Influence of TiO2 Particle Size on Dye-Sensitized Solar Cells

Article Cite This: J. Phys. Chem. C 2018, 122, 7051−7060

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Influence of TiO2 Particle Size on Dye-Sensitized Solar Cells Employing an Organic Sensitizer and a Cobalt(III/II) Redox Electrolyte Yoon Jun Son,†,‡,# Jin Soo Kang,†,‡,# Jungjin Yoon,§ Jin Kim,†,‡ Juwon Jeong,†,‡ Jiho Kang,†,‡ Myeong Jae Lee,†,‡,⊥ Hyun S. Park,*,∥ and Yung-Eun Sung*,†,‡ †

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826, Republic of Korea School of Chemical and Biological Engineering and §Global Frontier Center for Multiscale Energy Systems and Department of Mechanical and Aerospace Engineering, Seoul National University, Seoul 08826, Republic of Korea ∥ Fuel Cell Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea ⊥ School of Advanced Materials Engineering, Kookmin University, Seoul 02707, Republic of Korea

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‡

S Supporting Information *

ABSTRACT: Dye-sensitized solar cells (DSSCs) are highly efficient and reliable photovoltaic devices that are based on nanostructured semiconductor photoelectrodes. From their inception in 1991, colloidal TiO2 nanoparticles (NPs) with the large surface area have manifested the highest performances and the particle size of around 20 nm is generally regarded as the optimized condition. However, though there have been reports on the influences of particle sizes in conventional DSSCs employing iodide redox electrolyte, the size effects in DSSCs with the state-of-the-art cobalt electrolyte have not been investigated. In this research, systematic analyses on DSSCs with cobalt electrolytes are carried out by using various sizes of NPs (20−30 nm), and the highest performance is obtained in the case of 30 nm sized TiO2 NPs, indicating that there is a reversed power conversion efficiency trend when compared with those with the iodide counterpart. Detailed investigations on various factorslight harvesting, charge injection, dye regeneration, and charge collectionreveal that TiO2 particles with a size range of 20−30 nm do not have a notable difference in charge injection, dye regeneration, and even in light-harvesting efficiency. It is experimentally verified that the superior charge collection property is the sole origin of the higher performance, suggesting that charge collection should be prioritized for designing nanostructured TiO2 photoelectrodes for DSSCs employing cobalt redox electrolytes.

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INTRODUCTION On account of global energy and environmental crisis, development of alternatives for fossil fuels is one of the most important tasks that mankind is facing these days. Among various sustainable energy sources, solar energy is regarded as a promising candidate because of its large abundance and availability. There have been efforts to utilize incident sunlight by solar-to-electrical energy conversion using solar cells, and beside Si and thin-film photovoltaics, dye-sensitized solar cells (DSSCs) have drawn great attention because of their high performance and reliability in addition to economical advantages.1−8 In DSSCs, mesoscopic oxide semiconductor films are often used as photoelectrodes, and dye molecules adsorbed on the surface of the oxide serve as the light harvesters. This photoelectrode design enables effective photon absorption by large surface area for dye loading. TiO2 is widely used in DSSCs because of its favorable band positions, high stability, and low cost.9−12 TiO2 nanomaterials with various morphologies, such as nanoparticles (NPs),6,13 nanotubes,14−17 and nanowires,18−20 have been applied to the photoanode of © 2018 American Chemical Society

DSSCs; however, NPs manifested superior performance when compared with others because they provide larger surface areas for light harvesting. In DSSCs employing TiO2 NPs, the size of the particles has significant influences on photovoltaic performances of DSSCs and there have been a number of studies to unveil the size effect in DSSCs. Yanagida et al. reported that the size of TiO2 NPs affects electron diffusion and recombination processes in the TiO2 film, leading to a substantial difference in charge collection properties.21 Cao et al. revealed that larger TiO2 NPs show low electron transport resistance and high recombination resistance, which result in a superior charge collection efficiency.22 However, they insisted that increasing the size of TiO2 NPs over a certain level is detrimental to the overall performances, mainly because of the drop in the available surface area for dye loading and light harvesting. Received: December 11, 2017 Revised: March 2, 2018 Published: March 12, 2018 7051

