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Abstract
Copper indium gallium selenide (Cu(In,Ga)Se2; CIGS) solar cells with small thicknesses active layer have limits to show high efficiency owing to high carrier recombination and low reflection at the electrode/active layer interface. A passivation layer applied to the rear of the CIGS solar cell is regarded as one of the solutions. However, depending on the thickness of the passivation layer, the surface morphology of the thin CIGS absorber layer can be changed, affecting light absorption and photovoltaic properties. In this study, the optical electrical performances of CIGS solar cells with a thin enough layer of the local contact aluminum oxide (Al2O3) (LC-Al2O3) rear passivation were analyzed and demonstrated. The thin passivation layer with tens of nanometers resulted in higher increased efficiency because of improved open circuit voltage and short circuit current density. In addition, from the experiment, the optimal thickness of the thin passivation layer was derived. Too thin a layer causes a degradation of the performance, providing the surface scattering and parasitic resistance. Our results can be used to develop guidelines for designing high-performance CIGS solar cells with optimized passivation layers.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Solar cells are renewable energy sources that convert sunlight into electricity [1]. Owing to their high conversion efficiency, silicon solar cells are used in a majority of photovoltaic (PV) systems [2,3]. However, silicon solar cells have high raw material costs and complicated manufacturing processes [4]. To overcome these disadvantages, compound semiconductor-based thin film solar cell technologies are being developed. Copper indium gallium selenide (Cu(In,Ga)Se2; CIGS) solar cells are expected to replace silicon solar cells, due to high efficiency in both modules and cells [5–7], and simple process based on thin film deposition processes. Especially, the CIGS exhibit direct transition energy bandgaps, and owing to high light absorption at the wavelength of visible light, high-efficiency solar cells can be manufactured with a thickness of 1–2 µm [8,9].
Although the thin film shaped solar cells have various advantages in terms of cost, manufacturing, and form factors, high carrier recombination and low reflection at the electrode/ semiconductor interface can be obstacle to enhance the performance [10–12]. Many studies have recently reported solutions to this problem, hereby a passivation layer is applied between the electrode/semiconductor [13–15]. Because of the low surface recombination rate and high reflection at the rear, the passivation layer improves the open-circuit voltage (Voc) and short-circuit current (Jsc) of the solar cell [16].
A passivation layer can be deposited using diverse techniques, such as atomic layer deposition [17,18], sputtering [19,20], spin coating [21,22] and so on. Among them, sputtering is the most efficient method for mass-production, because it is simple and suitable for large-area deposition with various materials. To reduce the cost and processing time, the thickness of the passivation layer also become smaller. In addition, the passivation layer structure may affect the surface morphology of the upper side, i.e. whole of the thin CIGS solar cell. This can cause another optical or electrical issue compared to the flat CIGS solar cells. Hence, it is critical to optimize the thickness of the rear passivation layer in these cells, considering both productivity and performance.
In this study, we investigate the change in the characteristics of the CIGS solar cell as a function of the thickness of the local contact aluminum oxide (Al2O3) (LC-Al2O3) rear passivation layer, and attempted to determine the optimum thickness. Using optical and electrical simulations, the effects of the thickness of the LC-Al2O3 layer on CIGS solar cells were examined. To verify the performance of devices modeled using two-dimensional (2D) simulation, CIGS cells with varying passivation thicknesses were fabricated. In addition, the optical analysis was carried out to confirm the design factors for optimizing the thickness of the LC-Al2O3 layer. Based on this study, guidelines can be issued for the design of the LC-Al2O3 rear passivation layer on the CIGS cells, which will significantly contribute to the commercialization of large-area CIGS solar cells.
2. Experiments
2.1 Design of CIGS solar cells simulation
The effect of the LC-Al2O3 rear passivation layer on the CIGS solar cell was investigated, and the optimal thickness was determined. First, we calculated the power absorbed by each layer from a vertically incident air mass 1.5 (AM1.5) sun spectrum source irradiated with a plane wave ranging from 340 to 1200 nm using the 2D finite-difference time-domain (FDTD) method (FDTD Solutions, Lumerical Solutions Inc., Vancouver, BC, Canada). The photo-generation current can be calculated by assuming that all absorbed photons within the CIGS absorber layer generate electron-hole pairs. Second, the charge generation rate data calculated by the absorbed photons were imported into the 2D finite element Poisson/drift-diffusion method (CHARGE Solver, Lumerical Inc., Canada) to estimate the J-V characteristics and efficiency. The values of the refractive index and material characteristics of the molybdenum (Mo) [23,24], Al2O3 [25,26], CIGS [27,28], cadmium sulfide (CdS) [23], zinc oxide (ZnO) [29], and Al-doped zinc oxide (AZO) [30], which are components of the CIGS solar cell structure, were obtained from literature.
