Efficient Electron Collection by Electrodeposited ZnO in Dye

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C: Energy Conversion and Storage; Energy and Charge Transport

Efficient Electron Collection by Electrodeposited ZnO in Dye-Sensitized Solar Cells with TEMPO as Redox Mediator +/0

Raffael Ruess, Jonas Horn, Andreas Ringleb, and Derck Schlettwein J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b06796 • Publication Date (Web): 18 Aug 2019 Downloaded from pubs.acs.org on August 22, 2019

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The Journal of Physical Chemistry

Efficient Electron Collection by Electrodeposited ZnO in Dye-Sensitized Solar Cells with TEMPO+/0 as Redox Mediator Raffael Ruess, Jonas Horn, Andreas Ringleb and Derck Schlettwein* Institute of Applied Physics and Center for Materials Research, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany

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Abstract

The power conversion efficiency in established dye-sensitized solar cells (DSSCs) suffers from high overpotentials needed because of slow electron transfer kinetics. If redox couples are used that have a low reorganization energy fast dye regeneration can be achieved, but fast recombination reactions can barely be suppressed. If they become competitive to electron transport to the back electrode, solar cell efficiencies drastically drop. In this work, it is shown that electron transport is facilitated by substituting the commonly used photoanode material, nanoparticulate TiO2, by electrodeposited ZnO, which, albeit more complex surface reactions, provides electron transport by orders of magnitude faster than nanoparticulate TiO2. With TEMPO (2,2,6,6-tetramethyl-1piperidinyloxy) as redox mediator the dye is efficiently regenerated with overpotentials well below 0.2 V. We demonstrate that the external quantum efficiency with TiO2-based photoanodes is significantly limited by recombination, while it is maintained at high values for electrodeposited ZnO. It is thereby shown that redox couples with fast kinetics can be employed in DSSCs without drawbacks in quantum efficiency if sufficiently fast electron transport in the porous semiconductor network is provided.


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1. Introduction

Dye-sensitized solar cells (DSSCs) are considered a promising candidate for a sustainable power source. Since their basic working principle does not require starting materials at high purity or preparation steps at high temperatures, DSSCs can be offered at a significantly lower price than established silicon solar cells and also have a much lower energy amortization time despite lower cell efficiencies.1,2 The latter is caused by the need that electron transfer processes between different materials have to take place at reasonable rates to achieve efficient charge splitting.3 Thus, these electron transfer processes require a significant driving force or overpotential that manifests itself in substantially reduced photovoltages (Voc-loss).3 A main bottleneck of efficiency of classical DSSCs is given by irreversible kinetics of dye regeneration by the I3-/I- redox couple with an intermediate step via a I2-∙-radical.3,4 For efficient dye regeneration by I3-/I-, a minimum difference between the redox potentials of the sensitizer in ground state and the redox couple ΔG0r ≈ 0.5 – 0.6 eV has been found to be necessary.3,4 The core issue of recent research in DSSCs is to reduce the resulting Voc-loss during dye regeneration by using different redox couples that show fast electron transfer kinetics even at low overpotentials, i. e. reversible redox couples.3,5–9 However, first experiments showed that several new problems arise if the I3-/I- redox couple is simply replaced by, e. g. Co3+/2+-complexes.10,11 Especially, fast electron back-transfer (recombination) reactions occurred as a consequence of the faster kinetics of these reversible redox couples.12–14 Electrons injected into the semiconductor were found to be intercepted by the oxidized redox species within the time needed to diffuse to the back contact.12,13 The resulting low charge extraction efficiency lead to significantly reduced external quantum efficiency (EQE) and, thus, lower overall power-conversion efficiency compared to the energetically unfavorable I3-/I- redox couple.12,13 By passivating the semiconductor surface against


