High-Efficiency Colloidal Quantum Dot Photovoltaic Devices Using Chemically Modified Heterojunctions Randi Azmi,† Seung-Hwan Oh,‡ and Sung-Yeon Jang*,† †
Department of Chemistry, Kookmin University, 77 Jeongneung-ro, Seongbuk-gu, Seoul 136-702, Republic of Korea Radiation Research Division for Industry and Environment, Korea Atomic Energy Research Institute (KAERI), 29 Geumgu-gil, Jeongeup-si, Jeollabuk-do 580-185, Korea
‡
S Supporting Information *
ABSTRACT: High-efficiency colloidal quantum dot photovoltaic devices (CQDPVs) are achieved by improving the interfacial charge extraction via chemical modification of PbSCQD/ZnO heterojunctions. Simple treatment of the heterojunctions using a chemical modifier, 1,2-ethanedithiol, effectively reduces the interband trap sites of the ZnO nanoparticles (ZnO-np) by passivation of the notorious intrinsic oxygen-deficient defects. As a result, the interfacial bimolecular recombinations between (i) trapped electrons in the ZnO-np layers and the holes in the CQD layers and (ii) accumulated electrons in the CQD layers and the holes in the CQD layers are suppressed. Consequently, the power conversion efficiency of the chemically modified CQDPVs reached a certified power conversion efficiency of 10.14% with decent air stability. Notably, the entire device fabrication process, including chemical modification, could be performed at room temperature under ambient atmosphere.
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readily prepared at low temperatures by the in situ sol−gel conversion method8,11,12 or by deposition of preformed nanoparticles.13−15 It is known that the interfacial charge recombinations at the CQD/ETL heterojunction occur with much faster kinetics than the recombinations within CQD layers.16−19 Considering the fact that a major portion of charge generation and extraction occurs in the vicinity of the CQD/ETL heterojunction, control of the interfacial recombination is essential for enhancing the device performance. Typically, ZnO possess a high density of surface defects, which is the major origin of interfacial recombinations with impaired charge selectivity;20,21 thus, reduction of surface defects on ZnO is a key strategy for optimizing the device performance.22−24 The development of easy methods to reduce the surface defects of ZnO should be beneficial for the charge extraction properties and efficiency of the devices. In this work, high-efficiency PbS-CQD/ZnO heterojunction CQDPVs were developed by reducing the interfacial recombinations at the heterojunctions via simple chemical modification. The chemical modifier induced effective surface passivation of the chemically prepared ZnO nanoparticle (ZnOnp), which notoriously possesses an intrinsically high surface
olloidal quantum dots (CQDs) are promising photoactive materials for solar cells owing to their band gap tunability based on quantum size effects, along with their facile synthesis through wet chemistry. Notably, their near-infrared (NIR, λ > 800 nm) absorption provides the potential to utilize lower-energy photons, which has not been easily achieved with organic counterparts such as organic photovoltaic devices or dye-sensitized solar cells. Thus, the accessibility to the visible−NIR regime has been a crucial driving force for research on CQD-based photovoltaic devices (CQDPVs). Recently, devices with a power conversion efficiency (PCE) of >9% have been reported.1−3 Generally, CQDPVs are constructed by sandwiching photoactive CQDs between two metal electrodes. More often, charge-selecting layers are inserted between the CQD and electrodes to improve the quasi-Fermi level separation and charge extraction efficiency. PbS-CQDs have been widely used as photoactive layers because of their easy synthesis and processing,4 while wide band gap metal oxides have been dominantly used as n-type electron transporting layers (ETLs) for construction of p-n heterojunctions. The CQD/ETL heterojunctions are conventionally located near transparent electrodes for effective extraction of electrons; thus, the properties of the ETLs crucially influence the device performance.5−7 ZnO has offered attractive features, such as optical transparency, easy fabrication, appropriate work function, and high electron mobility.8−12 Furthermore, ZnO ETLs can be © XXXX American Chemical Society
Received: April 14, 2016 Accepted: May 9, 2016
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Figure 1. (a) Architecture of CQDPVs, (b) J−V characteristics under AM 1.5G one-sun illumination (100 mW cm−2), (c) external quantum efficiency (EQE) spectra, and (d) J−V characteristics under dark conditions.
