A Perovskite Photovoltaic Mini-Module-CsPbBr3 Photoelectrochemical Cell Tandem Device for Solar-Driven Degradation of Organic Compounds

Recently, halide perovskites have been widely explored for high-efficiency photocatalysis or photoelectrochemical (PEC) cells. Here, in order to make an efficient photoanode electrode for the degradation of pollutants, concretely 2-mercaptobenzothiazole (MBT), nanoscale cesium lead bromide (CsPbBr3) perovskite was directly formed on the surface of mesoporous titanium dioxide (meso-TiO2) film using a two-step spin-coating process. This photoelectrode recorded a photocurrent of up to 3.02 ± 0.03 mA/cm2 under standard AM 1.5G (100 mW/cm2) illumination through an optimization process such as introducing a thin aluminum oxide (Al2O3) coating layer. Furthermore, to supply high voltage for efficient oxidation of MBT without an external bias, we developed a new photovoltaic/PEC tandem system using a methylammonium lead iodide (MAPbI3) based mini-module consisting of three solar cells interconnected in series and confirmed its successful operation. This approach looks very promising due to its applicability to various PEC reactions.

E nergy production based on fossil fuels has caused many environmental issues, such as global warming and the release of pollutants.This has prompted many scientists to become interested in developing clean and sustainable new energy sources.Among them, solar energy is recognized as having the potential to replace conventional fossil fuels, because it is abundant enough to meet the global energy demand. 1 Recently, very promising results on halide perovskites in various solar energy conversion technologies have been reported exponentially.In particular, the use of lead halide perovskites in solar cells has resulted in very high photovoltaic conversion efficiencies of over 26% 2 due to their high light-harvesting efficiency, excellent charge transport, defect tolerance, and easily tunable band gap.−9 PEC systems, compared to photocatalytic systems, have the advantages of facilitating charge separation and collection as well as catalyst recycling by using photoelectrodes and an external bias. 7In the PEC cells, the photogenerated electrons and holes at the photoelectrode are separated and move along their respective paths, leading to various oxidation or reduction reactions, such as water splitting, 5,6,10−13 degradation of pollutants, 14 functionalization of C−H bonds, 15 and CO 2 reduction, 16 at the electrode−electrolyte interface.To design an effective PEC system, it is crucial to first find a light absorber with the following conditions: (1) it should be able to absorb light in the visible range, with a band gap energy of 1.5−2.5 eV, (2) the conduction band (CB) minimum and valence band (VB) maximum levels should provide the thermodynamic driving force to allow the desired reaction, and (3) it should possess sufficient stability in the electrolyte solution to ensure its long-term performance and durability.Keeping these conditions in mind, cesium lead bromide, CsPbBr 3 , halide perovskite has recently emerged as a promising candidate for PEC reactions due to its excellent photoelectrical properties and promising robustness. 17Some of us already demonstrated that colloidal CsPbBr 3 nanocrystals (NCs) synthesized by a hot-injection method exhibit a favorable energy band gap for hole injection to 2mercaptobenzothiazole (MBT) pollutant. 18−20 However, it is considered a potential human carcinogen and is known to be difficult to biodegrade. 