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the recombination reactions, the charge collection efficiency could be recovered to unity in DSSCs with Co3+/2+ complexes as redox couples.14–16 However, despite advantages compared to I3-/I-, Co3+/2+ complexes have shown to yield slower electron transfer kinetics compared to other reversible redox couples. The oxidation of Co2+ complexes requires a high-spin to low-spin transition, resulting in rather high inner-sphere reorganization energies.17–19 As a consequence, rather high ΔG0r ≈ 0.3-0.4 V are required for efficient dye regeneration.17 In order to further reduce the Voc-loss during dye regeneration, research has been spent to employ reversible redox couples with low reorganization energy. Promising candidates are copper complexes which have been shown to efficiently regenerate several dye molecules with ΔG0r < 0.2 V.5,8,20 The promising property of such copper complexes is the difference of reorganization energy for dye regeneration and charge recombination processes.21 The reduced copper complexes allow a fast dye regeneration, but upon oxidation the coordination sphere of Cu2+ is altered by other components in the electrolyte solution and the resulting oxidized complex shows slow recombination kinetics.22 This allows for high EQE at low dye regeneration overpotentials.8 However, due to the instability of the copper coordination sphere, rest potentials are significantly shifted by about 0.2 V to negative potentials with respect to the formal redox potentials.21 Consequently, the true overpotential for dye regeneration by copper complexes is debatable. Furthermore, copper complexes have some more disadvantages such as a high residual light absorption and slow mass transport kinetics.8,23


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Scheme 1. Chemical structures of (a) the redox couple TEMPO+/0 and (b) the sensitizer Y123.

Dye regeneration at ΔG0r < 0.2 eV has also been demonstrated with the organic oxoammonium/nitroxide-radical redox couple TEMPO+/0 (Scheme 1 a) (TEMPO = 2,2,6,6tetramethyl-1-piperidinyloxy).6 Most notably, TEMPO0 and related molecules are employed in addition to a primary redox couple as a tandem or binary redox mediator, leading to significantly enhanced dye regeneration rates and, thus, improved photovoltaic performance.24–26 However, in that case Voc is still determined by the primary redox couple. When used as a single redox mediator, TEMPO+/0 shows almost no light absorption in the visible spectrum, fast mass transport kinetics and no apparent irreversibility in the reduction/oxidation reactions in addition to the rapid electron transfer kinetics.27,28 However, chemical instabilities of TEMPO+ have been observed in solution, leading to a slow degradation over a few weeks29 which limits the technical applicability of TEMPO+/0, but it can serve as a good model for a mediator with very fast electron transfer kinetics also provided by alternative molecules like, e.g. AZADO+/0.28,30 As a major drawback of mediators with such fast electron transfer kinetics, fast electron recombination reactions were observed in DSSCs to significantly limit the diffusion length of electrons inside the TiO2 and, accordingly, the


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short-circuit photocurrent (jsc) and the EQE.6,27,31 Since electrons generated further away from the back contact cannot be extracted, only thin TiO2-films (< 2 µm) in combination with an additional light scattering layer and with dyes that show high molar extinction coefficients allow high values for jsc and EQE.28 However, the desired film thickness for the DSSC-photoanode would be far thicker to also collect the weakly absorbed light.32 Consequently, for the use of thick photoanode layers (> 2 µm) the diffusion length of electrons inside the semiconductor has to be extended. At rather fast recombination rate, this can be achieved by accelerating electron transport. In this work, we demonstrate that high values for jsc and EQE in DSSCs employing redox couples with fast electron transfer kinetics can be achieved by improving the electron transport rate in the semiconductor. As a model system, we use the TEMPO+/0 redox couple and replace the commonly used nanoparticulate TiO2 photoanode with an electrodeposited ZnO photoanode. While the former consists of high-temperature sintered nanoparticles that involve grain boundaries along the path of conduction, electrodeposited ZnO is prepared at low temperatures and consists of crystalline columns perpendicular to the substrate, i. e. in transport direction of electrons, which we will confirm leading to considerably faster electron transport.33,34 This finding fits to earlier observations that even as a bulk material, ZnO shows higher electron mobility than TiO2.35 Furthermore, we employ the sensitizer Y123 (Scheme 1 b) that has shown to yield high efficiencies in combination with TiO2 and kinetically fast redox couples.7,20 Sensitization of ZnO which such organic dyes that employ only one cyanoarcylic or carboxylic acid anchoring group have shown to result in a stable power output and high EQE.36–39


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2. Experimental Section

2.1. Materials TEMPOBF4 was chemically oxidized from TEMPO in presence of HBF4 and NaOCl solution (Roth, 12 % Cl) followed by recrystallization from water as described in detail in

40.

Unlike

otherwise noted, all materials were used as received from Sigma-Aldrich without further purification.

