The water-dipping effect of branched poly(ethylene imine) interfacial layers on the performance and stability of polymer:nonfullerene solar cells

Abstract

Here we report the water-dipping effect of polymeric interfacial layers on the performance of inverted-type polymer solar cells (PSCs) with polymer:nonfullerene bulk heterojunction (BHJ) layers. Aqueous solutions of branched poly(ethylene imine) (bPEI) were spun on indium-tin oxide (ITO)-coated glass substrates to make the interfacial layers with various thicknesses (0.1–5 nm) for the investigation of the bPEI thickness effect. The water-dipping process was carried out by immersing the 2 nm-thick bPEI layer-coated ITO-glasses in deionized water for various dipping times (0–90 min). The inverted-type PSCs were fabricated by spin-coating the BHJ layers on the water-dipped bPEI layers, followed by successive deposition of hole-collecting layers and silver top electrodes. The results showed that the open circuit voltage (V_OC) of PSCs could be greatly increased by the insertion of bPEI layers (all thicknesses) thanks to the formation of dipole layers. However, the limited thickness range (0.1–1.5 nm) of bPEI layers could only deliver an enhanced short circuit current density (J_SC) owing to the influence of high electrical resistance in the case of thicker bPEI layers. The water-dipped bPEI layers resulted in enhanced V_OC and J_SC due to the well-maintained work functions and reduced thickness, leading to 2.7-fold improved power conversion efficiency by water-dipping for 30 min.

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Introduction

Organic solar cells based on conjugated polymers, so-called polymer solar cells (PSCs), have been extensively studied for the last two decades and are still under the spotlight due to their expectation toward next-generation solar modules with lightweight and flexible features.^1–3 In particular, PSCs have an advantage in terms of manufacturing costs because continuous roll-to-roll wet-coating processes can be employed at room temperature under normal pressure.^4–6 Recently, it has been reported that the power conversion efficiency (PCE) of single-stack PSCs can reach ca. 18% thanks to nonfullerene acceptors and device optimization.^7,8 However, the lifetime of PSCs is not yet overcome and needs to be further improved from the viewpoint of organic materials and device structures.

Two types of device structures (normal-type and inverted-type) have been so far intensively studied for the fabrication of PSCs.^9–11 The normal-type PSCs need top electron-collecting electrodes with low work functions, such as aluminum (Al) and calcium (Ca), which can be easily oxidized leading to device degradation.^12,13 In contrast, for inverted-type PSCs, the electron-collecting role is played by the bottom electrodes (typically indium-tin oxide (ITO)) of which surfaces are modified with electron-accepting materials such as zinc oxide (ZnO), and relatively stable high-work function metals including silver (Ag) are usually used as the top electrodes.^14–16 However, the high-temperature processes for ZnO layers have been considered a demerit when it comes to the benefit of PSCs, so various approaches including nanoparticle coatings and hybrid layers have been attempted to reduce the process temperature of electron-collecting layers.^17–22

In particular, it has been reported that ultrathin polymeric dipole layers can change the work function of ITO electrodes suitable for electron collection in inverted-type PSCs.^23–25 The representative polymers for dipole layers include poly(ethylene imine) (PEI) derivatives, poly(2-ethyl-2-oxazoline) (PEOz), polyimide derivatives, etc.^26–29 Among these polymers, keen attention has been paid to the water-soluble PEI polymers that can provide eco-friendly processes.^30–32 A basic role of the PEI dipole layers, consisting of PEIE (with ethoxylated branches) and/or bPEI (with branches including primary amines), has been studied but the correlation between PEI thickness and device performance including stability still remains to be investigated when it comes to the challenging coating process of ultrathin PEI layers.

In this work, we investigated the water-dipping effect of bPEI dipole layers on the performance and stability of inverted-type PSCs. Ultrathin bPEI layers with thicknesses ranging from 0.1 to 5 nm were prepared on ITO-glass substrates via spin-coating of aqueous bPEI solutions with various concentrations. Thick bPEI-coated substrates were chosen and immersed in deionized water vessels by varying the dipping time at room temperature. On top of the water-dipped bPEI layers, polymer:nonfullerene bulk heterojunction (BHJ) layers were spin-coated for the fabrication of PSCs. The results showed that the PCE of PSCs could be improved by 2.7 times when the 2 nm-thick bPEI layers were water-dipped for 30 min. The PSCs with the water-dipped bPEI layers showed excellent stability upon storage for five weeks in spite of a very short half-lifetime under 1 sun conditions.