DOI: 10.1021/acs.jpcc.7b12206 J. Phys. Chem. C 2018, 122, 7051−7060

Article

The Journal of Physical Chemistry C

18TS, DN-GPS-22TS, and DN-GPS-30TS, Dyenamo) was cast onto FTO via a doctor-blading technique. After drying the deposited TiO2 films on the FTO glass at 60 °C for 5 h, a scattering layer (Ti-Nanoxide R/SP, Solaronix) was cast onto TiO2 films followed by heat treatment at 500 °C for 30 min in air. TiCl4 post-treatment was conducted by immersing the TiO2 electrodes in a 16 mM TiCl4 aqueous solution at 70 °C for 30 min, and the TiO2 electrodes were annealed once again at 450 °C for 30 min in air. Then, the TiO2 electrodes were immersed in a mixed solution of tert-butanol and acetonitrile (1:1 in vol ratio) containing 0.1 mM Y123 dye (DN-F05Y, Dyenamo) and 5 mM chenodeoxycholic acid at 30 °C for 24 h. For the preparation of Ru-DSSCs, the TiO2 films were sensitized by immersing them in an ethanol solution containing 0.5 mM N719 dye (Ru 535-bisTBA, Solaronix) at 30 °C for 24 h. Fabrication of DSSCs. Counter electrodes (CEs) for DSSCs were prepared by spin-casting 300 μL of isopropanol containing 50 mM H2PtCl6 followed by heat treatment at 400 °C for 30 min in air. The [Co(bpy)3]3+/2+ redox electrolyte was composed of 0.22 M Co(bpy)3(PF6)2 (DN-C01, Dyenamo), 0.033 M Co(bpy)3(PF6)3 (DN-C02, Dyenamo), 0.1 M LiClO4, and 0.2 M 4-tert-butylpyridine in acetonitrile solvent. The I3−/ I− redox electrolyte was composed of 0.6 M 1-butyl-3methylimidazolium iodide, 30 mM I2, 0.1 M guanidinium thiocyanate, and 0.5 M 4-tert-butylpyridine in a mixture of acetonitrile and valeronitrile (85:15 in vol ratio). The prepared photoanodes and CEs were assembled using thermoplastic sealants (Surlyn, DuPont), and the redox electrolytes were injected into the assembled cell through predrilled holes. Physicochemical and Photoelectrochemical Characterizations. The morphologies and sizes of TiO2 NPs were characterized by using a transmission electron microscope (TEM, Tecnai F20, FEI). The thickness of TiO2 films was measured using a scanning electron microscope (SEM, AURIGA, ZEISS). X-ray diffraction (XRD) patterns were obtained by using Rigaku D/MAX 2500-PC equipped with a Cu Kα radiation source. N2 sorption analyses were performed using Micromeritics ASAP 2020, and optical measurements were carried out using a spectrophotometer (LAMBDA 45, PerkinElmer). For the measurement of dye adsorption, Y123 dye-sensitized photoanodes (active area: 0.45 × 0.45 cm2) were dipped into 5 mL of a mixture of ethanol and toluene (1:1 vol ratio) containing 0.1 M NaOH and the mixed solution was stirred until the dye completely desorbed into the liquid. Transmittance and reflectance spectra of the prepared photoanodes were obtained using a UV-visible spectrophotometer (Cary 5000, Agilent Technologies). Photoelectrochemical Measurements. The photocurrent density−voltage (J−V) measurements were performed by using a solar simulator (XIL model 05A50KS source measure units, SERIC) and a potentiostat (MultiStat 1480, Solartron). One sun illumination condition was verified by a reference Si cell which was calibrated by National Institute of Advanced Industrial Science and Technology (AIST) in Japan. Incident photon-to-current conversion efficiency (IPCE) data were collected at the wavelength range of 350−800 nm with a spectral resolution of 10 nm by using a quantum efficiency measurement equipment (QEX7, PV Measurements). Photoluminescence (PL) spectra were obtained using a spectrofluorometer (Scientific FluoroMax-4, HORIBA). Electrochemical impedance spectroscopy (EIS) measurements were performed by using a Zahner Zennium Electrochemical Workstation with an ac amplitude of 10 mV. For intensity-

