J. Phys. Chem. C 2010, 114, 13433–13441
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Effects of Annealing Temperature on the Charge-Collection and Light-Harvesting Properties of TiO2 Nanotube-Based Dye-Sensitized Solar Cells Kai Zhu,* Nathan R. Neale, Adam F. Halverson, Jin Young Kim, and Arthur J. Frank* National Renewable Energy Laboratory, Golden, Colorado 80401-3393 ReceiVed: March 9, 2010; ReVised Manuscript ReceiVed: June 17, 2010
We report on the influence of annealing temperature (Ta) on the microstructure and dynamics of electron transport and recombination in dye-sensitized solar cells (DSSCs) incorporating oriented titanium oxide nanotube (NT) arrays. The morphology of the NT arrays was characterized by scanning and transmission electron microscopies and Raman and X-ray diffraction spectroscopies. Over the temperature range from 200 to 600 °C, the crystallinity, crystal phase, and structural integrity of the NT walls underwent pronounced changes whereas the overall film architecture remained intact. Increasing Ta from 200 to 400 °C transformed the as-deposited NT film from the amorphous phase to partially crystalline (300 °C) to fully crystalline anatase (400 °C). When the as-deposited NTs were detached from the underlying Ti substrate and then annealed, the anatase crystallites comprising the NT walls were stable to at least 600 °C in air. When the NTs remained attached to the substrate, thermal oxidation of the Ti metal initiated the growth and propagation of rutile crystallites in the NT walls at relatively low temperatures (ca. 500 °C). Once present in the NT walls, the rutile crystallites further catalyzed the anatase-to-rutile transformation, leading to partial degradation of the walls. The percent of rutile present in the TiO2 NT walls increased from 3% to 32% for samples annealed between 500 and 600 °C. Charge transport and recombination properties of dye-sensitized NT films were studied by frequency-resolved modulated photocurrent/photovoltage spectroscopies. Altering the microstructure of the NTs led to significant changes in the electron transport and recombination kinetics in DSSCs. At a fixed photoelectron density, the electron diffusion coefficient and recombination current density are found to change orders of magnitude in the opposite direction over the temperature range. DSSCs containing NT films annealed at 400 °C exhibited the fastest transport and slowest recombination kinetics. The various structural changes were also found to affect the light-harvesting, charge-injection, and charge-collection properties of DSSCs, which, in turn, altered the photocurrent density, photovoltage, and solar energy conversion efficiency. Introduction Dye-sensitized mesoporous nanocrystalline TiO2 solar cells (DSSCs; also known as Gra¨tzel cells) have received considerable attention as inexpensive and remarkably efficient solar devices.1 In traditional DSSCs, a film comprised of titania nanocrystallites is covered with dye molecules and the nanoporous film architecture is interpenetrated with a liquid redox electrolyte. The crystallite network is the recipient of injected electrons from optically excited dye molecules and provides a conductive pathway from the site of electron injection to the collecting electrode. Redox species in the electrolyte transport holes from the oxidized dyes to the counter electrode. Films normally used for DSSCs have substantial disorder associated with the individual crystallites (e.g., defects and size and shape nonuniformities) and the three-dimensional randomly packed particle network having a broad distribution of interparticle contacts.2 Such crystallite and network disorder can significantly retard electron transport. For instance, electron transport is 102-103 times slower in the nanoporous nanocrystalline films than in single-crystal TiO2 having a diffusion coefficient of about 10-1 cm2 s-1.3,4 One approach to make transport faster in DSSCs is to reduce both the morphological disorder and the dimensionality of the * To whom correspondence should be addressed. E-mail: Kai.Zhu@ nrel.gov and
[email protected].