2.2. Sample Preparation A compact ZnO-underlayer (ZnO-UL) was electrodeposited from an oxygen-saturated, aqueous zinc salt solution onto cleaned aluminum-doped zinc oxide (AZO) coated glass substrates (Kaivo, < 10 Ω □-1) followed by the electrodeposition of a porous ZnO film from the same solution with addition of 50 µM EosinY (Chemplex, > 88 %). The process was carried out as described in detail previously.37,41,42 The porous film covered a circular area with diameter 0.6 cm (0.283 cm2). The compact ULs and the porous films had a thickness of roughly 0.6 µm and 2.7 µm as determined by profilometry, respectively. A compact TiO2-underlayer (TiO2-UL) was spin coated from a commercial solution (TiNanoxide BL/SC, Solaronix) at 4000 rpm for 30 s onto cleaned fluorine-doped tin oxide (FTO) coated glass substrates (Kaivo, < 15 Ω □-1). The films were heated for 60 min at 500 °C in an oven in ambient air with a heating ramp of 10 °C min-1 and slow cooling afterwards. Subsequently, porous TiO2 was deposited from a commercial paste of TiO2 nanoparticles (18NR-T, Greatcell Solar) which was diluted in a ratio of 10:7.2:0.8 (paste:terpineol:ethyl cellulose). This diluted paste was then doctor bladed, followed by heating at 500 °C for 60 min at a heating rate of 10 °C min-1 and stop points at 180 °C, 320°C and 390 °C for 10 min each. After slowly cooling down to room


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temperature, the samples were immersed into an aqueous solution of 40 mM TiCl4 at 70 °C for 30 min. Thereafter, the films were heated again to 500 °C for 60 min. The porous films covered a quadratic area of 0.25 cm2. The thickness of the UL was determined to be < 100 nm (50-70 nm according to the providing company) and the thickness of the porous film was determined by profilometry to be around 3 µm. The photoanodes were sensitized by the organic dye Y123 (Dyenamo) by immersing into a 0.5 mM solution of Y123 (3-{6-{4-[bis(2',4'-dihexyloxybiphenyl-4-yl)amino-]phenyl}-4,4-dihexylcyclopenta-[2,1-b:3,4-b']dithiphene-2-yl}-2-cyanoacrylic acid) in 1:1 (volume) acetonitrile:tertbutanol for 60 min at room temperature. Counter electrodes for DSSCs were prepared by coating an FTO substrate with a PEDOT (poly-3,4-ethylenedioxithiopehe) layer by electropolymerization of the EDOT monomer in an aqueous solution of 0.1 M sodium dodecylsulfate and 0.01 M EDOT using a two-electrode setup with bare FTO as counter electrode and inducing a current of 0.2 mA cm-2 for 100 s.43 DSSCs were prepared by sandwiching the sensitized photoanode and the counter electrode with a hot-melting Surlyn® spacer (25 µm). Unlike otherwise noted, the DSSC electrolyte consisted of 0.7 M TEMPO, 0.07 M TEMPOBF4, 0.02 M LiClO4 and 0.5 M MBI (1methylbenzimidazole) in acetonitrile or, for the I3-/I--based reference electrolyte, 0.68 M PMII (3propyl-1-methylimidazolium iodide), 0.07 M I2, 0.02 M LiI, 0.5 M MBI. It was introduced into the cell through predrilled filling-holes, which were afterwards closed by a coverslip glass hotmelted with Surlyn® foil. For the photoelectrochemical characterization, the cells were masked by an aperture slightly larger than the area of the sensitized porous electrode as suggested in 44.


