Stabilizing the Ag Electrode and Reducing JâV Hysteresis through

Research Article Cite This: ACS Appl. Mater. Interfaces 2017, 9, 36338-36349

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Stabilizing the Ag Electrode and Reducing J−V Hysteresis through Suppression of Iodide Migration in Perovskite Solar Cells Jiangzhao Chen,† Donghwa Lee,*,‡ and Nam-Gyu Park*,† †

School of Chemical Engineering, Sungkyunkwan Univeristy (SKKU), Suwon 440-746, Korea Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea

‡

S Supporting Information *

ABSTRACT: Hysteresis and stability issues in perovskite solar cell (PSCs) hinder their commercialization. Here, we report an effective and reproducible approach for enhancing the stability of and suppressing the hysteresis in PSCs by incorporating a small quantity of two-dimensional (2D) PEA2PbI4 [PEA = C6H5(CH2)2NH3] in three-dimensional (3D) MAPbI 3 [MA = CH 3 NH 3 ] [d enoted as (PEA2PbI4)x(MAPbI3)], where the perovskite films were fabricated by the Lewis acid−base adduct method. A nanolaminate structure comprising layered MAPbI3 nanobricks was created in the presence of 2D PEA2PbI4. For x = 0.017, a power conversion efficiency (PCE) of as high as 19.8% was achieved, which was comparable to the 20.0% PCE of a MAPbI3-based cell. Density functional theory (DFT) calculations confirmed that iodide migration was suppressed in the presence of the 2D perovskite as a result of a higher activation energy, which was responsible for the significant reduction in hysteresis and the improved chemical stability against a Ag electrode as compared to the corresponding characteristics of its pristine MAPbI3 counterpart. An unencapsulated MAPbI3-based device retained less than 55% of its initial PCE in a 35-day aging test, whereas a (PEA2PbI4)0.017(MAPbI3)-based device without encapsulation exhibited a promising long-term stability, retaining over 90% of its initial PCE after 42 days. KEYWORDS: Perovskite solar cell, Hysteresis, Stability, Ion migration, Two-dimensional

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INTRODUCTION Since the groundbreaking report on solid-state perovskite solar cells (PSCs) with a power conversion efficiency (PCE) of 9.7%,1 organic−inorganic metal halide perovskites have rapidly emerged as promising light-harvesting materials for photovoltaic applications because of their excellent optical and electrical properties, which have attracted extensive attention in the photovoltaic community.2−11 Within an extremely short time, the PCEs of PSCs have soared to 22.1%12 at an unprecedented rate. Although high PCEs have been achieved, there are still several challenging problems to be addressed before the large-scale commercial application of PSCs, such as J−V hysteresis and instability. Significant hysteresis occurs in most PSCs, which has a considerable influence on the accurate determination of PCEs and long-term stability. In the past several years, possible origins of hysteresis have been explored intensively, mainly including ferroelectric effects,13−15 lowfrequency capacitance,16 high trap-state densities for electronic carriers at the perovskite surface,17 interfacial dipoles,18 and ion migration within the perovskite layer.19−24 To date, many efforts have been made to eliminate or reduce hysteresis. However, most of the research has primarily focused on the optimization of the charge-selective contacts,25−28 rather than the perovskite material itself. Recently, several reports19,20,23,24 © 2017 American Chemical Society

demonstrated that ion migration should be mainly responsible for hysteresis. In the methylammonium lead triiodide (CH3NH3PbI3, denoted as MAPbI3) material commonly used in PSCs, I− and MA+ ions are considered to migrate much more easily than Pb2+ ions.19 Furthermore, I− and MA+ ions at grain boundaries migrate faster than those in the bulk.19 Little attention has been paid to the inhibition of the migration of I− and MA+ ions. Moreover, it is well-known that Ag, as an important back-contact electrode that has been widely applied in mesoporous and planar solar cell configurations, suffers from severe corrosion by iodide ions from the perovskite layer, which is the main reason for the poor long-term stability of Ag-based PSCs. This corrosion is also due to the chemical reaction of Ag with iodide due to iodide-ion migration from the perovskite to the Ag electrode. Consequently, it is highly desired that I− and MA+ migration is inhibited or eliminated through grain boundary management. As mentioned above, for the practical application of PSCs to become possible, it is urgent that their instability be addressed, including thermal stability, photostability, and moisture Received: May 28, 2017 Accepted: September 27, 2017 Published: September 27, 2017 36338

DOI: 10.1021/acsami.7b07595 ACS Appl. Mater. Interfaces 2017, 9, 36338−36349

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ACS Applied Materials & Interfaces

Figure 1. (a) High-resolution top-view SEM image of a (PEA2PbI4)0.017(MAPbI3) film deposited on mp-TiO2/bl-TiO2/FTO/glass. (b) Crosssectional SEM image of the complete device employing (PEA2PbI4)0.017(MAPbI3).

and 3D perovskite is highly dependent on the concentrations of the precursor solutions. Moreover, the Ag electrode is much more stable in (PEA2PbI4)x(MAPbI3) than in MAPbI3, which indicates that the chemical reaction between Ag and iodide, often observed in MAPbI3, is significantly inhibited. The reduced hysteresis and improved stability against the Ag electrode are probably related to ion migration. Experimental and DFT calculation results confirmed that iodide migration is suppressed in (PEA2PbI4)x(MAPbI3), which is responsible for the chemical stability of Ag electrode and negligible hysteresis.

