Electrochemical Characterization of TiO2 Blocking Layers for Dye

Article pubs.acs.org/JPCC

Electrochemical Characterization of TiO2 Blocking Layers for DyeSensitized Solar Cells Ladislav Kavan,*,† Nicolas Tétreault,‡ Thomas Moehl,‡ and Michael Graẗ zel‡ †

J. Heyrovský Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Dolejškova 3, CZ-18223 Prague 8, Czech Republic ‡ Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, Swiss Federal Institute of Technology, Station 6, CH-1015 Lausanne, Switzerland S Supporting Information *

ABSTRACT: Thin compact layers of TiO2 are grown by thermal oxidation of Ti, by spray pyrolysis, by electrochemical deposition, and by atomic layer deposition. These layers are used in dye-sensitized solar cells to prevent recombination of electrons from the substrate (FTO or Ti) with the holeconducting medium at this interface. The quality of blocking is evaluated electrochemically by methylviologen, ferro/ferricyanide, and spiro-OMeTAD as the model redox probes. Two types of pinholes in the blocking layers are classified, and their effective area is quantified. Frequency-independent Mott−Schottky plots are fitted from electrochemical impedance spectroscopy. Certain films of the thicknesses of several nanometers allow distinguishing the depletion layer formation both in the TiO2 film and in the FTO substrate underneath the titania film. The excellent blocking function of thermally oxidized Ti, electrodeposited film (60 nm), and atomic-layer-deposited films (>6 nm) is documented by the relative pinhole area of less than 1%. However, the blocking behavior of electrodeposited and atomic-layer-deposited films is strongly reduced upon calcination at 500 °C. The blocking function of spray-pyrolyzed films is less good but also less sensitive to calcination. The thermally oxidized Ti is well blocking and insensitive to calcination.

1. INTRODUCTION The dye-sensitized solar cell (DSC) is a low-cost, highly efficient device to rival Si-based photovoltaics. It was discovered in 1991 by O’Regan and Grätzel,1 and their work triggered great academic and technological feedback during the next two decades.2−6 The generic concept of DSC is a liquid-junction photoelectrochemical cell with a dye-sensitized nanocrystalline TiO2 photoanode. It contacts electrolyte solution with redox mediator, which transports hole from the photooxidized dye toward the counterelectrode. An alternative of this device is the solid-state dye-sensitized solar cell (SSDSC) in which the photogenerated holes are transported by a solid conductive material, like 2,2′-7,7′-tetrakis(N,N-di-p-methoxyphenylamine)9,9′-spirobifluorene (spiro-OMeTAD).2 Recently, this concept attracted considerable interest in conjunction with a methylammonium lead iodide perovskite-sensitized solar cell7−9 achieving up to 15% solar conversion efficiency.9 The titania photoanode has been studied thoroughly, ranging from investigation of single crystals10,11 to advanced organized nanostructures.12,13 The nanocrystalline TiO2 (anatase) is usually deposited on transparent oxides, such as fluorine doped tin oxide (FTO), which is able to collect (or inject) electrons from (to) the conduction band of the TiO2. This mesoporous scaffold is covered by a dye- or pigment-light harvester capable of efficiently injecting electrons into the conduction band of TiO2. The back reaction (recombination) of photoinjected electrons with the oxidized form of mediator or with the hole-transporting medium is an undesired parasitic © 2014 American Chemical Society

process. It occurs both at the TiO2 surface and at the naked FTO areas, which are uncovered by the TiO2 nanoparticles. The recombination current over the FTO is relatively small in a liquid-type DSC, albeit not fully negligible,14 but it becomes crucial in the SSDSCs. For their proper function, a nonporous blocking underlayer of TiO2 must be deposited on top of FTO to prevent shunting of electrons from the FTO support to the hole-transporter. This underlayer is usually fabricated by spray pyrolysis. The method was introduced in 1995 by Kavan and Grätzel,15 and later on, it has been used in almost all works on SSDSCs, starting from the pioneering work2 up to the perovskite-based DSCs7−9,16−18 including even their variant without the hole-transporting medium.19 Despite the popularity of spray pyrolysis, these layers were never thoroughly examined for defects like pinholes, which would be detrimental to the blocking function. Furthermore, there are various other synthetic protocols toward dense titania layers, such as sol−gel,20 DC-magnetron sputtering,21 electrochemical deposition, 22 and atomic layer deposition (ALD).23−27 In particular, the last option is attractive not only for DSCs25,27 but also in the area of photoelectrochemical water splitting, where the ALD-made titania layers turned out Special Issue: Michael Grätzel Festschrift Received: October 19, 2013 Revised: January 3, 2014 Published: January 3, 2014 16408

