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. 2024 Feb 20;7(5):1792–1801. doi: 10.1021/acsaem.3c02775

Understanding the Internal Conversion Efficiency of BiVO4/SnO2 Photoanodes for Solar Water Splitting: An Experimental and Computational Analysis

Laura Geronimo †, Catarina G Ferreira †, Valentina Gacha †, Dimitrios Raptis †, Jordi Martorell †,‡, Carles Ros †,*
PMCID: PMC10934258  PMID: 38487269

Abstract

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This work aims to understand the spin-coating growth process of BiVO4 photoanodes from a photon absorption and conversion perspective. BiVO4 layers with thicknesses ranging from 7 to 48 nm and the role of a thin (<5 nm) SnO2 hole-blocking layer have been studied. The internal absorbed photon-to-current efficiency (APCE) is found to be nonconstant, following a specific dependence of the internal charge separation and extraction on the increasing thickness. This APCE variation with BiVO4 thickness is key for precise computational simulation of light propagation in BiVO4 based on the transfer matrix method. Results are used for accurate incident photon-to-current efficiency (IPCE) prediction and will help in computational modeling of BiVO4 and other metal oxide photoanodes. This establishes a method to obtain the sample’s thickness by knowing its IPCE, accounting for the change in the internal APCE conversion. Moreover, an improvement in fill factor and photogenerated voltage is attributed to the intermediate SnO2 hole-blocking layer, which was shown to have a negligible optical effect but to enhance charge separation and extraction for the lower energetic wavelengths. A Mott–Schottky analysis was used to confirm a photovoltage shift of 90 mV of the flat-band potential.

Keywords: BiVO4, spin-coating, water splitting, catalysis, hydrogen

Introduction

The increasing urgency to fight against the consequences of climate change, such as global warming, heavy rainfall, or droughts, has led to an exponential increase in research interest in renewable energy sources and, in particular, solar energy. However, despite the high-power conversion efficiencies achieved by photovoltaic cells,1,2 their operation is limited by daylight hours and ambient conditions. Because of that, there is high interest in developing technologies capable of storing solar energy in the form of molecular fuels. Hydrogen, produced from water splitting, is one of the most promising solar fuels, with high energy density.3

The photoelectrochemical (PEC) water splitting process is capable of converting solar energy directly into chemical energy by absorbing photons and generating enough potential to split H2O molecules.4 The PEC effect was first discovered on TiO2,5 and much research has been done with it,6,7 but its large band gap (3.2 eV) limits the absorption to ultraviolet light, which accounts for only 4% of the total solar spectrum. Some other metal oxides such as WO38 and Fe2O39 have also been intensively studied but lack either a shorter band gap or proper carrier mobility.10

Bismuth vanadate (BiVO4) is one of the most promising semiconductors for PEC water splitting due to its relatively narrow band gap of 2.4 eV, which allows for a theoretical maximum solar-to-hydrogen conversion efficiency of 9.1%.4,11 In addition, it is a nontoxic, inexpensive, and earth-abundant material that presents good stability in neutral-alkaline electrolytes.12 Nevertheless, bismuth vanadate is also known for suffering from high surface recombination of photogenerated electron–hole pairs and poor charge transfer,13 which results in a short carrier diffusion length (∼70 nm).10,11 This sets a maximum limit for the BiVO4 thickness to avoid significant resistivity but also restricts the photocurrent achievable due to the partial light absorption. In addition, it lacks a surface that is prone to the oxygen evolution reaction. Therefore, extensive research has been conducted on passivating its superficial states and enhancing its catalytic activity. This investigation involves the coordination of electrocatalysts, such as CoPi14 or NiFeOOH,15 to augment the material’s overall performance.

BiVO4 photoanodes can be fabricated following multiple deposition methods, such as spin-coating,16 pulsed laser deposition,12 spray pyrolysis,17 or electrodeposition,18 resulting in highly variable performances and film morphologies. Enhancements in the PEC performance of BiVO4 photoanodes are reported by different approaches, such as forming hetero- or semiconductor/electrocatalyst junctions, enabling an improvement of the charge transport and the transfer efficiencies simultaneously.19 It has been also demonstrated how a proper surface treatment can significantly enhance photocatalytic activity,20 and adding a charge-blocking layer can reduce surface recombination, resulting in higher photocurrents.21

Even so, a single BiVO4 absorber requires an external bias to perform the water splitting reaction, and normally, a wide range of the solar spectrum is lost.22 A tandem configuration consisting of a photovoltaic/photoelectrode (PV/PEC) device is a good option to overcome these problems,19,23 but this requires high transmittance for all wavelengths not absorbed by the photoanode and therefore compact non-nanostructured BiVO4 films. This way, the PV cell with band gap energy (Eg) lower than that of the BiVO4 will efficiently absorb the light transmitted from the photoanode and then generate enough voltage to assist the bias-free water splitting reaction.23 In this context, the most effective BiVO4 synthesis methods are intentionally omitted due to the undesirable highly scattering films they produce.24−26 Spin-coating, which yields intermediate-performance films characterized by both compactness and transparency, emerges as essential. This method plays a crucial role in the fabrication of compact tandem devices,23 enabling the realization of bias-free targeted reactions.