Experimental details

Materials and solutions

The bPEI solution (50 wt% in water) was purchased from Sigma-Aldrich (USA) and subjected to concentration control for obtaining bPEI layers with various thicknesses. Some solutions with higher concentrations (up to 30 mg ml⁻¹) were prepared for thickness measurement and calibration. Poly[(2,6-(4,8-bis(5-(2-ethylhexyl-3-fluoro)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione))] (PM6) and 2,2′-((2Z,2′Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-2-(2-ethylhexyl)-[1–3]triazole[3,4-e]thieno[2,3′:4′,5′]thieno-[2′,3′:4,5]pyrrolo[3,2-g]thieno[2′,3′:4,5]thieno-[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis-(5,6-difluoro-3-oxo-2,3dihydro-1H-indene-2,1-diylidene))dimalononitrile (Y11) were received from Solarmer Materials Inc. (China). PM6 and Y11 were dissolved in chlorobenzene at a concentration of 16 mg ml⁻¹ (PM6 : Y11 = 1 : 1.5 by weight) under inert conditions, followed by the addition of 1-chloronaphthalene (0.5 vol%) as an additive. The PM6:Y11 solutions were subjected to stirring at 50 °C for 3 h and then at room temperature for 48 h.

Fabrication of thin films and devices

The ITO-coated glass substrates were patterned to have the ITO electrode (8 mm × 12 mm), followed by wet cleaning steps using isopropyl alcohol and acetone for 30 min each in an ultrasonic bath. The cleaned ITO-glass substrates were further treated under ultraviolet-ozone (UVO) conditions using a UVO system (AC-6, Ahtech LTS). The bPEI solutions (0.1–5.0 mg ml⁻¹) were spun on the UVO-treated ITO-glass substrates at 4000 rpm for 60 s, followed by thermal treatment at 130 °C for 10 min. The final thickness of bPEI layers was determined by the extrapolation of thickness data from thicker films (see Fig. S1†). For the water-dipping experiments, the 2 nm-thick bPEI layers were spin-coated on the ITO-glass substrates and thermally treated at 130 °C for 10 min. The 2 nm-thick bPEI layer-coated ITO-glass substrates were immersed in the vessels (Petri dishes) that contain deionized water (20 ml). The dipping time varied from 10 min to 90 min. After taking out from the vessels, these samples were subjected to drying and treatment at 130 °C for 10 min. The bPEI layer-coated ITO-glasses including the water-dipped samples were moved into a nitrogen-filled glovebox for spin-coating of BHJ (PM6:Y11) layers at 1500 rpm for 60 s. The BHJ layers were dried at 40 °C for 5 min leading to a thickness of 100 nm. Finally, the BHJ layer-coated samples were transferred to an argon-filled glovebox and mounted on a shadow mask in a thermal evaporator. After pumping down the chamber pressure to ca. 1 × 10⁻⁶ torr, 10 nm-thick molybdenum oxide (MoO₃) and 80 nm-thick silver (Ag) electrodes were consecutively deposited on the PM6:Y11 layers.

Measurements and analysis

The optical absorption and transmittance of film samples were measured using a UV-Visible-NIR spectrometer (Lambda 750, PerkinElmer), while a surface profilometer (DektakXT-E, Bruker) was used for the measurement of the film and electrode thicknesses. Note that the thickness of bPEI layers was calibrated by extrapolation from the real thickness information of thick bPEI films (solution concentration up to 30 mg ml⁻¹) because the present bPEI layers were too thin to be properly measured with the present measurement system. The work function of the bPEI-coated ITO-glasses was measured using a Kelvin Probe System (APS01, KP Technology). The surface morphology of bPEI layer-coated ITO-glasses was measured using an atomic force microscope (AFM, model NX20, Park Science). The performance of solar cells was evaluated using a solar cell measurement system equipped with a solar simulator (92250A-100, Newport-Oriel, air mass 1.5G, 100 mW cm⁻²) and an electrometer (model 2400, Keithley Instruments). The incident light intensity from the solar simulator to PSCs was adjusted with a standard solar cell (BS-520, Bunkoukeiki Co. Ltd). All device measurements were carried out using a sample holder that is charged with argon at room temperature.