2.2 Preparation of the device
To obtain the LC-Al2O3 layer, Al2O3 layers were deposited on the Mo layer. Sputtering was used to deposit Al2O3 layers 10 nm and 80 nm thick. The pressure in the sputtering chamber was reduced to 10−7 Torr before depositing the Al2O3 thin film, and the deposition was carried out at 10 mTorr. A mixed gas of Ar and O2 was flowed at 100 sccm and 5 sccm, respectively, and an RF power of 500 W was applied for 5000 seconds. The Al2O3 layer patterning process consists of five steps: (1) Positive photoresist (PR, KL5302) was coated on the Al2O3-deposited substrate at 4000 rpm for 45 seconds. (2) The solvent was then evaporated by soft baking the substrate on a hot plate at 130 °C for 60 seconds. (3) A photomask was used to perform UV exposure at i-line wavelength (365 nm) on the PR-coated Al2O3 layer. The UV intensity was exposed for 10 seconds at a constant intensity of 12 mW/cm2. (4) A development process was used after exposure to achieve the desired pattern. To remove the unnecessary PR, this process was developed for 45 seconds with an AZ-300 MIF developer. (5) Finally, the Al2O3 layer was etched with a buffered oxide etch (BOE 6:1) solution at a rate of 0.8 nm/s. Wet etching is a low-cost, simple process with a high etch rate. Acetone was then used to remove the remaining PR. A dot-shaped LC-Al2O3 with a diameter of 3.9 µm and a pitch of 8 µm was formed using a photolithography process. CIGS absorber layers were on a without Al2O3 and with LC-Al2O3 rear passivation layer using a three-stage co-evaporation process. In the first stage, the indium (In) and gallium (Ga) were deposited for 20 min at 350 °C on a prepared substrate. In the second stage, the copper (Cu) and selenium (Se) were deposited, and the deposition temperature was kept constant at 441°C. The end point of the second stage was considered as the point where there was a 10 °C decrease in substrate temperature, as measured using a pyrometer. The third stage involved the deposition of In, Ga, and Se to develop the final Cu-poor CIGS layer.
A 40 nm-thick n-type CdS buffer layer was used to cover the CIGS absorber layer, and radiofrequency (RF) sputtering was used to deposit a 50 nm-thick intrinsic ZnO window layer and a 300 nm-thick AZO layer. To fabricate the solar cells, a transparent conducting oxide (TCO) layer was deposited on top of the buffer layer. Subsequently, a 1 µm-thick Al collection grid was deposited on top of the device using thermal evaporation.
2.3 Characterization of the CIGS solar cells
Various techniques were used to investigate the formation of the LC-Al2O3 rear passivation layer, the morphology of the fabricated CIGS thin film, and the effect on the PV properties of the CIGS solar cell by using LC-Al2O3. To confirm the pattern formation and thickness of the LC-Al2O3 layer, focused ions were observed using a microscope (Nikon) and a transmission electron microscope (FE-TEM, Hitachi, HF-3300) equipped with a focused ion beam (FIB) system (Hitachi, NB 5000). The PV characteristics of the solar cells were measured using a source meter (Keithley, 2400) and a solar simulator (Newport, 94022A) to simulate AM 1.5 solar irradiation.