This work studies the correlation between photon absorption and internal conversion efficiency and the growth process of BiVO4/SnO2 thin-film photoanodes. The samples were fabricated by a spin-coating deposition method to ensure a compact thin film with minimal light scattering, which results in a transparent material in the region below the band gap, that can be used in tandem PV/PEC configurations.23 Light conversion capabilities were studied for varying film thicknesses and the introduction of a SnO2 hole-blocking layer (HBL). The absorbed photon-to-current efficiency (APCE) was extracted from the experiments, and a correlation of the internal photon conversion rates for different thicknesses was obtained. This allows us to easily obtain one of the quantities (thickness or APCE) just by knowing the other one, at least for the thickness range obtained, for the first time in this study. The identification of this dependency between the internal conversion and the thickness was proven to help optimize the computational simulation of the incident photon-to-current efficiency (IPCE), based on a transfer matrix method, and will help in computational modeling of metal oxide photoanodes.

Materials and Methods

Materials

Bismuth(III) nitrate pentahydrate (Bi(NiO3)3·5H2O, 99.99%), vanadyl acetylacetonate (C10H14O5V, 97%), acetylacetone (C5H8O2, 99%), tin(II) chloride dihydrate (SnCl2·2H2O), iron(II) sulfate heptahydrate (FeSO4·7H2O, 99%), nickel(II) sulfate hexahydrate (NiSO4·6H2O, 98%), boric acid (H3BO3, 99.5%), potassium hydroxide (KOH, flakes, 90%), sodium sulfite (Na2SO3, 98%), zinc acetate dihydrate (Zn(CH3COO)2·2H2O), ethanolamine (NH2(CH2)2OH), 2-methoxyethanol (CH3O(CH2)2OH), and 1-chloronaphthalene (C10H7Cl) were all purchased from Sigma-Aldrich. Fluorine-doped tin oxide substrates (FTO glasses, 5 cm × 5 cm), with a sheet resistance of 15 Ω·cm–2, were obtained from Sigma-Aldrich.

Preparation of the BiVO4 Photoanodes

BiVO4 films were prepared on top of previously cleaned FTO-coated glass substrates using a layer-by-layer spin-coating deposition method. To fabricate the final bismuth vanadate solution, two precursor solutions of 0.2 M of Bi(NO3)3·5H2O and 0.03 M of C10H14O5V, both dissolved on acetylacetone, were mixed and sonicated for 15 min before and after the mixing. To get each layer, 20 μL of BiVO4 solution was spin-coated (Laurell Technology) twice in a row at 1000 rpm for 10 s, with an acceleration rate of 250 rpm·s–1, followed by a treatment in an oven at 500 °C in air for 5 min, for precrystallization. This process was repeated for 1, 5, and 9 layers, after which final annealing at 500 °C was performed for 2 h at a heating rate of 15 °C·min–1, for complete crystallization.

A SnO2 solution with a concentration of 30 μM was prepared from SnCl2·2H2O using isopropanol as the solvent, stirred for 5 h, and aged for 24 h at room temperature to increase its viscosity. The SnO2 layer was also deposited by spin-coating on top of some of the FTO substrates at 2000 rpm for 30 s.

BiVO4 photoanodes composed of 1, 5, and 9 spin-coating layers were fabricated both directly on top of bare FTO/glass and on substrates coated by a thin layer of SnO2. Bare FTO samples with and without the addition of a thin layer of SnO2 were made as references and equally studied.

Morphological and Optical Measurements

Samples’ morphological, cross-sectional, and grain size analyses were made using a Zeiss series Auriga FIB-SEM microscope operated at 5 kV. UV–vis measurements were performed with a PerkinElmer Lambda 950 UV/vis spectrometer, from which we obtained both specular transmittance and reflectance in a range from 300 to 800 nm, in steps of 5 nm. To estimate the band gap from the UV–vis data, Tauc’s27 direct and indirect band gap equation was applied.

