Effects of TiCl4 Treatment of Nanoporous TiO2 Films on Morphology

Oct 2, 2012 - The thickness of the TiCl4-treated, nanoporous TiO2 film was measured by the surface profiler (KLA Tencor Alpha-Step 500). The surface a...
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Effects of TiCl4 Treatment of Nanoporous TiO2 Films on Morphology, Light Harvesting, and Charge-Carrier Dynamics in Dye-Sensitized Solar Cells Sang-Wha Lee*,† and Kwang-Soon Ahn‡ †

Department of Chemical and Biochemical Engineering, Gachon University, 461-701 Bokjeong-dong, Soojung-gu, Seongnam, Gyeongi-do, Korea ‡ Department of Chemical Engineering, Yeungnam University, 214-1 Dae-dong, Gyeongsan, Gyeongbuk, Korea

Kai Zhu,§ Nathan R. Neale,§ and Arthur J. Frank*,§ §

Chemical and Materials Science Center, National Renewable Energy Laboratory, Golden, Colorado 80401-3393, United States ABSTRACT: We report on the effects of treating TiO2 nanocrystalline films with different concentrations of TiCl4 (5−500 mM) on the film morphology, chargecarrier dynamics, and performance of dye-sensitized solar cells. Transport and recombination in the TiCl4-treated films were studied by frequency-resolved modulated photocurrent/photovoltage spectroscopies. These studies showed that, at a low TiCl4 concentration (5 mM), the electron diffusion coefficient in the annealed film increased. At intermediate TiCl4 concentrations (15−50 mM), the surface area of the films increased, resulting in an increase of light harvesting and overall power conversion efficiency. At a high TiCl4 concentration (500 mM), light scattering in the film in the long wavelength region of the visible spectrum was enhanced, but the averaged pore size of the film became narrower, resulting in slower transport and loss of cell performance.

to improve the cell performance4−7 owing to an increased surface area of the films, which affords greater dye loading with a corresponding increase of the light harvesting efficiency.8 However, the extent to which the device performance improves is complicated by the effect that TiCl4 treatment has on the TiO2 conduction band edge position, charge injection, transport, and recombination.3,6,9−11 It has been shown that traps limiting transport and recombination are located predominately on the TiO2 surface.9,12 Thus, the TiO2 layer formed on the surface of the TiO2 nanocrystalline film from the TiCl4 treatment is expected to affect the distribution and density of surface traps and thereby influence the electron transport and recombination kinetics. Inasmuch as the recombination rate increases proportionally with an increase in the number of surface states,9 a higher roughness factor (surface area) of a film can lead to faster recombination. However, it has been reported that the TiCl4 treatment reduces the rate of recombination and increases the charge injection efficiency but has little effect on electron transport and light harvesting.6,11 We hypothesize that the various effects of TiCl4 treatment reported3,8,11,13−16 are, in part, due to the differences in the TiCl 4 treatment conditions, such as the TiCl 4

1. INTRODUCTION Dye-sensitized photoelectrochemical solar cells (DSSCs) based on highly porous nanocrystalline films of titanium dioxide are considered to be one of the promising solar cell technologies in the near future.1,2 The most extensively studied cell consists of a monolayer of a Ru−bipyridyl-based dye adsorbed to the surface of a thin nanocrystalline TiO2 film supported on a transparent conducting oxide (TCO) substrate. The nanocrystallites of the film are in contact with an electrolyte solution containing iodide and triiodide (I3−) ions as a redox relay and are sandwiched by a second plate of electrically conducting oxide substrate covered with platinum. Optically excited dye molecules on the surface of the nanocrystallites inject electrons into the conduction band of TiO2. The photoinjected electrons diffuse through the interconnecting network of TiO2 nanocrystals and are collected at the TCO substrate. The resulting oxidized dye molecules are reduced by I− ions, regenerating the original dye molecules. The oxidized iodide ions diffuse back to the Pt counter electrode as I3− ions, where reduction occurs to complete the oxidation−reduction cycle. The solar-to-electrical energy conversion efficiency of DSSCs has been enhanced by soaking nanoporous TiO2 films in aqueous TiCl4 solution and then annealing the films in air.3 Annealing converts species from the TiCl4 solution to TiO2 crystallites on the surface of the TiO2 nanocrystalline films. The TiO2 crystallites derived from the TiCl4 treatment are reported © 2012 American Chemical Society