network. The utilization of nanoporous films constructed of ordered one-dimensional nanowire5-10 and nanotube (NT)11-17 arrays, aligned perpendicularly to the underlying electroncollecting substrate, are examples of this approach. For instance, electrical conductivity measurements of ZnO films show that electron transport is 2 orders of magnitude faster in single-crystal NW arrays than in disordered nanoparticle films.5,7,8 Oriented arrays of TiO2 NTs are another oft-studied ordered film architecture for DSSCs. Unlike single-crystal ZnO NWs, recently we discovered that the transport rate in polycrystalline TiO2 NT arrays was surprisingly comparable to those in disordered nanoparticle films having similar average crystallite sizes.13 This unexpectedly slow transport was found to be related, in part, to orientational disorder in the array associated with the clustering of NTs created from capillary stress produced during the postgrowth cleaning/drying steps of the as-deposited NT films.18 When such disorder was prevented by modifying the conditions used for the postgrowth treatment, transport became faster, although it was still much slower than that measured for single-crystal TiO2 and still comparable to that in disordered nanoparticle films. These studies13,18 suggest that trap states associated with the polycrystalline nature of TiO2 NTs may slow, to some degree, the rate of electron transport. To a first approximation, it is expected that the average size of crystallites in the NT arrays should strongly affect the electron dynamics. This is the case for the traditional nanoparticle-based
10.1021/jp102137x 2010 American Chemical Society Published on Web 07/27/2010
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Figure 1. SEM images of titanium oxide NTs annealed at temperatures ranging from 200 to 600 °C. The length of the scale bar is 200 nm. Cracks result from sample preparation for SEM measurements.
films used in DSSC for which it has been shown that increasing the average particle size causes electrons to move faster through a film.19,20 Varying the annealing temperature is one strategy to induce change in the morphological properties of materials. Typically, films of oriented TiO2 NT arrays are prepared by electrochemically anodizing titanium metal in F-containing electrolyte21-24 and then annealing the resulting film. Annealing transforms the as-deposited amorphous titanium oxide phase to polycrystalline anatase TiO2.25-28 It has been shown that altering the annealing conditions can affect the crystal structure25,26,28,29 and photoresponse27,30 of the TiO2 NTs. The full extent to which the annealing temperature alters the morphology of NT films and the consequences of those changes on the electron dynamics and photoelectrochemical properties of DSSCs have not been investigated. In this paper, we examine the effect of annealing temperature Ta on the structural and electronic properties of TiO2 NT-based films. Elevating Ta from 200 to 600 °C is found to alter the crystallinity, crystal phase, and structural integrity of the individual NTs while having relatively little effect on the overall film architecture. The changes in the microstructure resulting from the annealing of the NT films are found to affect the dynamics of electron transport and recombination in significantly different ways. Changes in the microstructure of NT films used in the DSSCs also influence the light-harvesting, chargeinjection, and charge-collection properties as well as the cell performance.
drying treatments, the as-deposited NT arrays were annealed for 1 h in air at temperatures ranging from 200 to 600 °C (ramp rate 2 °C/min). The resulting films were immersed in 0.3 mM N719 dye (N719 ) [tetrabutylammonium]2[Ru(4-carboxylic acid-4′-carboxylate-2,2′-bipyridyl)2(NCS)2]) in ethanol for 24 h and then assembled into DSSCs as detailed previously.31 The semitransparent counter electrode consisted of TCO covered with a Pt catalyst. The cells were filled with an electrolyte composed of 0.8 M 1-hexyl-2,3-dimethylimidazolium iodide and 50 mM iodine in methoxypropionitrile. The amount of adsorbed N719 dye was determined by optical absorption of the desorbed dye.31 The structural properties of NT films were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Raman spectroscopy (using the backscattering geometry with a 488 nm laser excitation and 3-4 cm-1 resolution). The amount of adsorbed N719 was measured by optical absorption of the desorbed dye as detailed previously.31 Transport and recombination time constants were measured by intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) as described previously.32,33 For these measurements, 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 probe and bias light entered the cell from the counter electrode side. The details of the data analysis of these measurements are given previously.19,32,34,35 The setup for determining the photovoltaic characteristics under simulated AM1.5 solar irradiance is discussed elsewhere.31
