.315 V to 0.634 V after TiCl4 treatment (Table 1). As a result, the conversion efficiency is significantly improved by a factor of more than 14. The IPCE spectra in Fig. 2b show that the absolute IPCE is increased in the entire wavelength region after surface modification. The maximum IPCE at around 480 nm increases from 9.29% to 61.8%, corresponding to almost 7-fold increase, which is well consistent with the degree of photocurrent enhancement. The change in the amount of the adsorbed dye after TiCl4 treatment may be one of the reasons for the enormous increase in the photocurrent density. As mentioned in the Introduction section, the photocurrent density of the N719-sensitized SnO2 electrode was reported to be improved after surface treatment with metal oxide layers,20,22,28 which was mainly due to the increased dye loading. We prepare the N719-sensitized bare SnO2 and SnO2@TiO2 and compare their photovoltaic performance together with the amount of the adsorbed dye. The photocurrent density is found to increase by about two times from 5.07 mA cm2 to 10.97 mA cm2, as can be seen in Fig. 3. When comparing the amount of the adsorbed dye, the dye loading is increased to double after surface modification (Table 2), which is therefore mainly responsible for the increased photocurrent density. However, in the case of the MK-2 dye, it is noted that little change in the amount of the adsorbed dye is detected before (1.21 mmol cm2) and after (1.28 mmol cm2) TiCl4 treatment, as listed in Table 1, which indicates that the 7-fold increase in the photocurrent density is not closely related to the dye loading concentration. Fig. 4 shows clearly the correlation between the improved photocurrent and the amount of the adsorbed dye. In the case of MK-2 dye shown in Fig. 4a, a significant increase in the photocurrent density after TiCl4 treatment does not seem to be correlated with the dye loading amount. On the other hand, in the case of N719, the improved photocurrent after TiCl4 treatment

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Fig. 5 Fourier transformed infrared (FT-IR) spectra of (a) 0.3 mM MK-2 dye only and (b) MK-2 adsorbed SnO2 and SnO2@TiO2, where 0.3 mM and 0.03 mM of MK-2 solutions were used for dye adsorption.

seems to be clearly correlated to the increased dye loading (Fig. 4b). No further increase in the MK-2 dye loading after TiCl4 treatment indicates that the MK-2 dye is fully covered even on the bare SnO2 surface. The question here is then why does the photocurrent increase without increase in the amount of the adsorbed MK-2 dye. The dye aggregation effect may be one of the reasons for the photocurrent enhancement since aggregation or orientation of MK-2 may influence the electron transfer.30,33 We investigate the degree of aggregation of the MK-2 dye before and after TiCl4 treatment. Fig. 5 compares the FT-IR spectra of the MK-2 adsorbed SnO2 and SnO2@TiO2 depending on the MK-2 concentration. If there is aggregation, one broad and intense band in the fingerprint region between 1800 and 1000 cm1 is expected, instead of two narrow ones.34 The FT-IR spectrum of the 0.3 mM MK-2 dye solution in Fig. 5a shows two peaks in the fingerprint region, which is indicative of no aggregation of MK-2. As can be seen in Fig. 5b, no significant change in the aromatic vibration of two peaks at around 1400 cm1 is observed before and after TiCl4 treatment and is irrespective of the MK-2 concentration. This suggests that the aggregation effect on the photocurrent enhancement can be ruled out in our system. Fig. 6 compares time constants for electron transport (tC) and charge recombination (tR) along with the electron diffusion length (LD) and charge collection efficiency (ZCC) of MK-2sensitized 4 mm-thick bare SnO2 and SnO2@TiO2 films, where LD is determined from the electron diffusion coefficient (De), De E d2/2.35tC (d = film thickness),35 and tR, LD = (De  tR)1/2, and ZCC is obtained from ZCC = 1  (tC/tR).36 The tC of the SnO2@TiO2 is slightly higher than that of the bare SnO2, which underlines that electron transport rate is slightly decreased after

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Fig. 7 Transient photocurrent for the MK-2 dye-sensitized bare SnO2 and the TiCl4-treated SnO2 (SnO2@TiO2). The measurement was performed under 1 sun illumination.

Fig. 6 (a) Time constants for recombination, (b) the time constant for electron transport, (c) the electron diffusion length and (d) the charge collection efficiency as a function of the light intensity represented by the photocurrent density for MK-2 dye-sensitized 4 mm-thick bare SnO2 and SnO2@TiO2 films.