stability. Among these types of stability, the intrinsic moisture instability of PSCs is mainly caused by the instability of hybrid perovskite materials in the presence of water. Generally, three types of strategies have been proposed to solve the moisture instability problem. The first approach is to employ hydrophobic materials (such as carbon) in the device architecture composition (charge-transport material29−31 or back-contact electrode32,33) to protect the perovskite from being attacked by moisture. The second strategy is to incorporate hydrophobic molecules on the surface of the perovskite grains to functionalize the perovskite with hydrogen and ionic bonds.34 Although these two types of extrinsic methods can significantly improve the moisture stability of perovskites, the realization of long-term moisture stability is still a great challenge. Therefore, the third strategy, which emerges at the right moment as an intrinsic method, is to develop new hydrophobic perovskite materials through composition engineering; examples include MAPbI3−xBrx,35 MAPbI3−x(SCN)x,36 PEA2MA2Pb3I10 [PEA = C 6 H 5 (CH 2 ) 2 NH 3 ], 3 7 and BA 2 MA n − 1 Pb n I 3n + 1 [BA = CH3(CH2)3NH3, n = 2, 3 and 4].38,39 Although twodimensional (2D) perovskites with PEA or a long-chain alkyl ammonium enable the realization of excellent moisture stability through the presence of stronger Pb−I bonds in the 2D inorganic plane, the corresponding devices have been demonstrated to deliver much lower PCEs because of their prominently increased band gaps as compared to those of their three-dimensional (3D) counterparts (such as MAPbI3). Therefore, to overcome the disadvantage of 2D perovskites, Sargent and co-workers40 reported solar cells based on the quasi-2D perovskite PEA2MA59Pb60I181, obtaining a certified PCE of 15.3% with greatly improved longevity of performance. This indicates that the incorporation of a small quantity of 2D perovskite into a 3D perovskite can lead to PSCs that are simultaneously highly efficient and long-term-stable. Herein, simultaneously taking high efficiency and excellent stability into consideration, we report mesoscopic PSCs based on the highly stable mixed 2D and 3D perovskite (PEA 2PbI4)x(MAPbI3), where the perovskite layer was prepared by the Lewis acid−base adduct method.41 Compositional and processing engineering enabled us to find the optimal content of PEA2PbI4 in MAPbI3, namely, x = 0.017. Based on (PEA2PbI4)0.017(MAPbI3), we achieved a promising PCE of 19.84%. To the best of our knowledge, this efficiency is the highest reported for mixed 2D and 3D perovskite-based solar cells. Compared with a device based on 3D MAPbI3, the (PEA2PbI4)x(MAPbI3)-based device shows dramatically reduced hysteresis. However, the hysteresis in the mixed 2D

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RESULTS AND DISCUSSION The device employed in this work has a configuration of glass/ FTO/bl-TiO 2 /mp-TiO 2 /perovskite/spiro-OMeTAD/Ag (where FTO is fluorine-doped tin oxide; spiro-OMeTAD is 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene; and bl and mp denote blocking and mesoporous, respectively, as shown in the cross-sectional scanning electron microscopy (SEM) image in Figure 1b. Figure 1a presents a high-resolution top-view SEM image of (PEA2PbI4)0.017(MAPbI3) film deposited on mp-TiO2/blTiO2/FTO/glass. Here, the Lewis acid−base adduct method (formation of an MAI·PbI2·DMSO intermediate phase, where MAI is methylammonium iodide and DMSO is dimethyl sulfoxide)41 was used to prepare high-quality perovskite films for the purpose of constructing high-performance devices. The SEM image reveals that (PEA2PbI4)0.017(MAPbI3) has a nanolaminate structure comprising MAPbI3 bricks and PEA interlayers, similarly to the organic−inorganic layer-by-layer structure with aragonite bricks and organic protein found in the shells of abalone.42 Figure 1b shows a cross-sectional SEM image of the full device employing (PEA2PbI4)0.017(MAPbI3), where the surface morphology of the perovskite is preserved. From the morphological point of view, a small amount of 2D PEA2PbI4 is likely to passivate the surface of 3D MAPbI3. However, a quasi-2D structure due to PEA2PbI4 cannot be ruled out. Similarly to the n values in quasi-2D PEA2MAn−1PbnI3n+1, the x values in (PEA2PbI4)x(MAPbI3) also have a significant influence on device performance. Therefore, we first investigated the effect of the x value on the photovoltaic performance at the given concentration of 1.0 M precursor solution and an annealing temperature and time of 100 °C and 10 min, respectively, as presented in Figure S1 and Table S1 (Supporting Information). It is interesting to see that the opencircuit voltage (Voc) does not increase as the x value increases, which is different from the case of quasi-2D 36339


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Figure 2. (a) XRD patterns of MAPbI3 and (PEA2PbI4)0.017(MAPbI3) films spin-coated on FTO-coated glass substrate. (b) UV−visible absorption and (c) steady-state PL spectra of MAPbI3 and (PEA2PbI4)0.017(MAPbI3) films spin-coated on glass. (d) FTIR spectra of MAPbI3 and (PEA2PbI4)0.017(MAPbI3) films, PEA2PbI4, and PEAI. Perovskite films were prepared according to the optimal device conditions.

the MAPbI3 film in the range of 2800−3300 cm−1. Two weak peaks at about 2850 and 2920 cm−1 are attributed to C−H s t r e t c h in g v i b r a t i o n s , w h e r e t h o s e o b se r v e d in (PEA2PbI4)0.017(MAPbI3) are closer in shape to those in PEA2PbI4 than to those in MAPbI3. The broad peak at about 3160 cm−1, corresponding to N−H stretching vibrations,41,45,46 observed for both MAPbI3 and PEA2PbI4 with different intensities is split into two peaks in (PEA2PbI4)0.017(MAPbI3) because of the coexistence of PEA+ and MA+. Because we prepared the perovskite precursor solution by adding x mole of (PEAI + PbI2) to one mole of (MAI + PbI2), 2D PEA2PbI4 is expected to exist. Nevertheless, direct and clear evidence of 2D is difficult to obtain from spectroscopic studies, probably because of the tiny bit of 2D perovskiite with respect to the amount of MAPbI3. Considering both stability and efficiency, x = 0.017 [corresponding to the formula (PEA2PbI4)0.017(MAPbI3)] was chosen for further investigation on effect of precursor concentration at the given annealing temperature and time of 65 °C for 1 min and 100 °C for 10 min, as shown in Figure S2 and Table S2 (Supporting Information). Figure S3 (Supporting Information) shows statistical results of photovoltaic parameters for the devices depending on concentrations of precursor solution. It is found that short-circuit current density (Jsc) is increased, maximized and then decreased as precursor concentration increases. The highest Jsc is achieved from 1.2 M. Besides, when concentration is too high (2.0 M), all photovoltaic parameters significantly declined compared to lower concentrations, stemming from morphology and thickness. For other concentrations (0.8, 1.0, 1.2 and 1.5 M), morphologies are almost similar regardless of concentration as confirmed by top-view and cross-sectional SEM images in Figure S4 (Supporting Information). So when concentration is lower than 1.5 M, difference in Jsc mainly comes from different absorbance induced by different film thicknesses as evidenced by UV−visible absorption spectra in Figure S5 and incident

