Screening of Photoactive Dyes on TiO2 Surfaces Using Scanning

Jul 17, 2012 - used as a powerful yet low-cost screening method. A standard SECM ..... point and suitable electrochemical window. For the photo- ...
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Screening of Photoactive Dyes on TiO2 Surfaces Using Scanning Electrochemical Microscopy William Kylberg,* Andrew J. Wain, and Fernando A. Castro National Physical Laboratory, Hampton Road, Teddington, Middlesex TW11 0LW, United Kingdom S Supporting Information *

ABSTRACT: Screening the photoelectrochemical activity of dyes on TiO2 surfaces is becoming increasingly relevant as industry tries to expand commercialization of dye-sensitized solar cells (DSSCs). Here we demonstrate how a simplified version of scanning electrochemical microscopy (SECM) together with photoexcitation of dispensed arrays of dye spots can be used as a powerful yet low-cost screening method. A standard SECM configuration uses separate reference and auxiliary electrodes together with an ultramicroelectrode (UME) working electrode (WE) and the substrate as an optional second WE. In this work we evaluate different electrode configurations, measurement modes, and measurement parameters to optimize photocurrent image quality. We propose an innovative two-electrode configuration that is a close representation of a real DSSC in operation. This simplifies the required instrumentation (no bipotentiostat or reference/auxiliary electrodes) compared to the more conventional four-electrode approach, reducing costs and allowing the screening method to be readily used by industry. Using this approach, the simultaneous screening of different dyes is demonstrated, and the relative photocurrent of dye samples in the arrays is shown to be comparable to the relative performance of full devices.

1. INTRODUCTION Ever since the paper by Grätzel and O’Regan in 1991 in which a breakthrough conversion efficiency of above 7% for a photoelectrochemical photovoltaic cell was reported, there has been extensive investigation into this type of device.1 The dye-sensitized solar cell (DSSC), which typically consists of a nanoporous thin film of TiO2 that has been sensitized by a UV−vis light absorbing dye, a counter electrode, and an electrolyte, is expected to have a relatively low price in large scale manufacture. This is partly because the materials do not need to be of very high purity as they do in silicon-based PVs, for example, and production processes are potentially low cost. Through technological advances in the field of DSSCs such devices are rapidly approaching the stage of being mass produced using, for instance, printing methods over large areas. In order to measure the photoelectrochemical homogeneity of the applied thin TiO2 films with dye monolayers, new techniques that map the photoactive area on the millimeters to centimeters scale are required. As well as evaluating surface variations of larger areas, a method for rapid screening of new dyes would accelerate the development and optimization of new dyes and their combinations. In the DSSC the sensitized TiO2 photoanode is usually supported on a transparent conductive metal oxide glass substrate, commonly fluorine-doped tin oxide (FTO) and is combined with a platinised cathode. The electrodes are separated by a spacer with a thickness in the order of 50 μm, and an electrolyte containing a suitable redox mediator (usually I−/I3−) is injected between them. Upon illumination, a number of processes take place: Photons transmitted through the anode are absorbed by the dye (step 1). The excited-state electrons in © 2012 American Chemical Society

the dye are then injected into the TiO2 (step 2), and the resulting dye cations rapidly oxidize I− to I3− regenerating the dye (step 3). Meanwhile, the injected electrons diffuse through the TiO2 to reach the FTO electrode. If they do not reach the FTO, they can participate in undesirable back-reactions, for example, by reducing I3− ions (step 4). The I3− ions that are photogenerated by the dye diffuse to the cathode and are efficiently reduced back to I− (step 5). This combination of steps, summarized by the reactions below, result in a current that can be used to do electrical work in an external circuit. Note that the full I−/I3− reaction pathways are complex and involve more steps than stated below:2−4 D|TiO2 + hv → D*|TiO2

(1)

D*|TiO2 → e−(TiO2 ) + D+|TiO2

(2)

D+|TiO2 + nI− → D|TiO2 + z I3−

(3)

2e−(TiO2 ) + I3− → 3I− (recombination reaction)

(4)

I3− + 2e−(Pt) → 3I−

(5)