These observations suggested that the balance between charge collection and light harvesting is important for highly efficient power conversion, and in this respect, TiO2 NPs with the size of around 20 nm are often regarded as the optimum size to achieve the highest performance. Additionally, there have been investigations on the effect of particle size on electron injection from dye to TiO2 NPs. Yartsev et al. reported that charge injection is faster for larger NPs,23 but Tachiya et al. revealed that electron injection decreases from 90 to 30−70% when the size of NPs becomes larger than 50 nm when a conventional Ru sensitizer is utilized.24 On the other hand, Safari reported that in the case of DSSCs employing porphyrin sensitizers, larger TiO2 NPs are not only more efficient for charge injection but also beneficial for overall photovoltaic performance.25 In DSSCs employing TiO2 photoelectrodes, triiodide/iodide (I3−/I−) redox electrolytes are often used because of their favorable dye regeneration properties for conventional Rubased dyes and large charge recombination resistance at TiO2/ electrolyte interfaces.26 However, I3−/I− redox reactions occur at 0.35 V versus normal hydrogen electrode (NHE), which is an unnecessarily high energy level to reduce the oxidized sensitizers of which the highest occupied molecular orbital level is located at around 1.1 V versus NHE, limiting the photovoltage of DSSCs as a consequence.27 Moreover, I3−/I− has a number of drawbacks for use in DSSCs, such as significant light absorption in the visible light region and corrosive characteristic, which allow small choice for material selection in electrodes.28 As an alternative to the I3−/I− redox system, cobalt-based redox couples were introduced into DSSCs and substantial improvements were achieved by using organic donor−π−acceptor (D−π−A) dyes or porphyrin-derived sensitizers, which are compatible with cobalt redox electrolytes.5,6,29,30 Though these redox couples have the disadvantage of fast interfacial recombination kinetics with conduction band electrons at the TiO2 surface, the introduction of long alkyl chain to molecular sensitizers successfully addressed the recombination problems.31 Although there have been numerous studies on the size effect of TiO2 NPs on DSSCs employing the I3−/I− redox electrolyte, same kind of approaches have not been reported in the case of cobalt redox couples, though they have served as the state-ofthe-art redox electrolyte for DSSCs. In this study, systematic investigations were performed on the size effect of TiO2 NPs on the photovoltaic performances of the DSSC employing cobalt-based redox electrolyte. We used a typical sensitizer (Y123 dye) and an electrolyte (cobalt bipyridine ([Co(bpy)3]3+/2+) redox electrolyte) in this work, and the particle sizes were varied from around 20−30 nm. Unlike the case of I3−/I−-based DSSCs, the highest photovoltaic performance was achieved at 30 nm sized TiO2 NPs when the [Co(bpy)3]3+/2+ redox couple was utilized in DSSCs. The effect of particle size was then studied in terms of light harvesting, charge injection, charge collection, and dye regeneration using various optical and photoelectrochemical measurements and techniques.

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EXPERIMENTAL SECTION Preparation of TiO2 Photoanodes. All of the reagents were research grade products obtained from Sigma-Aldrich (otherwise stated) and were used without further purification. For the preparation of electrodes, a fluorine-doped tin oxide (FTO) glass (TEC-8, Pilkington) was ultrasonically cleaned using acetone, ethanol, and deionized water for 10 min each. Colloidal TiO2 paste with different particle sizes (DN-GPS7052


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Figure 1. TEM images of TiO2 NPs from (a) T1, (b) T2, and (c) T3. The size distribution of the NPs in the cases of (d) T1, (e) T2, and (f) T3, which was calculated based on 100 particles in the TEM images.

modulated photocurrent spectroscopy (IMPS) and intensitymodulated photovoltage spectroscopy (IMVS) analyses, a diode laser (λ = 680 nm) was used as the light source, and sinusoidally modulated light with the modulation depth of around 10% of the overall light intensity was illuminated on the photoanode side of the DSSC. IMPS and IMVS measurements were carried out at four different dc light intensities (25, 50, 75, and 100 W/m2) by using a Zahner Zennium Electrochemical Workstation and a Zahner PP211 potentiostat.

Table 1. Particle Sizes, Surface Areas, and Pore Characteristics of T1, T2, and T3

T1 T2 T3

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average particle size (nm)

surface area (m2/g)

cumulative pore volume (cm3/g)

mean pore size (nm)

porosity (%)

20.7 24.1 29.6

73.5 63.6 39.3

0.481 0.482 0.427

19.8 21.1 27.4

65.19 65.24 62.43

To measure the specific surface area and pore characteristics, N2 sorption experiments were performed and Figure S3 shows the isotherms for the TiO2 NPs in T1, T2, and T3. From the type-V behavior of the isotherm, it could be verified that the TiO2 electrodes are mesoporous. The Brunauer−Emmett− Teller (BET) surface areas were calculated as 73.5, 63.6, and 39.3 m2/g in the cases of T1, T2, and T3, respectively, and these values clearly show that there is a steep decrease in the specific surface area as the particle size increases. Pore volumes were also obtained by Barrett−Joyner−Halenda methods (see Table 1), and porosities of T1, T2, and T3 were calculated using the pore volumes and the following equation