Photoelectrochemical Measurements

Photoelectrochemical measurements were performed by using a 7 mL single-compartment cell with a 0.5 cm2 circular window. All the PEC measurements were performed under back-side illumination in a three-electrode setup made up of a Pt counter electrode (CH Instruments CHI115), a 1.0 M potassium chloride (KCl) Ag/AgCl reference electrode (CH Instruments CHI111), and the BiVO4 photoanode as the working electrode, the potential being controlled by a BioLogic SP-300 potentiostat. A 1.0 M potassium borate (KBi) solution, with pH corrected to pH 9 by the introduction of potassium hydroxide (KOH), and with the addition of 0.5 M sodium sulfide (Na2SO3) as a hole scavenger, was used as an electrolyte for the water splitting reaction.

Cyclic voltammetry measurements were performed under simulated AM 1.5G solar illumination (100 mW·cm–2) from a solar simulator (Sun 2000 class A, Abet Technologies), with a scan rate of 50.0 mV·s–1. Values of the series (Rs) and shunt resistance (Rsh) are determined as the inverse of the slope of the linear regions on the j–V curves at low (0.4–0.5 VRHE) and high (1.23 VRHE) applied potentials, respectively. The onset potential (Von) is defined at 0.01 mA·cm–2 photocurrent density (average of the forward and reverse scans), and the photogenerated voltage (Vph) is calculated as the potential between Von and 1.23 VRHE. IPCE measurements were carried out in the same three-electrode configuration, at a fixed potential of 1.23 VRHE, using a monochromator (Oriel 260 Cornerstone, Newport Instruments) illuminated with a 300 W xenon lamp (OPS-A500, Newport) and incorporating additional automated long-pass filters to cut out more energetic second-harmonic photons from the grating. To calculate the IPCE values from the experimental data collected, the following relation was used28

graphic file with name ae3c02775_m001.jpg 1

where Jph(λ) is the measured photocurrent density (mA·cm–2) and Pinc is the incident light power density (W·cm–2) for each wavelength considered, which was recorded with a calibrated S120VC photodiode (Thorlabs). The APCE was calculated as the ratio between IPCE and 1-T-R.

Mott–Schottky plots are obtained with the same photoelectrochemical conditions and equipment, and the relation between the capacitance of the space charge layer, the doping level, and the applied potential is obtained by the following equations29

graphic file with name ae3c02775_m002.jpg 2
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where ϵr is the relative permittivity of the semiconductor material (for BiVO4 ∼ 8630), ϵ0 is the permittivity at vacuum (ϵ0 = 8.854 × 10–14 F·cm–1), q is the charge of an electron, k is the Boltzmann constant (Inline graphic), VMPP is the voltage at maximum power, and ND is the donor density.

Computational Simulations

A transfer matrix approach, combining light incoherence at the thick glass substrate with coherence at the FTO/BiVO4 thin-layer stack, was considered to describe light propagation at the photoanode, which was assumed to be composed of flat layers, all parallel to each other.31,32 Refractive indexes of the different materials were determined by ellipsometry measurements. To estimate the thickness of the BiVO4 layer for the different photoanodes, a least means square method was used in combination with the transfer matrix approach to fit the experimental data for the transmittance and specular reflectance. Simulated IPCE curves were obtained by considering these estimated thicknesses together with the APCE values extracted experimentally.

Results and Discussion

In this work, BiVO4 photoanodes were deposited on top of FTO/glass substrates, with and without the addition of a thin SnO2 hole-blocking layer, using a layer-by-layer spin-coating deposition technique, schematically illustrated in Figure 1a and explained in more detail in the Materials and Methods section of this manuscript. To study the growth process of the BiVO4 and the influence of its thickness in terms of photoactivity of the photoanodes, different samples composed of 0, 1, 5, and 9 spin-coating layers of bismuth vanadate were deposited both on FTO/glass and on SnO2/FTO/glass substrates. The final samples obtained are pictured in Figure 1b, together with the nomenclature used to address them in this document. As clearly illustrated in this figure, the samples become more yellowish with the addition of BiVO4 layers, but they maintain high transparency regardless of the number of layers considered, which makes them very suitable for tandem photoanode–photovoltaic applications. At the nanoscale, a granular film is formed on top of the FTO polycrystalline film on a glass substrate, as shown in Figure 1c. The inclusion of additional spin-coating layers (>10L) induces a dewetting process in the BiVO4 film. This leads to the agglomeration of the material into large particles, resulting in pronounced scattering and, thus, minimal transmittance (Figure S1). Consequently, this hinders the applicability of these samples in tandem structures, a crucial consideration within the specific context of this study. Based on that, we limited the study to a maximum of 9 spin-coated layers.

Figure 1.