PEA2MAn−1PbnI3n+1.40 These results clearly indicate that a small amount of 2D PEA2PbI4 is not incorporated into the crystal lattice of 3D MAPbI3 because of the structural mismatch that possibly exists on the grain boundaries. Therefore, the formula (PEA2PbI4)x(MAPbI3) used in this article is reasonable. The effect of PEA incorporation on the crystal structure was investigated by X-ray diffraction (XRD) measurements, as shown in Figure 2a. The main characteristic diffraction peaks of the pure MAPbI3 film appear at the 2θ values of 14.12°, 28.42°, 31.90°, and 40.64°, corresponding to the (110), (220), (310), and (224) crystal planes, respectively, of tetragonal perovskite. No shift in the 2θ values and no new diffraction peaks appear for the (PEA2PbI4)0.017(MAPbI3) film in comparison with the MAPbI3 film, which is consistent with a previous report.40 This indicates that a tiny bit of 2D perovskite is hard to be detected by XRD. To investigate the effects of the incorporation of PEA into MAPbI3 on its optical properties, UV−visible absorption and steady-state photoluminescence (PL) spectra of MAPbI3 and (PEA2PbI4)0.017(MAPbI3) films were measured. From Figure 2b, it is clear that the absorption of (PEA2PbI4)0.017(MAPbI3) is blue-shifted compared with that of pure MAPbI3, which is consistent with the blue-shifted emission peak in the PL spectrum in Figure 2c. Because three layers of the 2D sheet of butylammonium lead bromide could be detected from the steady-state PL and its blue shift was observed as the number of layers decreased from 22 layers to 3 layers,43 a slight shift to lower wavelength along with the asymmetric peak in the PL spectrum are probably due to the presence of 2D PEA2PbI4 (the band gap is 1.55 eV for MAPbI344 and 2.36 eV for PEA2PbI444) despite the lack of a change in the lattice constants. Additionally, Fourier transform infrared (FTIR) spectra are compared because FTIR spectroscopy is sensitive to surface properties. The entire FTIR spectrum seems to be almost identical, but upon close investigation of FTIR spectra in Figure 2d, the (PEA2PbI4)0.017(MAPbI3) film looks somewhat different from 36340


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Figure 3. (a) J−V curves and (b) corresponding IPCE spectra of the best-performing devices employing MAPbI3 and (PEA2PbI4)0.017(MAPbI3) measured in reverse scan (from Voc to Jsc) at a scan rate of 150 mV/s under simulated AM 1.5G one sun illumination of 100 mW/cm2. The devices were prepared using 1.5 M precursor solution for MAPbI3 and 1.2 M precursor solution for (PEA2PbI4)0.017(MAPbI3). In panel b, the dashed lines indicate the current densities integrated from IPCE spectra. (c) J−V curves for the champion cell using (PEA2PbI4)0.017(MAPbI3) measured in reverse scan and forward scan (from Jsc to Voc) under standard AM 1.5G one sun illumination. (d−g) Statistical histograms of the photovoltaic parameters for 12 cells fabricated using (PEA2PbI4)0.017(MAPbI3) at the optimal concentration, annealing temperature, and time. The J−V curves were measured in reverse scan at a scan rate of 150 mV/s under AM 1.5G one sun illumination.

Figure 4. (a,b) J−V curves of (a) MAPbI3- and (b) (PEA2PbI4)0.017(MAPbI3)-based devices in forward and reverse scans at different scan rates. (c,d) J−V curves of (c) MAPbI3- and (d) (PEA2PbI4)0.017(MAPbI3)-based devices in forward and reverse scans at a scan rate of 150 mV/s (= voltage settling time of 200 ms) for 10 cells. (e,f) Statistical histograms of the PCEs for (e) MAPbI3- and (f) (PEA2PbI4)0.017(MAPbI3)-based devices with respect to the reverse and forward scan data observed from 10 cells.

photon-to-current conversion efficiency (IPCE) spectra in Figure S6 (Supporting Information). However, although films

based on 1.3 and 1.5 M show higher absorbance than that of 1.2 M, the 1.3 M- and 1.5 M-based devices exhibit relatively low 36341


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ACS Applied Materials & Interfaces Jsc, which could be attributed to worse charge extraction as confirmed by time-resolved photoluminescence (TRPL) spectra in Figure S7 (fitted results based on biexponential decay equation are presented in Table S3) combined with IPCE data in Figure S6 (Supporting Information). As we all know, Jsc is as result of integral of IPCE over the spectral range and IPCE is product of the light-harvesting efficiency, the charge injection yield from the perovskite into charge-transport layers, and the charge collection efficiency. Because device structure and perovskite composition are not changed except for perovskite precursor solution concentration, different Jsc is related to charge injection efficiency. So for 1.2 M-based device, better charge injection efficiency should be responsible for higher IPCE and Jsc compared with 1.3 and 1.5 M. Subsequently, we optimized annealing temperature (Figures S8 and S9 and Table S4) and annealing time (Figures S10 nd S11 and Table S5) for the case of 1.2 M precursor concentration. As shown in Figure 3a, a champion cell can be fabricated using (PEA2PbI4)0.017(MAPbI3) under optimal conditions (1.2 M, annealed at 65 °C for 1 min and 100 °C for 15 min), obtaining a PCE of 19.84% measured in reverse scan (from the Voc to Jsc, hereafter abbreviated as RS) at a scan rate of 150 mV/s under standard AM 1.5G illumination, exhibiting Voc, Jsc, and fill factors (FF) of up to 1.146 V, 22.69 mA/cm2 and 0.7632, respectively. This efficiency is comparable to 20.0% of MAPbI3-based device fabricated under same conditions except for the use of 1.5 M, achieving Voc, Jsc, and FF of up to 1.104 V, 23.58 mA/cm2 and 0.7685, respectively. Simultaneously, the corresponding IPCE spectra of both the best-performing devices are presented in Figure 3b. The integrated Jsc calculated based on IPCE data are slightly lower than the measured values, which indicates that the measured current densities are slightly overestimated due to spectral mismatch of solar simulator. In Figure 3c, the best-performing cell with (PEA2PbI4)0.017(MAPbI3) gave a PCE of 16.10% in forward scan (from Jsc to Voc, hereafter abbreviated as FS), producing Voc, Jsc, and FF of up to 1.121 V, 22.56 mA/cm2 and 0.6367, respectively. Figure 3d−g shows the statistical histogram of all of photovoltaic parameters for 12 cells for (PEA2PbI4)0.017(MAPbI3) fabricated under optimal conditions, and corresponding J−V curves and detailed photovoltaic parameters are given in Figure S12 and Table S6 (Supporting Information), respectively. Average Voc, Jsc, and FF of up to 1.126 ± 0.014 V, 22.07 ± 0.43 mA/cm2, and 0.7645 ± 0.01% are gained, respectively, corresponding to an average PCE of 19.00 ± 0.42%. The well-known hysteresis phenomenon often appears for most PSCs during the measurement of J−V curves, which results in discrepancies in efficiencies from forward and reverse scans or from different scan rates. This imposes a serious limitation on the evaluation of real PCEs and stabilities. Accordingly, we systematically investigated how to change depending on scan direction and rate for the PCEs of (PEA2PbI4)0.017(MAPbI3)-based devices compared with those of pristine MAPbI3-based devices. Figure 4a,b shows the J−V curves of MAPbI3- and 1.2 M (PEA2PbI4)0.017(MAPbI3)-based devices in FS and RS at different scan rates (300, 150, 60, and 30 mV/s); the corresponding photovoltaic parameters and hysteresis indexes (HI) are listed in Table S7 (Supporting Information). Meanwhile, J−V curves of 0.8, 1.0, and 1.5 M (PEA2PbI4)0.017(MAPbI3)-based devices in FS and RS at different scan rates (300, 150, 60, and 30 mV/s) are presented in Figure S13 (Supporting Information), and the corresponding

