Direct synthesis of two-dimensional MoS2 on p-type Si and application to solar hydrogen production

Transition metal dichalcogenides (TMDs) are promising two-dimensional (2D) materials, and MoS2 has been specifically utilized in electronic devices and integrated circuits. However, the direct synthesis of MoS2 on traditional semiconductors, such as silicon, remains challenging due to the hydrophobic surface of nonoxide wafers (e.g., Si, GaAs, and InP). Herein, a novel, facile, reliable, and one-step method for the direct synthesis of single-crystal MoS2 on a p-Si wafer via hybrid thermolysis is proposed. To demonstrate the applicability of the proposed method, a MoS2/p-Si heterojunction was fabricated and used for solar-driven hydrogen production. The as-fabricated n-MoS2/p-Si heterojunction exhibited a benchmark current density of −13.5 ± 1 mA/cm2 at 0 V and an onset potential of +0.02 V. This method reliably and efficiently produced high-quality MoS2 crystals on a wafer scale and is sufficiently simple to overcome the challenges associated with previous approaches. The method developed herein represents a tremendous advancement in the fabrication of 2D electronic devices. Solar panels that catalyze the splitting of water into hydrogen fuel and oxygen can now be fabricated using a simple deposition process. Interfaces between two-dimensional molybdenum disulfide (2D-MoS2) and electron-poor silicon can split water using light, but only when the naturally random crystallization patterns of 2D-MoS2 are inhibited. Researchers led by Ho Won Jang from Seoul National University and Soo Young Kim at Chung-Ang University, Seoul, have improved uniformity in this ultrathin material by initially coating silicon wafers with a molybdenum oxide precursor that adheres in smooth layers. By depositing a second, sulfur-rich coating and then heating the sample, the team produced uniform 2D-MoS2/silicon junctions down to 10-nanometer scales. The transparent device maintained a water-splitting photocurrent for over forty hours without degradation, thanks to the corrosion-resistant nature of high-quality 2D-MoS2 crystals. A direct synthesis method for high-quality MoS2 thin films on p-Si wafer is reported herein. To increase the hydrophilicity of the p-Si wafer, MoO3 was deposited before spin-coating. The (NH4)2MoS4 precursor was easily coated onto the MoO3/p-Si and was converted to a MoS2/p-Si heterojunction via thermolysis. This method has the potential to be used in 2D electronic device fabrication in the future.

Recently, TMDs such as MoS 2 , WS 2 , MoSe 2 , and WSe 2 have been introduced for use in different applications [10][11][12][13] . Among these TMDs, the n-type semiconductor MoS 2 has been most extensively studied owing to its remarkably tunable optoelectronic and photochemical properties with a direct band gap of 1.8 eV, making it a promising lowdimensional material for future optoelectronic devices 3,14 . For over a decade, a variety of synthesis methods for MoS 2 have been proposed using chemical vapor deposition (CVD), hydrothermal, sputtering, epitaxial growth, and thermolysis methods [15][16][17][18][19] . However, the fabrication of large-scale, uniform, and high-quality crystalline MoS 2 remains challenging.
The elementary building blocks of p-n junctions are vital for semiconductor electronic devices such as integrated circuits, photodetectors, solar cells, LEDs, diodes, and transistors [20][21][22][23][24] . Recently, the integration of lowdimensional TMD materials with single-crystal traditional semiconductors (e.g., Si, GaAs, and InP) has significantly impacted the development of functional electronic devices, including field effect transistors (FETs), diodes, photodetectors, and photoelectrochemical cells 21,[25][26][27] . However, poor-quality TMD-based heterojunctions and difficulty in large-scale fabrication restrict commercial applications. For example, so-called native oxides form easily on the Si wafer surface, resulting in poor contact with MoS 2 and deteriorated device performance. Therefore, the large-scale synthesis of superpristine MoS 2 on Si wafers has yet to be achieved.