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2.3. Sample Characterization Cyclic voltammetry was performed on as-deposited ZnO-UL and TiO2-UL. The TiCl4 treatment was performed on TiO2-UL as described above. Measurements were performed in a threeelectrode setup with an electrolyte consisting of 3 mM TEMPO, 3 mM TEMPOBF4 and 0.1 M tetrabutylammonium tetrafluoroborate in acetonitrile purged with N2 prior to the measurement. The UL-coated substrates served as working electrodes with a masked area of 0.283 cm2 and the auxiliary electrode was a coiled platinum wire with an immersed surface area of around 1 cm2. The reference electrode was a leak-free Ag/Ag+ electrode that was calibrated using ferrocene before each measurement series with E0(Fc+/0) = 0.64 V vs. SHE. The scan rate was 50 mV s-1. Scanning electron microscopy (SEM) images were taken by a Zeiss MERLIN microscope with acceleration voltage of 5 kV and emission current of 100 pA. Optical transmission spectroscopy was performed on as-sensitized photoanodes mounted in front of an integrating sphere (getProbe 5393 SET, getAMO) with a tec5 UV/vis spectrometer. Photoelectrochemical characterization of DSSCs was carried out by an IM6 potentiostat and a CIMPSpcs system (Zahner Elektrik). Current-Voltage (I-V) characteristics were measured in the dark or under illumination by an LS0106 Xenon arc lamp employing an LSZ189 AM 1.5G filter with the intensity calibrated to be 100 mW cm-2 by an ML-020VM pyranometer (EKO Instruments). The DSSCs were reproduced 4 times (ZnO) or 3 times (TiO2) and the measured I-V parameters showed deviations of less than 5% from the mean values. EQE measurements were performed either by illumination by a white light source and a monochromator (TLS02, Zahner Elektrik) at fixed intensity at the sample of 0.1 mW cm-2 or by monochromatic illumination with a 510 nm LED (CYR01, Zahner Elektrik) or 632 nm LED (RTR01, Zahner Elektrik) with intensities ranging from 0.05 to 15 mW cm-2. The intensity was controlled with a silicon


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photodiode (FDS100, Thorlabs). Electrochemical impedance spectroscopy (EIS) was performed under illumination by a 510 nm LED illumination with intensities ranging from 0.5 to 15 mW cm-2 while the DSSCs were kept at open-circuit bias with an amplitude of 0.01 V in a frequency range of 1 MHz to 0.3 Hz. The spectra were fitted with the ZView software (Scribner Associates) using the equivalent circuit shown in Figure S1.

3.

Results and Discussion

3.1. Photovoltaic Performance I-V characteristics were measured for the TiO2- and ZnO-based DSSCs with the TEMPO+/0 redox couple and are shown in Figure 1 a and Table 1. The characteristics measured for TiO2 are in very good agreement to a study with very similar sensitizer and redox electrolyte as presently used.6 One can clearly see that ZnO yields the higher jsc as also confirmed by the EQE measurements (Figure 1 b). However, the Voc of the ZnO-based DSSC is shifted to far less negative potentials compared to TiO2. The origin of the differences in jsc and Voc are discussed in detail in the following sections. The differences in jsc and Voc between cells based on either TiO2 or ZnO roughly compensate each other in the resulting PCE (power conversion efficiency) which is around 4-5 %. A higher voltage drop at ohmic resistances in the cell, mainly the diffusion resistance in the electrolyte (seen as difference of measured and color-shaded real parts of impedances in Figure 3 c), causes the lower FF (fill factor) of ZnO-based DSSCs (Table 1). This becomes apparent in the internal fill factor of cells FFint where the FF is corrected by the series resistance, diffusion resistance and charge transfer resistance that have been determined by EIS, respectively. FFint is similar for TiO2 and ZnO, indicating that the recombination mechanisms in the cells are widely identical for both photoanode


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materials. It is remarkable that despite these similarities the values of jsc and EQE are found considerably higher for ZnO-based cells, pointing at faster charge transport in ZnO compared to TiO2.

Figure 1. (a) I-V curves of a TiO2- (blue) or ZnO-based (red) DSSC with TEMPO+/0-based redox mediator. (b) EQE (solid lines) and light absorption (dashed lines) of the DSSCs shown in (a).

Table 1. I-V characteristics of multiple DSSCs with TiO2- or ZnO-based photoelectrode and TEMPO+/0 redox mediator. The mean values are shown with their standard deviation. jsc / mA cm-2