dx.doi.org/10.1021/jp4103614 | J. Phys. Chem. C 2014, 118, 16408−16418

The Journal of Physical Chemistry C

Article

to be successful for preventing corrosion of electrode materials such as Cu2O or silicon.28−31 Alternatively, the oxide layer grown spontaneously by thermal oxidation of Ti-metal is another option to consider for the growth of blocking layer. Here we present a systematic electrochemical study to evaluate some of these alternative blocking layers.

molecules were of the standard quality (p.a. or electrochemical grade) purchased from Aldrich or Merck and used as received.

3. RESULTS AND DISCUSSION 3.1. Cyclic Voltammetry with Model Redox Probes. To test the pinhole defects in our blocking layers, we used the aqueous solution of Fe(CN)63−/4− or dimethylviologen (MV2+) as the model redox systems. These couples were chosen as pHindependent redox probes with simple one-electron-transfer reaction. The redox potential of Fe(CN)63−/4− (0.24 V vs Ag/AgCl) is sufficiently positive to the flatband potential of TiO2 (anatase, rutile) at all practically accessible pH values in aqueous electrolyte solutions:10

2. EXPERIMENTAL SECTION FTO glass (TEC 15 from Libbey-Owens-Ford, 15 Ohm/sq) was cleaned ultrasonically in isopropanol and water, followed by treatment in UVO-Cleaner (model 256-220, Jelight). Surface-oxidized titanium metal was prepared using the grade 2, 60 μm thickness Ti foil from the G24i company. It was oxidized by heat treatment at 500 °C for 30 min in air. ALD was carried out using a Savannah 100 apparatus from Cambridge Nanotech. The Ti precursor was tetrakis(dimethylamino)titanium, TDMAT (99.99%, Aldrich), and the auxiliary reactant was either water (samples labeled ‘ALD’) or hydrogen peroxide (samples labeled ‘ALDP’). The substrate temperature was kept at 150 °C for the ALD samples and 200 °C for the ALDP samples. The ALDP samples have better crystallinity (anatase) than the ALD samples, which are regarded to be amorphous.30 Films were deposited under a nitrogen flow of 5 sccm with precursor and oxidant pulse lengths of 0.1 s and wait period of 10 s. The ALD samples were deposited under exposure mode, where the precursor was kept for 2 s in the chamber before the purge. The growth per cycle was determined by spectroscopic ellipsometry on a silicon wafer or gold-coated substrate and was found to be 0.53 Å/ cycle. Spray pyrolysis was carried out using ethanolic solution of di-iso-propoxy titanium bis(acetylacetonate) following the synthetic procedure described elsewhere14,15,32,33 with the difference that pure oxygen was used as carrier gas instead of compressed air. Electrochemical deposition employed the anodic oxidative hydrolysis of aqueous solution of TiCl3.22 The reaction was carried out potentiostatically by applying the voltage of 0.1 to 0.3 V between the FTO and the platinized FTO (prepared by thermal decomposition of H2PtCl6) electrodes. The reaction was carried out at pH 2.4 to 2.6; the growth rate was between 2 and 5 nm/mC/cm2 depending on the applied potential and pH, as detailed elsewhere.22 The actual layer thickness was determined ellipsometrically; to this purpose, the electrodeposited titania was grown on Au/Cr evaporated on glass. The thicker blocking layers were also measured by Alfa step profilometer (Tencor Instruments). In all cases, the electrodes with blocking layers were used either as made (sometimes dried at 100 °C overnight) or after subsequent calcination in air at 450 °C for 15 min, followed by 15 min at 500 °C, heating ramp 5°/min. Electrochemical experiments were carried out in a onecompartment cell using Autolab Pgstat-30 equipped with the FRA module (Ecochemie) controlled by the GPES-4 software. The reference electrode was Ag/AgCl (sat. KCl) for experiments in aqueous electrolyte solutions and the Ag-wire pseudoreference for aprotic media. In all cases, the electrolyte solutions were purged with Ar, and the measurement was carried out under an Ar atmosphere in the hermetically closed electrochemical cell. Impedance spectra were measured at varying potentials, which were scanned from positive to negative values (typically from 1.3 to −0.8 V vs Ag/AgCl in acidic media). Impedance spectra were evaluated using Zview (Scribner) software. Electrolytes, solvents, and redox-active

