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Effect of Rubidium-Incorporation on the Structural, Electrical, and Photovoltaic Properties of Methylammonium Lead Iodide-based Perovskite Solar Cells Ik Jae Park, Seongrok Seo, Min Ah Park, Sangwook Lee, Dong Hoe Kim, Kai Zhu, Hyunjung Shin, and Jin Young Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13947 • Publication Date (Web): 10 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017

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ACS Applied Materials & Interfaces

Effect of Rubidium-Incorporation on the Structural, Electrical, and Photovoltaic Properties of Methylammonium Lead Iodide-based Perovskite Solar Cells Ik Jae Park,† Seongrok Seo,‡ Min Ah Park,† Sangwook Lee,§ Dong Hoe Kim,∞* Kai Zhu,∞ Hyunjung Shin,‡ and Jin Young Kim†* †

Department of Materials Science and Engineering, Seoul National University, Seoul 08826,

South Korea ‡

Department of Energy Science, Sungkyunkwan University, Suwon 16419, South Korea

§

School of Materials Science and Engineering, Kyungpook National University, Daegu 41566,

South Korea ∞

Chemical and Materials Science Center, National Renewable Energy Laboratory, 15013 Denver

West Parkway, Golden, Colorado 80401, United States KEYWORDS methylammonium lead halides, solar cells, rubidium, trap density of states, conductivity

ACS Paragon Plus Environment

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ABSTRACT

We report the electrical properties of rubidium-incorporated methylammonium lead iodide ((RbxMA1–x)PbI3) films and the photovoltaic performance of (RbxMA1-x)PbI3 film-based p–i–ntype perovskite solar cells (PSCs). The incorporation of a small amount of Rb+ (x=0.05) increases both the open circuit voltage (Voc) and the short circuit photocurrent density (Jsc) of the PSCs, leading to an improved power conversion efficiency (PCE). However, a high fraction of Rb+ incorporation (x=0.1 and 0.2) decreases the Jsc and thus the PCE, which is attributed to the phase segregation of the single tetragonal perovskite phase to a MA-rich tetragonal perovskite phase and a RbPbI3 orthorhombic phase at high Rb fractions. Conductive atomic force microscopic

and

admittance

spectroscopic

analyses

reveal

that

the

single-phase

(Rb0.05MA0.95)PbI3 film has a high electrical conductivity due to a reduced deep-level trap density. We also found that Rb substitution enhances the diode characteristics of the PSC, as evidenced by the reduced reverse saturation current (J0). The optimized (RbxMA1-x)PbI3 PSCs exhibited a PCE of 18.8% with negligible hysteresis in the photocurrent-voltage curve. The results from this work enhance the understanding of the effect of Rb incorporation into organicinorganic hybrid halide perovskites and enables the exploration of Rb-incorporated mixed perovskites for various applications, such as solar cells, photodetectors, and light-emitting diodes.

Introduction


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Organic-inorganic hybrid halide perovskites (ABX3) – where A and B refer to monovalent (methylammonium (MA+ or CH3NH3+) and formamidinium (FA+ or CH(NH2)2+)) and divalent cations (Sn2+ or Pb2+), respectively, and X refers to halogen anions (I–, Br– or Cl–) – exhibit intriguing optical and electrical properties, such as superb light absorption coefficients (104–105 cm–1) under visible light (1.2–2.9 eV)1–3 and bipolar charge transport with extremely long charge diffusion lengths (over 100 µm in bulk single crystals and up to 1 µm in polycrystalline films).4–8 Among the diverse hybrid halide perovskites, MAPbX3 and FAPbX3 have been intensively studied as alternative light absorbing and/or charge transporting materials for various optoelectronic devices, such as perovskite solar cells (PSCs),9–11 light-emitting diodes,12–16 and photodetectors.17–19 However, each composition has its shortcomings; MAPbI3 has a slightly wide bandgap for it to fully absorb visible light, and FAPbI3 is prone to transform into the yellow non-perovskite phase (δ-FAPbI3). To overcome these shortcomings, several groups have studied compositional engineering, especially, mixing monovalent cations in the A site, creating organicorganic