18,21To investigate the PEC behavior of CsPbBr 3 NCs in the MBT oxidation reaction, a CsPbBr 3 NC film based photoanode was made by spin-coating of a CsPbBr 3 NC solution onto a very thin and compact titanium dioxide (TiO 2 ) film, and it showed a photocurrent of about 120 μA/cm 2 just as a proof-of-concept demonstration, 18 where the photocurrent was partially limited by the flat configuration of the substrate.In this study, in contrast to conventional perovskite bulk or NC films, the nanoscale CsPbBr 3 -sensitized photoelectrode was prepared by a two-step direct spin-coating of NC precursors onto a mesoporous-TiO 2 (meso-TiO 2 ) film, looking for an increase of effective surface by the use of a mesoporous electrode and an effective decoration of it by the direct growth of a halide perovskite on the TiO 2 surface.This is a very simple strategy, used in sensitized systems, to form effective nanoscale CsPbBr 3 photosensitizers directly on the surface of meso-TiO 2 film, and it allowed us to boost the photocurrent by more than 1 order of magnitude, achieving a high photocurrent of 2.34 ± 0.08 mA/cm 2 for the MBT oxidation by decoupling the actions of light absorption and charge transport.In the current structure of the CsPbBr 3 -sensitized electrode, the mesoporous metal oxide has the effect of increasing the surface area and improving charge separations and transport through electron injection into meso-TiO 2 , to enhance photocurrents.−29 Thus, it looks very promising and timely to test the in situ deposited nanoscale CsPbBr 3 as a photosensitizer for the target reaction in the PEC cells.As a further step to minimize the defect sites and improve the stability of the as-prepared CsPbBr 3 , a very thin layer of aluminum oxide (Al 2 O 3 ) has been deposited over the surface of the meso-TiO 2 /nano-CsPbBr 3 photoanode by an atomic layer deposition (ALD) technique and its passivation effect in the PEC system for oxidation of MBT was investigated and compared with those in previous studies. 7,14,30,31n addition, for an external bias free unassisted PEC reaction of MBT oxidation, a novel photovoltaic (PV)/PEC tandem device was devised by combining methylammonium lead iodide, MAPbI 3 , a perovskite-based mini-module, and the meso-TiO 2 /nano-CsPbBr 3 PEC system.In the well-known PV/PEC tandem system for water splitting, the photoinduced carriers (electrons or holes) are driven to the counter electrode through the PV cell to drive one of the half-reactions of water splitting, while the other carrier contributes to the complementary half-reaction. 32−34 In the case of our tandem device for the photodegradation of MBT, electrons and holes are photogenerated at the meso-TiO 2 /nano-CsPbBr 3 PEC electrode, and then the electrons are driven to the counter electrode through the mini-module, while the holes are responsible for the MBT oxidation.The mini-module fabricated by interconnecting three solar cells in series was used to supply enough voltage (>1.5 V) to lead to the desired MBT oxidation, and its role was confirmed by electrochemical measurements.The nanoscale CsPbBr 3 -sensitized photoanode was fabricated as described in Figure 1a.0.3 M lead(II) bromide (PbBr 2 ) with the same amount of 4-tert-butylpyridine (tBP) in N,N-dimethylformamide (DMF) and 0.03 M cesium bromide (CsBr) in methanol were used as precursor solutions, and they were sequentially spin-coated onto a meso-TiO 2 film with a thickness of approximately 1.6 μm.A relatively low concentration of PbBr 2 (0.3 M) was used compared to the high concentration (>1.0 M) required for CsPbBr 3 bulk films 29 (see the Supporting Information for further experimental details).The utilization of such a low concentration of precursors over a meso-TiO 2 film could enable the direct formation of a nanoscale CsPbBr 3 perovskite on the surface of the TiO 2 particulate film, a method that has been successfully proved in our recent works on nanoscale MAPbI x Br 3−x -or CsPbI x Br 3−x -sensitized solar cells. 28,35In this study, tBP was added to the PbBr 2 solution to enhance the crystalline quality of the CsPbBr 3 nanocrystals for optimal performances.The role of tBP is well-known on perovskites made by a 2-step deposition process, and it promotes a reaction with the second precursor by weakening the crystallinity of the first-deposited lead halide. 36,37The meso-TiO 2 /nano-CsPbBr 3 PEC electrode was completed by heating at 280 °C for crystallization of CsPbBr 3 , and then an expected yellow electrode was obtained, as shown in Figure 1a.