Table 1. Summary of Device Performance in Figure 1 and Figure S8 device pristine ZnO-np ACN-ZnO-np EDT-ZnO-np EDT-ZnO-np (certified)
VOC (V)
PCE (%) 9.45 (8.82 ± 0.56) 9.56 (8.95 ± 0.53) 10.26 (9.64 ± 0.50) 10.14 ± 0.22
0.67 0.67 0.67 0.68
(0.67 ± 0.02) (0.67 ± 0.02) (0.67 ± 0.01) ± 0.09
JSC (mA cm−2) 21.72 (21.42 ± 0.72) 21.52 (21.12 ± 0.89) 22.20 (22.10 ± 0.76) 22.19 ± 0.41
FF 0.65 0.66 0.69 0.68
(0.62 ± 0.04) (0.62 ± 0.04) (0.65 ± 0.04) ± 0.01
RS (Ω cm2)
RSH (Ω cm2)
JSC (IPCE) (mA cm−2)
16.78 17.21 15.28 −
601 652 861 −
21.62 21.47 22.13 −
nps. The thickness of the ZnO-np layers was ∼50 nm. The preparation of the ZnO-np dispersion was adapted from the literature with slight modification.26,27 For the chemical modification, 0.1 vol % EDT solution in acetonitrile (ACN) was spin-coated on the ZnO-np layers, followed by drying under ambient conditions for 15 min. The PbS-CQD active layers were deposited using a spin-coating-based conventional layer-by-layer (LBL) method.4,28 The thicknesses of the CQD layers was ∼300 nm as indicated at the cross-sectional scanning electron microscopy (SEM) images in Figure S1b. The band gap of the PbS-CQDs used in this study was ∼1.48 eV, which was determined from the first excitonic peak in the UV−vis spectrum (Figure S1). Device fabrication was completed by thermal deposition of the Au electrode (80 nm) at a low pressure. All fabrication processes were carried out at room temperature under air atmosphere. The CQDPV performance is pictured in Figure 1b−d, and the resulting figures of merit are summarized in Table 1. The
defect density compared to other metal oxides prepared by high-temperature annealing or sputtering methods.20,21,24,25 The changes in the surface defects of ZnO-np and the charge extraction properties of the fabricated devices, induced by chemical modification using 1,2-ethanedithiol (EDT), were investigated. Bimolecular charge recombinations were significantly suppressed at the chemically modified heterojunctions. Consequently, the PCE of the CQDPVs with the chemically modified heterojunctions reached 10.26% with a certified PCE of 10.14%. Notably, all device fabrication processes, including chemical modification, were performed at room temperature under air atmosphere. Figure 1a shows the architecture of the CQDPVs fabricated in this study. The devices were fabricated such that the electrons were collected at the front (illuminated) side and the holes were collected at the back (Au electrode) side. The ZnO ETLs were prepared on indium-doped tin oxide (ITO)/glass substrates by spin-coating from an aqueous dispersion of ZnO101
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Figure 2. TPV analysis results: (a) photovoltage decay curve and (b) τrec with respect to VOC. Alternating current IS results: (c) IMPS and (d) IMVS. The incident photon flux for the IMPS/IMVS measurements was 3.9 × 1016 cm−2 s−1.