Received: August 10, 2012 Revised: September 14, 2012 Published: October 2, 2012 21285

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TiCl4-treated, nanoporous TiO2 film was measured by the surface profiler (KLA Tencor Alpha-Step 500). The surface area and pore size distribution of the films were studied by using a nitrogen adsorption−desorption apparatus (model ASAP 2000, Micromeretics Instrument Corp.). X-ray diffraction (XRD) measurements were conducted using Bruker D8 Advance equipment following the Bragg−Brentano measurement method. The weight gain after the TiCl4 treatment was measured with a digital balance having a precision of 0.1 mg.

concentration. It is known that hydrolysis reactions (e.g., TiCl4treatment reaction) depend on specific reaction conditions. In this paper, we discuss the effects of varying the TiCl4treatment conditions on the TiO2 film morphology, light harvesting, charge-carrier dynamics, and performance of DSSCs. The nanoporous films were treated with different concentrations of TiCl4 solutions and then annealed in air. These studies reveal three TiCl4 concentration regions that determine the properties of DSSCs. At a low TiCl 4 concentration (5 mM), we observed that transport becomes faster, suggesting a decrease of the surface traps limiting transport and that the cell performance improves. At an intermediate TiCl4 concentration range (15−200 mM), competing factors, such as electron transport, recombination, and light harvesting, influence cell performance. The sample treated with 15 mM TiCl4 solution displayed the largest gain in photocurrent density and solar-to-electrical conversion efficiency. At the highest TiCl4 concentration (500 mM), the film pores narrowed, electron transport became slower, and cell performance declined.

3. RESULTS 3.1. Material Characterization. 3.1.1. XRD and SEM Analyses. Figure 1 shows the X-ray diffraction (XRD) patterns

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Dye-sensitized TiO2 solar cells were fabricated as described elsewhere.13,17 A paste of TiO2 nanocrystals was deposited by the doctor-blade technique on fluorine-doped transparent conducting oxide glass (FTO) substrate (F-doped SnO2 glass; 8 Ω/sq; Pilkington TEC8) and then annealed at 450 °C in air for 30 min. The films were about 6 μm thick and had a 60% porosity. The annealed films were immersed in an aqueous TiCl4 solution (containing 5− 500 mM TiCl4) at 70 °C for 30 min. After calcining the TiCl4treated TiO2 films, they were immediately immersed in acetonitrile/tert-butanol (50/50 v/v %) solution containing 0.3 mM N719 dye for 24 h at room temperature. The Pt counter electrodes were prepared by spreading a droplet of 5 mM H2PtCl6 in 2-propanol onto the FTO substrate and subsequently heating at 400 °C for 20 min. The Pt-covered counter electrode was placed over the TiO2 electrode, and the assemblage was sealed with 1.0 mm wide strips of a 25 μm thick Surlyn (Dupont grade 1702). The redox electrolyte consisted of 0.8 M 1-hexyl-2,3-dimethylimidazolium iodide and 50 mM iodine in methoxypropionitrile. The active cell areas were about 0.15 cm2. 2.2. Transport and Recombination Measurements. Intensity-modulated photocurrent spectroscopy (IMPS) at short circuit was used to measure the electron transport time constant, and intensity-modulated photovoltage spectroscopy (IMVS) at open circuit was used to measure the recombination time constant.9 The DSSCs were probed with a modulated beam of 680 nm light superimposed on a relatively large background (bias) illumination, also at 680 nm. The 680 nm laser pulses are only weakly absorbed by the dye and thus provided a relatively uniform photocharge density across the TiO2 film. A semiconductor diode laser provided both the bias illumination and the small sinusoidally modulated probe beam. The modulation frequencies were varied from 10 mHz to 10 kHz by using a lock-in amplifier (Stanford Research SR830). The amplitude of the modulated photocurrent density was maintained within 10% of the steady-state photocurrent density. 2.3. Materials Characterization. The TiCl4-treated TiO2 films were characterized by a profilometer, gas sorption analyzer, and X-ray diffractometer. The thickness of the