Voc / V

FF / %

FFint / %

PCE / %

TiO2

6.23 ± 0.14

0.981 ± 0.016

76.2 ± 3.9

78.6 ± 2.4

4.65 ± 0.11

ZnO

8.28 ± 0.34

0.735 ± 0.007

64.7 ± 3.5

80.0 ± 0.5

3.92 ± 0.10


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3.2. Charge Recombination and -Transport Cyclic voltammetry measurements on the underlayers (TiO2-UL and ZnO-UL) and the substrate layers (FTO and AZO) were performed in a TEMPO+/0 containing electrolyte solution (Figure 2 a) to determine the extent of back-recombination (leakage current) via these layers. As indicated by Yang et al.,6 the catalytic activity of FTO for the reduction and oxidation of TEMPO+/0 is high and leads to high leakage currents in the DSSCs if no UL is used. It was found that FTO and AZO produce slightly reduced peak currents and slightly increased peak separation compared to a platinum-coated electrode. This can be interpreted as a slight mitigation of TEMPO+/0 reduction and oxidation kinetics at the substrate layers. However, since this current shows up at potentials inside the energy band gap of TiO2 or ZnO, a direct contact of the electrolyte and the substrate layer would cause significant leakage currents and would result in an ohmic I-V behavior of DSSCs as observed by Yang et al..6 If a TiO2-UL is deposited onto the FTO substrate, the reductive and oxidative current peaks are more separated compared to the bare FTO substrate. However, one can still clearly see reduction and oxidation of TEMPO+/0 occurring at potentials well below the conduction band edge of TiO2.45 On the other hand, for the ZnO-UL almost no oxidative current is observed indicating perfect blocking of the oxidation of TEMPO0 and a reductive current only sets in at considerably more negative potentials close to the conduction band edge of ZnO.45 Consequently, the low Voc of the ZnO-based DSSCs cannot be explained by leakage currents via the substrate. Employing the model by Kavan et al.,46 the further separated current peaks observed with the TiO2-UL suggests that the reduction and oxidation kinetics are mitigated, rather than actual holes in the TiO2-UL, for example by spots of the FTO only covered by a thin layer of TiO2 where electron tunneling can occur or narrow pores in the TiO2 film allow for a slow approach of the redox species (defect type ‘B’ in


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46).

This conclusion is supported by the SEM images of the ULs (Figure 2 b) where no actual holes

can be observed in the layers and a very high portion of the substrate seems to be covered. The relative areas corresponding to the defect type ‘B’ are calculated from the oxidative peak ratio to be 55 % and 0.1 % for TiO2 and ZnO, respectively. Such high defective areas for TiO2 films are also observed for spray coated or electrodeposited TiO2 films after high temperature treatment.46

Figure 2. (a) Cyclic voltammetry at different substrate layers in contact with a TEMPO+/0 containing solution: Pt-coated FTO (dashed line), bare FTO (light blue), TiO2-coated FTO (dark blue), bare AZO (light red) and ZnO-coated AZO (dark red). (b) SEM-images of the TiO2 or ZnO ULs. (c) Dark currents of TiO2- (blue) or ZnO-based (red) DSSCs with TEMPO+/0-based redox mediator. Since the counter electrode can be assumed to be in equilibrium with the redox electrolyte, the applied voltage at the DSSCs corresponds to the potential vs TEMPO+/0.

Surprisingly, the reductive dark current in TiO2-based DSSCs (Figure 2 c) at potentials > -0.7 V vs. TEMPO+/0 is drastically reduced compared to the reductive current in cyclic voltammetry although the concentration of both TEMPO+ and TEMPO0 is by orders of magnitude higher in the


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DSSC. Either the deposition of the porous TiO2 film or the sensitization process lead to a compensation of the defects in the UL. This healing of defects is particularly advantageous since it enables DSSCs with low leakage currents via the back layer. The high oxidative current at positive potentials vs. TEMPO+/0 is caused by the ten-fold higher concentration of TEMPO0 in the DSSCs and by a higher diffusion coefficient compared to TEMPO+.6 Dark currents in the band gap region in the ZnO-based DSSCs are by orders of magnitude lower compared to TiO2-based DSSCs, because of the less defective ZnO-UL. The onset of high reductive currents occurs at significantly less negative potentials than for TiO2-based DSSCs corresponds to the lower Voc of the ZnO-based DSSCs as explicitly discussed in section 3.3.. In DSSCs, jsc and EQE are given by the light-harvesting efficiency ηlh, dye regeneration efficiency ηreg, electron injection efficiency ηinj and the electron collection efficiency ηcol:3 EQE = ηlh ηreg ηinj ηcol

(1)