To investigate the PEC performances for MBT oxidation, the photocurrents induced by the degradation of MBT were checked under various conditions using a configuration of three electrodes (Figure 1b); a meso-TiO 2 /nano-CsPbBr 3 photoanode, a nonaqueous Ag/Ag + electrode, and a platinum (Pt) wire were used as the working, reference, and counter electrodes, respectively.MBT is known to be oxidized between 0.47 and 1 V vs NHE depending on the experimental conditions. 38As shown in Figure 1b, CsPbBr 3 perovskites possess a valence band position suitable for injecting holes into MBT, allowing the easy degradation of MBT under illumination conditions.In our previous work, the band positions of CsPbBr 3 nanocrystals were determined by cyclic voltammetry and the total degradation of MBT was confirmed from the clear disappearance of the initial m/z (167.9937)characteristic peak provided by electrospray mass spectroscopy (ESI-MS) analysis. 18Indeed, upon comparison of the photocurrents of bare meso-TiO 2 and meso-TiO 2 /nano-CsPbBr 3 electrodes in an electrolyte of 0.1 M tetrabutylammonium hexafluorophosphate (Bu 4 NPF 6 ) in dichloromethane (DCM) containing 0.05 M MBT (Figure 1c), almost no photocurrent was observed in either electrode in the dark.However, under AM 1.5G (100 mW/cm 2 ) illumination, the bare meso-TiO 2 electrode exhibited a very low photocurrent of 0.25 ± 0.06 mA/cm 2 , while the meso-TiO 2 /nano-CsPbBr 3 photoelectrode showed a significantly higher photocurrent of 2.34 ± 0.08 mA/ cm 2 at 0.8 V (V vs Ag/Ag + ).All photocurrents were compared at 0.8 V (V vs Ag/Ag + ), where the highest photocurrent value was observed while the influence of dark current was minimized.This result demonstrates that the photocurrents are primarily coming from visible-light-absorbing CsPbBr 3 , not the meso-TiO 2 film, though the latter is UV-light absorbing and could contribute to MBT oxidation to some degree.Figure 1d represents the change in photocurrent with the concentration of MBT using a meso-TiO 2 /nano-CsPbBr 3 electrode.The photocurrent also increased when the concentration of MBT increased from 0.00 to 0.05 M but decreased at 0.07 M.This indicates that an increase of reactant concentration in the electrolyte leads to enhanced kinetics for photocurrent generation.However, beyond a certain threshold, an excessive reactant concentration actually blocks the mesoporous structure and slows down the reaction kinetics.A plausible hypothesis, considering the results of measurements with and without stirring shown below (see Figure 3d for details), is that the presence of a diffusion-controlled approach to the active site at the interface inside the mesostructure dominates until 0.05 M, but after that point, oxidized products come out too slowly from the mesostructure and accumulate for interference, which leads to a decrease of oxidation current.The averaged photocurrent values and their values corresponding to each MBT concentration were obtained using three different photoanodes to check the reproducibility, and they are summarized in Table S1.−41 Typically, when light enters through the glass side (back side illumination), even though the glass absorbs a certain portion of light, electron transfer to the FTO glass is facilitated, resulting in higher photocurrent values compared to the situation when light enters through the light absorber (front side illumination), indicating charge transport limitations. 41However, in the case of this meso-TiO 2 /nano-CsPbBr 3 PEC electrode, it shows nearly similar photocurrent values regardless of the direction of the incident light (see Figure S1 and Table S2), pointing out the absence of electron transport limitation through the meso-TiO 2 film.This fact allows one to take full advantage of the increased contact area with the electrolyte, which facilitates efficient hole transfer to MBT from in situ deposited nanoscale CsPbBr 3 photosensitizers without any long molecular ligands.This, in turn, contributes to the generation of overall high photocurrents exceeding 2.0 mA/cm 2 .
To confirm the influence of the thickness of the meso-TiO 2 layer on the PEC performance, meso-TiO 2 films with different thicknesses were produced by diluting a commercially available TiO 2 paste in different volumes of ethanol.Generally, it is expected that a thicker meso-TiO 2 film allows the immobilization of a higher fraction of light sensitizers, leading to higher photocurrents.However, when the electrode thickness exceeds the electron diffusion length, the photocurrent decreases. 42In this study, it was confirmed that a meso-TiO 2 thickness of about 1.6 μm was optimal (see Figure S2).Therefore, a meso-TiO 2 film with this thickness was used in all of the measurements shown hereafter.As shown in Figure 2, the morphology of the optimized meso-TiO 2 /nano-CsPbBr 3 film looks like that of the bare meso-TiO 2 film in the scanning electron microscopy (SEM) images of the surface and crosssection.However, transmission electron microscopy (TEM) measurements reveal the presence of a few nanometer-sized CsPbBr 3 photosensitizers on the surface of the larger TiO 2 particles.This observation is consistent with the results of our previous work conducted using a similar fabrication method. 35n order to further increase the performance of the system, an ultrathin Al 2 O 3 layer has been utilized in many solar energy conversion devices to protect the light-absorber material and minimize charge recombination at the interfaces.It has primarily been applied using the well-known atomic layer deposition (ALD) method. 