Figure 1. XRD patterns of TiO2 films treated at different TiCl4 concentrations. The letters A and R denote the anatase and rutile phases, respectively. The additional peaks denoted with an asterisk (*) are attributed to the FTO substrate.

for TiO2 electrodes that were treated with different TiCl4 concentrations and then annealed in air. When the TiCl4 concentration was increased from 50 to 500 mM, the intensity of the rutile peak increased. There was no significant change in the film thickness when the films were treated with TiCl4 concentrations up to 100 mM, suggesting that treating the TiO2 electrodes with TiCl4 in this concentration level leads to the growth of conformal TiO2 layers. In contrast, the 500 mM TiCl4-treated film exhibited about a 10% increase of film thickness (from 6.1 to 6.6 μm), suggesting that an overlayer of TiO2 was deposited on top of the TiO2 nanocrystalline films. Ambient hydrolysis of TiCl4 is reported4 to induce the formation of a rutile TiO2 overlayer on the SnO2 conducting glass. Rutile TiO2 displays stronger light scattering than anatase TiO2 because of the higher refractive index of rultile.7 Figure 2 shows scanning electron microscopy (SEM) images of TiO2 films before and after they were soaked in 15 mM and 500 mM TiCl4 solutions. Consistent with the results of XRD measurements, a comparison of the untreated and 15 mM TiCl4-treated film reveals no noticeable change in morphology as a result of the treatment (cf. Figure 2a, b). When a film was soaked in the much more concentrated 500 mM TiCl4 solution, however, the morphology changed dramatically, and an overlayer of TiO2 clusters formed on the top of the TiO2 nanocrystalline film (Figure 2c), which is an observation that is also consistent with XRD measurements. 21286

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Figure 2. SEM micrographs of temperature-annealed TiO2 films after treatment with (a) 0, (b) 15, and (c) 500 mM TiCl4 treatment.

3.1.2. Surface Area. Table 1 summarized the specific surface area, porosity, weight gain, and total surface area derived from

Figure 3 shows the diffusion coefficients as a function of photoelectron density for the DSSCs treated with different

Table 1. Morphological Changes of TiO2 Films after Treatment at Different TiCl4 Concentrations parameters 2

BET area (m /g) av pore width (nm) TiO2 mass gain (wt %)a total surface area (%)b

standard 5 mM 85.4 22.4 0.0 0.0

87.7 21.8 4.4 7

15 mM

50 mM

100 mM

95.5 19.6 13.2 27

73.9 15.3 33.1 15

64.9 12.2 58.2 20

a The TiO2 mass on electrode (1.5 × 1.5 cm2) has been determined based on a 4.5 μm TiO2 film with 60% porosity and 4.2 g/cm3 density. b The fractional increase of total surface area was calculated based on the untreated film.

Brunauer−Emmett−Teller (BET) analyses of the TiCl4-treated TiO2 films. Even though the TiO2 mass was higher when the TiCl4 concentrations were increased from 5 to 100 mM, the film thicknesses remained essentially unchanged. Over this concentration range, the average pore width decreased, indicating that the TiCl4 treatment lead to pore narrowing owing to the deposition of the TiO2 layer on the surface of the TiO2 nanocrystalline film. The specific surface area of TiCl4treated TiO 2 electrodes increased gradually when the concentration of TiCl4 solution was increased from 0 to 15 mM. This observation suggests that TiCl4 treatment leads to the roughening of the TiO2 surface. However, upon further increasing the TiCl4 concentration, the specific surface area decreased because of significant pore narrowing (Table 1). It should be noted that the total surface area of the films treated with TiCl4 at concentrations greater than 50 mM were still higher than that of the untreated standard film, despite the decreased pore width and BET specific surface area. Because the morphology of the 500 mM TiCl4-treated film differs significantly from the films treated at much lower TiCl4 concentrations (0−100 mM; Figure 2), the BET data are not included in Table 1. 3.2. Electron Transport and Recombination. 3.2.1. Electron Transport. Electron transport in dye-sensitized TiO2 films is generally described by continuum diffusion kinetics, which are governed by multiple trapping and thermionic release of electrons from traps having a distribution of residence times.18−20 With increasing electron density, deeper traps become filled, and trapping/detrapping events occur more frequently in shallower traps, leading to faster transport.21 The electron diffusion coefficient in DSSCs depends strongly on the film morphology (e.g., nanocrystallite size, crystalline phase, film porosity, and packing density), which is affected by the methods used for preparing the TiO2 films and subsequent film treatments.12,17,22,23