holes. The decay of additional photovoltage (ΔV) in the EDTZnO-np based devices was much slower than that in the unmodified control devices (Figure 2a). The plots of determined τrec as a function of VOC are shown in Figure 2b. The dependency on the τrec value of the EDT-ZnO-np based devices was 244 μs, which is ∼40% longer than that of the control devices (142 μs, Table S1). Because all devices used identical CQD active layers, the variation in the charge extraction properties must be influenced by the changes in the CQD/ZnO heterojunctions due to chemical modification. The charge extraction properties of the CQDPVs were also investigated by alternating current impedance spectroscopy (IS) analysis. The Nyquist plots in Figure S2a,b demonstrate the significant reduction of the total series resistance in the EDT-modified devices under all applied bias conditions. The charge transport time (τCT) and charge recombination lifetime (τR) were also obtained using intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS). The τCT values were estimated from the results of the IMPS analysis using the following relation: τCT = 1/2πf min (IMPS), where f min is the minimum current of the imaginary part of the lowfrequency range in the IMPS spectra (Figure 2c). The τR values were estimated from the results of the IMVS analysis (Figure 2d) using the following relation: τR = 1/2πf min (IMVS), where f min is the minimum voltage in the imaginary part of the lowfrequency range of the IMVS spectra. While the τCT values of the EDT-ZnO-np based devices decreased marginally compared to that of the control devices (0.46 μs vs 0.44 μs), the τR value was substantially higher under an incident photon flux of 3.9 × 1016 cm−2 s−1 (7.53 μs vs 4.53 μs, Table S1). These
PCE of the devices fabricated with the chemically modified ZnO-np layers (EDT-ZnO-np) was 10.26% (VOC = 0.67 V, JSC = 22.20 mA cm−2, and FF = 0.69), whereas that of the devices employing pristine ZnO-np layers was 9.45% (VOC = 0.67 V, JSC = 21.72 mA cm−2, and FF = 0.65). The device performance was unchanged by ACN treatment (ACN-ZnO-np), indicating that the performance enhancement is attributed to the EDT modifier (Figure 1b and Table 1). The chemical modification notably improved the FF of devices. The incident photon-tocurrent efficiency (IPCE) spectra presented in Figure 1c demonstrate that the slight enhancement of external quantum efficiency (EQE) of the devices in the visible range the chemical modification. The current densities were calculated from IPCE analysis were consistent with the results from the J−V analysis with negligible mismatch (Table 1). The charge selectivity was also improved in the case of the EDT-modified devices, as revealed by the dark J−V characteristics (Figure 1d). The EDTZnO-np based devices had a higher rectification ratio with reduced reverse saturation current density and increased forward output current density, compared to the control devices. Transient photovoltage (TPV) analysis is a direct method to investigate the charge recombination lifetime (τrec) by evaluating the charge-carrier decay dynamics in the presence of an electric field.29−31 TPV analysis was performed under open-circuit conditions over a range of light intensities under one-sun background illumination. A small perturbation of the VOC transiently generates additional electrons and holes, which leads to a small and momentary increase in the energy difference between the quasi-Fermi levels of the electrons and 102
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Figure 3. (a) Wide scan and (b) O 1s core level XPS spectra of pristine ZnO-np layer and EDT-ZnO-np layer. (c) PL spectra of pristine ZnOnp layer and EDT-ZnO-np layer (inset graphs indicate the UV−vis absorption spectra). UPS analysis of pristine ZnO-np layer and EDT-ZnOnp layer: (d) cutoff region and (e) valence band region. (f) Schematic illustration of the surface defect passivation of ZnO-np by chemical modification.