Experimental Section Titanium foils (Aldrich, 0.25 mm, 99.7% purity) were electrochemically anodized at 20 V versus a Pt counter electrode in a solution of 0.5 wt % NH4F (Aldrich, 99.99% purity) in glycerol (Alfa Aesar, 99% purity) to produce TiO2 NT arrays with the average length of about 2 µm (unless otherwise stated).13,23 After electrochemical anodization, the as-anodized NT films were first rinsed with water, then soaked sequentially in a bath of 20/80, 40/60, 60/40, 80/20, and 100/0 vol% ethanol/ water for 5 min each, and finally dried using a supercritical CO2 drying apparatus.18 Following the postgrowth cleaning and
Results and Discussion Morphological Characterization. Figure 1 shows SEM images of NT films annealed in air at temperatures Ta ranging from 200 to 600 °C. When Ta is increased from 200 to 400 °C, no significant changes in the NT morphology are observed. Analyses of these SEM images show that the average NT pore diameters, wall thicknesses, and center-to-center NT distances are 31, 8, and 55 nm, respectively. From these parameters, an intertube spacing of 8 nm and a film porosity of 63% were calculated.13 When the temperature is elevated to 500 °C, the
TiO2 Nanotube-Based Dye-Sensitized Solar Cells
Figure 2. TEM images of NTs annealed at temperatures ranging from 300 to 600 °C. The length of the scale bar is 50 nm.
apparent average pore opening of the NTs narrows and the intertube spacing decreases. At a temperature of 600 °C, portions of the NT walls become perforated and nanoparticles form on the NT surface. Despite these substantial changes in NT morphology, it is noteworthy that the overall NT array architecture is retained over the entire annealing temperature range. Figure 2 shows TEM images of closely packed NTs annealed between 300 and 600 °C. At annealing temperatures of 300 and 400 °C, the exterior walls of the NTs are relatively smooth and continuous, a characteristic feature of TiO2 NTs prepared in glycerol.13 However, on close inspection it can be seen that there are regions of individual NTs, where the interior pore diameter tapers and the wall thickens and display rib- or ring-like features giving the NT an overall segment-like appearance. Despite these features, high-resolution TEM images13 show that for NT films annealed at 400 °C the tube walls are continuous and fully crystallized with some crystallites having {101} facets that are at least 50 nm in length. The dark areas result from a varying crystallite density (overlap of NTs) and orientation. The breaks in the NTs of the 400 °C sample arise from sample preparation for TEM measurements. It is noteworthy that selective area electron diffraction data13 show diffraction rings, not single reflections, indicating that the crystallites are more or less randomly oriented within the NT walls. When the films are heated to 500-600 °C, the walls of the NTs begin to partially breakdown into individual nanoparticles. The microscopic changes associated with annealing can be understood more fully by examining the XRD patterns and Raman spectroscopy data discussed below. Figure 3 compares the XRD patterns of NT films annealed at 200-600 °C. When the as-deposited NT film is annealed at 200 °C, only the diffraction pattern from the Ti substrate is observed, indicating that at this annealing temperature the NT film is amorphous. Increasing the annealing temperature to 300 °C results in crystallization of the NT film as identified by the signature anatase TiO2 (101) reflection at 2θ ) 25.3°. This result is in accord with the observation25 that as-deposited amorphous TiOx NT films start to crystallize to the anatase phase at about 280 °C in a dry oxygen atmosphere. Anatase TiO2 is also the only crystalline phase observed in films annealed at 400 °C. However, when the temperature is raised to 500 °C rutile crystallites begin to form. The intensity of the rutile (110) diffraction peak at 2θ ) 27.4° increases significantly when the annealing temperature rises from 500 to 600 °C.
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Figure 3. X-ray diffraction patterns of 2 µm long NTs annealed at temperatures ranging from 200 to 600 °C: A, anatase; R, rutile; asterisk (*), Ti substrate.
Figure 4. (a) XRD patterns of TiO2 NTs on Ti substrates annealed at 600 °C; the average NT film thicknesses vary from 0 to 2000 nm. (b) The integrated intensity of anatase (101) and rutile (110) peaks in part a as a function of NT film thickness.