As shown in Figure 1 b, the light absorption and, correspondingly, ηlh is even higher for the TiO2-based cells and can therefore be excluded as the possible origin of the higher jsc and EQE of the ZnO-based DSSCs. Further, ηreg is unlikely to significantly differ between the TiO2- and ZnObased cells, since the same sensitizer and redox couple are used. A detailed discussion of the relevant energy levels and the dye regeneration overpotential is provided in section 3.3.. A higher ηinj for ZnO can also be excluded as origin of higher jsc and EQE since TiO2 is known to show a high ηinj > 90 % in combination with Y123.5,7


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Figure 3. (a) Diffusion length (circles) of TiO2- (blue) or ZnO-based (red) DSSCs with TEMPO+/0based redox mediator, calculated from the ratio of EQE (squares) when the cells are illuminated by a 510 nm LED from the substrate side EQESE and the electrolyte side EQEEE. The diffusion length was calculated according to

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with the absorption coefficient at 510 nm extrapolated for

TiO2 and ZnO as 1.40 µm-1 and 1.75 µm-1 from Figure S4, respectively, and the transmittance ratio between substrate and counter electrode was estimated to be 80 %. (b) EQE-ratio of the cells shown in (a) for illumination by a 632 nm LED. (c) Nyquist-plots of EIS measurements (points) and the respective fits (lines) of the cells shown in (a) at open-circuit and 5 mW cm-2 illumination by a 510 nm LED. The color-shaded part shows the fitted impedance at the semiconductor/electrolyte interface only. (d) τn (circles) and τtr (squares) of the cells shown in (a) derived from fitting the EIS data. The time constants of TiO2-cells cannot be determined separately as explained in the text and are found to lie within the blue-marked range.


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In order to confirm that a higher ηcol is the origin of the higher jsc in the ZnO-based DSSCs, we performed EQE measurements of cells at different illumination directions (from the substrate-side SE and the electrolyte side EE). Since light absorption in DSSCs is not uniform, the electron distribution in the semiconductor would vary between different wavelengths even at constant incident photon flux, resulting in a wavelength-dependent diffusion length.48,49 In order to determine ηcol or the corresponding electron diffusion length we performed measurements at a given wavelength at different light intensities and utilized the model presented in

47

to calculate

the electron diffusion length (Figure 3 a). It has to be noted that this model requires linear recombination kinetics which are typically not observed in DSSCs.47–50 However, the nonlinearity of the recombination reaction, represented as the parameter β, was determined from EIS measurements to be 0.38 and 0.43 for TiO2 and ZnO, respectively. Since β is reasonably similar for both materials, it can be assumed that although the values for the diffusion length are not quantitatively exact, they still serve a qualitative comparison. It can be clearly seen from the higher ratio of EQEEE/EQESE for ZnO-based cells that ηcol in the ZnO-based DSSC is higher than for TiO2, even at the highest absorbed photon flux leading to jsc close to that at AM 1.5G illumination. The higher values for the internal quantum efficiency (IQE) under illumination from either the SE- or EE-side confirm this more efficient charge collection by ZnO (Figure S5). Since both the TiO2 and the ZnO electrodes have a thickness of approximately 3 µm, thicker films would not result in a higher photocurrent, although more light would be absorbed. We have confirmed this with ZnO films deposited for a longer duration to achieve a thickness up to 5 µm (Figure S2). As expected, these films do not produce higher currents in DSSCs, which is due to a significantly reduced IQE at wavelengths where the sensitizer only has a low absorption coefficient. It is interesting to note that the diffusion length of electrons in ZnO


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is less sensitive to changes in the absorbed photon flux than that in TiO2. Thus, the EQE-ratio is significantly higher compared to TiO2 when the cell is illuminated with only weakly absorbed LED-light (Figure 3 b). This explains the spectral mismatch between EQE and light absorption in TiO2-based cells (Figure 1 b), where especially the weakly absorbed light is converted less efficiently. While the absorption spectra show qualitatively similar characteristics, the EQE of TiO2 increases at considerably lower wavelengths than that of ZnO. The intensity and wavelength dependence of EQE is influenced by the distribution of interband trap states (parameter α) and voltage-dependence of recombination rate (parameter β).51,52 As discussed in section 3.3., especially α shows a significant difference between TiO2 and ZnO. Since the absorption coefficient for the weakly absorbed light is quite susceptible to errors, e. g. due to the Stark-effect,53 scattering or interference (Figure 1 b), the diffusion length was not calculated for this case. For the calculation of the absorbed photon flux in Figure 3 b, an absorption coefficient for illumination with 632 nm was estimated from Figure S4. To differentiate between attenuated recombination rate and accelerated transport rate as origins of the higher diffusion length, we performed intensity-dependent EIS measurements at opencircuit conditions. Typical Nyquist-plots for TiO2- and ZnO-based DSSCs are shown in Figure 3 c. We realized that the data for the semiconductor/electrolyte impedance in TiO2-based DSSCs cannot be fitted accurately with a classical Warburg-type impedance, but with a Gerischer-type impedance. This phenomenon occurs when the electron transit-time exceeds the electron life-time (τn < τtr).51,52,54 The typical 45°-slope in the high-frequency regime that indicates diffusion processes in the semiconductor is not observed in this case.51,52 Although we only measured EIS at open-circuit conditions, we expect the condition τn < τtr to be true at any other voltage since the photogenerated current, and thus EQE, appear to be widely voltage independent (Figure 1 a). On