Figure 1

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(a) Schematic representation of the spin-coating process used to deposit the BiVO4 photoanodes on top of FTO/glass substrates. (b) Samples used in the experiments were classified by the number of deposited BiVO4 layers and the presence or absence of SnO2. (c) Cross-sectional SEM micrograph of the 9-layer BiVO4 + SnO2 sample.

Morphological Characterization

Morphological characterization of the different BiVO4 photoanodes was carried out by SEM analysis. The cross-sectional SEM micrograph of a 9-layer sample deposited on top of SnO2/FTO/glass is presented in Figure 1c and reveals that the BiVO4 has grown as a compact, homogeneously distributed grain film, without any signs of a multilayer deposition. This indicates that the final 2 h annealing treatment, performed on all samples, has probably resulted in a reorganization of the precrystallized films. The average layer thickness for this 9-layer BiVO4 sample was estimated from the SEM cross-sectional analysis to be 48 nm, as demonstrated in Figure S2. In addition, no signs of the SnO2 layer were detected, which indicates that its thickness may be lower than the cross-sectional SEM resolution (<5 nm).

Figure 2 shows the top-view SEM micrographs for all eight samples with an increasing number of BiVO4 layers in the presence or absence of a thin SnO2 layer. The first thing we observed from the analysis of these images was that the samples that contain one layer of SnO2 appear slightly blurrier than the ones with just BiVO4. This can have two main explanations: the photoanodes have slightly less conductivity than when in direct contact with FTO, which influences SEM resolution by slightly deflecting the electrons, or films have become smoother with the addition of a small SnO2 layer, as it was seen in some recent studies.21,33

Figure 2.

Figure 2

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Top-view SEM micrographs of BiVO4 and BiVO4 + SnO2 thin films with different numbers of layers. A scheme of the BiVO4 crystal development is included. Exposed FTO features are indicated with white arrows.

Despite that, for the same number of BiVO4 layers, the film morphology seems to be quite similar regardless of the presence of the SnO2 hole-blocking layer. In particular, when a single layer was deposited, top-view analysis resembled the one of the bare FTO/glass substrates but with blurrier grain boundaries. When the number of layers was increased to 5 and 9, rounded BiVO4 grains started to be visible and to cover the FTO surface. However, only partial coverage of the substrates was achieved in both cases, with multiple rounded grains of BiVO4 growing separated from each other and exposing the FTO grains beneath. So, it seems that the FTO rugosity is filled with BiVO4 grains, and the tips of the FTO’s tallest grains are more likely to be exposed. It can be seen how specific features of the FTO substrates, such as elongated grains (probably FTO grains with a specific crystallographic orientation), are found exposed more often than smaller rounded grains, suggesting that it is more difficult for BiVO4 to grow on top of these FTO crystalline orientations. This means that the underlying substrate will be exposed to the electrolyte.

Grain sizes of the different BiVO4 films, extracted from the analysis of the SEM micrographs, progressively increased with the addition of more layers. As shown in Figure 2 and the histograms of grain size in Figures S3 and S4, the average grain diameter goes from ∼85 nm for the 5-layer BiVO4 samples, up to 98 nm, for the 9-layer photoanodes, and it is not affected by the presence of SnO2. As commented previously, BiVO4 grains are not distinguishable in the 1-layer samples. Some previous works have reported that the SnO2 thickness, when higher than 40 nm, has a role in the BiVO4 crystallization.21 In our case, however, the hole-blocking layer is much thinner than that, which explains why the presence of SnO2 does not influence the growth of the BiVO4 grains. Moreover, the average film thickness of the 9-layer photoanode is about half the average grain width, which suggests a good contact between the BiVO4 film and the FTO and a favorable chemical surface tension during the thermal crystallization process at 500 °C.

Different grain sizes and grain orientations can have a significant effect as this can lead to a high number of structural defects that can act as carrier traps and recombination centers, leading to lower photoelectrochemical activity,14 or favor BiVO4 growing with some preferential crystalline orientation.16,34

Optical Characterization

UV–visible specular transmittance (T) and reflectance (R) measurements were obtained for the fabricated samples (Figure 3a,b). T values are observed to have a significant reduction for wavelengths <350 nm, directly related to the absorption of the FTO substrate, while wavelengths in the 350–550 nm range are less transmitted when increasing the amount of BiVO4 layers. Meanwhile, R values stay in the range of 15–30% for all wavelengths. SnO2 has no significant role in either T or R. With these, 1-T-R can be calculated (Figure 3c) and represents the wavelengths absorbed or scattered by the sample. 1-T-R values are observed to be directly correlated to the number of deposited layers (Figure 3) and, thus, the optical path through the material. By observing the sample’s 1-T-R spectra, it can be affirmed that both the bare BiVO4 samples and the ones having a SnO2 interlayer have similar absorption values and profile, meaning that BiVO4 thickness is very similar independently of the presence of SnO2 and that the latter has a minimal optical role.