(MA+-FA+)

or

organic-inorganic

(FA+-MA+-Cs+,

FA+-Cs+,

or

MA+-Cs+)

combinations.20–24 The incorporation of FA into MAPbI3 (or MAPb(BryI1-y)) has been reported to extend the light absorption edge up to 850 nm, and the incorporation of Cs into (FAxMA1x)Pb(BryI1-y)3

or FAPbX3 has been reported to improve the stability of the black perovskite phase

against inherent strain-induced phase segregation21,23 and humidity-induced decomposition.24 Recently, rubidium cations (Rb+) have been considered as candidates for mixed-cation perovskites in FA-based composition.25–28 The rubidium incorporation has led to improve the device performance and stability of perovskite materials. Zhang et al. demonstrated fabrication of perovskite films using the gas-quenching method. Rb-incorporated FA-based perovskites showed superior shelf-life in ambient conditions and device performance.25 Saliba et al. reported


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that (FA-MA-Cs-Rb)1Pb(Br-I)3 has a suitable tolerance factor in order to maintain its cubic perovskite structure and exhibits fair thermal stability and light stability under continuous 1 SUN irradiation.26 Park et al. also demonstrated effect of Rb-incorporation on FAPbI3 such as enhanced moisture stability and improved photovoltaic performance.27 In this work, we report the effects of Rb+ incorporation on the crystal structure and electrical properties of solution-processed MAPbI3 (MALI) films and on the photovoltaic properties of PSCs fabricated with these films. Rb+-incorporated MALI was prepared using a mixed precursor solution containing methylammonium iodide (MAI) and rubidium iodide (RbI). We found that the tetragonal perovskite phase segregates into an MA-rich tetragonal phase close to MALI and a RbPbI3 (RbLI) orthorhombic phase when the precursor solution contained over 10 mol% Rb+ (Rb10). Furthermore, 5 mol% Rb (Rb05) maintains a tetragonal perovskite phase and significantly reduces the electrical resistance and deep-level trap density, leading to an increase in both the short-circuit current density (Jsc) and open-circuit voltage (Voc) from 20.9 to 21.8 mA cm–2 and from 0.99 to 1.02 V, respectively, of p–i–n planar-type PSCs. An 18.8% PCE was achieved from the Rb05-PSC via optimization of the fabrication process.

Experimental Methods Device fabrication: Tin-doped indium oxide (ITO) glass was consecutively cleaned with diluted detergent, deionized water, acetone and ethyl alcohol. The ITO glass was dried with an N2 stream and treated with UV-ozone for 15 min to residual organic contaminants. Then, a 0.1 M nickel(II) acetate tetrahydrate (Sigma-Aldrich, ≥99.0% purity) solution in ethyl alcohol was spincoated on the ITO glass and annealed at 300 °C for 1 h to generate NiOx. After cooling, a 1.5 M


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mixed solution of PbI2 (Alfa Aesar, 99.9985%), methylammonium iodide (Dyesol), dimethyl sulfoxide (Sigma-Aldrich, anhydrous, ≥99.9% purity) in a 1:1:1 molar ratio in N,Ndimethylformamide (Sigma-Aldrich, anhydrous, 99.8% purity) was spin-coated at 4000 rpm for 30 sec on the NiOx/ITO substrate. During spinning, 1 mL of diethyl ether (Daejung, ≥99.0% purity) was dropped onto the substrate, which was then annealed at 65 °C for 1 min and then at 100 °C for 20 min. A rubidium-substituted perovskite layer was obtained by stoichiometric mixing of RbI (Sigma-Aldrich, 99.9%) and MAI in the perovskite solution. A [6,6]-phenyl-C61butyric acid methyl ester ([60]PCBM, nano-C, 99.5% purity) solution in chlorobenzene (20 mg mL–1) was spin-coated at 2000 rpm for 60 sec onto the perovskite layer. For polyethylenimine (PEIE, Sigma-Aldrich, 80% ethoxylated solution) modified devices, ~0.2 wt% PEIE solution diluted in methyl alcohol was spin-coated at 6000 rpm for 30 sec. Finally, a 120 nm-thick Ag electrode was deposited by thermal evaporation. Characterization: Structural analysis of (RbxMA1-x)PbI3 was performed by X-ray diffraction (XRD, New D8 Advance, Bruker) using Cu Kα radiation (λ=0.1542 nm). The bandgap energy of (RbxMA1-x)PbI3 was determined from the absorption spectrum collected using a UV-Vis-NIR spectrometer (Cary 5000, Agilent). Photoluminescence spectra of perovskite films were measured by Fluorolog3 spectrofluorometer (HORIBA Scientific) for samples excited at 530 nm. The morphology of (RbxMA1-x)PbI3 was observed using a field-emission scanning electron microscope (FESEM, SU70, Hitachi). TOF-SIMS 2D chemical distribution were measured using a TOF-SIMS measurement with a 30 kV 1 pA Bi beam (TOF.SIMS-5, ION-TOF). The J–V curves were measured with a potentiostat (CHI 608C, CH Instruments) under dark and AM 1.5 illumination (100 mW cm–2) using a solar simulator (Peccell Technologies, AAA graded) after calibration with a reference cell (PV Measurements). The masked aperture size (0.14 cm2) was