Results and discussion

As shown in Fig. 1a, the inverted-type PSCs were fabricated by placing ultrathin bPEI layers below the BHJ (PM6:Y11) layers (see the chemical structures of materials). The ultrathin bPEI layers coated on the ITO-glasses could not be easily identified with the naked eye, as compared in Fig. 1b, even though the 5 nm-thick BHJ films could be seen clearly. Hence, the thickness of bPEI layers was determined from the calibration curve that was obtained by extrapolation of thickness data for the thicker bPEI films made from high-concentration solutions (see Fig. S1†). Note that the 25 nm-thick bPEI films showed a noticeable absorption below 400 nm (wavelength) (see Fig. S2†). Therefore, compared to the BHJ layers coated, the 5 nm-thick bPEI layer-coated ITO-glasses could absorb a negligible number of photons over the wavelength range up to 1000 nm leading to high optical transmittance in the visible range (see Fig. 1c). As illustrated by the water-dipping process in Fig. 1d, the bPEI layer-coated ITO-glasses were immersed in the deionized water with various dipping times and taken out for drying. Note that the exact thickness of bPEI layers after the water-dipping processes could not be measured at the moment because of the above-mentioned limitations.

Fig. 1 (a) Illustration of inverted-type polymer solar cells with the bPEI interlayers and PM6:Y11 bulk heterojunction layers. (b) Photographs of ITO-glass and bPEI layer-coated ITO-glass (right top: ITO/PM6:Y11 and ITO/bPEI/PM6:Y11). (c) Optical absorption spectrum of bPEI layer-coated ITO-glass (inset: optical absorption spectrum of the PM6:Y11 film). (d) Illustration of the water-dipping process by immersing the bPEI layer-coated ITO-glasses in deionized water (DIW).

First, the performance of PSCs was measured according to the thickness of bPEI layers. As shown in Fig. 2a, the PSCs without the bPEI layers delivered a poor current density–voltage (J–V) curve featuring low short circuit current density (J_SC) and open circuit voltage (V_OC). Interestingly, the presence of only 0.1 nm-thick bPEI layers could remarkably improve the shape of J–V curves. Further improvement in J–V curves was made by the 0.5 nm-thick bPEI layers, which resulted in the highest J_SC and V_OC. However, additional thickness increases of bPEI layers led to a gradual roll-back of J–V curves. A sigmoidal-shaped J–V curve was measured in the case of 2 nm-thick bPEI layers, while the 5 nm-thick bPEI layers gave a very poor J–V curve without meaningful solar cell performances (see Fig. 2b on a semi-logarithmic scale). To understand the detailed trend of solar cell performances, the major solar cell parameters were extracted from the J–V curves in Fig. 2a. As displayed in Fig. 2c, the V_OC value suddenly jumped from 0.38 V to 0.84 V by the insertion of 0.1 nm-thick bPEI layers and further marginal increase to 0.86 V was achieved by the 0.5 nm-thick bPEI layers (see Tables 1 and S1†). However, the V_OC value was gradually reduced as the bPEI thickness increased further from 0.5 nm. A similar jump in J_SC (from 15.99 mA cm⁻² to 19.09 mA cm⁻²) was made by the presence of 0.1 nm-thick bPEI layers, but the J_SC value quickly dropped to 13.67 mA cm⁻² and 0.05 mA cm⁻² in the case of 2 nm and 5 nm-thick bPEI layers, respectively. The J_SC trend can be supported by the huge change in series resistance (see Table 1). The fill factor (FF) of solar cells was noticeably improved from 40.58% to 56.73% by the insertion of 0.1 nm-thick bPEI layers and further slightly increased to 62.24% in the case of 0.5 nm-thick bPEI layers. However, the thicker bPEI layers resulted in a largely reduced FF value, which greatly affected the trend of PCE (see the PCE trend in Fig. 2c). The highest (averaged) PCE of 10.54% (maximum PCE = 11.03%) was obtained in the case of 0.5 nm-thick bPEI layers.