RESULTS AND DISCUSSION Physical Characterization of TiO2 Electrodes. For the preparation of mesoscopic TiO2 photoelectrodes, colloidal TiO2 paste containing various sizes of NPs (DN-GPS-18TS, DN-GPS-22TS, and DN-GPS-30TS, Dyenamo) and a scattering layer (TiO2 particles of 200−400 nm; Ti-Nanoxide R/SP, Solaronix) were sequentially cast onto a FTO glass substrate and were thermally sintered at 500 °C. The thickness of the TiO2 films was controlled for accurate investigations on the size effects of TiO2 NPs. Figure S1 shows the crosssectional SEM images of the TiO2 electrodes. The thicknesses of the mesoscopic layers and scattering layers above them were around 5.8 and 3 μm, respectively. This thickness matched well with the optimized range for the usage of cobalt bipyridine redox electrolyte.31−33 Figure 1a−c shows the TEM images of the TiO2 NPs with different sizes. The particle sizes were measured based on the observations from TEM analyses, and the average diameters of three different TiO2 NPs calculated from 100 particles were 20.7 ± 4.4 nm (Figure 1d), 24.1 ± 3.1 nm (Figure 1e), and 29.6 ± 4.1 nm (Figure 1f), as summarized in Table 1. The TiO2 electrodes employing three different sizes of NPs were hereafter denoted as T1 (smallest particles), T2 (intermediate particles), and T3 (largest particles). The phase of TiO2 in the photoelectrodes was characterized by obtaining XRD patterns (Figure S2), and all of the signals besides those from the FTO substrate (JCPDS 41-1445) were able to be assigned as the peaks originated from the anatase TiO2 (JCDPS 21-1272).

P = Vp/(ρ−1 + Vp)

(1) −1

where Vp is the cumulative pore volume and ρ is the reciprocal of the density of anatase TiO2 (0.257 cm3 g−1).22 The calculated porosities of T1, T2, and T3 were 65.19, 65.24, and 62.43%, respectively. It was previously reported that the porosity of TiO2 NPs, which is related to their coordination number, has significant influences on the electron transport properties.34 A mesoscopic TiO2 film with a porosity of 50− 65% is known to have a coordination number of around 4, and this value is reported to be sufficiently high for effective electron transport.22 Given that the porosities of our TiO2 electrodes are within this range regardless of the particle size, it was expected that there are sufficient pathways for electrons to travel through the sintered TiO2 NPs. Therefore, we could exclude the effect of limitations in charge transport caused by the porosity of TiO2, and this indicates that T1, T2, and T3 are favorable for systematic studies on the size effects of TiO2 NPs 7053


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Table 2. J−V Parameters Obtained from the Results from Figure 2a

in DSSCs. The detailed properties of TiO2 NPs obtained from the TEM and N2 sorption experiments are summarized in Table 1. The effective surface areas of the TiO2 electrodes comprising T1, T2, and T3 were measured by dye adsorption experiments. Because this study was focused on the DSSCs employing [Co(bpy)3]3+/2+ electrolytes, Y123 dye that is well-suited to the cobalt bipyridine redox couple was used. Dye molecules were first chemisorbed on the surface of TiO2 followed by the detachment of the sensitizers using a mixture solution of ethanol and toluene containing NaOH. The amount of the adsorbed dye molecules was characterized by measuring the absorbance at 477 nm, and the results are displayed in Table S1 and Figure S4. The loading amounts of Y123 dyes on T1, T2, and T3 were 185, 122, and 69 nmol per unit geometric area of the TiO2 film (1 cm2), respectively. As observed in the BET analyses, the effective surface area became smaller in the case of larger TiO2 NPs. Size Effect of the TiO2 NP on DSSC Performance. The DSSCs employing T1, T2, and T3 were prepared by sandwichlike assembly of Y123 dye-sensitized photoelectrodes and Ptbased CEs, by using 25 μm thick thermoplastic sealants, and the acetonitrile-based [Co(bpy)3]3+/2+ redox electrolyte was then injected into the cell through predrilled holes. Figure 2a