φFB = −0.36 − 0.059· pH

(anatase, V vs Ag/AgCl) (1a)

φFB = −0.16 − 0.059· pH

(rutile, V vs Ag/AgCl)

(1b)

Although there is some controversy about the band edge positions in anatase/rutile,34−38 it is obvious that the redox potential of dimethylviologen (MV2+/MV+; −0.65 V vs Ag/AgCl) is approximately in the middle of the region of φFB values of TiO2 (anatase, rutile) in aqueous media (Scheme 1). Scheme 1. Flatband Potentials of TiO2 (anatase/rutile) Single-Crystal Electrodes in Aqueous Electrolyte Solution10 and the Redox Potentials of the Model Redox Probes

Hence, the redox reaction of dimethylviologen (MV2+/MV+) at the titania electrode in neutral-acidic media mimics that occurring at metallic electrodes, but in alkaline media, the TiO2 electrode can be switched to the rectifying regime for this redox couple. Titania behaves like an electrochemically silent dielectric material against the Fe(CN)63−/4− couple at all accessible pHs (cf. Scheme 1). In this case, the charge-transfer reaction is assumed to occur solely at the naked FTO surface, which is exposed to the electrolyte solution through pinholes.39 To avoid the pH dependence in eqs 1a and 1b (Scheme 1), we can also express the φFB against the reversible hydrogen electrode (RHE):26 for single crystal anatase, we obtain φFB = −0.16 V versus RHE.10 The described effects are demonstrated on the TiO2 layer, which is formed simply by thermal oxidation of Ti metal. Dense oxidic layers grow spontaneously on top of Ti metal treated in air at 500 °C for 30 min. The bare Ti metal shows the corresponding redox response to Fe(CN)63−/4−, albeit with a quite irreversible voltammogram, compared with that of FTO electrode under the same conditions (Figure 1). The irreversible voltammogram, observed on bare Ti electrode, can be ascribed to the presence of native oxidic layer on the Ti surface. The thermally grown oxide film, which manifests itself by a characteristic blue coloration of the Ti metal, fully blocks the redox reaction of Fe(CN)63−/4− (Figure 1). We observe 16409



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electrochemical rate constant40) at the coated FTO. The electrode was also heat-treated in air at the final temperature of 500 °C. (See the Experimental Section.) We observe a very small effect of heat treatment (Figure 3), which is expectable, because the spray pyrolysis deposition is already made at similarly high temperatures.15 To test the quality of blocking at more realistic conditions occurring in the SSDSCs, we alternatively used a solution of spiro-OMeTAD as the actual redox probe. From various possible variants of this experiment (either solutions of spiroOMeTAD in aprotic solvents, such as dichloromethane or odichlorobenzene containing various conductivity-promoting salts16,41 or solid films measured in propylene carbonate medium41), we have selected the solution containing 1 mM spiro-OMeTAD in CH2Cl2 + 0.3 M tetrabutylammonium hexafluorophosphate, which gave the best resolved and reproducible voltammograms. (In this context, we should note that solid films of spiro-OMeTAD41 are difficult to measure, as the oxidized form of spiro-OMeTAD spontaneously dissolves not only in propylene carbonate but also in other usual electrochemical solvents, for example, in acetonitrile.) Figure 3 (right chart) shows the blocking of spiro-OMeTAD at spray-coated FTO, including minor effect of calcination. The improved blocking may be caused by larger molecular volume of spiro-OMeTAD compared with that of Fe(CN)63−/4−; that is, the spiro-OMeTAD molecule cannot penetrate the very narrow pores in the blocking layer, which are still accessible for Fe(CN)63−/4−. To test the possible alternatives to spray pyrolysis for the fabrication of compact blocking layers, we explored the use of electrochemically deposited TiO2. This method is based on anodic oxidative hydrolysis of Ti(III) species in acidic electrolyte solutions:22