Fig. 2 Light (air mass 1.5G, 100 mW cm⁻²) J–V curves and parameters of PM6:Y11 solar cells with the bPEI interlayers: (a) J–V curves on a linear scale, (b) J–V curves on a semi-logarithmic scale, and (c) solar cell parameters as a function of bPEI thickness.

Table 1 Summary of solar cell parameters (averaged values from more than 10 devices) for the inverted-type PM6:Y11 solar cells with the bPEI layers according to the bPEI thickness

Parameter	bPEI thickness (nm)
	0	0.1	0.2	0.5	0.7	1.0	1.5	2.0	5.0
V OC (V)	0.38	0.84	0.86	0.86	0.86	0.85	0.85	0.84	0.61
J SC (mA cm^−2)	15.99	19.09	19.44	19.84	19.68	19.81	19.00	13.67	0.05
FF (%)	40.58	56.73	60.18	62.24	60.11	56.89	52.09	33.22	19.58
PCE (%)	2.49	9.05	10.05	10.54	10.12	9.62	8.39	3.79	0.01
R S (kΩ)	0.22	0.18	0.14	0.12	0.13	0.16	0.23	2.31	699.96
R SH (kΩ)	1.88	6.43	7.80	8.60	7.86	7.23	6.23	3.14	212.75

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The V_OC change can be explained by the work function shift for the ITO electrodes, which could be caused by the formation of dipole layers due to the interaction between the ITO surface and the bPEI polymer.³³ As shown in Fig. 3a, the work function of ITO electrodes was effectively lowered from −4.8 eV to −3.99 eV by the presence of 0.5 nm-thick bPEI layers. Thanks to the lowered work function in the PSCs with the bPEI layers, the built-in electric field could be increased leading to better transport of electrons from the BHJ layers to the ITO electrodes (see Fig. 3b and S3†). In the case of thick bPEI layers, however, the increased electrical resistance might be a limiting factor for electron collection in spite of still high built-in potential (high V_OC). Note that the thick bPEI layers showed a better surface coverage effect on the ITO electrodes (see the AFM images in Fig. S4†).

Fig. 3 (a) Work function as a function of bPEI thickness for the bPEI layer-coated ITO electrodes. (b) Flat energy band diagram for the PM6:Y11 solar cells with the bPEI interlayers (thickness = 0.5, 2.0, 5.0 nm). Note that the unit of energy (eV) in (b) is omitted to avoid crowding the diagram.

Based on the above results, the 2 nm-thick bPEI layers were chosen for the water-dipping experiment (see details in Fig. 1d). As shown in Fig. 4a, the shape of J–V curves was greatly improved by immersing the 2 nm-thick bPEI layer-coated ITO-glasses for 10 min only. Further improvement in J–V curves was observed in the case of 30 min. However, the extended dipping time led to a rollback of the J–V curve shape with a noticeable change in the short circuit region (J_SC values) but almost no change in the open circuit region (see Fig. 4b). Interestingly, the J–V curve in the case of 90 min was still similar to that of 10 min, indicating that the bPEI layers still remained on the surface of ITO electrodes.

Fig. 4 (a) Light (air mass 1.5G, 100 mW cm⁻²) J–V curves of PM6:Y11 solar cells with the bPEI interlayers that underwent water-dipping processes: (a) linear scale and (b) semi-logarithmic scale. The dipping time (T_DIP) is given on the graphs. Note that the initial thickness of bPEI layers was 2 nm.

The detailed change of solar cell parameters was plotted as a function of dipping time in Fig. 5. The V_OC value increased gradually from ca. 0.84 V (0 min) to ca. 0.88 V (30 min) but showed a stabilizing trend between 30 min and 90 min (see Tables 2 and S2†). However, the maximum J_SC value was measured at 30 min even though a similar gradual increase was measured between 0 min and 30 min. Further dipping after 30 min resulted in a slight but gradual reduction of J_SC values, which can be attributed to some nanoscale swelling or damage in the bPEI layers during continuous dissolving actions by water molecules.