Figure 3. Dependence of the electron diffusion coefficient on photoelectron density at short circuit for dye-sensitized TiO2 films treated at different TiCl4 concentrations.

TiCl4 concentrations. By assuming that the photoinjected electrons undergo exclusive random walks within traps having a power-law distribution of waiting times (WTD), one can derive an expression for the diffusion coefficient:23,24 D = C1(NT)−1/ α + 1/3 (Nsc)1/ α − 1

(1)

where NT is the total trap density, α is the WTD parameter (0 < α < 1), Nsc is the density of the photoinjected electrons, and C1 is a constant. The value D was calculated using the expression D = d2/(2.35τc),20 where d is the film thickness and τc is the transport time constant. The value of Nsc was determined from the equation Nsc = TnJscτc/qd(1 − P),9 where Tn is a thermodynamic factor, q is the elementary charge, Jsc is the short-circuit photocurrent density, and P is the film porosity. Equation 1 indicates that the diffusion coefficient D increases with an increase of Nsc with a power-law exponent of β = 1/α − 1. This expression is consistent with the general observation that the diffusion coefficient shows a power-law dependence on the photoelectron density.21 The best fits of the data in Figure 3 to eq 1 yield β = 0.66 ± 0.05, corresponding to α = 0.40 ± 0.03 for all samples. 21287

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When the TiCl4 concentration is increased from 0 to 5 mM, the diffusion coefficient increases by about 5-fold at a fixed photoelectron density. The much larger diffusion coefficient observed for DSSCs containing films treated with the 5 mM TiCl4 is likely due to a decrease of trap states resulting from the formation of a surface passivation layer.9,12 When the TiCl4 concentration is further increased to 15−200 mM, the electron diffusion coefficients decreased to a level that was similar to those of samples without the TiCl4 treatment. The smaller diffusion coefficient at increased TiCl4 concentrations is attributed to the increased surface area resulting from the formation of surface structures that give rise to surface defects acting as trap sites. At the highest TiCl4 concentration (500 mM), the treated DSSC exhibited a >10-fold smaller diffusion coefficient than that of the untreated DSSC. The significant decrease of the diffusion coefficient for the 500 mM TiCl4treated DSSC is consistent with the morphological change (e.g., pore-size narrowing and formation of the rutile TiO2 overlayer) of the TiO2 electrode treated at this TiCl4 concentration level. 3.2.2. Recombination Kinetics. Figure 4 shows the recombination rate R (R = Jsc/qd) as a function of the open-

Figure 5. J−V curves of TiCl4-treated cells at different TiCl4 concentrations.

circuit current density (J sc ), depending on the TiCl 4 concentration used for the film treatment. On the other hand, the fill factor (FF) and Voc of the TiCl4-treated DSSCs gradually decreased with increasing TiCl4 concentrations with the exception of the 5 mM TiCl4-treated sample, where the Voc was slightly larger (by about 20 mV) than that of the untreated sample. The DSSCs treated with 15−50 mM TiCl4 exhibited the largest power conversion efficiency, resulting primarily from the increased J sc . In the case of films treated with TiCl 4 concentrations in the range 200−500 mM, however, the cell efficiencies were lower than that of the DSSC containing the untreated film owing to the lower FF and Voc, which more than compensates for the higher Jsc. Taken together, the results of the various measurements indicate that the TiCl4 concentration has a significant influence on the density of transport-limiting traps, light harvesting (via the surface roughness factor), electron transport, and recombination.