Figure 3.

Figure 3

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UV–visible (a) transmittance (T) and (b) reflectance (R) and (c) 1-R-T spectra of the fabricated BiVO4 photoanodes with and without SnO2.

There is an onset near 470 nm, red-shifted when increasing the number of BiVO4 layers, and the 1-T-R values significantly increase for more energetic photons. The 330–470 nm region appears to be dominated by the absorbance of the BiVO4 deposited film, reaching 1-T-R values up to 60% for 9 layers. It presents two increasingly undulated steps that can be attributed to light interference on a different dielectric constant multilayer stack (BiVO4/SnO2/FTO/Glass). Meanwhile, in the 330–300 nm region, samples present 60–80% 1-T-R values, similar to bare FTO/Glass. For wavelengths greater than 470 nm, below the BiVO4 band gap energy, samples present non-negligible 1-T-R values. This can be explained by nonzero diffuse reflectance and transmittance light dispersion, not accounted for in 1-T-R, and by the presence of a tail of absorption states, probably due to imperfections in the BiVO4 crystallographic structure.35 Results obtained by the interpolation of the linear region following the Tauc plot based on the direct band gap case (Figure S5a) reveal band gaps in the range of 2.61–2.72 eV with increasing thickness, regardless of the inclusion of the SnO2 hole-blocking layer, which agrees with other reported values.19,36 The indirect band gap (Figure S5b) points at slightly smaller values (2.55 to 2.67 eV), although its determination is difficult due to light scattering and point defects in the photoanode.36−38

(Photo)electrochemical Characterization

Cyclic voltammetries, in a three-electrode system, obtained for the herein studied samples at dark and under back illumination are displayed in Figures S5, S6 and Figure 4, respectively. Using back-side illumination has positive implications on the PEC performance of materials that have bad carrier transport, such as BiVO4, because electrons can be directly injected from BiVO4 to the back FTO contact.39

Figure 4.

Figure 4

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(a) Cyclic voltammetry curves recorded in a three-electrode configuration using an aqueous solution of 1 M KBi with 0.5 M Na2SO3 as a hole scavenger (pH = 9) as the electrolyte, with a scan rate of 50.0 mV·s–1, under back-side simulated solar light illumination conditions (AM 1.5 G, 100 mW·cm–2) for 0-, 1-, 5-, and 9-layered BiVO4, with and without the addition of a SnO2 hole-blocking layer. (b) IPCE spectra of the same samples, measured in a similar configuration, using monochromatic illumination at 1.23 VRHE applied potential. (c) Calculated APCE conversion efficiencies for all samples based on 1-T-R (Figure 3c) and IPCE (Figure 4 b).

Table 1 summarizes typical j–V parameters of the fabricated photoanodes obtained in a way similar to a photovoltaic device curve. Both an increase in photocurrent density at 1.23 VRHE and a cathodic shift of the onset potential of the photoanodes are observed when adding BiVO4 layers. In particular, the 9-layer bare BiVO4 sample presents the highest photocurrent density at 1.23 VRHE, of 0.72 mA·cm–2, which is in the same order as other works for 40 nm thick layers.39 There is a small improvement in the short-circuit photocurrent density at 1.23 VRHE by the presence of SnO2, for any number of BiVO4 layers considered, but we can see a major effect of the SnO2 layer when looking at the profile of the curves: a better photodiode shape is obtained, with higher photocurrents obtained at similar potentials, together with lower hysteresis. These observations are in correlation with the electrical parameters determined with the addition of a SnO2 layer (Table 1): onset values of the potential are cathodically shifted compared to the samples with bare BiVO4, especially if we look at the 5- and 9-layered samples, which implies higher photogenerated voltage (Vph) for these photoanodes and also an observable cathodic shift of the potential at the maximum power point (Vmpp), to about 70 mV. In addition, a combined increase of the shunt resistance (Rsh) and a decrease in series resistance (Rs) with the addition of the SnO2 layer are registered, which results in a higher fill factor, clearly observed in the j–V curves of Figure 4a. All of these results point to SnO2 properly acting as a hole-blocking layer (HBL). Samples presenting a higher fill factor will be capable of extracting more power from the photoabsorbed light, thus enabling higher productivity in both single absorber and tandem PEC/PV devices.40

Table 1. Parameters Calculated from the Cyclic Voltammetries Presented in Figure 4aa.