7,30,31This method allows the precise control of the thickness of the Al 2 O 3 layer deposited on all of the effective surface of the target material.It has been widely used not only in bulk film type electrodes 7,31 but also in mesoporous electrode structures. 30To check the passivation effect of Al 2 O 3 on the PEC system for MBT oxidation, a very thin Al 2 O 3 layer was applied to the surface of the meso-TiO 2 / nano-CsPbBr 3 film by the ALD process as an increment of ∼0.9 Å thickness per cycle, and the photocurrent was measured as a function of the Al 2 O 3 layer thickness (Figure 3a).The photocurrent increased with the thickness of the Al 2 O 3 layer, and a remarkably high photocurrent of 3.02 ± 0.03 mA/cm 2 was recorded for samples prepared with 3 cycles (see Table S3).However, higher thicknesses (4 ALD cycles) led to a decrease of the photocurrent (2.71 ± 0.06 mA/cm 2 ) due to the higher transfer resistance induced by the Al 2 O 3 coating layer and the catalytic deactivation of the passivated surface, eventually inhibiting hole transfer to MBT. Figure 3b,c shows the absorbance spectra and X-ray diffraction (XRD) patterns, respectively, before and after 3 cycles of Al 2 O 3 ALD.Both results match the previously reported optical properties and XRD main peaks of CsPbBr 3 , 31,43,44 demonstrating that the deposition process shown in Figure 1a is suitable for the formation of CsPbBr 3 .In addition, the lack of any significant changes in the absorbance spectra and XRD peaks corresponding to CsPbBr 3 after 3 cycles of Al 2 O 3 ALD confirms that the deposition process of Al 2 O 3 did not affect the crystalline and optical properties of CsPbBr 3 .However, it is difficult to identify peaks related to Al 2 O 3 in the XRD pattern (Figure 3c) due to its ultrathin character (about ∼0.27 nm) and/or its amorphous character.To demonstrate the presence of the Al 2 O 3 layer, X-ray photoelectron spectroscopy (XPS) analysis was performed, and the results clearly showed peaks related to  3) initially showing a higher photocurrent compared to CsPbBr 3 /Al 2 O 3 (0).However, both exhibited similar photocurrents over time (Figure S4).This behavior became more evident when the measurement time was extended to 10 min (Figure 3d).In the absence of stirring, the photocurrent of CsPbBr 3 /Al 2 O 3 (3) started at a higher value than that of CsPbBr 3 /Al 2 O 3 (0) but decreased more rapidly.However, with stirring, the photocurrent of CsPbBr 3 /Al 2 O 3 (3) showed higher stability compared to that of CsPbBr 3 /Al 2 O 3 (0).To understand this phenomenon, the evolution of the photocurrent with the MBT concentration was measured without stirring.Figure S5a shows that the decrease in photocurrent is slower at an MBT concentration of 0.08 M compared to 0.05 M.This is because the consumption rate is faster than the supply rate of the reactant, MBT, at the PEC electrode surface.Therefore, CsPbBr 3 /Al 2 O 3 (3), which shows a higher photocurrent, can decompose MBT more rapidly than CsPbBr 3 /Al 2 O 3 (0), leading to a faster decrease in photocurrent.However, when we decrease the mass transport limitation, enhancing the supply of MBT to the electrode surface by stirring, the passivation effect of Al 2 O 3 enables more stable MBT oxidation.This is further supported by the lower intensity of the absorption peak at around 320 nm corresponding to MBT in the absorbance measurement of the electrolyte after a stability test of 30 min using CsPbBr 3 / Al 2 O 3 (3) with stirring (Figure S5b).Also, from the decrease of the MBT absorption peak after PEC oxidation, the removal efficiency of MBT could be estimated to be about 21% from the initial concentration of 0.05 M by using Beer's law.This result looks promising because we have used a relatively high concentration (0.05 M) of MBT when compared to a common value of a few or fewer micromoles employed in most degradation experiments by photocatalysts.Moreover, a larger cell volume was used here rather than the typical small volume of a cuvette, and most parameters were not optimized because we focused on the degree of maintaining PEC photocurrents in a relatively high concentration of pollutants by a newly designed electrode with perovskite sensitizers, not on the pollutant removal efficiency.Without stirring, the calculated removal efficiency was about 14% and was not as effective as in the case of stirring.But, when the measurement time was extended to 30 min, the photocurrent gradually decreased even with stirring (Figure S5c).Furthermore, when the electrolyte contained 0.08 M MBT, the effect of stirring was not observed since the concentration of MBT on the electrode surface was already sufficient, avoiding a mass diffusion limitation (Figure S5d).To check what happened to two-step spin-coated nanoscale CsPbBr 3 after the PEC operations, XRD patterns and photos of FTO/TiO 2 /CsPbBr 3 /Al 2 O 3 photoanode after a 30 min stability test were obtained, as shown in Figure S6.From these results, we could estimate that CsPbBr 3 nanoparticles deposited on the electrode were partially detached and some remaining parts were changed to CsPb 2 Br 5 during and parallel illumination modes (modes S and P), respectively.(b, e) J−V curves of a mini-module and CsPbBr 3 -PEC cell to predict operating points in modes S and P, respectively.The J−V curves of the PEC cells were obtained in a two-electrode configuration, and the CsPbBr 3 film was used as a filter for the mini-module to simulate mode S. (c, f) CA of the tandem device in modes S and P, respectively, without an external bias under chopped illumination.The light intensity for J−V and CA measurements was AM 1.5G (100 mW/cm 2 ), and the illuminated areas of the mini-module were 1.0 and 2.4 cm 2 in modes S and P, respectively.