TiCl4 treatment. On the other hand, tR for the SnO2@TiO2 is significantly increased by about 1.5 orders of magnitude, which is indicative of the enormous increase in the electron lifetime after TiCl4 treatment. Consequently, LD and ZCC are improved from B5 mm to B15 mm and from B38% to B96%, respectively. The increased LD after TiCl4 treatment is attributed to the significantly increased time constant for recombination. The preponderantly enhanced charge collection efficiency is mainly due to the prolonged electron lifetime, associated with the protection of the electron back reaction by the surface coated TiO2 layer. The doubled photovoltage as observed in Fig. 2 is also related to the significantly retarded recombination rate. We have found that the charge collection efficiency of the MK-2 dye-sensitized SnO2 solar cell is dominated by charge recombination kinetics. In contrast to the N719 dye, almost no change in the amount of the adsorbed MK-2 dye before and after TiCl4 treatment indicates that the MK-2 dye is likely to be fully covered on the bare SnO2 surface, as mentioned previously. Thus, it can be assumed that there is no free surface in direct contact with the redox electrolyte. Based upon this assumption, the recombination pathway is expected to occur directly from the SnO2 conduction band to the oxidized dye. The fast recombination in the bare SnO2 sensitized with the MK-2 dye is therefore anticipated to reduce the dye cation concentration whereas the slow recombination in the SnO2@TiO2 is likely to maintain the dye cation concentration right after the photo-electron injection. Thus, the probable difference in the dye cation concentration will have an influence on the dye regeneration efficiency.16 Fig. 7 compares the dye regeneration efficiency of MK-2 dye-sensitized bare SnO2 and SnO2@TiO2 electrodes, where photocurrent is measured with time under one-sun illumination.37 For the bare SnO2, a photocurrent of 5.62 mA cm2 after light illumination (turn on the Xe lamp) is decreased with time and saturated to 1.44 mA cm2, while almost no change in the photocurrent from

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9.8 mA cm2 to 9.7 mA cm2 is observed with time for the SnO2@TiO2. In Fig. 6, Jsat/Jmax is defined as the regeneration efficiency,37 which is estimated to be 26% and 99% for the bare SnO2 and the SnO2@TiO2, respectively. The difference in the dye regeneration efficiency seems to be related to regeneration dynamics. To probe the dye regeneration process directly, we performed nanosecond transient absorption spectroscopy of our samples (SnO2@TiO2 and bare SnO2 sensitized with the MK-2 dye). Fig. 8a shows the transient absorption (TA) spectra of MK-2 dyesensitized TiCl4-treated SnO2 (SnO2@TiO2) in the absence of the redox electrolyte at indicated delay times upon photoexcitation at 500 nm. In the TA spectra, a positive absorption band between 660 nm and 860 nm is originated from mainly MK-2 cation absorption38 and a negative absorption signal with the maximum at 540 nm indicates the ground-state bleaching. It also confirms that the decay time of the MK-2 cation matches exactly with the ground-state recovery time. However, in the presence of the electrolyte (Fig. 8b), the ground-state recovery time is faster than the cation decay time, due to the dye regeneration process by the electrolyte and the absorption bands of the intermediate species such as iodine radicals I2 .39 In the case of MK-2 bare SnO2 with electrolyte, the TA spectra are similar to those of SnO2@TiO2, but the decay and recovery time constants are quite different (see Fig. 8c). The bleaching recovery signals were best fitted with the biexponential rise functions. The rise time constants (relative amplitudes) are 0.43 ms (72%) and 3.9 ms (28%) for SnO2@TiO2 and 5.5 ms (66%) and 53.9 ms (34%) for bare SnO2. The bleaching recovery for MK-2 dye-sensitized SnO2@TiO2 (tave = 3.1 ms, red rectangles) is much faster than that for bare SnO2 (tave = 45.9 ms, blue circles) despite the absence of the recombination process due to the blocking layer of TiO2, which indicates rapid dye regeneration by the redox electrolyte. It is essential for higher efficiency dye-sensitized solar cells to have fast dye regeneration by the redox electrolyte because they could avoid recombination between the photo-oxidized dye and the injected electron.40 Here is a possible explanation of how the TiO2 layer enhances the dye regeneration rate. The regeneration from the electrolyte to the dye cation competes with the recombination from the injected electron to the dye cation. The recombination dynamics is dominant in bare SnO2, while recombination is relatively blocked in SnO2@TiO2 because of the electron blocking

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Fig. 8 Transient absorption spectra of MK-2 dye-sensitized TiCl4-treated SnO2 (SnO2@TiO2) at indicated delay times (a) in the presence and (b) in the absense of the redox electrolyte. Excitation wavelength was 500 nm. (c) Bleaching signals for MK-2 dye-sensitized bare SnO2 (blue circles) at 520 nm and SnO2@TiO2 (red rectangles) at 560 nm in the presence of the redox electrolyte after excitation at 500 nm. Biexponential fitted lines are black.