photovoltaic parameters and hysteresis indexes (HI) are also summarized in Table S7 (Supporting Information). To quantify the hysteresis degrees of devices, HI is defined according to a previous report as16 HI =

JRS (0.8 Voc) − JFS(0.8 Voc) JRS (0.8 Voc)

where JRS(0.8 Voc) and JFS(0.8 Voc) represent the Jsc values at 80% of Voc for the reverse and forward scans, respectively. As shown in Figure 4a,b, devices based on pristine MAPbI3 show severe hysteresis, whereas hysteresis is significantly reduced when 2D PEA2PbI4 is included in MAPbI3. It is interesting to see that the emerging kink shape in the J−V curves for MAPbI3-based devices in the FS near maximum power output disappears in (PEA2PbI4)0.017(MAPbI3)-based devices. The kink shape is frequently detected for planar or even mesoscopic PSCs during fast forward scans.47,48 The origin of this kink phenomenon was reported to be related to the modulation of interfacial barriers due to ion migration.20 The disappearance of the kink shape and reduction of the hysteresis in (PEA2PbI4)0.017(MAPbI3)-based devices are thus indicative of the suppression of ion migration. Scan-rate-dependent HIs are listed in Table S7 (Supporting Information), where the HIs of MAPbI3-based devices are reduced from 0.551 to 0.24 when the scan rate is decreased from 300 to 30 mV/s, whereas those of devices based on 0.8, 1.0, 1.2, and 1.5 M (PEA2PbI4)0.017(MAPbI3) decrease from 0.231 to 0.007, from 0.106 to 0.066, from 0.225 to 0.043, and from 0.345 to 0.138, respectively. This suggests that MAPbI3-based devices still show much more pronounced hysteresis than (PEA2PbI4)0.017(MAPbI3)-based devices even for very low scan rates. The J−V curves of 10 cells employing MAPbI3 and 1.2 M (PEA2PbI4)0.017(MAPbI3) in FS and RS for a fixed scan rate of 150 mV/s are compared in Figure 4c,d [photovoltaic parameters are presented in Tables S8 and S11 (Supporting Information)], and the corresponding statistical histograms of the PCEs are presented in panels e and f, respectively, of Figure 4. Simultaneously, J−V curves of 10 cells using 0.8, 1.0, and 1.5 M (PEA2PbI4)0.017(MAPbI3) measured in FS and RS at a scan rate of 150 mV/s are presented in Figure S14 (Supporting Information), and the corresponding photovoltaic parameters are listed in Tables S9, S10, and S12 (Supporting Information), respectively. Compared with the values for the MAPbI3-based devices (average HI = 0.368 ± 0.123), the HIs of the (PEA2PbI4)0.017(MAPbI3)-based devices are markedly reduced (average HI = 0.095 ± 0.038 for 0.8 M, 0.060 ± 0.034 for 1.0 M, 0.121 ± 0.030 for 1.2 M, and 0.256 ± 0.061 for 1.5 M). As mentioned previously, the significant reduction in hysteresis might be due to the suppression of ion migration due to the inclusion of 2D PEA2PbI4 in MAPbI3 or the likely presence of PEA+ ions on grain boundaries. The suppression mechanism is supported by our density functional theory (DFT) calculations, which is discussed in detail later in this section. Additionally, the extent of hysteresis in (PEA2PbI4)0.017(MAPbI3) was found to strongly depend on the concentration of the precursor solution. For concentrations as high as 1.5 M, HI increases to 0.256 ± 0.061, although this is still lower than that of MAPbI3, which is related to changes in grain size,16 as confirmed by SEM in Figure S4. The hysteresis between forward and reverse scans is a reflection of a slow response time of the cell to a change in load. Because photocurrent hysteresis was proposed to be a major 36342


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Figure 5. (a) Time-dependent short-circuit current density for MAPbI3- and (PEA2PbI4)0.017(MAPbI3)-based devices under 0 V bias. Open-circuit conditions under one sun illumination were maintained before measurement of the short-circuit current density. The devices were prepared using 1.5 and 1.2 M precursor solutions for MAPbI3 and (PEA2PbI4)0.017(MAPbI3), respectively. (b) Current density and power conversion efficiency as functions of time for perovskite solar cells employing MAPbI3 and (PEA2PbI4)0.017(MAPbI3). Data were obtained at the maximum voltage of Vmax = 0.857 V for MAPbI3 and Vmax = 0.914 V for (PEA2PbI4)0.017(MAPbI3) without pre-exposure to one sun illumination.