Generally, two procedures are used for the fabrication of p-n heterojunctions in 2D TMD materials and Si: (1) synthesis of TMDs on SiO 2 and subsequent transfer to arbitrary substrates such as Si, GaAs, and InP wafers and (2) direct synthesis of TMDs on an arbitrary substrate 25,27,28 . To the best of our knowledge, extensive research on the transfer process has been performed [29][30][31] . For example, Kwon et al. fabricated a p-n heterojunction using n-MoS 2 and p-Si for use in photoelectrochemical hydrogen production 27 . The fabrication process involved wet transferring, where MoS 2 was initially grown on SiO 2 / Si and subsequently transferred to p-Si using a supporting layer and removal process. Despite the efficient performance obtained, the fabrication of p-n heterojunctions is not easily reproducible because the transfer process is challenging and time-consuming for n-MoS 2 /p-Si heterojunctions. On the other hand, the only method for the direct synthesis of TMDs on Si is the sputtering method. Hao et al. fabricated MoS 2 /Si p-n junctions using the DC magnetron technique for use in photovoltaic cells 25 . However, the sputtering method for the direct synthesis of MoS 2 on p-Si results in poor MoS 2 crystallinity, and the formation of the MoS 2 film on p-Si is not easily controlled. Therefore, significant demand exists for the development of a reliable method for 2D p-n junction fabrication. The proposed method should be a waferscale, simple, fast (no transfer step), reproducible method that produces highly crystalline TMD films.
Herein, we report a novel method for the direct synthesis of 2D layered MoS 2 on a p-Si substrate, representing a tremendous advancement in the fabrication of 2D electronic devices, overcoming the challenges of previously reported techniques 27,32,33 . We demonstrate an approach for the direct synthesis of wafer-scale MoS 2 on p-Si using thermolysis to achieve perfect crystals. This method can be more broadly applied to other TMDs and single-crystal wafers. In addition, to show the applicability of the developed method, the as-fabricated n-MoS 2 /p-Si heterojunction was used for photoelectrochemical hydrogen production. Because of the direct formation of superpristine MoS 2 on p-Si and the high-quality p-n heterojunctions formed, the as-prepared photocathode exhibited efficient performance with a high current density of −13.5 ± 1 mA/cm 2 at 0 V and an onset potential of +0.02 V. The direct fabrication of p-n heterojunctions prepared using the proposed method offers a facile, reliable, time-saving fabrication process representing a remarkable development for the production of highquality semiconductor electronic devices.

Direct synthesis of MoS 2 on p-Si
Thermolysis of ammonium tetrathiomolybdate ((NH 4 ) 2 MoS 2 )) was used to form the MoS 2 thin film. Preparation of large-scale and high-quality MoS 2 by thermolysis is an efficient and facile approach that is well documented in the literature 27 To date, the decomposition of the (NH 4 ) 2 MoS 2 layer at high temperatures to form MoS 2 thin films has mostly involved the use of a SiO 2 /Si substrate. In this study, the (NH 4 ) 2 MoS 4 precursor was coated on a SiO 2 /Si substrate and subsequently annealed in the presence of sulfur to form MoS 2 /SiO 2 /Si. Coating of the (NH 4 ) 2 MoS 2 precursor on a substrate with a hydrophobic surface, such as p-Si, is impossible so we used a hybrid method to directly synthesize MoS 2 on the p-Si substrate. As shown schematically in Fig. 1a, MoO 3 with different thicknesses (5, 10, 15, and 20 nm) was deposited onto the p-Si substrate via evaporation to increase the hydrophilicity of the Si substrate. The (NH 4 ) 2 MoS 2 solution was then coated onto MoO 3 /p-Si via spin coating. Finally, the (NH 4 ) 2 MoS 4 / MoO 3 /p-Si was converted to MoS x /p-Si at 500°C under H 2 and N 2 gas flow via thermolysis (Fig. 1b). Finally, the temperature was raised to 900°C to transform the MoS x layer to a MoS 2 thin film and fabricate the MoS 2 /p-Si heterojunctions. It is clear from Fig. 1c that the contact angle of (NH 4 ) 2 MoS 4 with p-Si is quite high (55.5°), suggesting a poorly hydrophilic surface and the impossibility of coating the (NH 4 ) 2 MoS 4 solution on p-Si (Fig. 1c, left). On the other hand, the p-Si surface could be converted to a super-hydrophilic surface after MoO 3 deposition, allowing the (NH 4 ) 2 MoS 4 precursor to be easily coated on the substrate (Fig. 1c, right). To show the successful coating of the (NH 4 ) 2 MoS 4 precursor on p-Si, a video was recorded before and after MoO 3 deposition (Video S1, S2). Figure 1d shows the wafers of the directly fabricated MoS 2 /p-Si heterojunction, highlighting the efficiency and ease of the proposed method. Using this method, the large-scale fabrication of 2D-based semiconductor devices and integrated circuits could be achieved.