To determine whether the anatase-to-rutile TiO2 transformation originates at the Ti substrate, in the NT walls, or in both regions concomitantly, we grew NT films ranging in thicknesses L from 0 to about 2000 nm and analyzed by XRD the rutile content as a function of L. Figure 4a shows the XRD patterns of 600 °C annealed NT films with L varying from 0 (Ti substrate) to 2000 nm. The XRD patterns reveal that the annealed Ti substrate displays only the rutile (110) peak. The absence of the anatase phase in the oxidized Ti substrate is consistent with an accelerated phase transformation to rutile in the presence of a large number of oxygen vacancies.36 In
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contrast, the XRD patterns of NT films on the Ti substrate annealed at 600 °C exhibit both the anatase (101) and the rutile (110) peaks. Figure 4b shows plots of the respective integrated intensities IA and IR for anatase (101) and rutile (110) diffraction peaks as a function of L. The values of IA are seen to increase linearly with the NT film thickness, reflecting the proportional increase of the amount of anatase TiO2 with increasing L. The values of IR scale linearly with NT film thickness with a nonzero (positive) intercept at L ) 0, which represents the integrated peak intensity for rutile at the annealed Ti substrate. The linear increase of IR with NT film thickness signifies, as in the case of anatase, the proportionally larger amount of rutile with increasing L. It is notable that the value of IR is nearly 3-fold larger for the 2 µm thick annealed NT film than for the annealed Ti substrate, suggesting that by 600 °C the NT walls are the primary source of rutile in the annealed 2 µm thick NT film on the Ti substrate. The fraction of rutile (XR) comprising the TiO2 NT walls can be calculated from the XRD data for 500 (not shown) and 600 °C (Figure 4a) using the relation XR ) 1/(1 + 0.8 IA/IR).37 After accounting for the rutile formed in the annealed substrate,38 we estimate that the percent of rutile present in the TiO2 NT walls was 3% for films annealed at 500 °C and 32% for films annealed at 600 °C. In addition to the anatase-to-rutile phase transformation, the average anatase crystallite size, as determined by applying the Scherrer equation39 to the anatase (101) peak, increased only minimally, from 29 to 34 nm, about 17%, over the temperature range from 300 to 600 °C for samples annealed on the Ti substrate. This result would seem to conflict with the highresolution TEM observation noted above that some grains with {101} facets are longer than 50 nm. However, the Scherrer equation is known to emphasize larger sized particles of a population by calculating the volume-weighted average.40,41 The high-resolution TEM images suggest that the growth of the anatase crystallites occurs mainly along the length of NT walls while filling the breadth of the walls. It is reasonable to expect that even substantial lengthening of the crystallites will have a relatively minor impact on the crystallite volume and, therefore, on the average crystallite size as determined by the Scherrer equation. For example, doubling the length of crystallites in the NT wall would only correspond to about a 25% increase (assuming growth occurs exclusively along the length of the NTs and not laterally along the curved tube wall) in the average crystallite size based on the Scherrer equation. Two possible mechanisms could account for the transformation of anatase to rutile in the NT walls at temperatures above 400 °C: (1) the anatase crystallites could, independently, convert to rutile or (2) the rutile layer at the Ti substrate/anatase TiO2 NT interface could catalyze the phase transition. To determine which of the two mechanisms is most plausible, we examined by XRD NTs in the amorphous phase that had been scraped off the Ti substrate, deposited on glass substrates, and then annealed. Figure 5 shows the XRD pattern of such a NT sample annealed at 600 °C. Only the anatase (101) peak is observed; there is no trace of the rutile (110) peak. Thus, the XRD patterns from Figures 4 and 5 imply that the rutile phase formed in the substrate during annealing catalyzes the anatase-to-rutile phase transformation in the NT walls. The XRD patterns also indicate that in the absence of the Ti substrate the anatase-to-rutile phase transition temperature in the walls of TiO2 NTs is greater than 600 °C, which is consistent with the results of another study of TiO2 xerogels having