3. Results and discussion
The efficiency of CIGS solar cells is decreased because of the low light reflection and high surface recombination on the Mo/CIGS interface. On the Mo/CIGS rear surface, an LC-Al2O3 rear passivation layer can increase light reflection and suppress surface recombination [13–16]. To verify the effect of LC-Al2O3 rear passivation layer on the CIGS solar cell, the solar cell structure shown in Fig. 1 was designed. Figure 1 (a) shows the CIGS solar cell structures without Al2O3 (0 nm) and (b) with the LC-Al2O3 rear passivation layer. The model parameters used in the CHARGE Solver are shown in Table 1. The thickness of LC-Al2O3 was designed to be in the range of 10 to 120 nm. Because of the uneven surface between Mo and Al2O3 on the bottom, the top surface of CIGS may change if the Al2O3 layer is too thick. The passivation layer thickness that does not change the morphology of the CIGS was determined. In addition, the thin thickness of passivation layer can reduce process time and cost. Taking this into consideration, the thickness of the Al2O3 layer was set to a range of 0 nm to 120 nm, and an optimal thickness that improves the CIGS solar cell device was derived. The simulation of the structure thus formed was used to obtain the J-V curves of the CIGS solar cell, as shown in Fig. 1 (c). From the J-V curves, it can be seen that there is an improvement in the Voc and Jsc due to the LC-Al2O3 rear passivation when compared to without-Al2O3.
Fig. 1. Schematic diagram of CIGS solar cells. (a) without-Al2O3 (0 nm-Al2O3), (b) LC-Al2O3 rear passivation layer (10–120 nm-Al2O3). (c) Simulated current density (J)-voltage (V) curves according to the change in thickness of the LC- Al2O3 rear passivation layer
Table 1. Material parameters used in the CHARGE Solver simulation
To investigate the change in the characteristics of the CIGS solar cell caused by the LC-Al2O3 rear passivation layer, the changes in each parameter were examined in detail, as shown in Fig. 2. Figure 2 depicts the changes in Voc, Jsc, fill factor (FF), and efficiency of the CIGS solar cell caused by the LC-Al2O3 rear passivation layer, and Table 2 shows the characteristic values for each. First, the properties of CIGS solar cells with LC-Al2O3 thicknesses of 0 and 10 nm were compared. Voc increased by 0.99% from 740.03 to 747.39 mV, Jsc increased by 1.84% from 34.15 to 34.78 mA/cm2. FF was comparable, and efficiency increased by 2.94% from 16.803% to 17.297%.
Fig. 2. Simulated Electrical characteristics as a function of without-Al2O3 (0 nm) and LC-Al2O3 rear passivation thickness in the CIGS solar cells, as simulated, (a) open-circuit voltage (VOC), (b) short-circuit current (JSC), (c) Fill Factor (FF), and (d) Efficiency
Table 2. Simulated open-circuit voltage (VOC), short-circuit current (JSC), fill factor (FF), and efficiency (EFF) data for the fabricated CIGS solar cells without-Al2O3 (0 nm) with LC-Al2O3 rear passivation layer (10 nm to 120 nm)
The characteristics of CIGS solar cells without Al2O3 (0 nm) and with LC-Al2O3 having different thicknesses are compared. As the thickness of the LC-Al2O3 rear passivation layer increased, Voc increased by 1.12% from 740.03 (0 nm) to 748.31 mV (120 nm). Jsc increased by 2.31% from 34.15 (0 nm) to 34.94 mA/cm2 (100 nm) and then decreased to 34.935 mA/cm2 at 120 nm of Al2O3. FF was relatively the same, and efficiency increased by 3.45%, from 16.80 (0 nm) to 17.38% (100 nm, 120 nm).
The LC-Al2O3 rear passivation layer improved the Voc and Jsc of the CIGS solar cells, which also increased their performance as the LC-Al2O3 thickness increased. The reasons for the improvement in Voc and Jsc of the CIGS solar cells owing to the LC-Al2O3 rear passivation layer were investigated. To determine the reason for the change in Jsc, we analyzed the amount of light absorption in the CIGS absorber layer using the FDTD Solutions. Figure 3 depicts the spectral characteristics of the total absorbed power in a CIGS absorber layer as a function of the change in thickness of the LC-Al2O3 rear passivation layer. As the thickness of the LC-Al2O3 rear passivation layer increased from 0 to 80 nm, the light absorption in the three infrared wavelength regions improved and shifted to shorter wavelengths (blue dashed circles shown in Fig. 3). The light absorption in the CIGS absorber layer increased in the infrared wavelength region owing to the increased reflection at the Mo/CIGS interface caused by LC-Al2O3 rear passivation layer. It generates numerous electron/hole pairs in the CIGS absorber layer, which improves the Jsc.