Results and discussion
The uniformity of 2D-TMD materials is an important parameter that directly affects device performance. Methods such as sputtering, epitaxial growth, and thermolysis have been proposed for the fabrication of large-scale and uniform MoS 2 . Dumcenco et al. demonstrated the growth of monolayer MoS 2 via epitaxial growth 36 . However, randomly triangular-shaped MoS 2 with uncontrollable orientation limited the reliability of their process and prevented commercialization.
Atomic force microscopy (AFM) was used to examine the topological properties and thickness of the MoS 2 thin films. The inset of Fig. 2a shows the morphology of the asobtained thickness (11 nm) of the 10 nm MoO 3 deposited on p-Si, which is attributed to the incorporation of (NH 4 ) 2 MoS 4 and MoO 3 layers to form the MoS 2 thin film. The roughness of MoS 2 was 2.4 Å for the 10 nm MoO 3 deposited on p-Si (Fig. 2a), which is one of the smallest reported values to the best of our knowledge 27,37 . These results confirm that the MoS 2 thin film on p-Si obtained using this method is superpristine and ultraclean, resulting in high-quality p-n heterojunctions and enhanced device performance. Furthermore, AFM was performed to evaluate the roughness and thickness under various conditions ( Fig. S1) and confirmed the controllability of the developed method. The results demonstrate that the MoS 2 thickness can be easily controlled by the deposition of MoO 3 , obtaining MoS 2 thicknesses of approximately 8.5, 11, 16.8, and 25.2 nm for the 5, 10, 15, and 20 nm MoO 3 depositions on p-Si, respectively (Fig. 2b). In addition, the lowest roughness of 2.4 Å was obtained for the 11 nm MoS 2 film, which would likely provide the best device performance. From the AFM results, the film roughness can be optimized by adjusting the MoO 3 thickness on p-Si. Field-emission scanning electron microscopy (FE-SEM) was used to examine the morphology of the MoS 2 thin film on the p-Si substrate (Fig.  2c). From the FE-SEM images of the as-synthesized MoS 2 films, the films were uniform and continuous over the entire surface of the p-Si substrate. Moreover, Fig. S2a-d shows the as-synthesized MoS 2 on p-Si with different  Figure 2d shows the Raman spectra of the MoS 2 thin films with different thicknesses. Two vibrational modes (A 1g and E 1 2g ) were observed and attributed to the out-of-plane and in-plane stretching, respectively 38 . The distance between A 1g and E 1 2g increased with an increase in the thickness of the MoS 2 film, which was attributed to the tightening of the A 1g mode and moderation of the E 1 2g mode 27 . To confirm the formation of the MoS 2 thin film and the impossibility of MoS 2 /MoO 3 , Raman analysis was utilized for the assynthesized MoS 2 thin film and as-deposited MoO 3 layer (see Fig. S3). Two dominant peaks (A 1g and E 1 2g ) are located at 383 and 404 cm −1 in the as-prepared MoS 2 , while two dominant peaks are located at 820 and 997 cm −1 in MoO 3 . Therefore, the possibility of the existence of MoO 3 in the as-direct synthesized MoS 2 can be excluded.