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slightly smaller than the actual silver back electrode. The external quantum efficiency (EQE) was measured on incident photon-to-current conversion equipment (PV Measurements). Current and topological maps of the sample surfaces were measured simultaneously in ambient air using a commercial atomic force microscope (SPA-400, SII, Japan) and electrically conductive diamond-coated Si tips (CDT-FMR-10, Nanoworld, Inc.) with a typical resonant frequency of 13 kHz and a spring constant of 4.5 N/m. All images were acquired with a bias voltage of 1.5 V at a scan rate of 0.2 Hz. Positive biases were applied to the substrate while the tips were grounded. Mott-Schottky plots and admittance spectra (0 V bias, 20 mV AC amplitude) were measured using a potentiostat (Parstat MC, Princeton Applied Research) under dark conditions. The angular frequency (ω) was converted into energy ( = ln( / )), assuming an attempt-toescape frequency (ω0) of 1011 s–1.

Results and Discussion Figure 1a shows the X-ray diffraction patterns of the (RbxMA1-x)PbI3 films with various Rb contents. Here, we define x as the fraction of Rb+ ions (i.e., [Rb+]/{[Rb+] + [MA+]}) incorporated into the precursor solution for the film coating. Rb00, Rb05, Rb10, and Rb20 denote x=0, 0.05, 0.1 and 0.2, respectively. The crystal structure of MALI and RbLI (Figure S1) are tetragonal (I4/mcm) and orthorhombic (Pnma) at room temperature, respectively. The diffraction pattern of the tetragonal perovskite is clearly observed in all the films, indicating that the matrix of each film is MALI. The barely discernible peak at 12.7° results from a small amount of PbI2, which is a common residue of the solvent engineering process. It should be noted that the Rb10 and Rb20


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films show a weak peak at approximately 13.3°, which can be assigned to the (120) plane of orthorhombic RbLI, while the Rb00 and Rb05 films do not show this peak. The phase

Figure 1. (a) XRD patterns of (RbxMA1-x)PbI3 (x=0, 5, 10, and 20) and (b) narrow scan at the (110) peak position of perovskite. (c) The effective tolerance factor with various substituted Rb fractions. segregation indicates that the MALI-RbLI alloy is not a homogeneous solid solution system, which may result from the large difference in the ionic radius of MA+ and Rb+. The magnified XRD patterns show a peak at approximately 14.152° (Figure 1b), corresponding to the (110) peak of MALI, and undergo distinct peak shifts to larger angles when Rb is incorporated. The


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peak shift toward larger angles indicates the reduction in the lattice parameters, as determined, using Bragg’s equation, to be a=6.2530, 6.2420, 6.2443, and 6.2475 Å for Rb00, Rb05, Rb10, and Rb20, respectively. Interestingly, the (110) peak of Rb05 shifts the farthest (14.177°) from that of Rb00, and then the peak approximately returns to the initial position as the Rb incorporation increases (14.172° for Rb10 and 14.165° for Rb20). Therefore, in the Rb05 film, all the remaining Rb+ ions occupy the MA+ sites in the MALI matrix, as evidenced by the large peak shift without phase segregation, while in the Rb10 and Rb20 films, some Rb+ ions form the RbLI phase. Normalized PL spectra (Figure S2) also exhibit the largest peak shift for the Rb05 film, which is in a good agreement with the XRD patterns. Figure 1c gives the calculated effective Goldschmidt tolerance factor ( ) of (RbxMA1-x)PbI3 by assuming the complete, homogeneous substitution of Rb+ in the precursor solution for MA+ in the MALI matrix; was calculated using the effective (i.e., average) ionic radius ( ) as follows:29,30 = + (1 − ) =