Fig. 3. Simulated total absorbed power spectral characteristics in CIGS absorbers as the thickness of the LC- Al2O3 rear passivation changes. The inset is the area of the total absorbed power as a function of the passivation thickness. The figure on the right represents the spectral change of the part of a total absorbed power graph indicated by the blue dashed circles
When the LC-Al2O3 thickness was 10 nm, the total absorbed power value increased by 0.13% compared to the without-Al2O3 (0 nm), which resulted in 559.73. The increase in this value with LC-Al2O3 thickness decreased and then saturated. It saturated at an LC-Al2O3 thickness of 80 nm, which was a value of 562.30. The total absorbed power area values for LC-Al2O3 thicknesses of 80 and 100 nm were comparable. Thus, after a certain thickness, the passivation effect in terms of improving Jsc was limited.
To further examine the influence of LC-Al2O3 on CIGS solar cells, we set three thicknesses: without-Al2O3 (0 nm), 10 nm-Al2O3, which displayed an optical effect, and 80 nm-Al2O3, which exhibited saturated light absorption.
The surface recombination velocity of the general Mo/CIGS interface was reported to be 106 cm/s [18,31], which is known to influence the Voc. Figure 4 (a) shows the change in Voc of the CIGS solar cells as a function of the surface recombination velocity at the Mo/CIGS interface. The Voc values of the without-Al2O3 (0 nm) and with LC-Al2O3 were similar when the surface recombination velocity was <103 cm/s. However, when the surface recombination velocity was >103 cm/s, the LC-Al2O3 rear passivation layer exhibited higher Voc. Even at LC-Al2O3 thicknesses of 10 and 80 nm, the value of Voc differed. When the surface recombination velocity was >105 cm/s, the 80 nm-Al2O3 had a slightly higher Voc.
Fig. 4. Simulated results of (a) open circuit voltage as a function of surface recombination velocity at Mo/CIGS interface for CIGS solar cells without-Al2O3 and with LC-Al2O3 rear passivation thickness of 10 nm and 80 nm. Spatial profiles of (b) electron density and (c) hole density distribution with respect to the xy-plane in CIGS. In CIGS absorber layer (d) electron density and (e) hole density at the positions of the black dashed lines shown in Fig. 4 (b) and (c)
The carrier distribution inside the CIGS absorber layer was explored through simulations to investigate the reason for the change in Voc according to the recombination rate. Figure 4 (b) and (c) show the electron and hole distributions inside the CIGS absorber layer containing without-Al2O3 (0 nm) and with LC-Al2O3 thicknesses of 10 nm and 80 nm. Figure 4 (d) and (e) depict the electron and hole densities from Mo to the CIGS absorber layer (black dashed line). The number of electrons and holes increased from Mo to the inside of the CIGS absorber layer when comparing the without (0 nm) and 10 nm-LC-Al2O3. Because of the LC-Al2O3 rear passivation layer, more carriers diffused into CIGS. For 10 nm and 80 nm-LC-Al2O3, the carrier distribution within the CIGS was comparable. The LC-Al2O3 layer with a high density of fixed charges generates an electric field that pushes minority carriers into the CIGS [17]. Consequently, it was confirmed that it contributes to a better Voc by reducing the number of carriers that are highly likely to recombine at the Mo interface [17].
From our simulated results of CIGS solar cells with LC- Al2O3 passivation layer, it was found that only the 10 nm LC- Al2O3 passivation was very effective for the performance improvement of CIGS solar cells. Based on the simulation results, we attempted to validate the effect of the Al2O3 layer by fabricating CIGS solar cells without-Al2O3 and with LC-Al2O3 thicknesses of 10 and 80 nm. Microscopy and TEM analyses were used to confirm the formation of the LC-Al2O3 pattern with thicknesses of 10 and 80 nm in Fig. 5 (a)–(d). Figure 5 (e) and (f) show the well-formed CIGS on LC-Al2O3 rear passivation layer.