T1 T2 T3

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

0.788 0.789 0.794

12.2 12.5 13.4

69.2 69.6 68.5

6.64 6.89 7.29

I3−/I− redox electrolyte (Table S2 and Figure S5), which is the most widely studied system. According to the previous studies, around 20 nm sized TiO2 NPs show the highest performance in N719/iodide-based DSSCs2,4,35 and further increase or decrease in particle sizes results in poorer light harvesting (by less amount of dye loading) or inferior charge collection properties (due to the increase in the number of grain boundaries), respectively.15,19,36−38 From the photovoltaic parameters of the DSSCs (Table 2), it was observed that the open-circuit voltage (Voc) increased with increasing particle size. To understand this Voc trend, we conducted the J−V measurements under dark condition (Figure S6) and EIS analyses (Figure S7). As the particle size was increased, higher overpotential for dark currents in J−V and larger charge recombination resistances in EIS were observable, implying that the trend in Voc resulted from the superior charge collection characteristic of large-sized TiO2 NPs regarding the interfacial charge recombination. Nevertheless, we could deduce that the increase in PCE in the case of larger TiO2 NPs was mainly originated from the enhancement in the short-circuit current density (Jsc) rather than the other parameters: Voc and fill factor (FF). The difference in Jsc was further confirmed by IPCE spectra (Figure 2b). Jsc can be calculated from the IPCE spectra and the AM 1.5G spectrum using the following equation Jsc =

∫ IPCE(λ)eφph,AM 1.5G(λ) dλ

(2)

where e is the elementary charge and φph,AM 1.5G is the photon flux in AM 1.5G illumination. Because the elementary charge and the AM 1.5G spectrum are constant, the difference in Jsc could be regarded as a consequence of different IPCE values. In DSSCs, IPCE is generally expressed in terms of four efficiency factors as the following equation IPCE(λ) = LHE(λ) × φinj(λ) × φreg × ηcc(λ)

(3)

where LHE is the light-harvesting efficiency, φinj is the quantum yield for electron injection from the excited sensitizer to the TiO2 conduction band, φreg is the regeneration efficiency of oxidized dyes, and ηcc is the collection efficiency for photogenerated charge carriers. To understand the origin of different Jsc values in DSSCs employing TiO2 NPs with different sizes, we investigated the effect of particle sizes on these four efficiency factors separately. First, the effect of particle sizes on LHE was investigated. LHEs of photoanodes in DSSCs have often been studied based on transmittance; however, significant reflection occurs when the scattering layer is incorporated. Therefore, the LHE was calculated with considerations on reflectance, based on the following equations39,40 I0 = I(absorbed) + I(transmitted) + I(reflected) (4)

Figure 2. (a) J−V curves and (b) IPCE spectra of DSSCs employing T1, T2, and T3.

shows the J−V curves of the DSSCs measured under standard AM 1.5G illumination with a light intensity of 100 mW/cm2 (one sun condition), and the photovoltaic performance parameters are summarized in Table 2. The DSSC employing T3 manifested the highest power conversion efficiency (PCE) followed by those with T2 and T1. It was notable that the trend in PCE was opposite to the number of adsorbed dye molecules per unit geometric area because larger loading of sensitizers often leads to enhanced light harvesting. Our observation was also opposite to the trend in DSSCs employing N719 dye and

LHE = I(absorbed)/I0 = [I0 − I(transmitted) − I(reflected)]/I0 =1−T−R 7054

(5) DOI: 10.1021/acs.jpcc.7b12206 J. Phys. Chem. C 2018, 122, 7051−7060

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Figure 3. (a) Transmittance (dotted line), reflectance (dashed line), and sum of transmittance and reflectance spectra (solid line) and (b) LHE spectra of Y123 dye-sensitized photoelectrodes. (c) AM 1.5G solar spectrum. (d) Maximum photocurrent density spectra obtained using LHE and AM 1.5G spectra.