√!("#$ )

(1)

(2)

where % is the ionic radius of the iodine anion. The of pure MALI is 0.91, whereas that of pure RbLI is 0.77. In general, the cubic perovskite structure is stable when the tolerance factor ranges from 0.9–1.0, even though MALI has a slightly distorted cubic structure (i.e., tetragonal perovskite structure with tilted PbO6 octahedrons) at room temperature, and the orthorhombic perovskite structure is stable when the tolerance factor ranges from 0.71–0.9.29,30 It is worth noting that decreases with increasing Rb fraction and consequently descends into orthorhombic-stable territory when the substituted Rb fraction is increased to over 8.5 mol%. Therefore, it seems reasonable to associate the phase segregation in the Rb10 and Rb20 films


8

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(Figure 1b) with the low tolerance factors, although the Rb ions incorporated into the precursor are

Figure 2. (a) The photocurrent-voltage (J–V) curves of the planar structure solar cells with Rb00, Rb05, Rb10 and Rb20 (scan rate: 60 mV/s). (b) Absorption spectra and (αhν)2 vs. Energy plots (inset) of the (RbxMA1-x)PbI3 film for x=0, 5, 10, and 20. (c) Top-view SEM images of the perovskite films with different Rb contents. not likely to be completely incorporated into the final film, as evidenced by the existence of PbI2 (Figure 1a). We fabricated inverted-type (p–i–n junction) PSCs composed of ITO, NiOx, perovskite, PCBM, and Ag layers (inset of Figure 2a) using the Rb-incorporated MALI perovskite films to


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investigate the effects of Rb incorporation into MALI on the photovoltaic properties. Typical photocurrent density-voltage (J–V) curves of the (RbxMA1-x)PbI3-based PSCs are shown in Figure 2a. Rb05-PSC exhibits a higher PCE (16.5%) than Rb00-PSCs (15.6%). The higher PCE of Rb05-PSC is mainly due to the higher Jsc (21.8 mA cm–2) and Voc (1.02 V), compared to those of Rb00-PSC (20.9 mA cm–2, 0.99 V). The fill factor (FF) of both devices are similar at approximately 0.75. On the other hand, when the Rb concentration is increased to over 5 mol%, the device performance distinctly decreases, especially the Jsc (18.9 mA cm–2 for Rb20) and FF (0.62 for Rb20). Consequently, Rb20-PSC shows a significantly deteriorated PCE of 12.5%. We attributed the decreased solar cell performance in Rb10 and Rb20 to the segregated Rb-rich phase, which would have a large bandgap energy, a low electron-hole mobility due to the exceedingly flat band structure (i.e., small curvature) and a high exciton binding energy (~150 meV) for the RbLI matrix.31 In particular, considering the decreased visible light absorbance and the slightly blue-shifting absorption edge (Figure 2b) with increasing Rb incorporation, the decreasing photocurrent is mainly due to the light harvesting ability. The poor absorption ability may be due to a large portion of Rb-rich phase, which would have a large optical bandgap similar to that of the RbLI matrix (3.14 eV, Figure S3). The increase in the segregated Rb-rich phase is observed in the microstructures of the films. Figure 2c shows typical plan-view FESEM images of the (RbxMA1-x)PbI3 films on the ITO/NiOx substrates. It is noticeable that Rb00 and Rb05 show large grains with smooth surfaces, while Rb10 and Rb20 show additional microstructures and bright distorted sharp grains. Given that XRD peaks for the Rb-rich phase are observed only for Rb10 and Rb20 and that they increase with increasing Rb incorporation (Figure 1a), the bright grains are assigned to the Rb-rich orthogonal phase. The 2D


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compositional mapping images obtained by TOF-SIMS (Figure S4) also indicate that the Rb-rich phases are segregated from the MALI matrix. To understand the effects of the substitution of Rb cations in the MALI perovskite matrix, we further characterized the optoelectronic properties by measuring the EQE, c-AFM, dark current density, and trap density of states (tDOS). To avoid any interference effects from the secondary

