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Surface Photovoltage Spectroscopy Resolves Interfacial Charge Separation Efficiencies in ZnO Dye-Sensitized Solar Cells Manuel Rodríguez-Pérez, Esdras Canto, Rodrigo García-Rodríguez, Alexandra T. De Denko, Gerko Oskam, and Frank E Osterloh J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11727 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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The Journal of Physical Chemistry

Surface Photovoltage Spectroscopy Resolves Interfacial Charge Separation Efficiencies in ZnO Dye-Sensitized Solar Cells Manuel Rodríguez-Pérez,a,b,c Esdras Canto,a Rodrigo García-Rodríguez,a Alexandra T. De Denko,b Gerko Oskam,*a and Frank E. Osterloh*b a

Department of Applied Physics, CINVESTAV-IPN, Mérida, Yucatán 97310, México. Email: [email protected] Department of Chemistry, University of California, Davis. One Shields Avenue, Davis, CA, 95616, USA. Fax: (+1)530 752 8995; Email: [email protected] c Facultad de Ingeniería, Universidad Autonoma de Campeche, San Francisco de Campeche, Campeche 24085, México. b

KEYWORDS: Surface Photovoltage Spectroscopy, Dye-Sensitized Solar Cell, Built-in Potential, Kelvin Probe, Cobalt Redox Couple ABSTRACT: Any optimization of dye-sensitized solar cells (DSSC) must consider the energetics and charge transfer kinetics of the dye, substrate, and redox couple. Here we use surface photovoltage spectroscopy (SPS) to probe the energetics and photochemical charge transfer efficiency in fluorenyl-thiophene dye (OD-8) sensitized ZnO films. Discrete photochemical charge transfer events at the dye-ZnO interface and at the dye- I-/I3- or [Co(2,2'-bipyridyl)3]3+/2+ interfaces can be observed as negative photovoltage under dye excitation at 1.7 eV (460 nm). Without a redox couple, charge separation at the ZnO/dye interface is only 4% effective, likely due to the short electron hole separation distance. In the presence of the redox couples, charge separation approaches 26-54% of the theoretical limit, emphasizing the importance of the dye regeneration reaction via the redox couple. On the basis of the open circuit voltage, charge separation in fully assembled DSSC is 100% efficient with iodide, but only 61% efficient with the cobalt redox couple. This suggests that device improvements are possible by optimizing the dye regeneration reaction with the cobalt redox couple.