extends the optical path length within the TiO2 film, must have contributed to additional enhancement in light absorption at photoelectrodes, mitigating the particle size effect on LHE as a consequence.5,42−46 In short, it could be concluded that the effect of particle sizes on LHE was insignificant within the range of 20−30 nm. Second, to compare the kinetics of photoinduced electron injection in the cases of T1, T2, and T3, PL measurements were performed on dye-sensitized TiO2 photoanodes. During the PL measurements, dye molecules on the TiO2 electrode absorb the incident photons from laser, producing photoinduced charge carriers, which can either be injected into the TiO2 conduction band or release energy by radiative recombination.47 Excitation of dyes was carried out using a 463 nm laser, as this wavelength belongs to the absorption range of the Y123 dye.48 Figure 4a shows the PL spectra of Y123 dye-sensitized T1, T2, and T3, from where no notable difference was observable. Then, we measured the timeresolved photoluminescence (TRPL) decay measurement at the wavelength of maximum PL intensity (660 nm), to compare the electron injection kinetics when different-sized TiO2 NPs are used. TRPL decay curves of T1, T2, and T3, which are presented in Figure 4b, were fitted according to the third-order exponential decay model on the following equation

where I0 is the intensity of incident light and T and R are the transmittance and reflectance of photoanodes, respectively. We measured transmittance and reflectance in the wavelength region of 350−800 nm and calculated the LHE using eq 5, and the results are displayed in Figure 3a,b. Three photoanodes employing different sizes of TiO2 NPs had similar LHE in the measured wavelength region (Figure 3b), indicating that light harvesting shows saturated behaviors in the cases of T1, T2, and T3. We have to state here that there was a loss of photons in reflectance measurements possibly because of the effect of the scattering layer and the glass/air interface. This was confirmed by the transmittance and reflectance spectra of the FTO glass and the unsensitized TiO2 film with the scattering layer (Figure S8). Figure S8 shows that the sum of transmittance and reflectance values is lower than 100% even at wavelength ranges that are optically inert to FTO and TiO2, implying a substantial loss of photons by unavoidable reflection toward an incident beam source. Nevertheless, it was clear that the LHE was similar in the cases of T1, T2, and T3, and we performed further discussion based on the obtained LHE. The spectra showing the maximum photocurrent density with regard to the wavelength were obtained using the measured LHE and the AM 1.5G spectra in Figure 3c, and the results are depicted in Figure 3d. Additionally, the maximum Jsc value was calculated by integrating the ideal spectral photocurrent densities, and similar values of around 17.7 mA/cm2 were obtained for T1, T2, and T3 (see Table S3). This result is hard to understand when only the particle size and the corresponding dye-loading amount are considered because dye loading was significantly lower when larger TiO2 NPs were used. However, the measured similar LHE of three photoanodes can be explained by superior absorption properties (extinction coefficient) of the Y123 sensitizer compared to those of the typical Ru-complex sensitizer (e.g., N719 dye).33,41 In addition, the presence of the light-scattering layer, which

y(t ) = y0 + A1 e−t / τ1 + A 2 e−t / τ2 + A3 e−t / τ3

(6)

Among the parameters in the decay model, τ1 is the fast decay component term, which is related to the kinetics of charge injection.49 A lower τ1 value means less charge recombination in dye because of superior electron injection from the dye into the TiO2 electrode.50,51 The τ1 values obtained from T1, T2, and T3 were 0.57, 0.56, and 0.61 ns, respectively, and no significant difference could be revealed (detailed fitting results for other constants are summarized in 7055


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Figure 5. Transient photocurrent density of DSSCs employing T1, T2, and T3 at short-circuit condition.

insignificant gaps in regeneration efficiencies and Warburg impedances, we could additionally confirm that the difference in mass transport limitations regarding the particle sizes is negligible within the 20−30 nm range. This might be ascribable to the similar porosities (60−65%) of T1, T2, and T3, as presented in Table 1, which is a sufficiently high value for efficient mass transfer of [Co(bpy)3]3+/2+ redox couples.55,56 Another notable observation in time transient photocurrents is that the initial maximum value of photocurrent density (Jmax) was quite different as the size of TiO2 NPs were varied, on the order of T3 > T2 > T1. Jmax is not associated with mass transfer properties of the redox couples but rather dependent on other factors such as light harvesting, charge injection, and charge collection.31,55 Therefore, the effect of particle size on the dye regeneration term could be ruled out. Last, the effect of particle size in charge collection was taken into account. To compare ηcc in DSSCs employing T1, T2, and T3, electron dynamics in each TiO2 electrode was investigated by IMPS and IMVS measurements. IMPS and IMVS spectra were obtained at short-circuit and open-circuit conditions, respectively, using a 670 nm laser source with variations in light intensity: 25, 50, 75, and 100 W/m2. First, the electron transport time (τd), which shows the time for an injected electron to reach the current collector through a semiconductor film, and electron diffusion coefficient (Dn) within TiO2 were calculated from the IMPS results based on the following equations57

Figure 4. (a) Steady-state PL spectra and (b) TRPL decay curves of T1, T2, and T3. PL measurements were measured using a 463 nm laser source.