layers (#) j1.23VRHE(mA cm–2) Rsh (kΩ) Rs (kΩ) Von (VRHE) Vmpp (VRHE) Vph (V)
BiVO4
1 0.148 56.0 6.82 0.40 0.73 0.83
5 0.379 14.4 2.87 0.41 0.77 0.82
9 0.719 5.8 1.77 0.38 0.78 0.85
BiVO4 + SnO2
1 0.110 55.0 5.54 0.41 0.68 0.82
5 0.423 14.9 1.33 0.34 0.72 0.89
9 0.662 6.6 0.82 0.35 0.71 0.88
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a

Short-circuit current density at 1.23 VRHE (j1.23VRHE), shunt resistance (Rsh), series resistance (Rs), onset potential (Von), potential at maximum power point (Vmpp), and photogenerated voltage (Vph), defined as 1.23 VRHE–Von.

The j–V curves under dark conditions, displayed in Figure S6, reveal an anodic current appearing at a high anodic potential in the absence of the SnO2 layer, which is partially blocked by increasing the number of BiVO4 layers. This means that, without light, the semiconductor acts as a blocking diode, preventing electrons coming from the polarized substrate from oxidizing the electrolyte. This is eliminated with the introduction of a SnO2 layer, which points in the direction of SnO2 to introduce an enhanced electrical barrier.

Photon-to-Current Efficiency Characterization

The IPCE is a powerful technique to precisely characterize the response of a sample to incident light, independently of the light source.28 This is especially key in a field such as photoelectrochemistry, where experimental standardization of the conditions is less implemented compared to photovoltaics.41

As shown in Figure 4b, higher conversion efficiencies for the whole range of wavelengths are observed for an increasing number of BiVO4 layers, directly in accordance with the photocurrent density at 1.23 VRHE extracted from the j–V profiles of Figure 4a and with the undulated-like 1-T-R profile seen in Figure 3a. Nevertheless, if we compare each photoanode’s IPCE divided by the number of layers #L (Figure S7), we observe that the deposition of the first layer of BiVO4 results in the highest IPCE/#L ratio response (6%/L), triple the addition of subsequent layers (2%/L), which maintain a constant increment. This initial divergence is caused by the first BiVO4 layer being deposited directly on the rough surface of the FTO, filling its roughness depths. These depths capture more material during the spin-coating process, resulting in a thicker layer during the first deposition and consequently a higher IPCE/#L due to a longer optical path in the first layer. Increasing to 5 and 9 layers does not change the IPCE/#L ratio, meaning that after the first layer, each spin-coating cycle deposits the same amount of material. A slightly smaller IPCE/#L for the 1-layer BiVO4 + SnO2 sample points out that the surface has been slightly smoothed previously by the SnO2 < 5 nm film, as also indicated by the SEM micrographs of Figure 2.

In Figure S8, we also show the IPCE measurements of bare FTO and FTO/SnO2 substrates, presenting photon conversion for wavelengths in the 300–350 nm range (maximum of 0.05 and 0.25% at 300 nm for FTO and SnO2, respectively), corresponding to ∼3.50 eV, the band gap of SnO2,33 together with a tail of states down to 370–380 nm. This proves that FTO is not a completely degenerately doped metal oxide, presenting slight band bending capable of converting photons into current when in contact with water. The additional SnO2 thin film significantly increases the conversion capability, caused by the lower doping level compared with FTO.42

To better understand the IPCE curves, they must be compared to the 1-R-T measurements. This way, the transmitted, reflected, absorbed, and finally converted photons can be distinguished (Figure S9a–c). These last ones are described by the APCE, presented for every sample in Figure 4c. APCE reveals the internal conversion capability of the absorbent materials themselves, independent of the optical system used. As we have already seen in the Optical Characterization section, our samples present two light absorbance-increasing undulated steps, also visible in the IPCE curves (Figures 3a and 4b), but not in the APCE, meaning that these features were mainly optical. APCE presents three different regions: (1) The 300–330 nm one, dominated by a strong reduction of the conversion capability, caused by photons being first absorbed by the glass, FTO, and SnO2 layers. With thinner BiVO4 films (more separated BiVO4 nanoparticles, Figure 2), it will be less probable for higher energetic (small wavelengths) photons to interact with the BiVO4 nanoparticles, thus missing them; (2) the 350–450 nm one, with a relatively constant conversion rate over wavelengths, especially for a higher number of BiVO4 layers, with 19, 24, and 31% APCE at 400 nm for 1-, 5-, and 9-layer BiVO4 samples, respectively. The increase of internal conversion with thickness must be attributed to the increase of BiVO4’s grain size, allowing for larger band bending6 and thus better charge separation and extraction, which translates into larger photocurrents; and (3) the 450–500 nm one, corresponding to the absorption edge of BiVO4, which is shifted to lower energies with the increase in BiVO4 thickness.