Fig. 5 Solar cell parameters as a function of dipping time (T_DIP) for the PM6:Y11 solar cells with the bPEI interlayers.

Table 2 Summary of solar cell parameters (averaged from more than 10 devices) for the inverted-type PM6:Y11 solar cells with the water-dipped bPEI layers according to the dipping (immersion) time (T_DIP) of bPEI layers (initial thickness = 2.0 nm)

Parameter	T DIP (min)
	0	10	30	60	90
V OC (V)	0.84 (0.85–0.82)	0.87 (0.88–0.87)	0.88 (0.89–0.86)	0.88 (0.88–0.87)	0.88 (0.88–0.87)
J SC (mA cm^−2)	13.67 (15.32–12.59)	20.06 (20.81–19.49)	20.14 (21.62–19.02)	19.92 (20.67–19.00)	19.80 (20.61–19.05)
FF (%)	33.22 (36.72–29.86)	52.67 (53.45–51.26)	57.96 (61.73–53.92)	55.66 (58.13–52.47)	54.04 (54.93–53.06)
PCE (%)	3.79 (4.42–3.11)	9.22 (9.58–8.66)	10.25 (10.76–9.79)	9.70 (10.36–9.33)	9.39 (9.71–9.12)
R S (kΩ)	2.31 (5.13–1.08)	0.24 (0.25–0.22)	0.15 (0.21–0.01)	0.19 (0.22–0.15)	0.23 (0.25–0.20)
R SH (kΩ)	3.14 (3.54–2.61)	5.80 (6.90–4.23)	8.86 (16.50–5.82)	8.10 (13.53–5.28)	7.70 (10.08–5.72)

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From the V_OC and J_SC trends, it is considered that some upper part of the bPEI layers was dissolved by water but the bottom part facing the ITO surfaces still remained even after the immersion process of up to 90 min. As a consequence of both V_OC and J_SC, higher FF and PCE values could be achieved by the water-dipping process. Here it is noted that the work function change by the water-dipping was not so large between −3.71 eV and −3.95 eV (see Fig. 6a). Therefore, the electron transport from the BHJ layer just to the bPEI layers might be similar in terms of built-in potential (see Fig. 6b). Conclusively, the major limiting factor affecting the performance of PSCs can be the thickness of bPEI layers, which is directly related to the electrical resistance (see the series resistance trend in Fig. 5).

Fig. 6 (a) Work function as a function of dipping time (T_DIP) for the water-dipped bPEI layer-coated ITO electrodes. (b) Flat energy band diagram for the PM6:Y11 solar cells with the water-dipped bPEI interlayers (T_DIP = 0, 30, 60 min). Note that the unit of energy (eV) in (b) is omitted to avoid crowding the diagram.

Finally, the shelf lifetime was tested by storing the best PSCs with the water-dipped bPEI layers (T_DIP = 30 min) inside an argon-filled glovebox (inert conditions). As displayed in Fig. 7a, the J–V curves of devices were almost similar without noticeable changes even after 5 weeks. The detailed analysis in Fig. 7b discloses that all the parameters were well maintained and showed no large deviations according to the storage time. This result reflects that the huge reduction of J_SC and FF (even for V_OC) under 1 sun conditions in Fig. S5† can be ascribed mainly to the photo-induced degradation of BHJ layers but not seriously caused by the interfacial reaction between the primary amine units of bPEI and polar units (such as the ketone part) of Y11 under dark conditions. This may be supported by the changed optical absorption spectra of PM6:Y11 BHJ layers that were exposed to 1 sun conditions for 10 h in spite of almost no change in the case of storage for 5 weeks in the dark (see Fig. 8 and S6†). Note that the device stability under continuous 1 sun conditions (100 mW cm⁻²) was tested but all the PSCs exhibited very quick degradation even though marginal difference was measured according to the dipping time (see Fig. S5a†). This quick degradation can be basically attributed to the degradation of BHJ layers.^34–36 The detailed change of solar cell parameters is plotted as a function of exposure time to understand the influence of bPEI dipping time. As shown in Fig. S5b,† all the PSCs exhibited an almost linear reduction of V_OC with the exposure time. After 10 h exposure, the percentage of V_OC drop was ca. 17.1% at T_DIP = 10 min and ca. 10.0% at T_DIP = 30 and 60 min. However, the J_SC values showed a relatively large drop (ca. 63–68% after 10 h exposure) with the light exposure time irrespective of the dipping time. The FF values were also gradually and considerably reduced with the light exposure time and the most significant FF drop (ca. 46.2%) was measured at T_DIP = 10 min. Both J_SC and FF drops can be supported by the noticeably increased series resistances in Fig. S7† (see the dark J–V curves in Fig. S8†). Keen attention is paid to the formation of a sigmoidal shape in the J–V curves in the case of T_DIP = 10 min, even though no such sigmoidal J–V curves were measured for relatively longer dipping times (T_DIP = 30–90 min). This result implies that the thickness of bPEI layers may be correlated with the stability of the PSCs with the PM6:Y11 BHJ layers under 1 sun conditions. According to the previous reports on the negative effect of amine units, it is assumed that the relatively higher number of primary amines in the thicker bPEI layers might have a greater influence on the reaction with the electron-acceptor molecules (Y11 here) leading to such a relatively larger decay of V_OC and FF.³⁷ Consequently, as shown in Fig. S5b,† the PCE of devices was significantly reduced with the light exposure time.