The defect density of the ZnO layers can also be analyzed more directly by using photoluminescence (PL) spectroscopy (Figure 3c). Generally, ZnO shows two PL peaks; the sharp peak in the UV region (∼365 nm) is attributed to the bandedge emission by radiative annihilation of excitons, and the broad peak in the visible region (∼545 nm) is due to defectrelated emission.38,39 The significant reduction of the PL of the EDT-ZnO-np layer confirmed passivation of the ZnO surface defects due to EDT modification. Similar surface defect passivation33 and the reduction of the intragap bands of ZnO40 by thiol molecules has been reported, and the present results are in good agreement with the reported data (refer to the scheme in Figure 3f). The band structure of the ZnO-np layers was studied using ultraviolet photoelectron spectroscopy (UPS). The work function (WF) values of the pristine ZnO-np layer and EDTZnO-np layer obtained from the cutoff region were 3.68 and 4.28 eV (Figure 3d), while the valence band edges relative to the Fermi level were 2.84 and 2.00 eV, respectively (Figure 3e). The optical band gap of the ZnO-np ETLs obtained from the UV−vis absorption spectra (inset of Figure 3c) was 3.14 eV. EDT modification induced a shift of the Fermi level of the ZnO-np layers toward the center of the band gap, suggesting a reduction of the electron concentration by suppression of the interband trap states.24 Charge recombination in CQDPVs is generally dominated by bimolecular recombinations. Geminate charge recombination is negligible because of the relatively low exciton binding energy in PbS-CQD at room temperature.41 The typical bimolecular recombination processes occurring in CQDPVs are
results indicate enhanced charge transport and reduced charge recombination in the EDT-ZnO-np based devices. X-ray photoelectron spectroscopy (XPS) analysis revealed the presence of chemical changes on the surface the ZnO-np layers due to EDT modification. The wide scan XPS spectra in Figure 3a clearly show the appearance of S 2p and S 2s peaks located at ∼160 eV and ∼225 eV (inset of Figure 3a) in the profile of EDT-ZnO-np, which indicates the formation of covalent bonds between Zn and S.32,33 The presence of S was also confirmed by energy dispersive X-ray spectroscopic analysis combined with field-emission scanning electron microscopy (see Figure S3). The shift of the Zn 2p3/2 peak to lower binding energy (∼0.18 eV) in the case of EDT-ZnOnp (Figure S4) indicates higher electron densities around the Zn atoms due to the presence of S. These results confirmed the formation of Zn−S bonds, which can replace the original Zn− O bonds in the surface hydroxyl groups or carboxylate groups.33 The O 1s core level spectra of ZnO can generally be deconvoluted into two peaks (Figure 3b); the lowerbinding-energy peak (∼530.05 eV) is associated with the oxygen atoms in the ZnO matrix (i.e., Zn−O bonding), and the higher-binding-energy peak (∼531.40 eV) is attributed to oxygen-deficient defects, such as oxygen vacancies and hydroxyl O−H groups.8,34−37 The intensity ratio of the high-energy peak/low-energy peak (IA/IB in Figure 3b) was significantly decreased for the EDT-ZnO-np layers compared to that of the pristine ZnO-np layers, indicating that the oxygen-deficient defects on the surface of the ZnO-np layers were passivated by Zn−S bonds due to EDT treatment. 103
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Figure 4. (a) JSC and (b) VOC with respect to light intensity. The intensity was varied from 100 to 11 mW cm−2. Schematic illustrations of charge extraction at PbS-CQD/ZnO-np heterojunctions with (c) pristine ZnO-np ETL and (d) EDT-ZnO-np ETL. Process 1 indicates the bimolecular recombination during charge sweep-out. Process 2 indicates the interfacial bimolecular recombination induced by trapped charges in ZnO-np layers. Process 3 indicates the improved extraction of accumulated charges through a new intragap band formed by defect reduction.
been reported in earlier literature by Greenham and coworkers.24 In that report, the interfacial charge recombination was reduced through nitrogen doping of ZnO preparation by atomic layer deposition (ALD).24 In our work, the reduction of trap densities and bimolecular recombination were achieved by the simple chemical modification of heterojunctions. Furthermore, there was no significant change in built-in potentials and the width of the depletion layers at the heterojunctions by chemical modification as indicated by the Mott−Schottky plots in Figure S5. Based on the characterization results, the interfacial charge recombination in the EDT-modified heterojunctions was significantly reduced because of following reasons: (i) Removal of the ZnO trap sites can reduce the chance of bimolecular recombination between the trapped electrons in the ZnO layers and the holes in the valence band of PbS-CQDs (process 2). (ii) The decrease in the accumulated charges at the heterojunctions can reduce the chance of recombination between the electrons in the intragap state of PbS-CQD and the holes in the valence