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INTRODUCTION

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Twenty years after the publication of the first dye-sensitized solar cell (DSSC, 7 %), 1 there is an increasing interest in ZnO as n-type electron collector material.2-5 ZnO is an attractive support because it is transparent in the visible region of the solar spectrum 6-8 and because its electron mobility (200 cm2V-1s-1) 9 is about two orders of magnitude higher than for TiO2.2 ZnO films can be synthesized via sol-gel, 10-11 hydrothermal,12-13 electrodeposition technqiues,3, 14-15 among others.16 Using a ruthenium complex as a sensitizer (N719) on a hierarchically structured ZnO, a 7.5 % efficient DSSC was achieved.17 However, the performance of ZnO DSSC often lags behind that of comparable TiO2 based devices. The reason for this is that electron injection from the dye into the ZnO conduction band is generally two orders of magnitude slower than for TiO2. 18 This has been attributed to a lower density of conduction band states for ZnO,2 and the presence of a multistep injection mechanism.19 Another possible reason for slow electron injection is the presence of dye agglomerates at the ZnO surface. 20 These agglomerates result from the reaction of the dye with Zn2+ ions released during the sensitization process. 21 Overcoming these obstacles requires a better understanding of the photochemistry of dyes at interfaces. 22 Here we employ surface photovoltage spectroscopy (SPS) to study energetics and photochemical charge transfer kinetics in ZnO DSSC. SPS is a well-known tool for the characterization of charge transfer processes at sensitized TiO2 and FTO interfaces.23-28 Most of the earlier SPS work on DSSC employs the ‘fixed capacitor arrangement’, 29-32 where differential photovoltage values are recorded under chopped illumination, as a function of illumination wavelength. While differential photovoltage values do provide information about majority carrier types, carrier dynamics and bandgaps, they are difficult to relate to the thermodynamic properties of the device. In this study, we use the oscillating Kelvin probe arrangement under constant sample illumination 33 on ZnO films sensitized with the commercial dye OD-8 to obtain absolute photovoltage information about the charge separation at each interface. Comparison of the measured photovoltages to the theoretical photovoltages of each contact allows calculation of the charge transfer efficiencies. OD-8 is a metal free dye 34 that contains a fluorenylthienyl unit coupled to a diarylamine donor and a cyanoacrylic acid acceptor (Figure 1A). Derivatives of this dye have achieved energy conversion efficiencies of up to 52% when coupled to a TiO2 substrate,35 but as we show below, the corresponding ZnO devices are less efficient. The SPS measurements were conducted under vacuum on ZnO coated FTO substrate in the absence or presence of the dye and in contact with I-/I3- or cobalt redox couples [Co(2,2'bipyridyl)3]3+/2+. This yields effective band gaps of ZnO, the HOMO-LUMO gap of the dye. Based on a comparison of the measured photovoltage to the theoretical thermodynamic driving force for charge transfer at each interface, we find that charge separation at the ZnO/dye interface is only 4% effective, while up to 54% efficient charge separation is achieved after adding the cobalt redox couple. While for the cobalt redox couple good agreement exists between the measured photovoltage value and the open circuit voltage of the DSSC, for iodide, discrepancies are attributed to the slower kinetics of the iodide oxidation

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reaction under the conditions of the SPS experiment. Overall, the combined SPS and device measurements suggest that significant efficiency improvements are possible with OD-8/ZnO DSSC by improving electron injection from the dye into the ZnO substrate and by improving the dye regeneration reaction with the cobalt redox couple. Also, this work shows the advantages and limitations of the oscillating Kelvin probe configuration for assessing charge separation in films sensitized with molecular dyes.

RESULTS AND DISCUSSION 70

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Figure 1 shows details of the DSSC. The device is composed of a mesoporous ZnO film (Wurtzite structure type) that is grown on top of FTO substrate by electrodeposition, followed by calcination at 450 °C. The OD-8 dye (Everlight Chemicals) binds to the ZnO via the carboxylate group. Dye sensitization is performed through repeated soaking/ heating cycles, using acetonitrile/t-butyl alcohol as a solvent, and 0.5 mM of chenodeoxycholic acid (CDCA) to prevent dye aggregation. The cell is completed by a Pt / FTO counter electrode and filled with an electrolyte composed of either [Co(bpy)3](PF6)3 and [Co(bpy)3](PF6)2 or a mixture of iodine and 1,2-dimethyl-3propylimidazolium iodide in acetonitrile/ 4-tert-butylpyridine. Full details of the DSSC fabrication are given in our earlier publication. 36

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system, as described before. 37-38

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Figure 2. A) Optical absorption spectrum of OD-8 dye. B) I/V curve for OD-8 DSSCs with iodide/tri-iodide or cobalt tris-bipyridyl redox couple.

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Figure 1. A) Schematic view of dye-sensitized solar cell, incl. commercial OD-8 dye (Z)-2-cyano-3-(5-(9,9-dihexyl-7-(dihexylamino)9H-fluoren-2-yl)thiophen-2-yl)acrylic acid. The electrolyte consists of 0.22 M [Co(2,2´-bpy)3]2+, 0.05 M [Co(2,2´-bpy)3]3+ and 0.2M 4-tertbutylpyridine in acetonitrile. Alternatively, an acetonitrile solution of 0.05 M iodine, 0.2 M TBP, and 0.27 M 1,2-dimethyl-3-propylimidazolium iodide was used. B) Photo of assembled DSSC.