Figure 6. (a) Chamber for moisture stability testing including a hygrometer and a bottle with deionized water. The chamber was covered with a black curtain to eliminate light effects. (b,c) UV−vis absorption spectra of (b) MAPbI3 and (c) (PEA2PbI4)0.017(MAPbI3) films as functions of exposure time. (d,e) Optical images of (d) MAPbI3 and (e) (PEA2PbI4)0.017(MAPbI3) films as functions of exposure time. The relative humidity (RH) level was about 85%. Each film was prepared by spinning a 1.2 M precursor solution onto blank glass under the same conditions to exclude potential effects of film thickness on moisture stability.

a final stable output Jsc value (∼22.3 mA/cm2) in excellent agreement with the Jsc value (22.34 mA/cm2) obtained from the J−V curve, compared with the large difference between the final stable output Jsc value (∼21.9 mA/cm2) and the Jsc value (24.12 mA/cm2) for the control device. This result also implies that the introduction of PEA into 3D MAPbI3 can improve photostability. To further check the reliability of our efficiency measurements and study the effects of hysteresis on photovoltaic performance, stabilized current densities and PCEs at the voltages at the maximum output power [Vmax = 0.857 V for MAPbI3 and Vmax = 0.914 V for (PEA2PbI4)0.017(MAPbI3)] were measured (Figure 5b). As for the MAPbI3-based device, the photocurrent density increases continuously and is stabilized at 21.2 mA/cm2 after about 150 s, producing a stabilized PCE of ∼18.2%, which is ∼98% of the RS efficiency (18.54%) observed from the J−V data. In contrast, the photocurrent density of the (PEA2PbI4)0.017(MAPbI3)-based device rises quickly to a maximum and stabilizes at ∼19.5 mA/

problem in the precise characterization of device efficiency, it is indispensable for PSCs to measure steady-state power output at the maximum power point. Figure 5a shows the timedependent Jsc values for MAPbI3- and (PEA2PbI4)0.017(MAPbI3)-based devices measured under 0 V bias. The two types of devices show significantly different change trends before about 60 s. In the case of the MAPbI3based device, Jsc decays continuously and stabilizes at about 21.9 mA/cm2 after 60 s. Conversely, the Jsc value of the (PEA2PbI4)0.017(MAPbI3)-based device decays rapidly from 23.3 to 21.6 mA/cm2 within the initial 5 s and then increases continuously and stabilizes at about 22.3 mA/cm2 after 60 s. This faster current stabilization correlates with low hysteresis and ionic movement in the perovskite.49 Although the (PEA2PbI4)0.017(MAPbI3)-based device has a lower Jsc value than the MAPbI3-based device in the J−V data, the former obtains a higher stable output value of Jsc than the latter. Interestingly, the (PEA2PbI4)0.017(MAPbI3)-based device shows 36343


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ACS Applied Materials & Interfaces cm2 within a few seconds, producing a stabilized PCE of ∼17.8%, which is ∼95% of the RS efficiency (18.78%) obtained from the J−V data. As mentioned previously, for the (PEA2PbI4)0.017(MAPbI3)-based device, the faster current and PCE stabilization in the electric field generated by the bias voltage might be related to the suppression of I− ion migration (discussed in detail in relation to the DFT calculations later in this section). We tested the moisture stabilities of MAPbI 3 and (PEA2PbI4)0.017(MAPbI3) films deposited on nonconductive blank glass substrates using the same fabrication method as used for the devices. Figure 6a shows a homemade chamber used for testing moisture stability, which contains a hygrometer and a bottle with deionized water. Relative humility was controlled in the range of 60−90% at room temperature. To rule out the effects of film thickness and light, the same concentration (1.2 M) of precursor solution was utilized, and the chamber was covered with a black curtain. Compared with the MAPbI3 film, the (PEA2PbI4)0.017(MAPbI3) film exhibited significantly enhanced moisture stability according to UV−vis absorption spectra (Figure 6b,c), where the absorbance corresponding to MAPbI3 decreased within 10 days and was not detected after 15 days for the MAPbI3 film, whereas minimal change in absorbance was observed for the (PEA2PbI4)0.017(MAPbI3) film. It was also observed from optical images as a function of time (Figure 6d,e) that MAPbI3 film started to degrade seriously after only 5 days, whereas the (PEA2PbI4)0.017(MAPbI3) film remained unchanged even after 20 days. The MAPbI3 film was nearly decomposed into PbI2 after 20 days, whereas the 20-day-aged (PEA2PbI4)0.017(MAPbI3) film was still the same color as the initial film. This indicates that a small amount of 2D PEA2PbI4 in 3D MAPbI3 can play an important role in impeding the decomposition of MAPbI3 in the presence of moisture. According to the moisture stability test results, PEA2PbI4, stable against moisture, is likely to locate at the grain boundaries of MAPbI3. If PEA2PbI4 were located randomly in the MAPbI3 grains, moisture stability might not be preserved because MAPbI3, which is unstable against moisture, would be exposed directly to humid air. In addition to the moisture stabilities of the perovskite films in the dark, the long-term stabilities of full devices were also investigated. For this purpose, devices without encapsulation were stored in a nitrogen-filled glovebox in the dark, and their J−V curves were measured in reverse scan at a scan rate of 150 mV/s under standard AM 1.5G one sun illumination at an interval of 1 week. Figure 7 displays the time evolution of photovoltaic performances of three devices for MAPbI3 and (PEA2PbI4)0.017(MAPbI3). The corresponding J−V curves and detailed photovoltaic parameters are given in Figure S16 and Table S13 (Supporting Information), respectively. For the MAPbI3-based device, the PCEs of the three cells decreased markedly from 18.47%, 18.20%, and 18.66% to 6.42% after 4 weeks, 9.51% after 3 weeks, and 9.18% after 5 weeks, retaining only ∼35%, ∼52%, and ∼49%, respectively, of their initial performances. In contrast, a slight degradation from 17.85%, 18.00%, and 17.63% to 16.27%, 17.035, and 15.97%, respectively, was observed for the (PEA2PbI4)0.017(MAPbI3)based devices after 6 weeks, corresponding to ∼91%, ∼95%, and ∼90%, respectively, of the initial PCEs. When considering the device storage conditions of a nitrogen atmosphere in the dark, degradation of the MAPbI3-based device is likely related to corrosion of Ag electrode due to the chemical reaction of Ag