Figure 4. Dependence of recombination rate on the open-circuit voltage for dye-sensitized TiO2 films treated at different TiCl4 concentrations.

4. DISCUSSION Treating TiO2 films with different TiCl4 concentrations influences the electron transport and recombination kinetics, the film morphology (scattering overlayer and the pore size), and the J−V characteristics. Several competing factors (e.g., surface passivation, light harvesting, and morphological properties) determine the overall power conversion efficiency of DSSCs as discussed below. 4.1. Faster Electron Transport in the Low TiCl 4 Concentration Region. The 5 mM TiCl4-treated DSSC exhibited the largest diffusion coefficient. Several studies have shown that surface traps limit electron transport.9,12 However, if one considers only the effect of surface area on charge transport, the small increase in surface area (ca. 7%) for the 5 mM TiCl4-treated sample would lead to slower transport than the standard, untreated device.9 Thus, we attribute the improved transport to the TiCl4 treatment producing a TiO2 surface deposit that reduces the surface defect (trap) density. The 5 mM TiCl4-treated cell also displayed a small increase in Voc, which is likely due to the small increase of the quasiFermi level of nanocrystalline TiO2, resulting from the decrease of the surface-state density as discussed above. At higher TiCl4 concentrations, Voc gradually decreases, which is consistent with the faster recombination rate associated with an increased number of surface recombination sites.22,24

circuit voltage (Voc) for dye-sensitized TiO2 films treated with different concentrations of TiCl4 solutions. The recombination rate increases with increased Voc owing to the higher density of photoinjected electrons. At a fixed Voc, the recombination of electrons in the TiO2 films with oxidized redox species in the electrolyte also progressively becomes faster with at higher TiCl4 concentrations. The faster recombination is in accord with the observed increase of the surface area of the TiO2 films and the expected increase of recombination centers9 resulting from the higher TiCl4 concentrations (Table 1). Based on the transport and recombination results shown in Figures 3 and 4, the 5 mM TiCl4-treated sample should have the most favorable charge-carrier dynamics for charge collection. However, it is worth noting that the highest cell efficiency was obtained for the 15 mM TiCl4-treated DSSC, suggesting that other factors also affect the overall power conversion efficiency. 3.3. Solar Cell Characteristics. Figure 5 shows the photocurrent density−voltage (J−V) curves for the DSSCs treated with different TiCl4 concentrations; Table 2 summarizes the key J−V characteristics of DSSCs. The TiCl4 treatments resulted in about 10−40% improvement of short21288

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Table 2. Changes in J−V Parameters after Treatment at Different TiCl4 Concentrationsa parameters Jsc (mA/cm2) Voc (V) FF efficiency (%)

standard 8.074 0.687 0.644 3.578

± ± ± ±

0.007 0.004 0.006 0.011

5 mM

15 mM

50 mM

200 mM

± ± ± ±

11.488 ± 0.050 0.664 ± 0.003 0.596 ± 0.002 4.709 ± 0.006

11.706 ± 0.105 0.6401 ± 0.005 0.566 ± 0.004 4.467 ± 0.062

10.879 ± 0.046 0.639 ± 0.173 0.496 ± 0.002 3.566 ± 0.007

8.693 0.706 0.584 3.718

0.053 0.001 0.021 0.112

500 mM 9.366 0.619 0.474 2.894

± ± ± ±

0.021 0.006 0.007 0.022

Standard indicates the dye-sensitized TiO2 film without TiCl4 treatment. J−V measurements were conducted on the cell (ca. 0.16 cm2) without scattering layer under a light intensity of 0.95 sun. All the data were obtained at least three times by 2 min intervals between measurements. a