Figure 2A shows optical absorption spectra of the ZnO film before and after loading with the dye. The dye absorbs light down to 550 nm, indicating a HOMO-LUMO separation of 2.25 eV. Based on its 3.2 eV band gap, ZnO is expected to block light below 400 nm from reaching the dye. Figure 2B presents current - voltage curves under AM 1.5 standard illumination for DSSCs with the [Co(bpy)3]3+/2+ redox couple or with the I-/I3- redox couple. It is found that the triiodide/iodide redox couple gives a lower JSC value (5.24 mA cm-2) but a slightly higher VOC (0.57 V) than the cobalt redox couple (5.6 mA cm-2 and 0.55 V, see table 1). Because of the lower fill factor (FF) for the I-/I3- solar cell, the power conversion efficiency is significantly smaller (1.03%) than for the cell with the cobalt redox couple (1.48%). These performance differences are typical of ZnO/OD-8 cells made with these redox couples. 36 In order to gain insight into the charge transfer dynamics of the OD-8/ZnO DSSC, surface photovoltage spectroscopy (SPS) measurements were performed on electrodeposited ZnO films onto FTO substrates, sensitized ZnO films, and sensitized ZnO films using a Kelvin probe in a vacuum

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Table 1: Photovoltage and efficiency data for thin films and devices measured here. Sample Theoretical photovoltage ∆CPDth / V Photovoltage ∆CPD / V

ZnO/dye/I-/I3(or I-/I2- )

ZnO/dye/ Co(bpy)33+/2+

–0.25… –0.75

–0.85

–1.64 (–0.98)

–1.30

–0.18

–0.04

–0.42

–0.71

VOC (theory) / V

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26 % (43 %) 0.56

VOC (exp) / V

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0.57

0.55

% of theory

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100 5.24 0.35

61% 5.6 0.48

% of theory

ISC / A cm-2 FF

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FTO/ZnO ZnO/dye

24-90% 4.70%

54 % 0.9

The spectra are shown in Figure 3 and the corresponding processes are visualized in the energy scheme in Figure 4 for the FTO/ZnO configuration. A negative photovoltage develops at 3.2 eV (section III in the Figure) near the ZnO absorption edge (Figure 2). The voltage is due to excitation of ZnO and migration of photoelectrons towards the FTO substrate (Figure 4). A negative photovoltage is typical for films of n-type

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semiconductors, where charge separation is due to the diffusion of electrons (majority carriers) and trapping of holes in surface states. 37, 39-40 The maximum photovoltage of -0.18 V occurs at 3.7 eV and then diminishes at higher photon energy, due to the limited light penetration depth of ZnO. The small feature at 2.1 – 2.6 eV is attributed to the excitation of oxygen vacancies in ZnO which are commonly associated with the green photoluminescence of that material.41 Figure 3b shows how the SPS scan changes after sensitization with the OD-8 dye. The main feature begins at 1.9 eV and reaches its –0.04 V maximum value at 2.7 eV (460 nm), near the absorbance maximum of the dye (see absorption spectrum in Figure 2a). Hence this signal can be ascribed to excitation of the dye and subsequent electron injection into the ZnO conduction band (Figure 4). The second photovoltage signal at 3.2 eV is again due to the band gap excitation of ZnO. This feature is much smaller (–6 mV) than before because of the light shading effect from the dye. In addition, back electron transfer from ZnO occurs into the dye molecule, which still contains the photohole from the 2.7 eV excitation. This process is responsible for the reversal of photovoltage in section IV of the spectrum. Figure 3c shows the photovoltage spectrum of the sensitized ZnO film after addition of an acetonitrile solution of iodine/1,2-dimethyl-3propylimidazolium iodide followed by drying in air. The