Scanning electrochemical microscopy (SECM) is a scanning probe microscopy technique that enables local electrochemical interrogation on a micrometer scale. In SECM a piezoelectric positioner controls the movement of an ultramicroelectrode (UME) probe over a substrate immersed in an electrolyte. The activity of electrocatalysts and other functionalized surfaces may Received: May 11, 2012 Revised: June 22, 2012 Published: July 17, 2012 17384

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Figure 1. Four different modes for detecting the photocurrent from illuminated dye sensitized TiO2 in electrolyte containing I− and I3− ions with SECM technique. The species written in red is the dominant or sole species in the bulk solution before illumination. Adapted from reference 17.

surface the electrogenerated I− may be subsequently oxidized by the photoexcited surface to regenerate I3−. At tip sample separation distances of d when the ratio d/re is about 3 or less, the diffusion of I3− to the electrode will start to be blocked, but if the surface catalyzes the I− → I3− reaction, a positive feedback current is detected. This type of feedback mode has been demonstrated for estimating the dye regeneration rate constant of photoexcited dye cations at the semiconductor/electrolyte interface by measuring approach curves toward dye-sensitized ZnO or TiO2 surfaces in I3− electrolyte.12,14 Mode 2: In TG/SC the tip generates the species that will react with the surface and the substrate collects the induced current. In the case of photosensitized TiO2 and an electrolyte containing I3− ions, I− ions are electrogenerated at the UME tip (I3− + 2e− → 3I−), and the I− ions then induce a photocurrent at the photosensitized TiO2 that can be measured. Mode 3: The mode most similar to an operational DSSC is the SG/TC mode in which the UME detects the photogenerated products (normally I3−) diffusing from the surface. In this case, since the UME reduces the photooxidized species (i.e., I3− + 2e− → 3I−), it is essentially mimicking the cathode of a DSSC. Although SG/TC imaging in four-electrode SECM of photosensitized metal oxides has been suggested previously, it has only been demonstrated for small illuminated spots and not on arrays of dye.12,16 Mode 4: The photoelectrochemical performance could also be measured in competition mode by setting the tip potential to oxidize I− in competition with the photosensitizer. This mode has been demonstrated for imaging noble metal catalyst spots of submillimeter size5 but not for observing photoelectrochemical processes. In this work we explore the suitability of the four different modes described above and the experimental factors influencing the application of SECM to photoelectrochemical imaging and propose a two-electrode SG/TC mode that enables rapid screening of printed dye arrays. This novel approach simplifies

be imaged by SECM in a variety of ways, and its extension to photoelectrochemical imaging has recently been demonstrated.5−9 For example, a screening method that can be used for metal oxide photocatalysts or for dye-sensitized metal oxides has previously been developed where the UME tip holder is used to position an optical fiber with a coupled light source and the photocurrent is measured through the substrate.10,11 Other studies have measured the photocurrent in collection and feedback modes at the UME of dye-sensitized TiO2 films. Those results were used to measure diffusion coefficients within the TiO2 pores and electron transfer coefficients.12−14 While electrochemical screening by SECM over dye-sensitized metal oxides has been suggested in the literature, there has not yet been a full evaluation or demonstration of screening printed dye arrays.15,16 As noted in a recent SECM review,17 there are at least four different SECM modes to measuring electrocatalytic activity on a substrate. In Figure 1 they are presented as (mode 1) feedback, (mode 2) tip generation/substrate collection (TG/ SC), (mode 3) substrate generation/tip collection (SG/TC), and (mode 4) competitive mode. In principle, all four modes could be useful in examining photoelectrochemical activity of dye-sensitized TiO2 surfaces if a suitable redox mediator such as the I−/I3− couple is used. At a planar UME the steady-state current I for a diffusion-controlled electrochemical process is related to the concentration of reactant, c*, by I = 4nFc*Dre, where n is the number of electrons transferred, F is the Faraday constant (96 485 C mol−1), D is the diffusion coefficient, and re the UME tip radius. The current at a small disk reaches a steady state in a relatively short time (∼ re2/D), so for a UME of radius 5 μm and diffusion coefficient of 5 × 10−6 cm2 s−1 this translates to about 50 ms.18 We will discuss each of the SECM modes in Figure 1 in turn. Mode 1: In feedback mode the UME potential is biased such that it reduces I3− to I− under diffusion control. Close to the 17385