Table S4). This means that the quantum yield of charge injection at the dye/TiO2 interface was almost independent of the size of TiO2 NPs. Therefore, we could conclude that the TiO2 particle size does not affect charge injection properties in the range of 20−30 nm. Third, the effect of particle size on dye regeneration efficiency was investigated. In DSSCs, redox couples are responsible for the regeneration of oxidized dye molecules, and mass transfer properties of the redox couples dictate the dye regeneration kinetics when the same dye and redox electrolytes are used. Cobalt-based redox couples often suffer from poor mass transfer within the mesoporous TiO2 film, mainly because of the steric hindrance originated from their large molecular size. This leads to sluggish dye regeneration and low photocurrent generation.52,53 Therefore, it was expected that any difference in φreg in the cases of T1, T2, and T3 would be solely influenced by the mass transfer of [Co(bpy)3]3+/2+ redox couples. To compare the mass transport properties, we performed photocurrent transient measurements by measuring the time transient Jsc value with on and off of one sun illumination. It has been reported in the previous publications that the initial value of photocurrent (measured instantly after the light absorption) decreases to a saturated value as the redox species near the surface of photoanodes are continuously consumed.54 The mass transfer properties can be compared by observing the magnitude of photocurrent decay after the initiation of light illumination. As depicted in Figure 5, DSSCs employing T1, T2, and T3 showed an instant drop of photocurrent densities for around 150 ms until reaching saturated values. However, the magnitudes of photocurrent decay for three DSSCs were very small compared to their saturated Jsc values (Jsat) as well as comparable to each other, revealing that the effect of mass transport limitation would be too trivial to bring about a significant difference in φreg. Furthermore, we quantitatively calculated regeneration efficiency, which is defined as the ratio of saturated photocurrent density to initial peak current density,55 and also obtained the Warburg impedance values from the EIS spectra measured under the forward bias of 0.7 V (Table S5). From the

τd = 1/2πfmin (IMPS)

(7)

Dn = d 2/2.5τd

(8)

where f min(IMPS) is the characteristic frequency at the minimum of the imaginary component and d is the thickness of the TiO2 film, which is around 5.8 μm for all TiO2 films in this work. The calculated τd and Dn values of DSSCs employing T1, T2, and T3 are, respectively, displayed in Figure 6a,b. It was clear from the results that as the light intensity increases, τd decreases and Dn increases, indicating that electron diffusion is facilitated in TiO2 under larger photon flux. This observation matches well with those in the previous reports that at a high light intensity, the traps in TiO2 are filled with electrons and the trapping/detrapping process is thereby restrained, making electron transport faster as a consequence.58 Regardless of the light intensity, it was notable that DSSCs employing larger NPs had shorter τd and larger Dn than those with smaller NPs, suggesting that electron transport is faster in the TiO2 electrode when the size of NPs is increased. This can be attributed to the 7056


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Figure 6. (a) Electron transport time, (b) electron diffusion coefficient, (c) electron lifetime, and (d) electron diffusion length in TiO2 photoelectrodes of DSSCs employing T1, T2, and T3 obtained by IMPS and IMVS measurements.

different number of defect sites; the mesoscopic film with smaller NPs, which are nanocrystals, has a larger number of grain boundaries for electrons to pass through until they arrive at the current collector.21,59 Trapping of charge carriers at the defect sites in grain boundaries retard the transport of electrons in mesoscopic TiO2 films.60 Then, the electron lifetime (τn), which is the time it takes for an electron in the semiconductor film to recombine with a hole/electron acceptor at the semiconductor/electrolyte interface, was calculated from the IMVS results using the following equation58 τn = 1/2πfmin (IMVS)

TiO2 particles was suggested as the origin of the increased τn.22 On contrary to the case of iodide redox electrolyte, usage of cobalt bipyridine species leads to significant interfacial charge recombination with electrons in TiO2 because of their fast electron transfer kinetics as an outer-sphere redox couple.31,61 Therefore, the effect of charge recombination resistance seems to be larger than that of the chemical capacitance in our system, and this difference between iodide and cobalt redox electrolytes is expected to be the origin of opposite trends in τn when different redox species are utilized. On the basis of the Dn and τn obtained from IMPS and IMVS analyses, respectively, electron diffusion length (Ln), which indicates the average travel distance of electrons within TiO2, was calculated based on the following equation58