Thickness Estimation and IPCE Simulation

To better understand and also to predict the PEC behavior of the different BiVO4 samples, computer simulations were performed in which a transfer matrix formalism was employed to describe the light propagation inside the photoanodes. This method is very accurate in describing multilayer systems in which the layers are all flat and parallel to each other and their thicknesses are in the order of the wavelength of the incident light,31,32 as is the case of the compact BiVO4 photoanodes herein fabricated.

The refractive indexes considered for the FTO and BiVO4 layers are presented in Figure S10. The SnO2 layer was not included in the simulations, as its thickness is very small and its optical role was found to be irrelevant, as demonstrated in Figure 3.

Using these refractive indexes, the thicknesses of the different photoanodes prepared were estimated by fitting the transmittance and reflectance spectra experimentally measured. The best fits obtained are shown in Figure S11, and according to the results, thicknesses of 7, 23, and 48 nm were estimated for the 1-, 5-, and 9-layer BiVO4 samples, respectively. This agrees with the values extracted from the cross-sectional SEM analysis of Figure 1c, for the case of the 9-layer sample, and it is also in accordance with our prior observations that the first layer deposited is thicker than the remaining ones.

To estimate the IPCE response of the photoanodes, both the thickness of BiVO4 and the absorbed photon-to-current efficiency must be used as input for the computational simulations. Interestingly, as observed in Figure 5a, the APCE values experimentally determined at 400 nm seem to follow a linear trend with the thickness of the BiVO4 layer. This allows us to easily obtain one of the quantities (thickness or APCE) just by knowing the other, at least for the thickness range obtained in this study.

Figure 5.

Figure 5

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(a) APCE values at 400 nm wavelengths, from Figure 4c, as a function of the thickness simulated from the T and R measurements, where an almost linear correlation can be easily seen. (b) Simulated IPCE (dashed line) overlapped to the experimental IPCE for all samples, fitted with thicknesses of 7, 23, and 48 nm. IPCE was acquired in a three-electrode configuration, using an aqueous solution of 1 M KBi with 0.5 M Na2SO3 as a hole scavenger (pH = 9) as the electrolyte, using monochromatic illumination at 1.23 VRHE applied potential.

The IPCE curves simulated using the transfer matrix model for a different number of BiVO4 layers are plotted in Figure 5b, together with the experimental responses obtained for the same cases, both with and without the addition of the SnO2 hole-blocking layer. A good agreement between experimental and simulated curves was obtained, and only small differences in the absorption onset position are observed, which can be attributed to slight differences between the refractive indexes considered for the BiVO4 absorbing layer and real ones. This shows that it is possible to estimate the sample thickness just by knowing its IPCE, provided that the samples are fabricated following an equal synthesis process.

Role of the SnO2 Interlayer

Finally, voltage-dependent IPCE and Mott–Schottky (M–S) plots were acquired to understand the role of the SnO2 intermediate layer. Figure 6a presents the variation of the normalized IPCE spectra upon applied potential. As can be seen, both the 9L and the 9L + SnO2 samples present almost identical IPCE profiles down to 0.4 VRHE, similar to the ones presented in Figure 4b. The main difference is the capacity of the sample having the intermediate SnO2 layer to extract more efficiently the photons in the 350–450 nm range. This directly points at the SnO2 layer helping to increase the electron–hole separation and transport closer to the BiVO4/FTO interface, a larger penetration depth where lower energy photons have more probability to be absorbed. This proves its HBL capabilities, creating an increased charge separation and extraction potential, also seen in the cyclic voltammetry (Figure 4a) as a better fill factor. At 0.3 VRHE, the effect is more pronounced, although the generated photocurrents are small at this potential, close to the flat-band condition. The higher HBL effect presented for the lowest applied potential can be explained by the fact that at equal applied potential, the sample with the SnO2 layer has a more cathodic onset potential (Table 1), thus really having added extraction potential. All IPCE spectra for different voltages are presented in Figure S12, where a decrease of the total IPCE can be observed with more cathodic potentials, following the same trend as Figure 4a voltammetries.

Figure 6.

Figure 6

Open in a new tab

(a) Normalized IPCE measurements of 9L samples, with and without the SnO2 interlayer, as a function of the applied potential, from 0.7 to 0.3 VRHE. IPCE was acquired in a three-electrode configuration, using an aqueous solution of 1 M KBi with 0.5 M Na2SO3 as a hole scavenger (pH = 9) as the electrolyte, using monochromatic illumination. (b) Mott–Schottky plots of nine-layer BiVO4, with and without the addition of a SnO2 layer, measured in an electrolyte solution of 1.0 M KBi with 0.5 M of Na2SO3 as a hole scavenger (pH = 9) at 1 kHz under dark conditions.