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substrate tilt (Figure S1). The approach curves were set to approach until the current reached 90% of the steady-state current. In an ideal negative feedback approach curve, 90% equates to approximately d/re = 3, or 10−15 μm in this case.19 We note that ideal approach curves are modeled for nonporous substrate surfaces, but in experiments with porous TiO2 this may well be complicated by subsurface and lateral diffusion processes. The solvent 3-MPN was chosen for its high boiling point and suitable electrochemical window. For the photoelectrochemical imaging, a solution of 500 mM lithium iodide (LiI) in 3-MPN (0.1 M TBAP) was used as the redox electrolyte. The SECM experiments were undertaken in a twoelectrode configuration where the potential was set between the UME and FTO substrate or in four-electrode configuration with a separate reference electrode (10 mM Ag/Ag+ in 3-MPN, 0.1 M TBAP, +0.544 V vs NHE) and Pt counter electrode. The I3− solution, which was used in feedback mode, was prepared in 1 mM concentration by mixing 1 mM LiI and 1 mM I2 in 3MPN (0.1 M TBAP) electrolyte. The association constant of I− + I2 ↔ I3− in 3-MPN is very high (log Keq ∼ 5) so the solution is considered to consist of almost exclusively I3−.12,20 2.3. Light Source. To apply uniform illumination over the entire area of the TiO2 substrate (∼1 cm2), an LCD back light (white light LED) with diffuser from a commercial display was prepared to be used with the SECM. A Keithley source meter 2440 was used to control the lamp intensity. Currents between 10 and 100 mA were applied for a range of irradiances but unless stated were run at an operation current of 60 mA, which corresponds to a total irradiance of about 2.2 mW cm−2 as measured by a calibrated Hamamatsu spectrometer. The spectral irradiance spectrum of the lamp was analyzed with a Hamamatsu spectrometer TM-CCD-A series with fiber optic coupled to an integrating sphere for a range of different currents, and the spectrum at 60 mA can be seen in Figure S2 together with absorbance spectra of N719 and MK-2 dyes on TiO2 after equal adsorption time. After 15 min the light spectrum and radiance were steady, and measurements in different areas within a square of 2 × 2 cm of the display showed a radiance variation of less than 10%. The temperature of the light source was measured and did not increase to more than 25 °C.

the measurement while improving the signal-to-noise ratio and allows a comparative assessment of dye performance.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. Glass with fluorine-doped tin oxide (FTO, kindly donated by Pilkington Glass) was cleaned in detergent (Hellmanex), water, and ethanol. A thin film of titanium dioxide (Solaronix TiO2 paste Ti-Nanoxide T) was applied by doctor blading with a single layer of Scotch tape as spacer and sintered at 450 °C for 20 min. Film thicknesses of 3−4 μm were produced as measured by a confocal microscope (Olympus LEXT OLS3000) and stylus profilometer (Taylor Hobson Precision) on selected samples (Figure S1). The dye spots were produced by dispensing 0.2 mM EtOH (Fisher) solutions of the dye through a piezoelectric jetting device of a CHI1550A solution dispenser. Two different commercially available dyes were used for the experiments: ditetrabutylammonium cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′dicarboxylato)ruthenium(II) (N719) and 2-cyano-3-[5‴-(9ethyl-9H-carbazol-3-yl)-3′,3″,3‴,4-tetra-n-hexyl[2,2′,5′,2″,5″,2‴]quaterthiophen-5-yl]acrylic (MK-2), both obtained from Aldrich. Arrays or lines of dye spots were printed. The nozzle of the jetting device was sensitive to the solvent in use and to the solute and its concentrations so in order to minimize repeatability problems, solutions using the same solvent and concentration were used for all preparations. Each deposition consisted of a number of droplets and by varying the number of drops per deposition the diameter of the printed spots could be controlled. When running repetitive runs, a 3 s pause in between runs allowed for complete evaporation of droplet solvent. Repeated runs (50−150 times) were applied to produce spots of sufficient concentration. The films were rinsed in EtOH after the printing was completed and dried with compressed air. The substrates were inserted into the sample holder, and a silver wire was used to make electrical contact to the FTO. UV−vis absorbance spectra of dye-sensitized TiO2 films were measured on a PerkinElmer Lambda 850 spectrometer. 2.2. Electrochemistry and SECM Experiments. All electrochemistry was performed with a UME either in bulk solution or in solution close (5−160 μm) to the substrate surface. The following experiments were performed either with I− or I3− ions in solution, as these are the components of the preferred redox mediator in DSSC electrolytes, or with ferrocenemethanol, FcMeOH (Fe2+/Fe3+), in SECM experiments where topographical information was the priority. The experiments were performed over bare TiO2, fully dyesensitized TiO2, or TiO2 with printed dye spots. A scanning electrochemical microscopy system (CHI 900b) with a UME of 10 μm diameter was used to measure the photoelectrochemical activity of the TiO2 films. UME tips were polished, rinsed, and checked under a microscope before use. The estimated ratio of the metal disk to entire electrode radius (the so-called RG value) was observed to be about 15, and although precise feedback measurements for fitting with theory require RG below 10 for reliable data, higher RG values are acceptable for comparative measurements in the amperometric SG/TC mode. Before the SECM measurements of the dye arrays, the sample surface was properly leveled by performing approach curves in FcMeOH solutions (1 mM) in 3-methoxypropionitrile (3MPN, Fisher) with 0.1 M tetrabutylammonium hexafluorophosphate (TBAP, Sigma-Aldrich) supporting electrolyte and then scanning in X and Y directions while adjusting the