4.2. Improved Light Harvesting in the Intermediate TiCl4 Concentration Region. In the intermediate TiCl4 concentration range (15−100 mM), the treatment produced a 15−20% increase in the total surface area resulting from TiO2 surface coverage of the nanocrystalline TiO2 films. Even though the TiCl4 treatment influenced the charge-collection process, the charge-collection properties of these cells alone cannot explain the increased power conversion efficiencies. For instance, the 15 mM TiCl4-treated cell exhibited the highest overall power conversion efficiency, whereas the 5 mM TiCl4treated sample showed the best charge-carrier dynamics for charge collection. The electron diffusion coefficients in DSSCs incorporating films treated with 15−50 mM TiCl4 were comparable to that in the DSSC containing the untreated film (Figure 3). On the other hand, the DSSC containing the 15 mM TiCl4-treated film displayed the highest overall power conversion efficiency, which was mainly due to the relatively high photocurrent density. The sample treated with 15 mM TiCl4 exhibited a significantly increased specific surface area (Table 1), which favors a higher dye loading and greater light harvesting. Therefore, the improved power conversion efficiency of the DSSCs treated in the intermediate TiCl4 concentration range is due predominantly to the enhanced light-harvesting property (dye loading) resulting from the larger surface area of the films associated with the TiCl4 treatment. 4.3. Morphological Changes in the High TiCl 4 Concentration Region. When treated with 500 mM TiCl4, the device exhibited the lowest diffusion coefficient (Figure 3) and the highest recombination rate (Figure 4), implying relatively poor charge collection. This result is consistent with the observed morphological changes (i.e., the pore-size narrowing and formation of defective TiO2 overlayer).4,25 The incident photon-to-current conversion efficiency (IPCE) measurements show that the TiCl4 treatment (Figure 6) affects the shape of IPCE curves. The DSSC with the 500 mM TiCl4treated film exhibited the largest increase in the IPCE spectral response in the 600−800 nm wavelength range. The increase in the IPCE spectral response is likely due to the increased light harvesting resulting from light scattering associated with the formation of the higher refractive index rutile TiO2 overlayer covering the anatase film (Figure 2). However, despite the increased light scattering in the long wavelength region of the visible spectrum, the overall power conversion efficiencies of these devices were less than that of DSSC containing the untreated film. This can be understood in terms of the morphological changes in the TiO2 film induced by treating the film with the high TiCl4 concentration (500 mM). Table 1 shows that the pore size was significantly narrowed at the high TiCl4 concentration, leading to possible diffusion-limited kinetics in the electrolyte. Electron transport in electrolytefilled TiO2 films can be described as ambipolar diffusion such that the electron motion in the porous TiO2 network is electrostatically coupled to ionic motion in the electrolyte.26

Figure 6. Normalized IPCE curves of dye-sensitized cells treated at different TiCl4 concentrations.

Thus, the narrowed pore size in films treated with the high TiCl4 concentration could retard the diffusion of I3−/I−, leading to a smaller electron diffusion coefficient (Figure 3). As a result, the overall power conversion efficiency of cells with films treated with a high TiCl4 concentration is dominated by the narrowed pore sizes, leading to slow electron diffusion and poor cell performance.

5. CONCLUSIONS We examined the consequences of treating nanoporous TiO2 nanocrystalline films in DSSCs with TiCl4 concentrations covering two orders of magnitude. Intensity modulated photocurrent/photovoltage spectroscopies and J−V measurements revealed three distinct concentration regions that have markedly different effects on the film morphology, light harvesting, charge-carrier dynamics, and overall power conversion efficiency. Depending on the particular TiCl 4 concentration range, transport and recombination can become either faster or slower, and light harvesting and cell efficiency can either increase or decrease. Changes in film morphology are found to be the cause of these effects. Understanding the TiCl4 concentration effects of surface treatment on the TiO2 film morphology and electrical properties will be valuable for developing more effective nanostructured electrodes for a variety of electrochemical applications, such as sensitized solar cells, batteries, electrochromics, and photoelectrolysis of water.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (S.-W.L.); Arthur.Frank@nrel. gov (A.J.F.). Notes

The authors declare no competing financial interest. 21289

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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (grant NRF-2010-C1AAA001-2010-0028958). This work was also funded by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences (N.R.N., A.J.F.), and the Division of Photovoltaics, Office of Utility Technologies, (K.Z.), under contract no. DE-AC36-08GO28308.

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REFERENCES

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