Figure 3. (a) EQE spectra (left-axis) of the PSCs and corresponding integrated Jsc values for Rb00 and Rb05. (b) Topological and c-AFM images measured at different position with and without Rb substitution (scale bar: 500 nm). phase (i.e., the Rb-rich phase), we compared only the Rb00 and Rb05 samples, which have a homogenous perovskite phase. Figure 3a presents the EQE spectra and integrated Jsc values of


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Rb00-PSC and Rb05-PSC. Both devices show similar spectra from 300 nm to 800 nm. The integrated Jsc value calculated from the EQE curves for Rb00 and Rb05 are 20.1 and 20.9 mA cm–2, respectively, which are consistent with the J–V curves shown in Figure 2a. It should be noted that Rb05-PSC exhibits a slightly higher quantum efficiency than Rb00-PSC at wavelengths from 450 nm to 750 nm, despite the slightly inferior absorption properties (Figure 2b) of Rb05 to Rb00, i.e., lower absorbance and slightly wider bandgap (Figures 2b, S5). Therefore, considering

Figure 4. (a) J–V curves under dark conditions for the PSCs with Rb00 and Rb05. Plots of (b) dJ/dV against voltage, (c) dV/dJ against 1/(J+Jsc–GSHV/A), and (d) ln(J+Jsc–GSHV/A) against VJRSA with fitting lines.


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the results comprehensively, such as the superior quantum efficiency, Jsc, and even Voc (Figure 2a), the charge collection properties of the samples should be investigated to understand the main reason for the superior photovoltaic performance of Rb05-PSC compared to Rb00-PSC. Figure 3b shows the current and topology mapping images of the Rb00 and Rb05 films, collected using c-AFM under 1.5 V bias in the dark. The topological images show that both films have similar morphologies with slightly different average grain sizes, 187 nm for Rb05 and 165 nm Rb00, when the grain size is determined by the line intercept method using the AFM and SEM (Figure 2c) images. In stark contrast, c-AFM shows significantly different images for Rb00 and Rb05. The Rb05 film contains much brighter and larger domains than the Rb00 film over the entire mapping region, which indicates that a larger vertical current flows through Rb05 than Rb00. The average current value of Rb05 (0.253 nA), obtained from the line profiles of the c-AFM image, is 2.5 times higher than that of Rb00 (0.107 nA). The higher current indicates the higher electrical conductivity of the thin films, as the film thicknesses are all the same (see Figure S6). As the MA and Rb cations have the same valence, it is likely that the substitution would not generate excess net charge. Therefore, the increased electrical conductivity resulting from Rb substitution may be due to the increased charge mobility (σ=nqµ, where n is the charge concentration, q is the elementary charge, and µ is the carrier mobility), and hence the photoexcited charges in the Rb05 film can be carried more efficiently than those in Rb00 film. The device junction properties of the PSCs were analyzed by their dark current curves (Figure 4a). Figures. 4b–4d show the reverse saturation current (J0), shunt resistance (RSH) and series resistance (Rs) of the PSCs, respectively, which were determined by fitting the dark current curves. Each parameter was determined using the diode equation:32–35


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& = & 'exp +

,(-./01 2) 6− 345

17 +

89: ; 2

− &? = +

/ 345 6 ln( /AB @ C

+ 1)

(4)


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where Jph is the photocurrent density. Because a lower J0 indicates a higher Voc, it is clear that Rb substitution results in an enhancement in the diode characteristics, which leads to an increase in Voc. To study the origin of the enhanced diode characteristics upon Rb substitution, the tDOS of the Rb00 and Rb05 perovskite materials was evaluated by admittance spectroscopy of their respective PSCs, as shown in Figure 5a. The tDOS (NT) was derived from the measured capacitance (C) as a function of the angular frequency (ω) using the following equation:24,36–38 -

IJ

#G DE (F ) = − @H I 45

(5)

where Eω is the difference between the trap energy level and the valence band edge, Vbi is the built-in potential, and W is the depletion width. Vbi and W are obtained from the Mott-Schottky plots in Figure 5b and are summarized in Table S2. The tDOS of both samples at low F below 0.375 eV (shallow trap states) are similar. Interestingly, however, their deep-tDOS values at high F above 0.375 eV are significantly different. Rb05 has a much lower deep-tDOS than Rb00, as

much as one order of magnitude, at F ≈0.5 eV. The trapped charge (electrons or holes) at a shallow level can be easily detrapped with a small activation energy, while deep-level trapped charges will annihilate or recombine with an opposite charge to emit phonons, leading to an increase in the recombination loss and an increase in the leakage current at the diode junction.39– 41