band of PbS-CQD (process 1). (iii) Suppression of the Fermi-levels (i.e., WF) by removal of the trapped electrons can improve the extraction of electrons in the intragap state of PbS-CQDs to ZnO (process 3).24 (iv) The widths of the depletion regions in the PbS-CQD active layers remain intact despite the chemical modification. This reduced charge recombination is the origin of the improved performance of the chemically modified CQDPVs, as confirmed in the statistical analysis results of 50 devices (Figure S6). The long-term stability was assessed without any encapsulation in air in the dark without humidity control (Figure S7). The devices were stable for 60 days without significant
depicted in Figure 4c (processes 1 and 2). In order to gain more insight into the charge recombination at the PbS-CQD/ ZnO heterojunctions, we measured the changes in the JSC and VOC of the CQDPVs under various illumination intensities. The power law dependence of JSC upon the illumination intensity can generally be expressed as JSC ∝ Iα, where I is the light intensity and α is the exponential factor.42,43 Given that the α value is close to unity, bimolecular recombination during the charge sweep-out under short-circuit conditions is negligible.43−45 The α value of the EDT-ZnO-np based devices was 0.98, whereas that of the pristine ZnO-np based devices was 0.95 (Figure 4a). This result indicates a reduction of the bimolecular recombination in the EDT-modified devices due to the decreased space charges at the heterojunctions (process 1 in Figure 4c). Figure 4b shows the change in the VOC values as a function of the illumination intensity. The slope of VOC versus the light intensity gives kT/q, where k, T, and q are the Boltzmann constant, temperature in Kelvin, and the elementary charge, respectively.45,46 The larger the kT/q value, the greater the probability of trap-assisted recombination.47 As shown in Figure 4b, the slope of the plot for the EDT-ZnO-np based devices was lower (1.58 kT/q) than that of the pristine ZnO-np based devices (1.69 kT/q), which indicates reduced trapassisted bimolecular recombination due to EDT modification (process 2 in Figure 4c). It is widely known that the PbS-CQDs intrinsically possess a relatively high density of intragap states;48 the increased number of trap densities in ZnO can influence various charge recombination mechanisms at the PbS-CQD/ ZnO heterojunctions. Similar effects of reduced trap-density in ZnO layers on the charge recombination in CQDPVs have 104
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(2) Lan, X.; Voznyy, O.; Kiani, A.; García de Arquer, F. P.; Abbas, A. S.; Kim, G.-H.; Liu, M.; Yang, Z.; Walters, G.; Xu, J.; et al. Passivation Using Molecular Halides Increases Quantum Dot Solar Cell Performance. Adv. Mater. 2016, 28, 299−304. (3) Kim, G.-H.; García de Arquer, F. P.; Yoon, Y. J.; Lan, X.; Liu, M.; Voznyy, O.; Yang, Z.; Fan, F.; Ip, A. H.; et al. High-Efficiency Colloidal Quantum Dot Photovoltaics via Robust Self-Assembled Monolayers. Nano Lett. 2015, 15, 7691−7696. (4) Tang, J. A.; Sargent, E. H. Infrared Colloidal Quantum Dots for Photovoltaics: Fundamentals and Recent Progress. Adv. Mater. 2011, 23, 12−29. (5) Pattantyus-Abraham, A. G.; Kramer, I. J.; Barkhouse, A. R.; Wang, X. H.; Konstantatos, G.; Debnath, R.; Levina, L.; Raabe, I.; Nazeeruddin, M. K.; Gratzel, M.; et al. Depleted-Heterojunction Colloidal Quantum Dot Solar Cells. ACS Nano 2010, 4, 3374−3380. (6) Liu, H.; Tang, J.; Kramer, I. J.; Debnath, R.; Koleilat, G. I.; Wang, X. H.; Fisher, A.; Li, R.; Brzozowski, L.; Levina, L.; et al. Electron Acceptor Materials Engineering in Colloidal Quantum Dot Solar Cells. Adv. Mater. 2011, 23, 3832−3837. (7) Willis, S. M.; Cheng, C.; Assender, H. E.; Watt, A. A. R. Transitional Heterojunction Behavior of PbS/ZnO Colloidal Quantum Dot Solar Cells. Nano Lett. 2012, 12, 1522−1526. (8) Sun, Y. M.; Seo, J. H.; Takacs, C. J.; Seifter, J.; Heeger, A. J. Inverted Polymer Solar Cells Integrated with a Low-TemperatureAnnealed Sol-Gel-Derived ZnO Film as an Electron Transport Layer. Adv. Mater. 2011, 23, 1679−1683. (9) Yang, T. B.; Cai, W. Z.; Qin, D. H.; Wang, E. G.; Lan, L. F.; Gong, X.; Peng, J. B.; Cao, Y. Solution-Processed Zinc Oxide Thin Film as a Buffer Layer for Polymer Solar Cells with an Inverted Device Structure. J. Phys. Chem. C 2010, 114, 6849−6853. (10) Schumann, S.; Da Campo, R.; Illy, B.; Cruickshank, A. C.; McLachlan, M. A.; Ryan, M. P.; Riley, D. J.; McComb, D. W.; Jones, T. S. Inverted organic photovoltaic devices with high efficiency and stability based on metal oxide charge extraction layers. J. Mater. Chem. 2011, 21, 2381−2386. (11) Liang, Z. Q.; Zhang, Q. F.; Wiranwetchayan, O.; Xi, J. T.; Yang, Z.; Park, K.; Li, C. D.; Cao, G. Z. Adv. Funct. Mater. 