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50 mV from ZnO excitation is seen. This value also is 10 times larger than in Fig. 3b, suggesting reaction of the photoholes in ZnO with iodide. Reversal of the photovoltage in section IV of the spectrum indicates that back electron transfer from ZnO to triiodide and oxidized dye also occur, as seen before in Fig.3b. At the end of the scan, some oxidized dye/iodide remain, based on the –0.14 V residual voltage. Lastly, Figure 3d shows SPS data for the [Co(bpy)3]3+/2+ redox couple in contact with the ZnO/dye interface. This time the photovoltage from dye excitation is nearly twice as high (–0.71 V) as with iodide. Interestingly, the negative photovoltage from excitation of ZnO (feature III) is less pronounced (–20 mV ∆CPD instead of –50 mV ∆CPD in Figure 3c). The reason for this is not initially clear. It might be that the photovoltage in section II is simply dominated by the reversal of the electron injection from the dye to ZnO, or that the [Co(bpy)3]3+/2+ redox couple is less prone to photo-oxidation by ZnO, or that the ZnO process is suppressed by shading from the cobalt redox couple. At the end of the scan, when the illumination intensity and light penetration depth of the film approach zero, the photovoltage returns to baseline. This means that the electrons flow back from the ZnO to the cobalt redox couple, reversing the light-induced charge transfer. A quantitative analysis of photochemical charge transfer in the DSSC is possible by combining the SPS data with the energetics

Figure 3. Surface photovoltage spectra of (A) electrochemically deposited ZnO film, (B) ZnO film with OD-8 dye, (C) ZnO film-OD-8- I-/ I-3, and (D) for ZnO film-OD-8-Co(bpy)33+/2+. 25

spectrum is very similar to that in Figure 3b, except that the photovoltage values are approximately 10 times larger than without the redox couple: at 2.7 eV, the ∆CPD maximum is –0.42 V. This voltage boost is due to the transfer of the photohole from the dye to the redox couple. Again, a feature of approximately –

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of the device. Figure 4 summarizes the Fermi level of FTO, the conduction and valence bands of the ZnO nanoparticle layer, the HOMO-LUMO positions of the dye, and the electrochemical potentials of the redox couples. Numerical SPS data is given in Table 1 for the respective ZnO/OD-8/electrolyte configurations.


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For each interface, the thermodynamic driving force for charge transfer is given by the difference of the donor and acceptor energy levels. Dividing this value by the electron charge yields the theoretical photovoltage ∆CPDth which can be expected if charge separation were 100% efficient For example, ∆CPDth(ZnO/OD-8) = ECB(ZnO)/e – LUMO(OD-8)/e = –0.85 V. The sign of this voltage is negative because electrons are moving away from the Kelvin probe. By comparing the measured photovoltage with this theoretical photovoltage ∆CPDth in Table 1 we define the charge transfer efficiency η for the respective charge transfer process as shown in Eq. 1. Eq. 1

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η = ∆CPD / ∆CPDth

For electron transfer across the ZnO/dye interface, ∆CPD = –0.04 V out of a theoretical –0.85 V, i.e. the charge transfer efficiency is 4.7%. Electron injection from the dye into ZnO is very ineffective because of rapid back-electron transfer from ZnO to the oxidized dye. This recombination process is fast in the absence of a redox couple because of the short electron-hole separation distance at the ZnO-dye interface. Indeed, back electron transfer to the dye is known to be one of the two main performance-limiting factors in DSSC.42-44 Charge separation is greatly improved upon adding the cobalt redox couple. Experimentally, the photovoltage rises to –0.71 V, almost twenty times the value without the redox couple. The theoretical photovoltage of the ZnO/dye/redox configuration is ∆CPDth = – 0.85 – 0.45 V = –1.30 V. Based on this, photochemical separation at the ZnO/dye/cobalt interface is 54% efficient. According to Figure 4, iodide is more reducing than Co2+, and consequently, a larger theoretical photovoltage ∆CPDth = –0.85 (electron injection) – 0.79 V (hole injection) = –1.64 V is calculated. However, only –0.42 V photovoltage is observed experimentally, which corresponds to 26 % of the theory. This finding is surprising considering that the thermodynamic driving force for iodide oxidation is much higher than for oxidation of the cobalt redox couple. The low photovoltage can be explained by assuming that under the vacuum conditions of the SPS experiment, iodide is oxidized to the di-iodide radical, not to the tri-iodide radical.