Fig. 5. Optical microscope images of LC-Al2O3 rear passivation thickness of (a) 10 nm and (b) 80 nm. TEM cross-section images for fabricated CIGS solar cells with LC-Al2O3 rear passivation thickness of (c) 10 nm and 80 nm (scale bar 500 nm). (e) and (f) are enlarged TEM images in Fig. 5 (c) and (d), respectively (scale bar 100 nm
The electrical properties of the fabricated CIGS solar cells were also investigated. Figure 6 depicts the J-V curve for different LC-Al2O3 thicknesses and Table 3 shows the characteristic values for each. As suggested by the simulation results there is an improvement in the Voc and Jsc in the fabricated device. The LC-Al2O3 layer has been shown to improve the properties of CIGS solar cells. When the 0 nm and 10 nm-Al2O3 were compared, the average Voc increased from 622.75 to 626.00 mV, an improvement of 0.52%, and the average Jsc increased from 31.07 to 31.42 mA/cm2, an improvement of 1.13%. When the 0 and 80 nm-Al2O3 was compared, the average Voc increased from 622.75 to 628.75 mV, an improvement of 0.96%, and the average Jsc increased from 31.07 to 32.02 mA/cm2, an improvement of 3.06%. As the result of improvements in Voc and Jsc, the CIGS solar cell efficiency of 80 nm-Al2O3 increased by 4.32%, from 13.19 to 13.76.
Fig. 6. Electrical characteristics as a function of fabricated CIGS solar cells without-Al2O3 (0 nm) and with LC-Al2O3 passivation thickness of 10 nm and 80 nm, (a) current density (J)-voltage (V) curves (b) open-circuit voltage (VOC), (c) short-circuit current (JSC), (d) Fill Factor (FF), and (e) efficiency
Table 3. Open-circuit voltage (VOC), short-circuit current (JSC), fill factor (FF), efficiency (EFF), series resistance (RS), and shunt resistance (RSh) data for the fabricated CIGS solar cells without-Al2O3 (0 nm) with LC-Al2O3 rear passivation layer (10 nm to 80 nm)
However, in the case of the fabricated LC-Al2O3 with a thickness of 10 nm, the improvement, when compared to the simulation results, was not significant. Voc improved by 0.99% and Jsc by 1.84% in the simulation results, but the results for the fabricated device showed that Voc and Jsc improved by 0.52% and 1.13%. Additionally, while there was no change in the FF due to the LC-Al2O3 layer in the simulation, the results of the fabricated solar cell decreased. It fell by 1.16%, from 66.96 to 66.18%, resulting in a 0.15% decrease in efficiency from 13.19 to 13.17%. Furthermore, the calculated RSh value for devices with an Al2O3 thickness of 10 nm was 25.12%, lower than the other devices. The RSh value can decrease due to high leakage current through the shunt path [32].
This different device performance of CIGS solar cells compared with a simulated result could be closely related with the real complicated device structure. In order to understand the cause of disappointing device performance of CIGS with 10 nm LC-Al2O3, we have calculated the light absorption of CIGS according to the roughness of Al2O3 passivation layer. To understand the causes of poor performance in a CIGS solar cell with 10 nm-Al2O3, we investigated the change in light absorption based on the roughness of the Mo surface. The calculated light absorption in the CIGS absorber layer with smooth and rough 10 nm-Al2O3 is shown in Fig. 7. Figure 7 (a) shows the TEM cross-sectional image of 10 nm-Al2O3 formed on Mo. The Mo layer has a surface roughness from 30 nm to 50 nm, which is larger than the thickness of the upper thin layer. Although the 10 nm thick of Al2O3 was deposited conformally, the Al2O3 layer cannot be formed in a continuous media as well as a flat film layer.
Fig. 7. (a) TEM cross-section image for Al2O3 thickness of 10 nm formed on Mo. (b) Total absorbed power spectral characteristics in CIGS absorbers with smooth and rough LC-Al2O3 rear passivation layer of 10 nm. (c) Simulated results of the spatial profiles for the power absorbed per unit volume with respect to the xy-plane at a wavelength of 1052.76 nm in CIGS absorbers with smooth and rough LC-Al2O3 rear passivation layer of 10 nm
Figure 7 (b) depicts the calculated spectral characteristics of the total absorbed power in the CIGS absorber layer with smooth and rough 10 nm-Al2O3. Compared to the smooth LC-Al2O3 rear passivation layer, the total absorbed power area value of the rough LC-Al2O3 rear passivation layer decreased by 0.43%. This indicates that the rough LC-Al2O3 rear passivation layer reduced the amount of light absorption in the CIGS absorber layer at infrared wavelengths. The spatial profiles of the power absorbed per unit volume with respect to the xy-plane at a wavelength of 1052.76 nm are shown in Fig. 7 (c). In the CIGS absorber layer, the absorbed power of the smooth LC-Al2O3 rear passivation layer was distributed uniformly. On the other hand, the absorption power distribution for the rough LC-Al2O3 rear passivation layer was distorted owing to surface scattering on the Mo/CIGS and Mo/Al2O3/CIGS layers. The calculated average value of power absorbed per unit volume along the y-axis from 0.2 µm to 2.13 µm was 3.96 × 1010 W/m3 in the case of rough LC-Al2O3 rear passivation layer. This was an 8.5% decrease in value compared to that of the smooth LC-Al2O3 rear passivation layer, which was calculated as 4.33 × 1010 W/m3. Reduced light absorption can result in low JSC values because it decreases the number of electron-hole pairs within the CIGS absorber layer.