3. RESULTS AND DISCUSSION 3.1. Mode of Operation. To evaluate and select the simplest and most effective method of screening dyes for DSSC with SECM, different electrode configurations and measurement modes were tested. The four modes of measuring the photoelectrochemical activity that were presented in the introduction (Figure 1) were all evaluated as techniques for screening. To probe the feasibility of SECM to measure photoelectrochemical oxidation of I− ions, approach curves were performed in feedback mode (mode 1) over a fully dyesensitized TiO2 film. With increasing light intensity there was indeed an increased positive feedback contribution to approach curves that result from increasing electron transfer rates, as has been previously reported (Figure S4).21,12 While feedback mode yields kinetic and transient information and indicates the photoactive properties of the surface, it was not found to be suitable for array imaging because of the small tip−surface distance required for good contrast. This mode was therefore not investigated further. Of the other modes, mode 2 (TG/SC mode) is intrinsically not suited to large samples and higher resolutions due to the high background currents, as was 17386

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previously found,17 and mode 4 (competition mode) only yields relatively low current contrasts. Instead, the SG/TC mode (mode 3) was found to be the mode of preference for our photocurrent experiments. To detect the photocurrent, manifested as an increased local concentration of I3− from the photooxidized I−, the UME tip was biased to reduce I3−. Upon illuminating the dye-sensitized film, an increased cathodic current was detected at the locally positioned UME. In our experiments with the SG/TC mode a concentration of 500 mM I− could be used to record a good signal without the side effect of a very large background current. This concentration is also close to that typically employed in DSSC devices. A further practical consideration that favors the electrolyte composition of mode 3 is the transparency of the iodide solution in the visible part of the spectrum as compared to the optical absorbance of I3− ion and I2 in solution.22 This may allow for simultaneous spectroscopic investigation of the electrolyte and substrate. This, however, is beyond the scope of this paper. For the SG/TC mode, two configurations of electrodes (two- or four-electrode) were investigated (Figure 2a,b). The

mode the bias potential at the UME should be in the region where I3− is reduced to I− with a diffusion-limited current. Based on the above, a potential of −0.3 V vs Ag/Ag+ (+0.244 V vs NHE) was selected as the UME bias potential in the fourelectrode measurement and 0 V for the FTO substrate. For comparative purposes, the potential bias of the UME in the two-electrode measurement was set to −0.3 V vs the substrate. In Figure 3, steady-state currents at the UME positioned at 20 μm above the surface over fully N719 sensitized films were

Figure 3. Steady-state currents at UME with light switched on at t = 0. Measured 20 μm above a fully N719 dye-sensitized TiO2 film.