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Data for the conduction band edge of ZnO (-0.25 V vs NHE),45 the standard reduction potentials for Co(bpy)33+/2+ (+0.65 V vs NHE), 46 the I– /I3– pair (+0.31 V vs. NHE in acetonitrile) 47 and the I–/I2–* pair (+0.93 V vs NHE in organic solvents), 48 are taken from the literature. The HOMO position of the OD-8 dye was approximated from EOx ~ +1.1 V vs NHE in tetrahydrofurane.35 The Fermi level of FTO is reported to vary between 0 and +0.56 V vs NHE, depending on surface treatment. 49 Built-in potentials are shown in red.

Di-iodide radical formation has been previously observed in DSSC using nanosecond-laser spectroscopy and pseudo-steadystate photoinduced absorption spectroscopy. 48, 50-51 The formation of this radical is generally attributed to low iodide concentrations, but in the present case it seems to be associated with the low iodide mobility of in the dried sample film. In water, E0(I2–* /I–) = +1.03 V vs NHE while in organic solvents E < +0.93 V vs NHE. This reduces the driving force for iodide oxidation to 0.13 eV, and the theoretical photovoltage of the ZnO/dye/redox configuration to –0.98 V, below the value for the cobalt redox couple (Figure 4). Using this corrected value, the charge transfer efficiency for the ZnO/dye/iodide configuration is estimated as 42.9%. However, this value is likely also an underestimation, considering the effect of the dry measurement conditions on the rate of the two electron transfer reactions. To obtain further insight into the charge transfer dynamics involving the iodide and cobalt redox, couples, photovoltage measurements under transient illumination were conducted for the ZnO/dye/redox configuration under fixed 2.7 eV illumination, near the dye absorption maximum. The data in Figure S1 shows that photovoltage features are reversible over the course of the experiment and achieve maxima of -0.41V and -0.66 V, for iodide and cobalt redox couples, respectively. These values are similar to the values observed in Figure 3. Voltage formation and baseline recovery occur on the 100s time scale for both redox couples. The slow timescale suggests that under the conditions of the experiment, photovoltage formation is limited by electron diffusion through the films, and not by the charge transfer kinetics at the dye/ redox couple interface. To observe the latter, time resolved SPS using the capacitor arrangement may be required. However, such measurements are outside the scope of this study.

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Figure 4. Energy diagram for the FTO/ZnO/OD-8/iodide/tri-iodide/ Co(bpy)33+/2+ Kelvin probe configuration with direction of illumination.

Lastly, we discuss the factors that limit the performance of the fully assembled DSSC. In theory, the maximum Voc of a DSSC is determined by the gap of the dye, but the need for electron hole separation reduces this value to the electrochemical potentials of the electron and hole acceptors as shown in Figure 1B. Using (EC − Ered)/e, the theoretical open circuit voltage VOC(th) equals 0.90 V for the DSSC with the cobalt redox couple and 0.56 V for the DSSC with iodide/tri-iodide. The experimental VOCs in Figure 2B (0.55 V for cobalt and 0.57 V for iodide) correspond to 61% and 100% of the theory, respectively. This shows that the performance of the iodide DSSC is limited mainly by the slow redox kinetics at the iodide/dye contact, which causes the lower photocurrent of the iodide device and its lower fill factor. This interpretation agrees with the observation of incomplete oxidation of iodide under the vacuum conditions of the SPS experiment. Here, charge transfer across the dye-iodide interface is hampered by the slow mobility of the reagents in the dry ZnO/dye/I3–/I– film. For DSSC assembled with the cobalt redox couple the



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situation is different. These devices only achieve a fraction of their theoretical VOC (0.55 V instead of 0.90 V). This is attributed to low photochemical charge separation efficiency at the dyecobalt redox couple interface. This is mirrored by the low charge separation efficiency of 54% for this interface, as determined in the SPS experiment.