Fig. 10 Density of state (DOS) distribution of MK-2 sensitized bare SnO2 and SnO2@TiO2.

Fig. 9 (a, b) Capacitance (Cm) and (b, c) recombination resistance (Rrec) of MK-2 sensitized SnO2 and SnO2@TiO2. Applied potential (Vapp) was converted to the corrected potential (VF) and VF was converted to the equivalent conduction band position (Vecb).

layer on SnO2. Thus, the regeneration is dominant in SnO2@TiO2. Since the recombination is suppressed, faster regeneration is expected for SnO2@TiO2 than for bare SnO2. The results of the 15-fold increase in the dye regeneration time match well with the 14-fold enhancement of the conversion efficiency after TiCl4 treatment. To understand the nearly double increase in Voc, EIS is performed. Fig. 9 shows chemical capacitance (Cm) and charge recombination resistance (Rrec) as a function of the applied potential (Vapp) represented by the corrected potential (VF), Vapp = VF  jRs, where j is the current and Rs is the total series resistance at the applied voltage.41 Rrec decreases with the increasing VF due to the rise of the Fermi level in SnO2 (Fig. 9c) and Cm increases with the increasing VF (Fig. 9a) because the SnO2 film is filled with electrons.41 Since Cm is closely related to the conduction band position,42 the difference in Cm before and after TiCl4 treatment is indicative of change in the conduction band position. Thus, it is better to compare Rrec as a function of

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the equivalent conduction band position (Vecb). Vecb is obtained using the relationship Vecb = VF  DEC/q,43 where q is the electron charge and DEC is the difference in the conduction band between the MK-2 sensitized SnO2 and the MK-2 sensitized SnO2@TiO2. Rrec increases by about one order of magnitude after the TiCl4 treatment (Fig. 9d) in spite of the same Cm (Fig. 9b), which is responsible for the significant increase in Voc. We further investigate the change in the density of states (DOS) before and after TiCl4 treatment. The DOS is calculated using DOS = Cm/[qAL(1  p)], where q, A, L and p are the electron charge, cell area, film thickness and film porosity, respectively.44–46 The capacitance measured at the applied bias potential is used for determining Cm. A, L, and p are 0.291 cm2, 4 mm and 0.30 for SnO2 and 0.366 cm2, 4 mm and 0.19 for SnO2@TiO2. The DOS distribution is depicted in Fig. 10, in which the DOS of SnO2 becomes narrow after surface modification with TiCl4. When considering the little change in the electron density before and after TiCl4 treatment, as shown in Fig. 9a, a higher Voc is expected because a similar electron density in a narrow DOS for SnO2@TiO2 can lead to a Fermi level higher than that of bare SnO2 with a wide DOS.

4. Conclusion The photovoltaic performance of MK-2 dye-sensitized bare SnO2 and core–shell SnO2@TiO2 electrodes was compared, where the core–shell structure was prepared by treatment of the SnO2 electrode with TiCl4. Almost no change in the amount of the

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adsorbed dye was observed before and after TiCl4 treatment, while a 14-fold increase in photocurrent and a 2-fold increase in photovoltage were observed after TiCl4 treatment. Such an enormous increase in photocurrent was found to be mainly due to a much prolonged electron lifetime after the formation of the TiO2 passivation layer on the SnO2 surface, associated with more than 2-fold enhanced charge collection efficiency. The increase of photocurrent was found to be related to the 15-fold faster regeneration dynamics as observed by TA spectra. From our observation, it is concluded that the ‘‘thin’’ amorphous oxide layer on the SnO2 surface plays an important role in retarding the charge recombination rate, accelerating dye regeneration, and changing the DOS distribution. The MK-2sensitized bare SnO2 and SnO2@TiO2 electrodes are proposed as a model system to investigate the role of the interfacial blocking layer because two electrodes have the same amount of the adsorbed dye.

Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Ministry of Science, ICT & Future Planning (MSIP) of Korea under contract No. NRF-20100014992, NRF-2012M1A2A2671721, NRF-2012M3A7B4049986 (Nano Material Technology Development Program) and the NRF-2012M3A6A7054861 (Global Frontier R&D Program on Center for Multiscale Energy System).

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