recorded as a function of time as a chopped light was applied. The figure shows the response for two- and four-electrode configurations. In both cases the increase in cathodic current represents the tip collection of I3− induced by illuminating the photosensitized TiO2. It is clear from Figure 3 that the twoelectrode configuration results in a significantly greater current at the UME. A contributing factor is likely the absence of an auxiliary counter electrode competing with the UME for current collection. It is reasonable to consider the two-electrode configuration more similar to an operational DSSC than the configuration using four electrodes. In fact, applying a potential to the UME in the two-electrode case is equivalent to measuring the current at a specific point on the I−V (current−voltage) curve of a DSSC. Similar results to those in Figure 3 were observed when the UME was positioned over printed dye spots, although the magnitude of the current was, not surprisingly, less and the rise time longer. Considering the strong electrochemical signal in the two-electrode configuration and the fact that it makes for a simpler system where cheaper source meters can be used in place of more specialized potentiostats, this approach was prioritized as an innovative method to screen dyes. To investigate how the potential bias between UME and photosensitized TiO2 affects the measured photocurrents, linear sweep voltammetry (LSV), CV, and lateral amperometric SECM scans over single printed spots of N719 dye with a range of different potentials were recorded. In Figure 4, an LSV plot for a UME 10 μm above a printed dye spot in the absence and presence of light shows how the cathodic current increases due to the photogenerated I3− ions. The optimum bias region, at least for probing above the printed spots, appears to be between about 0 and −0.4 V, where any increase in I3− after photoexcitation is easily detected. In order to minimize background currents it is desirable to use an

Figure 2. Schematic depiction of (a) two- and (b) four-electrode configurations. Adapted from references 13 and 16.

four-electrode configuration consisted of UME, reference electrode, auxiliary electrode, and the FTO substrate which could be selectively biased as a second working electrode. Alternatively, connecting the UME as a working electrode and the TiO2−FTO substrate as counter/reference electrode formed a simplified two-electrode configuration. 3.2. Electrochemistry. Cyclic voltammetry (CV) (Figure S3) was undertaken at a UME submerged in solutions of either I3− or I− ions in 5 mM concentrations in 3-MPN (100 mM TBAP) solutions with a Ag/Ag+ reference electrode. In the I− solution two oxidative electrochemical processes were observed: one at E ∼ 0 V (I− → I3−) and another at E ∼ +0.5 V (I3− → I2). In the I3− solution only the latter was observed primarily. For measuring the photocurrent in SG/TC 17387

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Figure 4. Linear sweep voltammetry of UME in two-electrode cell in the dark and with illumination 10 μm above a printed N719 dye spot on TiO2 in LiI (500 mM) solutions in 3-MPN (100 mM TBAP).

Figure 5. Photocurrent profile over two N719 spots on a TiO2 film (two-electrode) in LiI solution of 500 mM concentration and 100 mM TBAP.

electrolyte containing only I− ions, but as is seen in the dark current there will always be some oxidized I− resulting in small quantities of both I2 and I3− in the electrolyte. When measuring above a fully dye-sensitized TiO2 film, the voltammetry is complicated by the shifted potential of the reference potential as the TiO2-substrate quasi-Fermi level is shifted to a more negative potential. Another important variable to consider is the required preconditioning. Upon light soaking the TiO2 will slowly fill up with photoinjected electrons and cutting off the light will lead to a slow discharge of the electrons in the TiO2. Measuring a photocurrent transient with a chopped light, the time for discharge was estimated to about 200 s. The photoactivity of unsensitized TiO2 was also considered. A control experiment was undertaken with bare TiO2 being illuminated in the two-electrode configuration in SG/TC mode. A very small photocurrent (assumed to be an increase in I3−) was observed that was more than an order of magnitude smaller than that of the photosensitized TiO2 so it has only negligible interference on the measurements. This is probably due to the blue part of the LED spectrum (Figure S2) having a weak overlap with the TiO2 bandgap absorbance (absorbance edge ∼380−400 nm) creating holes in the valence band that oxidize the I−.23 Adding a filter to block light below 495 nm did remove the photocurrent from bare TiO2. 3.3. Array Imaging. Arrays of printed dye spots were prepared as described in the Experimental Section. Single spots, lines, or arrays were then imaged in one or two dimensions using SECM in two-electrode configuration. In all of the following measurements the UME was first positioned at ∼10 μm from the substrate surface using approach curves in FcMeOH solution (1 mM) and comparing to theory for negative feedback.19 To begin with, lateral scans were performed over two dye spots in one direction as is shown in Figure 5. The lateral scans indicate that the UME current is slightly sensitive to topographical variations of the sample. For example on the far left of Figure 5 the edge of the TiO2 film is seen as a step-up in the current (at ∼250 μm). More importantly, however, there is a significant increase in measured current as the UME passes over the two dye spots as a result of their photoactivity. In the volume of electrolyte around the spots there will be a diffusion field of photogenerated species. To gather information