Figure 1. Cyclic voltammograms at Ti metal (Ti) and that oxidized thermally in air at 500 °C (TiOx), scan rate 0.1 V/s. The voltammogram at FTO electrode is shown for comparison. The electrolyte solution was 0.5 mM K4Fe(CN)6 + 0.5 mM K3Fe(CN)6 in aqueous 0.5 M KCl, pH 2.5.

solely the electron accumulation below φ FB and the corresponding reduction of Fe(CN)63− at this rectifying interface, that is, without the counterpart of the anodic oxidation. This conclusion is reproduced also for the dimethylviologen redox probe. In mildly acidic solution, the TiO2 film behaves electrochemically like FTO (Figure 2 left chart), but the anodic reaction is fully blocked at pH 11.5 (Figure 2, right chart). Subsequent calcination at 500 °C has no apparent effect on the blocking function of the thermally oxidized Ti. This is significant in view of the fact that the growth of mesoporous titania on top of the blocking layer usually requires a calcination step in the standard protocol of preparation of DSC photoanodes. Hence, the thermally oxidized Ti would be an attractive electrode for SSDSCs if the illumination of this solar cell from the back (counterelectrode) side was possible. The handicap of Ti-supported photoanode, requiring backside illumination, is avoided in optically transparent blocking layer made by spray pyrolysis. This is the most popular technique for fabrication of various types of SSDSCs.2,7−9,16,17 Figure 3 (left chart) shows the cyclic voltammograms of Fe(CN)63−/4− measured at the 60 nm thick film of TiO2 made by spray pyrolysis. The blocking is not complete: we detect solely the decrease in the peak current densities accompanied by considerably enhanced peak-to-peak separation indicating larger charge-transfer resistance (smaller

Ti 3 + + H 2O → TiOH2 + + H+

(2a)

TiOH2 + + H 2O − e− → TiO2 + 3H+

(2b)

The electrodeposition seems to be the ideal method for the growth of dense blocking layers on FTO because the galvanic reaction 2b occurs directly at the electrochemical interface; that is, the naked areas of FTO are covered preferentially.22 Another advantage of electrochemical deposition consists of accurate control of the film thickness simply by the passed charge and pH; the latter defines the faradaic efficiency of the reaction 2b.22 The as-grown film was heat-treated at 100 °C overnight.

Figure 2. Cyclic voltammograms at Ti electrode thermally oxidized thermally in air at 500 °C (TiOx), scan rate 50 mV/s. The voltammogram at FTO electrode is shown for comparison. The electrolyte solution was 1 mM dimethyl viologen in aqueous 0.5 M KCl, pH 5.3 (left chart) or 11.5 (right chart). 16410



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Figure 3. Cyclic voltammograms at bare FTO electrode and that covered by 60 nm thick film of TiO2 made by spray pyrolysis (labeled ‘spray’). The electrode that was heat-treated in air at the final temperature of 500 °C is labeled ‘spray & HT’. Scan rate 20 mV/s. Left chart: The electrolyte solution was 0.5 mM K4Fe(CN)6 + 0.5 mM K3Fe(CN)6 in aqueous 0.5 M KCl, pH 2.5. Right chart: The electrolyte solution was 1 mM spiroOMeTAD in CH2Cl2 + 0.3 M tetrabutylammonium hexafluorophosphate. Scan rate 50 mV/s.