X-ray diffraction (XRD) measurements were used to confirm the ultrahigh purity of the 2H-MoS 2 single crystal on p-Si (Fig. 3a). The corresponding peaks were sharp and located at 14.5 and 63.8°, arising from the (002) and (008) planes, respectively. Patil et al. evaluated the crystal structure of bulk MoS 2 and thin film MoS 2 15 . The bulk MoS 2 showed various peaks, whereas the as-prepared MoS 2 showed only a (002) peak, and the other peaks were diminished, suggesting a highly pristine MoS 2 singlecrystal layer. High-resolution transmission electron microscopy (HR-TEM) was used to confirm the quality of the MoS 2 crystal, and the result is shown in the inset of Fig. 3a. The 2H-phase of MoS 2 was observed mainly with [002] and [008] orientations (inset of Fig. 3a), which was completely confirmed by the XRD results. From the XRD data, three sharp domain peaks were observed and were ascribed to the highly crystalline MoS 2 film on p-Si. X-ray photoelectron spectroscopy (XPS) was used to determine the elemental composition, and the atomic ratios of the samples are shown in Fig. 3b. The XPS spectra are shown in Fig. 3b, confirming the presence of Mo and S and successful formation of the MoS 2 film. In addition, the ratio of Mo to S was 31:69, which approximately agrees with the MoS 2 structure (inset of Fig. 3b). The highresolution XPS peaks of S 2p and Mo 3d are shown in Fig. 3c. To study the effects of the (NH 4 ) 2 MoS 4 precursor on the formation of single-crystal MoS 2 on p-Si, XPS was performed on the MoS 2 prepared with and without the (NH 4 ) 2 MoS 4 precursor. Because of the very weak bonds between the deposited MoO 3 layer and the p-Si substrate, no peaks originating from Mo or S were observed without the precursor (see Fig. 3c, red and green curves). On the other hand, the Mo 3d and S 2p peaks were strongly apparent after deposition of the (NH 4 ) 2 MoS 4 precursor on MoO 3 /p-Si. The (NH 4 ) 2 MoS 4 precursor assists in the formation of MoS 2 and likely acts as a protective layer to prevent evaporation of the MoO 3 layer during thermolysis. In previous studies, this phenomenon has been attributed to the higher O-S bonding energy compared to that of the Si-S bond 27,39 . However, we believe that the poor hydrophilicity of the Si wafer is the main reason underlying the impossibility of direct growth of MoS 2 crystals on nonoxide wafers. The UV-vis normalized absorbance of the different thicknesses of the astransferred MoS 2 thin films on glass are shown in Fig. 3d. The optical band gaps were evaluated via prolonging the line of the initial absorbance peak and increased from 1.39 to 1.65 eV as the layer thickness decreased from 25.2 to 8.5 nm. Additionally, the astransferred MoS 2 layers with different thicknesses on glass are shown in the inset of Fig. 3d. The films uniformly covered the glass substrate, and the film color changed from transparent to dark brown with an increase in Fig. 3 a XRD spectra and HR-TEM measurements (inset) of the as-fabricated MoS 2 (11 nm)/p-Si sample; b wide range XPS spectra of the as-fabricated MoS 2 (11 nm)/p-Si sample and the corresponding atomic ratio; c high-resolution Mo and S peaks for the MoS 2 /p-Si samples prepared with and without the (NH 4 ) 2 MoS 4 layer; and d absorbance spectra of the as-prepared MoS 2 thin films on glass with different thicknesses thickness from 8.5 to 25.2 nm. These results confirmed that the optical and electronic properties can be easily tuned via the proposed method, which is more reliable and reproducible than previously developed methods 29,31-33 .
The crystalline structure of the vertical cross-section of the MoS 2 film was examined by TEM (see Fig. 4). TEM with energy-dispersive X-ray spectroscopy (TEM-EDS) mapping of the as-direct synthesized MoS 2 thin film showed Mo and S constituting the MoS 2 crystalline structure were uniformly distributed throughout the film. However, there was some O due to air oxidation of the sample. Therefore, the results confirm the formation of MoS 2 /p-Si, and the possibility of MoS 2 /MoO 3 /p-Si is excluded.
To confirm the applicability of this method, a MoS 2 /p-Si photocathode was fabricated, and it is clear that the most difficult and time-consuming step (the transfer process) was removed to improve upon previously reported methods 27,29,31 . The as-fabricated photocathode was used as a working electrode in a three-electrode electrochemical cell filled with 0.5 M sulfuric acid as an electrolyte and was placed under simulated solar irradiation (100 mW/cm 2 ). Figure 5a shows the photocurrent onset potential (RHE) vs. current density of the sample under different conditions, confirming the successful fabrication of a MoS 2 /p-Si heterojunction that showed p-n junction behavior. The dark current is shown as the violet-color line. The cost-effective fabrication procedure of bare p-Si, as well as its suitable band gap and crystallinity, are particularly suited for solar hydrogen production 27,40 . However, to absorb H 2 onto the p-Si surface, a very large voltage must be applied. Therefore, the integration of a catalyst, such as 2D-TMDs, with p-Si must be performed to achieve efficient hydrogen production performance. MoS 2 (11 nm)/p-Si showed the highest performance