on the profile of the diffusion fields over the dye spots, lateral probe scans over the dye spots were measured at different tip− substrate separations over the TiO2 surface. Lateral scan measurements over a pair of dye spots (350 μm in diameter) were performed every 10 μm up to a tip substrate distance of 120 μm (Figure 6). It can be seen that the area with a strong current flux is confined to a relatively narrow field above the dye spots, but even at distances of 100 μm the spots are still highly resolved albeit with lower absolute photocurrents. The diffusion field measurement was also performed in a fourelectrode configuration over two adjacent N719 spots and results were very similar to the two-electrode measurement. In Figure 6, it can be seen that there is a maximum overlap of the diffusion fields when the tip distance is about 20 μm from the surface. This overlapping of diffusion fields remains to some extent even at spot separations of 1000 μm and under the conditions of our experiments could only be fully neglected at separations above 1500 μm. As well as undertaking scans in one dimension, SECM can be used to map the photoelectrochemical activity in X- and Y-axis to yield 2-dimensional area maps of dye-sensitized TiO2 films and dye arrays. Among the parameters that can be varied with the SECM software when recording the image are the increment distance and increment time that combine to set an overall speed. It was noted that the increment time should be at least 100 ms to ensure a good image without too much noise, while the distance increment determines pixel resolution. At high scanning speeds (>200 μm/s) for the mapping of the dye spot arrays, a smearing effect of the imaged spots grew in the direction of the scans. This effect, which can be reduced by slower scanning, is likely due to convective forces resulting from the UME movement. When comparing different dye spots, the smearing effect is not disruptive if the direction of the UME sweep is perpendicular to the sequence of the dyes. Measuring at high speed can in fact circumvent slow current gradients when the substrate has not been sufficiently preconditioned. Spot diameters ranged between 350 and 650 μm, and this resulted in the photoactivity maps having sharp current peaks where the dye spots are positioned. The maximum peak currents were therefore used as a measure of relative photoactivity. SECM imaging with a small step size, e.g., 5 μm, as is seen in Figure 7, can reveal dye spot activity in very fine detail. From 17388

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Figure 6. Diffusion field profile over two N719 dye spot under illumination. The spots were about 350 μm in diameter and 1000 μm apart. The red bars below the figure represent the 350 μm spots.

Figure 7. SECM image of two N719 dye spots recorded at a speed of 50 μm/s over a length of 2000 μm. The tip−substrate distance was 10 μm, and the step size was 5 μm. The total scan time was 2.5 h.

Figure 8. A photocurrent SECM map of 2 × 2 N719 array with spots of different sizes (two-electrode configuration). The figure with the circles on the right represent the number of drops per jetted dispense and the relative sizes of the dye spots as determined by optical microscopy.

optical microscopy imaging it is possible to measure the real size and shape of the printed spots and compare these with the SECM images. The diameters of the spots from the SECM image are approximately 10−15% larger than measured from the optical microscope images, which is reasonable when considering the electrochemical diffusion field. Wash marks where some N719 has made a faint ring around the dye spot are observed under an optical microscope (not shown) and are also detected as photoelectrochemically active (Figure 7), demonstrating the sensitivity of this measurement. To evaluate the measurement reliability, different array formations of printed dye spots were prepared. Given the nature of the jetting method of dispensing dye spots, the major

issue to address is one of dye coverage and how this is influenced by the dispensing parameters (Figure S5). The piezoelectric device dispenses roughly 100−200 pL for each drop. With the concentration of the solution known it is possible to calculate what the nominal projected coverage (moles per projected area, as opposed to the real area of the porous TiO2) is and compare this to literature values for fully covered films. For the 0.2 mM dye solutions, the calculated nominal projected coverage resulting from 100 drops is ∼4.2 nmol/cm2. This is compared to a projected coverage of the order of 100 nmol/cm2 typically reported in the literature.24 Based on this and taking into account thickness variations in the TiO2 layers and assuming similar porosities, the dye spots 17389