Therefore, it can be understood that Rb substitution lowers the deep-tDOS, leading to superior

diode characteristics (i.e., lower J0 and higher RSH) and consequently a larger Voc. In addition, the low deep-tDOS of Rb05 may facilitate the charge transport, which results in the high electrical conductivity confirmed by c-AFM.40,42 The exact reason why Rb substitution


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Figure 6. Photovoltaic characterization of the PSCs with PEIE (scan rate: 60 mV). (a) J–V curve of the best performing solar cell with Rb05 perovskite, depending on the scan direction. (b) Stabilized photocurrent density and PCE of Rb05-PSC measured at maximum power (0.87 V). lowers the deep-tDOS needs to be studied in more detail in the near future. Tentatively, we suggest the reason to be the accelerated grain growth by Rb substitution, as evidenced by the larger grain size of Rb05 compared with that of Rb00, which was possibly caused by the fast diffusion of Rb ions in the MALI matrix due to its small ionic radius. Finally, we optimized the device fabrication process, including the interfacial engineering, to obtain high-efficiency solar cells using Rb05. Figure 6 shows the photovoltaic properties of Rb05-PSC, which employed polyethyleneimine ethoxylated (PEIE). The extremely thin PEIE


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layer was inserted at the interface between the PCBM and Ag layers to modify work function and thus, to reduce the barrier energy between PCBM and the Ag metal electrode.43,44 Consequently, Rb05-PSC showed a significantly enhanced fill factor. The J–V curve, in forward and reverse scanning directions, of the best performing Rb05-PSC is presented in Figure 6a. The best device exhibits a PCE of 18.8% (Jsc of 22.8 mA cm–2 and Voc of 1.02 V) with negligible hysteresis depending on the scan direction. The statistics of the solar cell parameters are presented in Figure S7. The steady-state photocurrent density at maximum power (0.87 V) is plotted as a function of time in Figure 6b. A stabilized PCE of 18.3% with negligible PCE degradation (< 3%) was achieved within ~500 sec under 1 SUN illumination. To investigate the humidity stability of solar cells, Rb00- and Rb05-based devices were exposed to air (RH~40%) without encapsulation. The PCE variation with time in Figure S8 shows that the efficiency was reduced at a similar rate. It seems to be no significant difference in stability.

Conclusion In summary, we studied the effect of monovalent Rb-cation incorporation in MAPbI3-based solar cells. We revealed that MALI-RbLI is not a homogeneous solid solution system, and 5 mol% Rb incorporation does not induce phase segregation to the MA-rich tetragonal phase (Rb-doped MALI) and RbLI orthorhombic phase, which is relevant to the effective tolerance factor. The increased electrical conductivity of the perovskite thin film upon Rb substitution increases the EQE and Jsc of the PSCs. In addition, the diode characteristics were enhanced upon Rb substitution, which results in an increase in the Voc. We found that Rb substitution into MALI reduces the density of deep-level trap states, which is considered to be the reason for the reduced


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J0, increased RSH, and increased electrical conductivity. Through the optimization of the device structure and fabrication process for the Rb0.05MA0.95PbI3 perovskite-based PSCs, a high PCE of 18.8% was achieved with negligible hysteresis. ASSOCIATED CONTENT Supporting Information. Crystal structure of the perovskite, PL and absorption spectrum of RbPbI3, TOF-SIMS 2D chemical distribution maps, typical cross-sectional SEM images of the PSCs, statistics of the solar cells parameters, device parameters measured from the dark current analysis, the built-in potential and depletion width of Rb00 and Rb05. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] ACKNOWLEDGMENT This work was supported by the Nano·Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2012M3A7B4049989, 2016M3A7B4909369). This work was supported by the Korea Institute of Energy Technology Evaluation and Planning(KETEP) and the Ministry of Trade, Industry & Energy(MOTIE) of the Republic of Korea (20163010012580). REFERENCES


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