Figure 7. Normalized (a) Jsc, (b) Voc, (c) FF, and (d) PCE values as functions of storage time in a nitrogen-filled glovebox. Black and red data correspond to MAPbI3 and (PEA2PbI4)0.017(MAPbI3), respectively. Three devices were tested.

with iodide migrated from the perovskite layer. Therefore, the better stability of the (PEA2PbI4)0.017(MAPbI3)-based device sufficiently demonstrates that the introduction of a small quantity of 2D PEA2PbI4 into 3D MAPbI3 significantly improves the stability toward the Ag electrode. Similarly, the reduced hysteresis also indicates that the underlying presence of 2D PEA2PbI4 at the grain boundaries inhibits iodide migration from MAPbI3 to Ag. It is well-known that the ion migration rate is closely related to the external bias, temperature, and illumination intensity. Recently, Huang and co-workers23 reported that light could greatly reduce the energy barrier (activation energy) for ion migration. They found that a typical ion drift velocity in MAPbI3 polycrystalline films was 1.2 μm/s under 1 sun illumination, which was greatly reduced to 0.016 μm/s under a low light intensity of 0.02 sun. To explore the effects of I− ion migration on device stability, we repeatedly measured J−V curves of one device for 30 cycles at a reverse scan rate of 150 mV/s under continuous AM1.5G simulated one sun illumination, in which the (n + 1)th J−V curve was measured right after the nth curve. Figure 8a shows that the J−V curves of the MAPbI3-based device became misshapen after only several cycles. It is noted that the Jsc values were almost invariable with the number of cycles, but the abnormal shape occurs at bias conditions, which indicates that the unusual curve shape is influenced by the internal electric field formed by the external bias and related to the metallic properties of the Ag electrode. It is expected that the migration of I− ions is accelerated under bias conditions and that there is probably an interaction between Ag and iodide. It is widely recognized that I− ions migrating to the Ag electrode can react easily with Ag to produce AgI, ultimately affecting the metallic properties of the Ag electrode. The J−V curves of the (PEA2PbI4)0.017(MAPbI3)based device exhibit a surprisingly normal shape regardless of the number of cycles, and the PCE is changed slightly from 17.56% to 15.56%, as displayed in Figure 8b. As shown in Figure 8c, a huge PCE deviation is observed in the MAPbI3based device as compared to the much smaller deviation in the (PEA2PbI4)0.017(MAPbI3)-based device. This indicates that the 36344


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Figure 8. (a,b) J−V curves of perovskite solar cells employing (a) MAPbI3 and (b) (PEA2PbI4)0.017(MAPbI3) measured repeatedly for 30 cycles in reverse scan at a scan rate of 150 mV/s under continuous AM 1.5G one sun illumination. The (n + 1)th cycle was measured right after the nth cycle. (c) Statistical PCE histogram from the J−V curves in panels a and b, where the box size indicates the standard deviation, the open square in the box represents the mean value, and the data are presented on the right of the box (curves with the data show the distribution of the data, associated with the standard deviation).

Figure 9. Energetics of iodide vacancies in MAPbI3 and PEA2PbI4 using first-principles density functional theory (DFT) calculations: (a) Schematic view of two symmetrically inequivalent I sites (apical and equatorial) in MAPbI3 and PEA2PbI4. (b) Relative energetics of MAPbI3 (red squares) and PEA2PbI4 (blue triangles) during the migration of an I vacancy from an equatorial to an apical position. (c) Relative energetics of MAPbI3 (red squares) and PEA2PbI4 (blue triangles) during the migration of an I vacancy between two equatorial positions.

the other hand, for PEA2PbI4, only the equatorial positions share their vertices with neighboring octahedrons, whereas the apical positions are isolated. This structural difference makes a significant difference in the energetics of the I vacancies. For MAPbI3, the I vacancies preferentially sit on equatorial rather than apical positions. Specifically, our DFT calculations predict that an I vacancy sitting on an equatorial position is 0.11 eV lower in energy than an I vacancy on an apical position. (See the blue line in Figure 9b.) In contrast, the apical position is energetically more favorable site for I vacancies in PEA2PbI4. The energetic preference of I vacancies for apical positions is 0.69 eV in PEA2PbI4. (See the red line in Figure 9b.) Thus, the I vacancies will preferentially move to apical positions in PEA2PbI4. However, the movement of I vacancies might be kinetically limited by energy barriers. Therefore, we further performed nudged-elastic-band (NEB) calculations to explore the kinetic

Ag electrode in the (PEA2PbI4)0.017(MAPbI3)-based device is quite stable against repeated J−V measurements under continuous illumination. This stability of the Ag electrode is obviously due to the presence of a small amount of 2D PEA2PbI4 and presumably the location of PEA2PbI4 at the grain boundaries of MAPbI3 to inhibit further iodide migration. To understand the physical origin of the suppressed migration of I ions in (PEA2PbI4)x(MAPbI3), we further performed first-principles density function theory (DFT) calculations. In this study, we built two structures, namely, MAPbI3 and PEA2PbI4, which are the two extreme cases of (PEA2PbI4)x(MAPbI3). For both MAPbI3 and PEA2PbI4, two symmetrically inequivalent I sites (apical and equatorial) are available, as shown in Figure 9a. The apical and equatorial positions represent the vertices along the longitudinal and planar (equatorial) directions. For MAPbI3, the apical and equatorial positions are each shared by two octahedrons. On 36345