Figure 4. Cyclic voltammograms at bare FTO electrode and that covered by 60 nm thick film of TiO2 made by electrochemical deposition (labeled ‘electrodep.’). The electrode that was heat-treated in air at the final temperature of 500 °C is labeled ‘electrodep. & HT’. Left chart: The electrolyte solution was 0.5 mM K4Fe(CN)6 + 0.5 mM K3Fe(CN)6 in aqueous 0.5 M KCl, pH 2.5. Scan rate 50 mV/s. Right chart: The electrolyte solution was 1 mM spiro-OMeTAD in CH2Cl2 + 0.3 M tetrabutylammonium hexafluorophosphate. Scan rate 100 mV/s.

Figure S1 (Supporting Information) shows that this mild heat treatment causes pronounced improvement of blocking function, particularly for the film thicknesses of several nanometers. The performance of the thus prepared 60 nm thick electrodeposited film in contact with our model redox probes is demonstrated in Figure 4. After heat treatment at 500 °C, the film thickness decreased to 45 nm in accord with ref 22. The film made by electrodeposition exhibits excellent blocking. The only redox process, which is apparent in the voltammogram of the noncalcined film, is the electron accumulation in titania occurring at potentials negative to ca. −0.7 V versus Ag/AgCl, that is near the flatband potential, φFB expected for this electrode/electrolyte solution interface.42 More precisely, we see the reduction of Fe(CN)63−, which is triggered at this potential due to the quasi-metallic nature of TiO2 occurring in the accumulation regime. The reverse process, that is, the oxidation of Fe(CN)63−, is fully hindered because it requires potentials above 0.2 V when TiO2 is switched back to its insulating state. However, the blocking is considerably reduced upon heat treatment at 500 °C. Increasing the film thickness up to 140 nm did not bring any significant improvement of the blocking function of the calcined electrodes. Hence, the electrodeposited underlayer does not seem to be the most suitable for DSC if the mesoporous titania requires calcination step during the photoanode fabrication, which is usually the case.

Further insight into the blocking effect is obtained for titania layers made by ALD. Figure 5 shows the behavior of layers made from TDMAT and H2O as precursors at the substrate temperature of 150 °C. (See the Experimental Section.) The blocking was tested using Fe(CN)63−/4− as the model redox couple. The corresponding test with spiro-OMeTAD is presented in Figure 6. The as-made electrodes with 3−6 nm thick layers exhibit excellent blocking of the Fe(CN)63−/4− redox reaction, but thinner layers (1 and 2 nm) do not. The ALD 1 nm layer shows significant charge transfer occurring at the FTO/electrolyte solution interface. In this case, we can estimate the effective pinhole area simply from the peak current, Ip, which is given by the Randless−Sevcik equation: Ip = k·n3/2 ·A ·c·D1/2 ·v1/2

(3)

k is a constant (k = 2.69 × 105 C mol−1 V−1/2), n is the number of electrons appearing in the half-reaction for the redox couple, A is the electrode area, D is the diffusion coefficient, and ν is the scan rate. Equation 3 assumes linear diffusion toward the FTO/electrolyte solution interface, which may or may not be the right model for insulating layer with pinholes. Depending on the size of pinholes and their mutual distances, specific diffusion fields can develop with different geometries, up to the microelectrode-like one, which is characterized by spherical diffusion and sigmoidal-shaped voltammogram.43,44 An intermediate situation occurs when the spherical diffusion fields of the small pinholes merge to form, eventually, a single 16411



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various layers made by ALD (cf. Figures 5−7). The sigmoidal steady-state current was not observed in our voltammograms.

Figure 5. Cyclic voltammograms at bare FTO electrode and that covered by ALD made layers of various thicknesses from 1 to 6 nm. The layers were deposited using TDMAT and H2O as precursors, substrate temperature 150 °C. The electrode with 6 nm thick layer, which was heat-treated in air at the final temperature of 500 °C, is labeled ‘ALD 6 nm & HT’. Scan rate 50 mV/s. The electrolyte solution was 0.5 mM K4Fe(CN)6 + 0.5 mM K3Fe(CN)6 in aqueous 0.5 M KCl, pH 2.5. The voltammograms (except FTO and ALD 6) are offset for clarity.