The characteristics of the fabricated device with an Al2O3 thickness of 80 nm are well correspond to the simulated results. On the other hand, the case of the LC-Al2O3 with thickness of 10 nm showed poor performance, unlike the expected value. The JSC value of the fabricated device with a 10 nm-Al2O3 thickness showed lower improvement than the simulated results. In case of the FF value, the value decreased compared to that of the reference device without the LC-Al2O3 layer. In the Al2O3 thickness of 10 nm, the reason for the differences between simulated and fabricated devices is originated from the surface morphology of the fabricated device. Due to the thinner Al2O3 thickness compared to the roughness of the Mo layer, the Al2O3 can be partially covered. And this may not be enough to form the intrinsic properties of the Al2O3 material. A partially covered Al2O3 thin film on Mo decreases the FF value due to insufficient rear passivation effect. Furthermore, the absorbed power within the CIGS absorber layer decreased due to scattering on uneven Mo/CIGS and Mo/Al2O3/CIGS surfaces, resulting in lower JSC values. The solar cell characteristics may be affected if the Al2O3 thin film is partially covered with a thickness of less than 10 nm due to the rough surface of real Mo. As a result, optimizing the thickness of the LC-Al2O3 rear passivation layer is critical for high-efficiency thin CIGS solar cells.
To compete with silicon solar cells with high efficiency and low cost, CIGS solar cells must improve power conversion efficiency and reduce absorber layer thickness. However, reducing the thickness of the absorber layer results in insufficient absorption and detrimental effects of rear surface recombination. In this work, we showed that the passivation layer increased light reflection and reduced recombination loss, though the efficiency improvement was not significant. The light absorption occurs in the direction from the incidence surface to the rear electrode, and the amount of the absorption decreases exponentially from the incident surface. As the absorber layer becomes thinner, more amount of the light can arrive at the rear interface. In this case, the effects of the local contact as mentioned above could become larger. Considering the low efficiency of the PVs with thin absorber layer, our work will help to realize competitive thin solar cells by applicating the optimal passivation layer.
4. Conclusion
We demonstrated that an optimally thin LC-Al2O3 rear passivation layer is effective in improving the performance of CIGS solar cells. A simulation was used to examine the passivation layer effect, and the performance of the CIGS solar cells was investigated for different thicknesses of the LC-Al2O3. Devices were fabricated based on simulation results to demonstrate the effectiveness of LC-Al2O3. When the LC-Al2O3 layer was applied to the CIGS solar cell, the performance of the cell improved compared to that upon the application without Al2O3. However, when the LC-Al2O3 thickness was 10 nm, it was demonstrated that the performance of the solar cell could be negatively affected due to surface scattering and parasitic resistance caused by the substrate. The efficiency increased by 4.32% when the LC-Al2O3 thickness was 80 nm owing to improvements in Voc and Jsc. This observation indicates that it is critical to design a thin passivation layer of optimal thickness for high-efficiency CIGS solar cells. Our results provide design guidelines for fabricating high-efficiency CIGS solar cells with a local contact rear passivation layer.
Funding
Daegu Gyeongbuk Institute of Science and Technology (ICT (22-CoE-ET-01)); National Research Foundation of Korea (ICT (2022M3J1A1085371)); National Research Foundation of Korea (Grant NRF-2022R1I1A3070928).
Acknowledgments
This work was supported by the Basic Science Research Programs through the National Research Foundation of Korea (NRF), Ministry of Education, under Grant NRF-2022R1I1A3070928, by the program of Phased development of carbon neutral technologies through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (2022M3J1A1085371) and by the DGIST R&D programs of the Ministry of Science, ICT (22-CoE-ET-01).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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