the stability test for 30 min.Thus, it seems to be reasonable to conclude that the very thin layer of Al 2 O 3 applied could not fully protect CsPbBr 3 , but some uncovered parts degraded gradually during MBT oxidation in a less polar solvent, DCM.At the current stage, though this durability in PEC cell looks encouraging for further enhancement, it will be necessary to do more careful checks to extend the working time to a few hours in the next step to higher stability.
To drive the unassisted photodegradation of MBT in a PEC system without an external bias, a photovoltaic mini-module was used as the voltage supply source.The mini-module was fabricated by interconnecting three solar cells in series based on MAPbI 3 perovskite, following the architecture shown in Figure S7.−49 The characteristics of this mini-module are compatible to those of our PEC system, where a high voltage of at least 1.5 V is required and the photocurrent of the mini-module does not limit the MBT degradation.The combination of the PEC system and the mini-module can have two possible different configurations.The first configuration is a serial tandem one where a narrow-band-gap MAPbI 3 mini-module is placed behind the wider-band-gap CsPbBr 3 -PEC electrode, allowing light to pass through CsPbBr 3 and enter MAPbI 3 (Figure 4a), both systems consequently sharing the 1 sun incident light.Consequently, most of the short-wavelength radiation is absorbed by the electrode while the long-wavelength radiation is mostly absorbed by the mini-module.In the second configuration, the PEC electrode and the mini-module are placed parallel to each other for incident light harvesting (Figure 4d).Consequently, in this configuration the electrode and mini-module both are illuminated with the full 1 sun spectra.The former can be called the tandem serial illumination mode (mode S) and is generally used in various PV/PEC tandem devices 32−34 due to its efficient light utilization and minimal space constraints.The latter looks similar to the parallel illumination mode (mode P) of a PEC tandem cell. 32,50When the mini-module and the CsPbBr 3 -PEC cell are arranged in mode P, each device is able to utilize its maximum efficiency, since both devices are exposed to the same light irradiation.However, a higher effective area for light incidence is required.To verify the operation of the tandem device in mode S, a mask with the same active area as the CsPbBr 3 -PEC cell was used on the mini-module, ensuring that the light passes through both devices with the same area.To achieve high voltage in the mini-module, it is necessary to illuminate all three active layers connected in series.Therefore, the mask was placed so that all three active layers fit within the mask area coinciding with the area of the CsPbBr 3 -sensitized electrode.To predict the operating point of the mini-module/ CsPbBr 3 PEC cell tandem device arranged in mode S, the current density−potential (J−V) curve of the CsPbBr 3 PEC cell was obtained by using a two-electrode configuration (Figure 4b).For the J−V curve of the mini-module, the CsPbBr 3 film was placed in front of the mini-module like a filter to simulate mode S (the blue line in Figure 4b).Figure 4c shows the current density (∼0.8 mA/cm 2 ) obtained from the tandem device in mode S, showing a good match with the operating point predicted from Figure 4b.Samples with and without alumina coating, with a higher performance for the former, were analyzed (Figure 4b,c).To check the perform-ance of the tandem device in mode P, a different MAPbI 3 perovskite mini-module was used without a mask, and a current density of over 1.0 mA/cm 2 was obtained (Figure 4e,f).Here, we focused more on the optimization process of the CsPbBr 3 photoanode electrode for more efficient MBT oxidation.Additional efforts to optimize mini-modules are beyond the scope of this work, as even mini-modules with a low photocurrent are sufficient to avoid current limitation and produce the proof of concept presented here in terms of reaching the necessary photovoltage.Therefore, it is expected that photocurrent enhancement can be achieved through a further optimization process of the integrated PV/PEC architecture.