Figure 6b compares the capacitance behavior of the 9-layer photoanodes, and as both samples have similar morphologies (Figure 2), thicknesses, and optical properties (Figure 3), it can be assumed that the differences between the two curves are due to the presence or absence of SnO2. Both samples present a positive slope in the 0.3–0.4 VRHE region (a sign of n-type semiconductors), a similar slope (doping level), and a clear shift to lower potentials when a SnO2 layer is introduced, implying an extra space charge region is formed.

The flat-band potential (Vfb), the donor density (ND), and the depletion region width (W) can be estimated by the analysis of the M–S measurements shown in Figure 6 and are presented in Table 2. High values of donor densities suggest that both photoanodes fabricated in this study are highly doped semiconductors.29 Moreover, our samples present W ≤ photoanode thickness, which is desirable since the electric field generated in the depletion region will be maximum, enhancing the charge separation and extraction.6 The presence of a SnO2 layer induces a down-shift of 90 mV in the flat-band potential, similar to the ∼70 mV measured in the cyclic voltammetries (Table 1), improving charge separation and enhancing the fill factor and thus the overall performance of the photoanodes, as observed in Figure 4a.

Table 2. Parameters Obtained from the Mott–Schottky Plots of 9-Layer Samples (Figure 6)a.

sample Vfb(VRHE) ND(cm–3) W (nm)
BiVO4 0.25 2.9 × 1018 34.8
BiVO4 + SnO2 0.16 3.1 × 1018 38.9
Open in a new tab
a

Flat-band potential (Vfb), donor density (ND), and depletion region width (W).

Conclusions

BiVO4 photoanodes were fabricated by the spin-coating method, varying the number of deposited layers and introducing a thin (<5 nm) SnO2 hole-blocking layer. Spin-coating is proven capable of depositing thin films with minimal light scattering, which is ideal for tandem PEC/PV devices. Thicker samples increased the BiVO4 grain size, simultaneously enhancing light absorption and generating larger photocurrents. The deposition process is found to deposit thicker BiVO4 layers in the first coating cycle due to the filling of the FTO substrate’s rugosity cavities, which is revealed by a larger IPCE per layer (IPCE/#L) ratio. SnO2 intermediate <5 nm layer is found to smooth FTO’s rugosity and to induce a 70–90 mV cathodic shift to the flat-band potential, acting as a hole-blocking layer and helping to achieve better fill factor of the photoanodes, without modifying the charge carrier density in the BiVO4. The SnO2 intermediate layer was also found to enhance charge separation and extraction for the lower energetic wavelengths.

The APCE revealed that the internal photon conversion efficiency increases when thicker samples are fabricated, up to 31% in our case, a sign of better internal charge separation and extraction together with a slight reduction of the BiVO4 effective band gap. Finding a linear dependency of the internal APCE conversion with film thickness is key to incorporating this parameter into the computational simulation of light propagation based on the transfer matrix method. This enabled accurate matching of the predicted IPCE with experimentally obtained measurements. This establishes a first approach method to obtain the sample thickness just by knowing its IPCE, accounting for variating internal APCE conversion. This states, for the first time, the correlation of the internal conversion efficiency of BiVO4 photoanodes with the material thickness, paving the way for precise computational modeling of light conversion efficiency. These findings establish a robust foundation for simulating tandem PV/PEC structures, fast-tracking the development of bias-free synthetic solar fuel devices.

Acknowledgments

ICFO acknowledges financial support from the LICROX and SOREC2 EU-funded projects (Codes: 951843 and 101084326), the BIST Program, and the Severo Ochoa Program. This work was partially funded by the CEX2019-000910-S (MCIN/AEI/10.13039/501100011033 and PID2020-112650RB-I00), Fundació Cellex, Fundació Mir-Puig, and Generalitat de Catalunya through CERCA. C.R. acknowledges support from the MCIN/AEI JdC-F Fellowship (FJC2020-043223-I) and the Severo Ochoa Excellence Postdoctoral Fellowship (CEX2019-000910-S).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsaem.3c02775.

  • UV–vis transmittance and reflectance spectra, cyclic voltammetries, IPCE, and APCE supporting images are supplied in an additional document (PDF)

Author Contributions

The manuscript was written through the contributions of all authors. All authors have approved the final version of the manuscript.

The authors declare no competing financial interest.

Supplementary Material

ae3c02775_si_001.pdf (1.6MB, pdf)

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Supplementary Materials

ae3c02775_si_001.pdf (1.6MB, pdf)

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