The PEC performance of the samples was evaluated by chopped light measurements in three different electrolytes. The first electrolyte has near neutral pH adjusted with a borate buffer (similar to the conditions in 4.3 and used by Plate171) and sulfite as hole scavenger to avoid kinetic limitations of the OER. The second electrolyte consists of the same buffer but ferro-/ferricyanide as hole scavenger. Finally, the conditions of Yan et al. are used with pH 13 and ferro-/ferricyanide as hole scavenger.56 All measurements were executed under front side illumination.
Figure 45 shows typical cyclic voltammetry (CV) and chronoamperometry (CA) plots of measurements in the first electrolyte. The CV shows an onset for light and dark current at about 0.9 V vs. RHE, which is a typical value.55 Unfortunately the light current density is only marginally larger than the dark current density. This and the almost simultaneous onset of both indicates a rather weak photovoltage and -current.
Figure 45: Chopped light photoelectrochemical measurements of Mn2V2O7 in a borate buffer (pH 9.2) with sulfite hole scavenger: (a) Cyclic voltammetry; (b) Chronoamperometry at 0.89 V vs. RHE.
Indeed the current densities are so low that a possible contribution of the FTO substrate has to be considered. To exclude this substrate contribution the present measurement is compared to the photoresponse of an FTO substrate with only the MnO sacrificial layer in Figure S 12. It clearly shows that such a contribution does not exist and the photoresponse indeed originates from the Mn2V2O7 ALD film.
A subsequent chronoamperometry measurement was executed at ECA = 0.89 V vs. RHE, the dark current onset, to quantify the photocurrent density. This measurement was done with a decreased chopping frequency as shown in Figure 45b. A clear photocurrent is visible but it is very small. The contribution of the dark current should therefore be considered. Hence the measured photocurrent densities will be expressed as ΔJ = Jlight – Jdark.
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There are significant spikes in current density when the shutter is opened and closed. Such transients usually indicate a limitation of the charge injection into the electrolyte i.e. the oxidation of SO32- in this study.60
Similar CA measurements were performed using the other two electrolytes and examples are shown in Figure S 13 and Figure S 14. The results of the measurements for the three different samples (1:7, 1:8 and 1:9 cycle ratio) are summarized in Figure 46.
Figure 46: Measured photocurrent densities ΔJ in dependence of the manganese to vanadium ratio of the samples in the three different electrolytes: borate buffered (pH 9.2) and sulfite scavenger (black), borate buffered
(pH 9.2) and [Fe(CN)6]3-/4- scavenger (red), and 0.1M KOH (pH 13) and [Fe(CN)6]3-/4- scavenger (blue).
All measured current densities are very low, the largest value barely exceeds 25 µA/cm2. The current densities employing sulfite as scavenger (black) are significantly lower than the ones using ferro-/ferricyanide. This and the transients mentioned above indicate that sulfite is not a suitable hole scavenger in the present case. Not all available charges carriers contribute to the sulfite oxidation and thereby to the measured photocurrent.
The pH on the other hand does not affect the photocurrent density significantly. The measured photocurrent densities barely differ between pH 9.2 (red) and 13 (blue). One reason to prefer the borate buffer could be the milder conditions possibly leading to an increased stability of the film.
Finally, the influence of the stoichiometry on the measured photocurrent density is considered. The two samples with an excess of vanadium show larger photocurrent densities.
The difference in current density between those is rather low. Interestingly the sample with the almost ideal stoichiometry Mn2V2.06O7.12 gives the lowest photocurrent density.
The reason for this counterintuitive behavior cannot be fully resolved. The higher carbon content (5.52 at.% compared to 3.44 and 3.48 at.%) could increase the number of defects in the film facilitating recombinations. Or the other way around an increase in vanadium and/or
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oxygen content could reduce the amount of defects like oxygen vacancies, increasing the overall conductivity.
However, all measured current densities are extremely low compared to the absorbed photon flux of 3 mA/cm2 leading to overall absorbed-photon-to-current efficiency (APCE) of less than 1%. The investigations of Yan et al. showed larger current densities of about 100 µA/cm2, even though they do not report a film thickness.56 Publications on other ALD-grown photoabsorbers report photocurrent densities of 90 – 1200 µA/cm2 indicating that ALD is in principle suited to deposit photoabsorbers.54,88,89 Maybe this is not the case for Mn2V2O7. Moreover it is possible that Mn2V2O7 is an intrinsically bad material for PEC applications.
The absorbed photon flux could facilitate 3 mA/cm2 and the scavenger inhibits kinetic limitations, but still very few charge carriers contribute to the photocurrent. Therefore major losses have to occur during the charge transport within Mn2V2O7 i.e. the photogenerated carriers are unable to reach the Mn2V2O7 interfaces. Time-resolved microwave conductivity (TRMC) is a suitable technique to investigate these charge carrier dynamics. Unfortunately this technique requires stronger microwave absorption than the ALD samples provide to achieve signals above the noise level. To still get a principle idea of the charge carrier dynamics TRMC measurements of PLD grown Mn2V2O7 films were evaluated.162 These were deposited and investigated in a separate project. A more detailed look on TRMC measurements is given in 2.2.3 and 5.1.5.
The measurement in Figure 47 shows a very weak signal. Despite the poor signal to noise ratio, an exponential decay of the photoconductivity could be fitted to identify the peak photoconductivity øΣµmax = 3.8·10-6 cm2/Vs and the TRMC decay time τ = 111 ns. Combined they account for a charge carrier diffusion length of 1.0 nm. This diffusion length is very low even on metal oxide scale with values in the range of 2 – 200 nm.57 Such a short diffusion length, mainly caused by the exceptionally low peak photoconductivity, explains the low photocurrent densities. Only a small fraction of the excited charge carriers can reach the Mn2V2O7 interfaces and thereby contribute to the photocurrent. Especially the low peak photoconductivity characterizes the PLD-Mn2V2O7 as a poor photoabsorber as well. While charge carrier diffusion length in the range of 10 – 100 nm could be compensated by nanostructures it becomes increasingly difficult with extremely thin layers e.g. in terms of suitable nanostructures.
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Figure 47: TRMC measurement on Mn2V2O7 grown by PLD measured at 6.50x1014 photon/pulse·cm2 and 410 nm excitation irradiation.
When only 1.0 nm are considered the absorbed photons could facilitate a photocurrent density of about 150 µA/cm2. Factors further lowering this current density can be a weak charge separation, trapping of charge carriers at the Mn2V2O7-MnO or Mn2V2O7-FTO interface and tunneling of charge carriers throughout the thin film to recombine.
Furthermore the stability of the films still is an issue as Figure 48, a photograph of the investigated samples after the discussed PEC measurements, illustrates. The brighter cycles clearly show the area exposed to the electrolyte during the measurements with at least parts of the Mn2V2O7 film dissolved. However, it cannot be distinguished which measurement contributed to the dissolution to which extent. The Pourbaix diagram in Figure S 15 suggests that especially the pH 13 electrolyte will dissolve the Mn2V2O7 film.56 Apparently even the specifically introduced MnO sacrificial layer does not adequately stabilize the Mn2V2O7 thin film.
Figure 48: Photograph of Mn2V2O7 samples after PEC measurements, from left to right: 9:1, 8:1, 7:1 cycle ratio.
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