In summary, a nanoscale CsPbBr 3 perovskite photosensitizer can be formed in situ on the surface of meso-TiO 2 film by a two-step spin-coating method using a low concentration of precursor solutions (<0.3 M).The meso-TiO 2 /nano-CsPbBr 3 photoelectrode architecture is advantageous for the MBT oxidation reaction, since a higher density of catalytically active sites is available by increasing the contact area with the electrolyte and facilitating the electron transfer from CsPbBr 3 to the meso-TiO 2 film.Indeed, the PEC behavior of the meso-TiO 2 /nano-CsPbBr 3 photoanode measured under various conditions confirmed that the photocurrent was obtained from the MBT oxidation by photoexcited CsPbBr 3 .Through the optimization process, a high photocurrent of 2.34 ± 0.08 mA/cm 2 was obtained at 0.8 V (V vs Ag/Ag + ) under standard AM 1.5G (100 mW/cm 2 ) illumination.In addition, a thin layer of Al 2 O 3 for the passivation effect was introduced on the surface of the meso-TiO 2 /nano-CsPbBr 3 photoanode cell by the ALD method, and the highest photocurrent of 3.02 ± 0.03 mA/cm 2 was obtained with 3 cycles of ALD.Consequently, the addition of perovskite boosts in most of the cases 1 or 2 magnitudes of the reaction current compared to previously reported PEC-degradation systems based on oxide electrodes for various organic pollutants, as shown in Table S4.This result looks promising for further enhancements by optimizing nanoscale interfaces, though the typical bulk-film-derived PEC currents are higher at the current stage as summarized in Table S5.Interestingly, the Al 2 O 3 -deposited electrode showed improved stability when it was stirred during the CA measurement.This is because stirring facilitated the supply of MBT to the surface of the meso-structured electrode, leading to a continued reaction.Furthermore, the meso-TiO 2 / nano-CsPbBr 3 PEC cell and MAPbI 3 -based mini-module were combined for an unassisted MBT oxidation reaction driven directly by sunlight.The performance of this PV/PEC tandem device was evaluated in two different configurations, tandem serial and parallel illumination modes, yielding photocurrents of about 0.8 and 1.0 mA/cm 2 , respectively.The combination of such mini-modules and photosensitizer-based photoelectrodes opens up promising perspectives for the exploitation of halide perovskite based systems for several PEC reactions.
Experimental details of the photoelectrode preparation, Al 2 O 3 ALD process, mini-module fabrication, characterizations, additional results and information including statistical photocurrents of PEC cells, LSVs for

Figure 1 .
Figure 1.(a) Scheme of the two-step deposition process showing the direct formation of CsPbBr 3 nanocrystals on the meso-TiO 2 film with an active area of 1.0 cm 2 and (b) its application to the PEC cell for MBT oxidation.(c) Linear sweep voltammograms (LSVs) of bare meso-TiO 2 and meso-TiO 2 /nano-CsPbBr 3 photoanodes in 0.1 M tetrabutylammonium hexafluorophosphate (Bu 4 NPF 6 ) in dichloromethane (DCM) with 0.05 M MBT.(d) LSVs of the meso-TiO 2 /nano-CsPbBr 3 photoanode depending on the concentration of MBT in the electrolyte.All LSVs were obtained in a three-electrode configuration under AM 1.5G (100 mW/cm 2 ) illumination.

Figure 4 .
Figure 4. (a, d) Schematic diagrams of MAPbI 3 -based mini-module/CsPbBr3 PEC cell tandem devices for MBT oxidation with tandem serial and parallel illumination modes (modes S and P), respectively.(b, e) J−V curves of a mini-module and CsPbBr 3 -PEC cell to predict operating points in modes S and P, respectively.The J−V curves of the PEC cells were obtained in a two-electrode configuration, and the CsPbBr 3 film was used as a filter for the mini-module to simulate mode S. (c, f) CA of the tandem device in modes S and P, respectively, without an external bias under chopped illumination.The light intensity for J−V and CA measurements was AM 1.5G (100 mW/cm 2 ), and the illuminated areas of the mini-module were 1.0 and 2.4 cm 2 in modes S and P, respectively.