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ACS Applied Materials & Interfaces aspects of I vacancy migration. Figure 9b shows the energy barrier for I vacancy migration from an equatorial position to an apical position, and Figure 9c presents the relative energetics for I vacancy migration from an equatorial position to another equatorial position. The energetics for the two different systems are indicated by red squares (PEA2PbI4) and blue triangles (MAPbI3). For PEA2PbI4, the migration of an I vacancy from an equatorial to an apical position has an energy barrier of only 0.01 eV, which means that the migration along the pathway occurs spontaneously. On the other hand, migration between two equatorial positions presents a much higher energy barrier (0.67 eV), indicating that the migration process between two equatorial positions is significantly limited by the energy barrier in PEA2PbI4. Because the migration barriers along the two pathways in MAPbI3 are more than two times lower in energy (0.29−0.30 eV, blue triangles), one can imagine how quickly I vacancies move to apical positions and how slowly the migration process can occur between equatorial positions in PEA2PbI4 compared to MAPbI3. Thus, it is reasonable to conclude that I vacancies will rapidly move from the bulk to apical sites in (PEA2PbI4)x(MAPbI3), whereas migration along the equatorial pathway is considerably limited. These energetics can explain t he supp ressio n o f I mig ratio n in (PEA2PbI4)x(MAPbI3). As I vacancies preferentially move to apical positions, apical positions act as sinks of I vacancies. As a result, the number of I vacancies in the bulk will continuously decrease in proportion to the number of apical positions of PEA2PbI4. Because the migration of I ions srequires a neighboring vacancy site, the reduced number of I vacancies will definitely decrease the migration of I ions. In addition, the high migration barrier along the equatorial pathway also reduces the migration of I ions in PEA2PbI4. Thus, the migration of I ions is suppressed, and so, the stability of the Ag electrode in (PEA2PbI4)0.017(MAPbI3) is enhanced. Based on the better stability of the Ag electrode observed for (PEA2PbI4)0.017MAPbI3 in Figure 8 and the higher activation energy for iodide migration for 2D PEA2PbI4 compared to 3D MAPbI3 as shown in Figure 9, the small amount of PEA2PbI4 introduced into MAPbI3 is likely to be located on the surface of 3D MAPbI3, rather than forming a 2D + 3D mixture. If perovskite grains were composed of 2D and 3D materials with the 2D material randomly distributed in the 3D grains, it would be hard to explain the better stability of the Ag electrode because iodide migration in such a 2D + 3D model is expected to be similar to that in 3D MAPbI3, as can be seen in Figure 10a. Therefore, the surface-modified 3D model in Figure 10b is more persuasive in explaining the better stability of the Ag electrode because of the deactivation of iodide migration at the surface of the perovskite grains due to the presence of 2D perovskite with a higher activation energy for iodide migration. One might question whether the small amount of PEA2PbI4 (0.017 mol with respect to MAPbI3) would be sufficient to cover all of the grain boundaries. To address this question, we estimated the required amount of PEA2PbI4. We assumed that PEA2PbI4 homogeneously covered the grain boundaries of MAPbI3. Considering that the thickness of a PEA2PbI4 monolayer is about 0.5 nm, we set the grain boundary thickness as 1.0 nm. Models with two different geometries for MAPbI3 grains were utilized to estimate the molar ratio between PEA2PbI4 and MAPbI3: “rhombus” and “equilateral hexagon”. (The grain size was set to 100 nm on the basis of the SEM images.) The calculations for the two models led to minimum amounts of PEA2PbI4 of 2.00% and 2.02%,

Figure 10. (a) Perovskite grains comprising mixed 2D PEA2PbI4 and 3D MAPbI3 showing iodide migration from perovskite grains to Ag electrode by means of spiro-MeOTAD. (b) 3D MAPbI3 whose surface was passivated with 2D PEA2PbI4 showing that iodide migration was suppressed at the surface because of the higher activation energy of 2D PEA2PbI4.

respectively, which is close to the amount of PEA2PbI4 that we incorporated into MAPbI3 (1.7%).

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CONCLUSIONS In conclusion, we have successfully demonstrated a novel and robust type of grain boundary treatment involving the incorporation of a small quantity of 2D PEA2PbI4 into 3D MAPbI3 to yield (PEA2PbI4)x(MAPbI3). Significantly reduced hysteresis along with improved chemical stability of the Ag electrode were observed in (PEA2PbI4)0.017(MAPbI3)-based devices compared with MAPbI3-based devices. Eventually, a (PEA2PbI4)0.017(MAPbI3)-based device without encapsulation achieved a promising long-term stability under nitrogen conditions in the dark, retaining over 90% of its initial PCE value after 42 days. The presence of 2D PEA2PbI4 was found to be beneficial to the suppression of iodide ion migration due to the higher energy barrier for ion migration in the 2D material than in the 3D material, as confirmed by DFT calculations. In this work, we propose that 2D PEA2PbI4 is likely to be located on the grain boundaries of MAPbI3 to inhibit iodide migration into the Ag electrode, which is associated with improved chemical stability of the Ag electrode and moisture stability, and to deactivate iodide migration at the grain boundaries, which is as s oci at ed w it h a redu ct ion i n hy s t eresis. The (PEA2PbI4)0.017(MAPbI3)-based cell delivered a PCE of as high as 19.84%, the highest performance reported for a 2Drelated PSC. This work is expected to provide important insights into hysteresis-free and stable perovskite solar cells. Finally, an in-depth understanding of the effects of ion migration on the photovoltaic properties of perovskite solar cells is further required to achieve commercially viable PSCs.

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EXPERIMENTAL SECTION

Synthesis of CH3NH3I and PEAI. Methylammonium iodide (MAI, MA = CH3NH3) was synthesized according to the literature method.11,41 Hydroiodic acid (30 mL, 57 wt % in water, Aldrich) was added to a methylamine (27.8 mL, 40 wt % in methanol, TCI) solution, and the mixture was stirred at 0 °C for 2 h. A dark brown precipitate was obtained by evaporating the solvent on a rotary evaporator at 50 °C. The resulting precipitate was washed with diethyl ether several times until the color of the precipitate changed to white. The product was purified by recrystallization from ethanol. After filtration, the white precipitate was collected and dried at 50 °C under a vacuum for 24 h before use. Phenylethylammonium iodide (PEAI) was synthesized according to the method described elsewhere.40 HI aqueous solution (57 wt %, 22.4 mL, 170 mmol) was added to 36346