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the other hand, the EIS data for the ZnO-based DSSCs can be nicely fitted with the classical Warburg model, indicating that the condition τn > τtr is fulfilled. We note that the respective Gerischer and Warburg behavior for the TiO2- and ZnO-based DSSCs is consistent for the EIS measurements at all investigated light intensities. Extracting exact values for τn and τtr out of the Gerischer-type impedance is not possible since they are linearly dependent on each other in this case.12 Therefore, we utilized the respective diffusion lengths at a given LED intensity determined in Figure 3 a that resemble the ratio between τn and τtr, enabling their quantitative determination for the TiO2-based cells. The values from this fitting are shown in Figure 3 d. Since EIS is determined at open circuit, the diffusion lengths determined at short-circuit by EQE would represent a lower limit, because the electron concentration at open circuit is generally higher. Hence, the data points of TiO2 shown in Figure 3 d for τn represent a lower boundary and those for τtr an upper boundary. Concluding from the fact that a Gerischer-type impedance behaviour is observed (τn < τtr), the exact values for τn and τtr have to be placed within the blue-marked range. Independent extraction of τn and τtr from fitting the EIS data for ZnO-based DSSCs is straightfoward since a Warburg-type impedance is observed.12 The values of τn and τtr are plotted versus the common equivalent conduction band voltage Vecb that corrects for any energy shifts between the materials and resembles an approximately constant charge carrier concentration at a given voltage. The τtr-values for both nanoparticulate TiO2 and electrodeposited ZnO are in a range that is typically observed for both materials.42,55–58 Fast recombination of electrons from the semiconductor to the TEMPO+ species leads to a significantly lower τn than for other redox couples such as I3-/I- or [Co(bpy)3]3+/2+.6,27 As evident from Figure 3 c, electron recombination occurs at about the same rate for TiO2 and ZnO. However, due to the much faster transport of electrons


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through the highly crystalline electrodeposited ZnO electrode, τn > τtr is still fulfilled and leads to efficient charge collection and, consequently, a high EQE and jsc, unlike for the nanoparticulate TiO2 photoelectrode.

3.3. Energy Levels and Surface Charge The Voc in a DSSC is represented by the difference between the quasi-Fermi level in the electrolyte (= redox potential) and in the semiconductor. The latter is confined by the conduction band edge potential. Thus, in order to compare the Voc of the TiO2- and ZnO-based DSSCs, the conduction band edge potential has to be determined. In the bulk material, both band edges are situated at -0.5 V vs. SHE.45 This position can be assumed to be the same as in contact to the I3-/Ireference electrolyte, since in this case no shift was observed relative to an inert electrolyte.53 The chemical capacitance Cµ measured by EIS can be used to determine the conduction band position relative to a reference sample.51,52 Cµ of cells with TEMPO+/0 and I3-/I-, are shown in Figure 4 a and are fitted with exponential functions corresponding to lines. Their slope is characteristic of trap distributions in the band gap and horizontal shifts are indicative of shifts in the conduction band edge.51,52 Cµ of the TiO2-based DSSCs are on the same line for both TEMPO+/0- and I3-/I--based electrolytes indicating a constant conduction band position. However, for the ZnO-based cells Cµ is shifted towards positive potentials for TEMPO+/0 relative to I3-/I-. Despite an apparent change in slope (i. e. trap distribution parameter α), this shift can be attributed to an approximate downward shift of the conduction band edge potential by around 0.28 V, determined at 10-4 F where the Fermi-Potential should be close to the conduction band edge. The observed differences in Voc of the TEMPO+/0-based DSSCs between TiO2 and ZnO of around 0.25 V (Table 1) correspond well to this downward shift of the ZnO conduction band potential.