with a current density benchmark of −13.5 ± 1 mA/cm 2 at 0 V and an onset potential of +0.02 V, which is significantly improved over that of bare p-Si (0.2 mA/ cm 2 at 0 V and an onset potential of −0.38 V). To investigate the mechanism and catalytic activity of the MoS 2 layers, current density-potential polarization curves were translated to the logarithmic value of the current density as a function of the overpotential (Fig. 5b). From the photoelectrochemical (PEC) measurement results, the MoS 2 (11 nm)/p-Si sample is expected to exhibit the highest catalytic activity. MoS 2 (11 nm)/p-Si exhibited the lowest Tafel slope of 65 mV/dec, which corresponds to the Volmer-Heyrovsky electrochemical mechanism 41 . These results show that the optimization of the MoS 2 layer thickness is essential for achieving the highest performance of the resulting devices. Since the proposed fabrication method is a controllable process, the resulting MoS 2 thin film can be easily optimized. Electrochemical impedance spectroscopy (EIS) is another method that can be used to evaluate photocatalytic activity. The small circle in the EIS Nyquist plot represents the figure of merit for the HER activity, as shown in Fig. 5c. From these EIS results, the smallest circle was obtained for the MoS 2 (11 nm)/p-Si sample, as expected. The fastest movement of electrons between the electrolyte and working electrode was obtained using the optimized MoS 2 (11 nm)/p-Si photocathode. The inset of Fig. 5c shows the Randles equivalent circuit including the active electrolyte resistance (R S ), charge-transfer resistance (R ct ), and capacitor. The value of R ct was determined to be 66.5 Ω for the optimized MoS 2 (11 nm)/p-Si photocathode, agreeing with the current density results. Because the MoS 2 thin film is relatively transparent, the incident solar light passed through the film and was absorbed by p-Si, generating hole-electron pairs and current induction via electrons moving to the MoS 2 thin film and electrolyte. The stability of the as-optimized MoS 2 (11 nm)/p-Si photocathode was assessed for 20 cycles to clarify the durability of the device for the photoelectrochemical HER (see Fig. 5d). We found a negligible current density shift after 20 cycles (ΔJ = ± 1 mA/cm 2 ), which confirmed the excellent stability of the as-fabricated MoS 2 (11 nm)/p-Si photocathode for solar hydrogen production. For the optimized MoS 2 /p-Si photocathode, the current density was stable after 40 h without notable degradation, indicating that the synthesized MoS 2 thin film acts as not only a catalyst for hydrogen production but also an excellent passivation layer that prevents the p-Si photocathode from experiencing severe photocorrosion (see Fig. S4). Therefore, the optimized device prepared by the direct synthesis process can be applied for long-term HER activity in acidic solutions under solar illumination. Ultraviolet photoelectron spectroscopy (UPS) measurements were performed to obtain the band diagram and energy levels, and the energy diagram of the optimized MoS 2 /p-Si heterojunction is shown in Fig. 5e. From the UPS results, the work functions were determined to be 4.67 and 4.48 eV for the optimized MoS 2 /p-Si and p-Si, respectively. From Fig. 5e, it is clear that for the optimized MoS 2 , the Fermi level was located at 0.23 eV less than the conduction band (E c ), demonstrating n-type semiconductor behavior. The junction between the n-and p-type semiconductors resulted in band bending and Fermi level equalization. Therefore, the photogenerated carriers (electrons) were easily transferred from p-Si to the n-MoS 2 layer. In addition, the proximity of the E c in MoS 2 to the hydrogen reduction potential (E H+/H2 = 4.53 eV) facilitated electron transfer from MoS 2 to the electrolyte without any significant obstacles.
The results show that the proposed method can provide a reliable and facile procedure for the fabrication of 2D material-based p-n junctions, which can be applied in an efficient manner. Table S1 indicates a comparison between the device performance of the as-direct synthesized MoS 2 /p-Si photocathode with that of previously reported similar structures, current densities at 0 V vs. RHE and overpotentials for our MoS2/p-Si photocathode and other photocathodes based on 2D material /p-Si.

Conclusions
In conclusion, we developed a controllable method for the direct synthesis of MoS 2 on p-Si. Thermolysis is an efficient method for synthesizing MoS 2 , but a superhydrophilic surface is required for the deposition of the (NH 4 ) 2 MoS 4 precursor onto a nonoxide substrate. Therefore, we added an additional preliminary step (MoO 3 deposition) to convert the p-Si wafer from a poorly hydrophilic to a superhydrophilic surface for the precursor coating. The as-fabricated MoO 3 /(NH 4 ) 2 MoS 4 / p-Si was successfully converted to MoS 2 /p-Si via thermolysis. In addition, the MoS 2 /p-Si heterojunction was used for solar hydrogen production to confirm the applicability of the proposed method. The significant advantages of this controllable wafer-scale production process for the reproducible generation of highly crystalline products have the potential to revolutionize the 2D electronic industry in the near future.