Figure 7. Cyclic voltammograms at bare FTO electrode and that covered by ALD made layers of various thicknesses from 1 to 6 nm. The layers were deposited using TDMAT and H2O2 as precursors, substrate temperature 200 °C. The electrode with 6 nm thick layer, which was heat-treated in air at the final temperature of 500 °C, is labeled ‘ALDP 6 nm & HT’. Scan rate 50 mV/s. The electrolyte solution was 0.5 mM K4Fe(CN)6 + 0.5 mM K3Fe(CN)6 in aqueous 0.5 M KCl, pH 2.5. The voltammograms (except FTO and ALDP 6) are offset for clarity.

Hence, eq 3 provides a realistic background for evaluation of the fraction coverage of FTO by the titania blocking layer in most cases studied here. If jp is the peak current density (referred to the projected electrode area, A0) measured at the actual blocking electrode with Au being the uncovered area of FTO and jp,FTO is the current density measured at bare FTO electrode, then the effective pinhole area equals:

A u /A 0 = jp /jp,FTO

(4)

The actual values are listed in Table 1. To better see the peak currents of spiro-OMeTAD, the voltammetric measurement was repeated as shown in Figure S2 (Supporting Information). Closer inspection of cyclic voltammograms reveals that the blocking of FTO by dense titania film manifests itself by the decrease in jp, which may (i) or may not (ii) be accompanied by a marked increase in the peak-to-peak separation, ΔEpp. The second case (later termed ‘defect A’) is assumed to be a simple feedback of naked FTO; that is, the partially blocked electrode behaves like a ‘clean’ FTO but with a relatively smaller effective area (eq 4). An example of this kind of defect is the ALD 1 nm film in Figure 5. (We should note, however, that the ΔEpp values for the blocked FTO and the bare FTO are hardly identical under the usual experimental conditions. Hence, we propose an arbitrary criterion that the relative enhancement ΔEpp is smaller than 3 in the defect A.) The first case (i, termed ‘defect B’) represents a more complex situation, where the defect causes not only the delamination of the titania film from FTO (eq 4) but also the slowdown of charge transfer kinetics (accompanied by strong increase in ΔEpp). According to the classical work of

Figure 6. Cyclic voltammograms at bare FTO electrode and that covered by ALD made layers of various thicknesses from 1 to 6 nm. The layers were deposited using TDMAT and H2O as precursors, substrate temperature 150 °C. The electrode with 6 nm thick layer, which was heat-treated in air at the final temperature of 500 °C, is labeled ‘ALD 6 nm & HT’. Scan rate 50 mV/s. The electrolyte solution was 1 mM spiro-OMeTAD in CH 2 Cl 2 + 0.3 M tetrabutylammonium hexafluorophosphate. The voltammograms (except FTO and ALD 6) are offset for clarity.

planar diffusion layer. In general, the applicability of eq 3 is corroborated by (i) the scan-rate (v1/2) dependence of Ip and (ii) the Nernstian peak-shaped voltammogram.43 This kind of voltammogram, as well as the Ip ≈ v1/2 dependence, was observed for the spray-pyrolyzed layers (cf. Figure 3) for the heat-treated electrodeposited layers (cf. Figure 4) and also for 16412



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Table 1. Effective Pinhole Areas for Two Different Redox Probesa

a

electrode (layer thickness & HT if any)

Au/A0 (%) [Fe(CN)63−/4−]

Au/A0 (%) [spiro-OMeTAD]

defect type [Fe(CN)63−/4−]

Ti (native oxide) TiOx (thermally oxidized Ti) spray-coated 60 nm spray-coated 60 nm & HT electrodeposited 60 nm electrodeposited 60 nm & HT ALD 1 nm ALD 2 nm ALD 3 nm ALD 4 nm ALD 5 nm ALD 6 nm ALD 6 nm & HT ALDP 1 nm ALDP 2 nm ALDP 3 nm ALDP 4 nm ALDP 5 nm ALDP 6 nm ALDP 7 nm ALDP 6 nm & HT

30

Electrochemical Characterization of TiO2 Blocking Layers for Dye

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