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Figure 4. (a) Cµ derived from fitting the EIS data of TiO2- (blue) or ZnO-based (red) DSSCs with a TEMPO0/+-based redox electrolyte (dark colors) or I3-/I--based reference electrolyte (light colors). (b) Schematic illustration of semiconductor conduction band potentials (CB), sensitizer HOMO and LUMO potentials and formal redox potential of the redox couple determined in this work.

Since both the TEMPO+/0 and the I3-/I- redox mediator solutions contain the same concentration of additives and the same solvent, it can be concluded that the TEMPO+/0 redox couple is responsible for this downward shift of the conduction band edge of ZnO. It has been observed in preliminary work that adsorbed cations induce a significant downward shift of the ZnO conduction band energy.37,38,53 In TiO2-based DSSCs such shifts are not observed if alkaline molecules are


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present in the redox electrolyte solution, such as the presently used MBI.53 These molecules control the amount of surface adsorbed cations that would otherwise stabilize energy levels also in TiO2.59,60 Since the surface of ZnO is alkaline by itself,35 the control of surface adsorbed cations by alkaline molecules is mitigated.53 Therefore, we conclude that TEMPO+ cations adsorb onto the ZnO surface, stabilize the electronic levels, consistent with changes in the trap distribution and lead to the observed downward shift of the conduction band (Figure 4 a), while on TiO2 the adsorption of TEMPO+ is hindered by the MBI. The formal redox potential of the dye in ground state, when adsorbed onto the TiO2 or ZnO surface were measured by cyclic voltammetry (Figure S3) and serve as estimate for the HOMO potentials of the sensitizer EHOMO.20 The respective LUMO potentials were calculated from the approximation ELUMO = EHOMO - E0-0 with E0-0 determined from the onset of the absorbance spectra (Figure S4).20 We find that these potentials of Y123 are quite different between TiO2 and ZnO, with the energy levels of Y123 being somewhat destabilized on ZnO. Similar observations of a blue-shifted absorption were made by Chandiran et al..61 The sensitizer Y123 binds onto the semiconductor via its COOH-moiety. The alkaline ZnO is more likely to deprotonate the carboxylic acid which then leads to destabilization of electrons in the sensitizer.61 Especially, the LUMO is affected by the deprotonation since it is intentionally situated close to the COOH-moiety while the HOMO is located further apart from the semiconductor surface for this type of sensitizer.62 Interestingly, these shifts narrow ΔG0r down to around 0.15 eV for ZnO which still seems to be sufficient to achieve an IQE close to 90 % (Figure S5).


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Conclusions

Quantum efficiency of dye-sensitized solar cells with a TEMPO+/0-based redox mediator were found to be negatively influenced by a low charge carrier diffusion length. Electrons from the TiO2 recombine in a shorter time than they need for the diffusion to the back contact. Electrodeposited ZnO electrodes show electron transport by orders of magnitude faster than TiO2 and allow for efficient electron collection in combination with the TEMPO+/0 redox couple. Although overall efficiency of ZnO-based devices is limited by unfavorable energy shifts in presence of such cationic redox couples, the present work demonstrates a pathway to profit from fast dye regeneration by kinetically fast redox couples when using a semiconductor material with high electron mobility.

Associated Content Supporting Information. Equivalent circuit for EIS fitting, effect of film thickness on DSSC characteristics, measurements of redox potentials, absorption spectra, IQE of DSSCs depending on light intensity and direction of illumination.

Author Information Corresponding Author * Phone: +49-641-99-33400. Fax: +49-641-99-33409. E-mail: [email protected]. ORCIDs Raffael Ruess: 0000-0002-9274-4714 Derck Schlettwein: 0000-0002-3446-196X


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Acknowledgements The authors acknowledge financial support by the Deutsche Forschungsgemeinschaft (DFG) via the project SCHL340/19-1 and via the GRK (Research Training Group) 2204 "Substitute Materials for Sustainable Energy Technologies".

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Efficient Electron Collection by Electrodeposited ZnO in Dye

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