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CONCLUSION

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In summary, we have shown that SPS with a vibrating Kelvin probe can be used to measure absolute photovoltage at interfaces. By combining the photovoltage information with thermodynamic data on the band edges, it is possible to estimate charge separation efficiencies in dye sensitized solar cells. Maximum photochemical charge separation occurs at 2.7 eV (460 nm) near the absorbance maximum of the OD-8 dye. Without a redox couple, charge separation at the ZnO/dye interface is very ineffective despite the large built in potential of –0.85 V resulting from the dye LUMO and the ZnO conduction band edge. This is likely due to the short electron hole separation distance at the ZnO/dye interface. Photochemical charge separation is improved in the presence of the redox couples. Based on the comparison of theoretical and observed photovoltage, charge separation is 54.3% efficient with the cobalt redox couple. This explains why the open circuit voltage for the cobalt DSSC is only 61% of the theoretical value. For the iodide redox couple, SPS determines 42.9% efficient charge separation, while the open circuit voltage of the assembled device suggests 100% efficient charge separation. This discrepancy can be explained with the limited mobility of the iodide under the vacuum conditions of the SPS experiment, and its slower two-electron redox chemistry. Overall, these results show that surface photovoltage spectroscopy is a useful tool for the observation of photochemical charge transfer events with molecular dyes. The photovoltage data obtained from it complements information from other techniques, such as transient optical spectroscopy 43, 52 and electrochemical impedance. 53 Furthermore, the study suggests that better DSSC performance with cobalt redox dyes should be possible after improving the charge transfer efficiency at the ZnO/dye/cobalt interface.

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EXPERIMENTAL 40

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Chemicals: ZnCl2 (Sigma-Aldrich; ≥98%), KCl, (Aldrich, 99%), cetyltrimethylammonium bromide, (CTAB, Aldrich; 99.9%), Co(II) tris(bipyridyl) tetracyanoborate (Co-200; Everlight Chemical 98%), Co(III) tris(bipyridyl) tetracyanoborate (Co-300; Everlight Chemical 98%), 1,2-dimethyl-3-propylimidazolium iodide (DMPII; solaronix), 1,2-dimethyl-3-propylimidazolium iodide (IonLic DMPII, Solaronix), chenodeoxycholic acid (CDCA; Sigma-Aldrich; ≥97%), Lithium iodide (Mallinckrodt, 99.8%), Polyvinylpyrrolidone (PVP40, Sigma-Aldrich), t-butyl alcohol (Sigma-Aldrich; ACS ≥99%), ethanol, anhydrous (J.T Baker; 99%), Methanol (Sigma-Aldrich; 99.8%), acetonitrile (Aldrich, 99.93%), , 4-tert-Butylpyridine (Aldrich,99%), H2SO4 (Aldrich; 98%), HCl (Reproquifin; 36.5-38%), Oxygen (Praxair; 99%) Platisol T (Solaronix), Meltonix 1170-60, (Surlyn 60 µm thick, Solaronix), and water (18 MΩ•cm resistivity). All the solvents and chemical were used in normal conditions (room