(9)

where f min(IMVS) is the characteristic frequency at the minimum of the imaginary component. A larger τn means a slower recombination rate of photoelectrons at the semiconductor/electrolyte interface, which is favorable for efficient charge collection. As depicted in Figure 6c, τn was the largest in the case of T3 followed by those of T2 and T1, which matches well with the trend observed in charge recombination resistances in EIS analyses (Figure S7). This means that smaller TiO2 NPs are more vulnerable to interfacial charge recombination of conduction band electrons. This also seems to be related to the different number of grain boundaries in TiO2 films employing TiO2 NPs of various sizes because charge recombination occurs more often in the defect sites, which are densely located in grain boundaries. In the previous report by Cao and co-workers, DSSCs employing conventional N719 dye and iodide redox electrolyte manifested smaller charge recombination resistance at the TiO2/electrolyte interface when smaller TiO2 NPs were used (unless too small for mass transfer of iodide species in the electrolyte); however, τn, which can be expressed by the product of charge recombination resistance and chemical capacitance, was larger in relatively small-sized NPs, and larger chemical capacitance in smaller

Ln = (Dn × τn)1/2

(10)

Ln determined by the competition between electron transport and recombination is directly correlated with ηcc, and a larger Ln represents superior charge collection efficiency in mesoscopic TiO2 electrodes.62,63 Figure 6d shows that Ln increases as the size of NPs becomes larger, and it could thereby be verified that the different Jsc values in DSSCs employing T1, T2, and T3 were originated from the difference in charge collection properties, which were significantly affected by the size of TiO2 NPs. Furthermore, to quantitatively compare the relationship between Jsc values and charge collection properties, charge collection efficiency was calculated using the following equation4 ηcc = 1 − (τtr /τn)

(11)

From the calculated ηcc values in Table S6, it was confirmed that the ratio of change in Jsc and ηcc from T1 to T3 almost coincides with each other under four different illumination conditions, adding credibility to our result that the difference in 7057



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Jsc values is mainly determined by different charge collection properties of three electrodes.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b12206. Additional experimental results of SEM images, XRD characterization, BET characterization, dark J−V, and EIS analysis (PDF)

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REFERENCES

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CONCLUSIONS In this work, we investigated the size effect of TiO2 NPs on the DSSC employing a cobalt-based redox electrolyte. Mesoscopic TiO2 photoelectrodes with 20−30 nm sized NPs were introduced to DSSCs employing [Co(bpy)3]3+/2+ redox electrolyte and Y123 dye, and a reversed trend in the photovoltaic performance was observed when compared with conventional systems based on I3−/I− redox couples and Rucomplex sensitizers; the highest PCE was obtained in the case of 30 nm NPs. The size effects were further examined by verifying the influences of NP sizes on four different efficiency factorslight harvesting, charge injection, charge collection, and dye regenerationusing various optical and electrochemical techniques. From these investigations, it was verified that larger TiO2 NPs with smaller surface area do not show any notable drop in LHE, but significant improvement in charge collection was observed. Given that charge injection and dye regeneration were not affected by the size of TiO2 NPs, we could conclude that the higher performance at 30 nm was ascribable to the enhanced charge collection within TiO2 by a reduced number of grain boundaries. Because our study is the first investigation on the effect of TiO2 NP sizes on DSSCs employing a cobalt-based redox electrolyte, and our observation strongly demonstrates that improving charge collection is the key to enhance photovoltaic performances, it is strongly anticipated that this work will provide significant insights into the design of mesoscopic TiO2 photoelectrodes.

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.S.P.). *E-mail: [email protected] (Y.-E.S). ORCID

Jin Soo Kang: 0000-0001-8894-2630 Hyun S. Park: 0000-0002-7960-9729 Yung-Eun Sung: 0000-0002-1563-8328 Author Contributions #

Y.J.S. and J.S.K. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Y.-E.S. acknowledges that this work was supported by the Institute for Basic Science (IBS) in Republic of Korea (Project Code: IBS-R006-A2). H.S.P. acknowledges supports from the National Research Foundation of Korea (2016M3D1A1021142) funded by the Ministry of Science, ICT & Future Planning of Korea. PL spectra and UV−vis optical measurements were performed at the Global Frontier Center for Multiscale Energy Systems in Republic of Korea. 7058


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Influence of TiO2 Particle Size on Dye-Sensitized Solar Cells

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