Fig. 7 Shelf-lifetime of the PM6:Y11 solar cells with the water-dipped bPEI interlayers (T_DIP = 30 min) upon storing for 5 weeks under dark conditions (argon-filled glovebox): (a) J–V curves and (b) solar cell parameters as a function of storage time.

Fig. 8 Optical absorption spectra of the PM6:Y11 films (glass/ITO/bPEI/PM6:Y11) that were stored for 5 weeks in the dark (blue solid lines) and exposed to 1 sun conditions for 10 h (red dashed lines): (a) T_DIP = 30 min and (b) T_DIP = 60 min.

Conclusions

The water-dipping effect of bPEI layers was investigated by fabricating inverted-type PSCs with PM6:Y11 BHJ layers. The initial examination of bPEI thickness effects found that the insertion of 0.1 nm-thick bPEI layers greatly enhanced V_OC (from 0.38 V to 0.84 V) and J_SC (from 15.99 mA cm⁻² to 19.09 mA cm⁻²). A very similar level of J_SC and V_OC was measured in the thickness range between 0.1 nm and 1.5 nm, which can be considered an effective thickness range leading to the best-optimized performances. The thicker bPEI layers could keep pretty high V_OC levels but resulted in poor J_SC values. Consequently, the 0.5 nm-thick bPEI layers could deliver the highest PCE (from ca. 2.49% to 10.54%). The enhanced PCE was attributed to the lowered work function caused by the formation of dipole layers between the bPEI layers and the ITO surfaces. The water-dipping process by immersing the 2 nm-thick bPEI layer-coated ITO-glasses led to remarkably improved J_SC (from ca. 13.67 mA cm⁻² to 20.14 mA cm⁻²) and FF (from ca. 33.22% to 57.96%). The water-dipping for 30 min could deliver about 2.7 times improved PCE due to the enhanced FF but a longer time dipping (>30 min) adversely influenced the performance of PSCs. Although the PSCs with the water-dipped bPEI layers showed quick performance degradation under 1 sun conditions, they exhibited very stable performances upon storage for 5 weeks under dark conditions. Finally, the present water-dipping process is expected to contribute to the process development for ultrathin nanoscale interfacial layers that cannot be easily coated for large-size or film-type substrates with conventional wet-coating technologies.

Author contributions

Hyunji Son: data curation, formal analysis, investigation, methodology, validation, visualization, writing – original draft. Woongki Lee: data curation, formal analysis, investigation, methodology, validation, visualization, writing – original draft. Sooyong Lee: methodology. Hwajeong Kim: project administration. Youngkyoo Kim: conceptualization, funding acquisition, supervision, writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Research Foundation (NRF) of Korea (2021R1I1A3A04037494, 2021R1I1A1A01060041, and Basic Science Research Program_2018R1A6A1A03024962) and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) – the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (KETEP-MOTIE_20224000000150).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3se00831b

‡ These authors contributed equally to this work.

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