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temperature and atmospheric pressure), without further purification. Preparation of Electrodeposited ZnO Films Porous films of ZnO were obtained through the use of a galvanostatic method using a Gamry potentiostat/ Galvanostat/ ZRA 3000 with Ag/AgCl (3 M NaCl) as a reference electrode, and Pt as counter and SnO2:F- coated glass substrate (FTO, TEC 8Ω/sq, Pilkington) as a working electrode. A 0.5 cm2 area was masked with polyester tape (Cole-Parmer), and activated with 2 M H2SO4 followed by rinsing with deionized water. The electrolyte was prepared with 0.01 M ZnCl2, 0.1 M KCl and 0.10 mM PVP40. The solution was bubbled with O2 for 20 minutes before use. Electrodeposition was carried over a 8400 s period using a current density of -0.5 mA/cm2. All the films were sintered at 450 °C for 1 h after electrodeposition. Preparation of dye solutions and sensitizing of ZnO films: The dye used in this work was the (Z)-2-cyano-3-(5-(9,9-dihexyl-7(dihexylamino)-9H-fluoren-2-yl)thiophen-2-yl)acrylic acid (OD8 from Everlight Chemical). 0.5 mM OD-8 and 0.5 mM CDCA solutions were prepared in a mixture of acetonitrile/t-butyl alcohol (1:1 v/v). ZnO films were heated at 90 °C for 20 min and immediately soaked in the dye solution (described above) for 4 h in order to sensitize them. After 4 h, the films were removed from the dye and carefully washed with the same mixture of acetonitrile/t-butyl alcohol. Preparation of electrolyte solutions: All the electrolyte solutions used were prepared in a 5 mL volumetric flask of a 1:1 (v:v) mixture of tert-butanol:acetonitrile. The cobalt redox couple consists of 0.22 M [Co(2,2´-bpy)3](PF6)2 (0.8329 g), 0.05 M [Co(2,2´-bpy)3](PF6)3 (0.218 g) and 0.2 M 4-tert-butylpyridine (0.1352 g). The I-/I3- redox couple was prepared at the same molar ratio as the cobalt electrolyte: 0.05 M iodine (0.0634 g), 0.2 M TBP, and 0.27 M 1,2-dimethyl-3-propylimidazolium iodide (0.3592 g). The thickness of the films was determined using a KLA Tencor D-120 profilometer. UV/Vis diffuse reflectance spectra were recorded on films using a Thermo Scientific Evolution 220 UV Vis spectrometer. Photovoltaic characterization of dye-sensitized solar cells was performed using methods described in CantoAguilar et al. 36 Surface photovoltage spectroscopy (SPS) measurements were conducted using the vibrating Kelvin probe configuration described in earlier publications. 37, 40, 54 Samples consisted of FTO substrates coated with ZnO films, or dyesensitized ZnO films, or dye-sensitized ZnO films treated with a few drops of the electrolyte solution. After drying in air, samples were mounted inside of the measurement chamber and placed under vacuum (1.6 × 10-4 mBar). A vibrating gold Kelvin probe (Delta PHI Besocke) served as the reference electrode. Samples were illuminated with monochromatic light from a 150 W Xe lamp filtered through an Oriel Cornerstone 130 monochromator with light intensity range of 0.1 to 0.3 mW cm-2. SPS data is plotted as ∆CPD versus photon energy. Here, ∆CPD corresponds to the change of the contact potential difference (CPD) between the sample and the gold Kelvin reference electrode, going from dark to light conditions. Surface photovoltage spectra were acquired during 30 min scans (358 data points, 5 sec per data


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point) and corrected for drift effects by subtracting a dark scan. No correction for the variable light intensity from the Xe lamp was performed. Based on repeat measurements, the spectra are reproducible within an error margin of 8.9%. The emission spectrum of the Xe lamp is given as Figure S2.

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ACKNOWLEDGMENT

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We thank Adam Moulé for use of the profilometer. This material is based upon work supported by a grant from the University of California Institute for Mexico and the United States (UC MEXUS) and the Consejo Nacional de Ciencia y Tecnología de México. The authors also acknowledge CONACYT, SENER and IER-UNAM for funding through the Mexican Center for Innovation in Solar Energy (CeMIE-Sol), Project P-27. We acknowledge the Department of Energy (DE-SC0015329) for financial support of this work. M.R.P. thanks CONACYT of Mexico for a postdoctoral fellowship.

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REFERENCES

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Surface Photovoltage Spectroscopy Resolves ... - ACS Publications

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