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was printed together with the commercially available organic dye MK-2 (Figure 10). The two dyes were dispensed from solutions of 0.2 mM in ethanol and both with 100 drops each. In Figure 10a, the photocurrents of two neighboring spots of dye have overlapped, and the image has been smeared (horizontally) due to the relatively high scan speed (2000 μm/s). In the case of the MK-2 dye this was required due to the time-sensitive nature of the dye film that will be discussed below. The much greater photocurrents from the MK-2 dye is likely due to its greater extinction coefficient (∼38 440 at λmax for MK-2 and ∼13 600 at λmax for N719)30,31 and to a certain degree a greater surface coverage. UV−vis absorbance spectra of the two dyes adsorbed onto 1 cm2 TiO2 films (4 h adsorption in 0.2 mM EtOH dye solution) showed ∼4 times higher absorbance peak for the MK-2 dye over the N719 dye which, considering a higher extinction coefficient by a factor of 2.8, suggests a higher coverage (Figure S2). The higher coverage of MK-2 is likely to stem from its smaller molecular size, N719 having a relatively larger footprint due to its two carboxylate anchoring groups. For the thin TiO2 films in our experiments the extinction coefficients will have a larger influence on the photocurrent from the dye spots than the fully sensitized films because the low coverage results in an optical absorbance that is far from being saturated. From measurements taken at different times it was observed that while N719 is rather robust, the organic dye is not very stable on the TiO2 surface and the photoelectrochemical signal decays significantly within an hour. In control experiments it can be seen that the dye either bleaches or desorbs after this time. However, fully sensitized TiO2 films with MK-2 were immersed in 3-MPN with and without light for 2 h, and the dye did not desorb or bleach within this time. Similar experiments on TiO2 films with incomplete dye coverage did very clearly result in bleached dye but no desorption. This suggests the fully sensitized MK-2 samples had a much better stability over time than the printed MK-2 spots (Figure S7). According to the literature, the MK-2 dye is stable if UV light is minimized.32,33 A filter to block out the LED light below 495 nm was able to slow the decay of the half-sensitized films but not completely (Figure S7). To compensate for the lower light intensity with the filter, the current source of the light was adjusted so that the cell had the same short circuit current with and without filter for these control experiments. In order to verify the higher performance of the MK-2 dye, current−voltage curves of TiO2 films (area = 1 cm2) with complete dye coverages of N719 and MK-2 were recorded. These samples were connected in twoelectrode configurations with an auxiliary Pt wire as counter electrode. A redox electrolyte consisting of 500 mM LiI and 50 mM I2 (common composition for DSSC electrolyte) in 3-MPN was employed. The resulting I−V curves are depicted in Figure 10b where it can be seen that the fully dye covered samples exhibit a higher short-circuit photocurrent for the MK-2 dye than N719, consistent with the SECM map in Figure 10a. The I−V curves from Figure 10b compare well with measurements from the literature if the lower light intensity is taken into account.28 It should be noted that while the open-circuit voltage and hence overall conversion efficiency are not measured in the SECM imaging, the photocurrent generation capability of the dyes is mapped very efficiently (see Supporting Information for further discussion).

are estimated to have less than 10% complete coverage after 100 drops. In the above considerations we assume that the dye coverage is approximately homogeneous throughout the spot volume, which is reasonable given the capillary forces within the TiO2 porous film structure. It is difficult to predict the exact impact of coverage on the magnitude of the photocurrent, but it has previously been observed that there is a coverage threshold where the incident photon to current efficiency (IPCE) of DSSC devices experiences an abrupt increase above 30−50% of full dye coverage.25,26 One reason is believed to be that lateral hole conduction between neighboring dye molecules in the adsorbed monolayers facilitates the regeneration of oxidized dye.25,27 Furthermore, it has been observed that, in the case of N719, areas of the TiO2 that are not covered by dye are not protected against some of the recombination reactions between conduction band electrons in the TiO2 and the I3− in the electrolyte that decrease the photocurrent and the open-circuit voltage.28,29 In Figure 8 an array has been prepared with a different number of drops at each position. As is seen from the SECM image and the spot contours from optical microscopy (only outlines shown), a higher number of drops result in larger spots, which is expected from the greater spread before evaporation. Despite this, the dye coverage per projected area (concentration) still increases with the number of drops (Figure S6a), and this is reflected in an increase in the maximum photocurrents measured by SECM (Figure S6b). Next, in order to rule out effects due to spot size variation, an array was prepared in which the concentration of dye spots was controlled while maintaining roughly the same spot diameters (∼350−400 μm); two rows with different amounts of single drops were dispensed. The top two spots in Figure 9 (in this