Materials
All three chemicals used for the three-step cleaning process had purities exceeding 99.99% and were purchased from Sigma Aldrich Company. Hydrofluoric acid (HF, 10 vol%), MoO 3 powder (99.97% trace metals basis), ammonium tetrathiomolybdate (99.97% trace metals basis) and sulfur powder (99.98% purity) were purchased from Sigma Aldrich Company. Commercially available p-Si wafer, where p-Si specified with (100) orientation and p-type boron dopant, was used.

Fabrication of the MoS 2 /p-Si heterojunction
First, p-Si wafers were cleaned ultrasonically using a conventional three-step process, followed by immersion in acetone, isopropyl, and deionized water. Next, the p-Si wafers were immersed in 10% hydrofluoric acid (HF, 10 vol%) to remove the native oxide. Subsequently, different thicknesses (5, 10, 15, and 20 nm) of MoO 3 were deposited onto p-Si using thermal evaporation. Next, a 10 mM solution of ammonium tetrathiomolybdate (NH 4 ) 2 MoS 4 dissolved in glycol ethylene was spin coated onto the MoO 3 /p-Si wafers at 4000 rpm for 60 s. Afterward, the samples were placed on a hot plate at 50°C for 15 min to remove any remaining solvent. Subsequently, the MoO 3 /p-Si wafers were placed in a CVD chamber to initiate thermolysis, and N 2 (200 cm 3 /min) and H 2 (40 cm 3 /min) gases were flowed in the chamber. Initially, the temperature was raised to 500°C and was maintained for 30 min. The chamber pressure was maintained under the same N 2 /H 2 gas flow at 1.2 Torr. Afterward, the chamber temperature was increased to 900°C and maintained for 1 h. Finally, sulfurization was initiated using 0.5 g of sulfur powder (Sigma-Aldrich, 99.5% purity) in another heating zone at 350°C for 1 h. Figure 1 schematically illustrates the preparation process.

Fabrication of the MoS 2 /p-Si photocathode
To achieve better contact, the backs of the MoS 2 /p-Si samples were scratched using a blade, and an InGa alloy (Sigma-Aldrich, 99% purity) was subsequently coated on the scratched samples. Next, a copper wire was connected to the back of the MoS 2 /p-Si samples using silver paste and dried on a hot plate at 50°C for 1 h. Finally, epoxy resin was used to passivate the sample for subsequent PEC measurements. Figure S5 schematically shows the fabrication process. Characterization X-ray photoelectron spectroscopy (XPS) was performed using a K-alpha plus (ThermoFisher Scientific, USA) instrument under a vacuum of 1 × 10 −5 mbar using Mg Kα radiation (1250 eV) and a constant pass energy of 50 eV. The crystal structures of the thin film samples were determined by X-ray diffraction (XRD) analysis (New D8-Advance, Bruker-AXS, Germany) with a Cu Kα target at 0.1542 nm. Raman spectra (LabRAM HR, Horiba Jobin Yvon, Japan) were obtained at an excitation wavelength of 515 nm. The field-emission scanning electron microscopy (FE-SEM, Zeiss 300 VP) images were obtained at an acceleration voltage of 50 kV. Transmission electron microscopy (TEM) was performed using a JEOL-2100F (Japan) instrument. Atomic force microscopy (AFM, XE-100/PSIA) was used to evaluate the roughness and thickness of the prepared thin films. The transmittance spectra of the thin films were examined by a UV-vis spectrophotometer (V-670).

Photoelectrochemical measurements
Electrochemical measurements were performed in 0.5 M H 2 SO 4 using a three-electrode quartz electrochemical cell connected to a potentiostat (Ivium 5612, Netherlands). An Oriel 150 W solar simulator was used and calibrated to an output of 100 mW/cm 2 under AM 1.5 G 100 mW/cm 2 illumination. A scan rate of 10 mV/s was used for the 15 linear sweeps. EIS was conducted by applying a constant potential of −0.65 V relative to the open circuit potential with a sweeping frequency ranging from 250 kHz to 0.1 Hz with a 10 mV AC dither.
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