2012, 22, 2194− 2201. (12) Park, H. Y.; Ryu, I.; Kim, J.; Jeong, S.; Yim, S.; Jang, S. Y. Effects of the Morphology of a ZnO Buffer Layer on the Photovoltaic Performance of Inverted Polymer Solar Cells. J. Phys. Chem. C 2014, 118, 17374−17382. (13) Luther, J. M.; Gao, J. B.; Lloyd, M. T.; Semonin, O. E.; Beard, M. C.; Nozik, A. J. Stability Assessment on a 3% Bilayer PbS/ZnO Quantum Dot Heterojunction Solar Cell. Adv. Mater. 2010, 22, 3704− 3707. (14) Gao, J. B.; Luther, J. M.; Semonin, O. E.; Ellingson, R. J.; Nozik, A. J.; Beard, M. C. n-Type Transition Metal Oxide as a Hole Extraction Layer in PbS Quantum Dot Solar Cells. Nano Lett. 2011, 11, 1002−1008. (15) Chuang, C. H. M.; Brown, P. R.; Bulovic, V.; Bawendi, M. G. Improved Performance and Stability in Quantum Dot Solar Cells Through Band Alignment Engineering. Nat. Mater. 2014, 13, 796− 801. (16) Zhitomirsky, D.; Voznyy, O.; Levina, L.; Hoogland, S.; Kemp, K. W.; Ip, A. H.; Thon, S. M.; Sargent, E. H. Engineering colloidal quantum dot solids within and beyond the mobility-invariant regime. Nat. Commun. 2014, 5, 3803. (17) Zhao, N.; Osedach, T. P.; Chang, L. Y.; Geyer, S. M.; Wanger, D.; Binda, M. T.; Arango, A. C.; Bawendi, M. G.; Bulovic, V. Colloidal PbS Quantum Dot Solar Cells with High Fill Factor. ACS Nano 2010, 4, 3743−3752. (18) Osedach, T. P.; Zhao, N.; Geyer, S. M.; Chang, L. Y.; Wanger, D. D.; Arango, A. C.; Bawendi, M. C.; Bulovic, V. Interfacial Recombination for Fast Operation of a Planar Organic/QD Infrared Photodetector. Adv. Mater. 2010, 22, 5250−5254. (19) Kemp, K. W.; Labelle, A. J.; Thon, S. M.; Ip, A. H.; Kramer, I. J.; Hoogland, S.; Sargent, E. H. Interface Recombination in Depleted
degradation. We shipped a device to an accredited laboratory (Newport) after 25 days of air storage to obtain certified PCE. The device was measured in air under standard AM1.5G conditions (ASTM Standard E948-15), displaying a PCE of 10.14 ± 0.22% (VOC = 0.6771 V, JSC = 22.186 mA cm−2, and FF = 0.676). This result was in good agrement with the values obtained in our laboratory. The published certificate is shown in Figure S8. In summary, high-efficiency CQDPVs were developed by improving the interfacial charge extraction at the PbS-CQD/ ZnO-np heterojunction via simple chemical modification. Chemical modification of the heterojunction using EDT induced effective reduction of the intrinsic defects in the chemically prepared ZnO-nps, which are hard to avoid otherwise. The alteration of the energy levels and the reduction of the intraband trap sites in the ZnO-np ETLs substantially improved the charge extraction properties. Bimolecular recombination processes at the heterojunctions were substantially suppressed, while the width of the depletion regions remained intact. CQDPVs with a certified PCE of 10.14% were achieved using the chemically modified heterojunction. This study suggests a useful strategy to improve the interfacial properties of p-n heterojunctions via simple chemical modification.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00070. Experimental section, UV−vis absorption spectrum, cross-sectional SEM images, EIS Nyquist plot, TPV analysis results, EDS spectra, Zn peak analysis from XPS, Mott−Schottky analysis, histogram of PCE devices, stability test of the devices, certified PCE from Newport, hysteresis of J−V curve by forward and reverse scan, and summary of charge transport property analysis (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge support from the New and Renewable Energy Core Technology Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) in the form of a grant from the financial resources of the Ministry of Trade, Industry and Energy, Republic of Korea (S.-Y.J., 20133030000210) and the Global Scholarship Program for Foreign Graduate Students at Kookmin University in Korea (R.A.). A portion of this research was supported by National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (S.-H.O. No. 2012M2A2A6013183).
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REFERENCES
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DOI: 10.1021/acsenergylett.6b00070 ACS Energy Lett. 2016, 1, 100−106
Letter
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DOI: 10.1021/acsenergylett.6b00070 ACS Energy Lett. 2016, 1, 100−106