Figure 9. Photocurrent SECM maps (two-electrode, scan speed 400 μm/s) of 2 × 2 array of N719 spots with different concentrations. The two top spots have been deposited with 150 drops each and the bottom two with 100 drops each.

case shown in color to improve current contrast) have been deposited with 150 drops and the lower two with 100 drops. Again, the maximum photocurrents in the array are observed to increase with concentration (see Figure S5b for plot of data from Figures 8 and 9). Finally, to demonstrate the utility of this technique to comparing the photoactivity of different dyes, the N719 dye 17390

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Figure 10. (a) A two-electrode SECM map of 2 × MK-2 dye spots (top) and 2 × N719 dye spots (bottom) scanned at a speed 2000 μm/s. (b) Corresponding I−V curves of 1 cm2 fully sensitized TiO2 films with same illumination (LED light). (c) Molecular structures of the two dyes N719 and MK-2 are depicted.

4. CONCLUSIONS Scanning electrochemical microscopy has been used to image photoinduced electron injection of dye-sensitized TiO2 by probing the steady-state current of I3− ions generated by photooxidation of I − ions. Various different electrode configurations were tested, and the highest contrast and most repeatable imaging resulted from a two-electrode configuration in which a potential of −0.3 V was applied between the UME and the TiO2/FTO substrate. Arrays of dye spots printed from a picoliter dispenser with different spot sizes, dye concentrations, and dye materials were probed with the SECM, and we have demonstrated that this method is a feasible technique for the rapid screening of printed dyes to evaluate their suitability in DSSC devices. These measurements are at present only qualitative, but such a relative comparison of dye performance nevertheless remains a powerful and informative tool for the discovery and development of new dye materials. While the technique does have limitations such as uncertainties in dye coverage and no measure of fill factor and open circuit voltage, this method may be complemented by subsequent characterization of full devices. The instrumental setup has the potential to be built with relatively cheap and simple equipment and has been shown to yield insight into not only the photoelectrochemical performance of dye materials but also photobleaching and dye adsorption phenomena as well as local effects of mediator diffusion. The methodology described herein could be extended to examine the effect of dye coadsorbents and to screen local counter electrode performance, and indeed the

application of this technique to the mapping of nonuniformities in dye-covered TiO2 surfaces will be the subject of future work.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Profiles of TiO2 film as measured by stylus profilometry and SECM; UV−vis absorbance spectra of dyes on TiO2 and emission spectrum of the LED light are presented; cyclic voltammetry of electrolyte in bulk and close to dye-sensitized TiO2 is demonstrated; SECM approach curves toward dyesensitized TiO2 under illumination; a description of dye spot characterization; data plots of dye concentration against spot size and against maximum photocurrent; graph showing the change in maximum current with time under illumination; discussion on the theoretical considerations of the photocurrent detection. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author

*Phone +44 20 8943 6692; e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Neil Lockmuller and Claudiu Giusca (NPL) for their kind help in measuring the profile of the TiO2 films. Dr. Paul Miller (NPL) is thanked for help in determining the spectrum 17391

dx.doi.org/10.1021/jp304599v | J. Phys. Chem. C 2012, 116, 17384−17392

The Journal of Physical Chemistry C

Article

(33) Katoh, R.; Yaguchi, K.; Furube, A. Chem. Phys. Lett. 2011, 511, 336−339.

and irradiance of the LED light source. We are grateful to Jim Banks (NPL) for the kind assistance with the wiring of the LED lamp. Dr. Alan Turnbull (NPL) is acknowledged for helpful discussions. This work was supported by the UK Department for Business, Innovation and Skills.

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