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Theoretical Calculations. All DFT calculations were performed with pseudopotential approaches utilizing the projector augmented wave (PAW) method and the generalized gradient approximation of Perdew, Burke, and Ernzerhof (GGA-PBE) for the exchangecorrelation potential as implemented in the Vienna ab Initio Simulation Package (VASP) code.50,51 The lattice parameters a and c of the tetragonal MAPbI3 system were obtained as 8.86 and 12.66 Å, respectively, for four unit cells, and the lattice parameters of the orthorhombic PEA2PbI4 system were identified as 8.65, 9.07, and 30.90 Å for a, b, and c, respectively, for four unit cells. Monkhorst− Pack k-point sampling with a grid of 4 × 4 × 1 was used for Brillouin zone integration.52 An energy cutoff of 500 eV was used for the planewave representation of the wave functions, and atomic structures were relaxed until all Hellman−Feynman forces were below 0.01 eV/Å. The climbing-image nudged-elastic-band method with 10 images was used for locating minimum-energy pathways.53

phenylethylamine (10.0 mL, 80 mmol) dissolved in 10 mL of ethanol in an ice bath with vigorous stirring, and then the mixture was allowed to react for another 20 min until a colorless precipitate appeared. The precipitate was filtered under a vacuum and washed repeatedly using cold diethyl ether. The crude product was collected; recrystallized from ethanol twice to obtain thin, plate-like white crystals; and dried at 50 °C under a vacuum for 24 h before use. Device Fabrication. FTO glass (Pilkington, TEC-8, 8 Ω/sq) was etched with a laser; ultrasonically cleaned for 15 min with detergent, deionized water, and ethanol, respectively; and then treated with ultraviolet−ozone (UVO) for 20 min. The TiO2 blocking layer (blTiO2) was deposited on the FTO-coated glass by repeatedly spincoating a solution of titanium diisopropoxide dis(acetylacetonate) (Sigma-Aldrich, 75 wt % in isopropanol) in 1-butanol (Sigma-Aldrich, 99.8%) three times at 2000 rpm for 20 s and dried at 125 °C for 5 min. A mesoporous TiO2 (mp-TiO2) layer was deposited on top of the blTiO2 layer by spin-coating a dilute TiO2 paste (50-nm-sized TiO2 nanoparticles, terpineol, ethylcellulose, and lauric acid with a nominal composition of 1.25:6:0.9:0.1 in weight percentages) in anhydrous 1butanol (0.1 g/mL) at 2000 rpm for 20 s, which was dried at 125 °C for 5 min and then annealed at 550 °C for 1 h. After the sample had been cooled to room temperature, the mp-TiO2 film was exposed to UVO for 20 min before being treated with 0.02 M aqueous TiCl4 (Sigma-Aldrich, ≥ 98%) solution at 90 °C for 10 min. The TiCl4treated film was cleaned with deionized water and annealed at 500 °C for 30 min. The mp-TiO2 film was treated again with UVO for 20 min prior to deposition of the MAPbI3 film. A 1.5 M solution of 0.2385 g of MAI and 0.6915 g of PbI2 dissolved in 0.106 mL of DMSO (>99.5%, Sigma) and 0.894 mL of dimethylformamide (DMF, anhydrous, 99.8%, Sigma-Aldrich) was spin-coated on the mp-TiO2 film at 4000 rpm for 25 s. Diethyl ether (0.5 mL) was dropped quickly onto the substrate at 15 s before finishing spin-coating. The resulting transparent adduct films (MAI·PbI2·DMSO) were heated at 65 °C for 1 min and then at 100 °C for 5−25 min. (PEA2PbI4)x(MAPbI3) (x = 0.011, 0.014, 0.017, 0.023, 0.034, 0.071, and 0.250) films were prepared using the same procedure as used for the MAPbI3 film except for the use of different concentrations (0.8, 1.0, 1.2, and 1.5 M) for the (PEA2PbI4)0.017(MAPbI3) precursors. After the samples had been cooled to room temperature, 20 μL of 2,2′,7,7′-tetrakis(N,N-di-pmethoxyphenylamine)-9,9-spirobifluorene (spiro-MeOTAD) solution was spin-coated on top of the perovskite layer at 3000 rpm for 30 s. The spiro-MeOTAD solution was prepared by mixing 72.3 mg of spiro-MeOTAD (Merck), 28.8 μL of 4-tert-butylpyridine (t-BP), and 17.5 μL of lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) stock solution [520 mg of Li-TSFI in 1 mL of acetonitrile (Sigma-Aldrich, 99.8%)] in 1 mL of chlorobenzene. Finally, ∼120 nm of Ag electrode was deposited on top of the spiro-MeOTAD-coated film by thermal evaporation at a constant evaporation rate of 0.3 Å/s. Characterization. Photocurrent density−voltage (J−V) curves were measured using a solar simulator equipped with 450-W xenon lamp (Newport 6279 NS) and a Keithley 2400 source meter. Light intensity was adjusted to AM 1.5G one sun (100 mW/cm2) illumination with an NREL-calibrated Si solar cell with a KG-5 filter. A metal mask with an aperture area of 0.125 cm2 was applied on top of the cell. The IPCE spectral measurements were performed on an IPCE system (PV measurements) under dc mode, where the monochromatic beam was supplied by a 75-W xenon lamp (USHIO). Scanning electron microscopy (SEM) images were obtained on a fieldemission scanning electron microscope (JSM7000F). UV−visible spectra were obtained with a UV/vis spectrometer (Lambda 45, PerkinElmer). Steady-state and time-resolved photoluminescence (PL) spectra were measured using a fluorescence lifetime spectrometer (QuantaurusTau C11367-12, Hamamatsu) with the excitation of a 464-nm laser (PLP-10, Hamamatsu) pulsed at a frequency of 10 MHz for state−state PL spectra and 500 kHz for time-resolved PL spectra. Time-resolved PL was detected using a time-correlated single-photon counting (TCSPC) technique to measure the spontaneous photoluminescence decay. X-ray diffraction (XRD) spectra were obtained by Bruker AXS (D8 Advance, Bruker Corporation) using Cu Kα radiation at a scan rate of 4° min−1.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications Web site. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b07595. Optimization of the x ratio in (PEA2PbI4)x(MAPbI3), precursor concentration, annealing temperature, and time; SEM images of perovskite films deposited from (PEA2PbI4)0.017(MAPbI3) precursor solutions with different concentrations onto mp-TiO2/bl-TiO2/FTO/glass; UV−vis spectra of (PEA2PbI4)0.017(MAPbI3) films prepared using precursor solutions of different concentrations on mp-TiO2/bl-TiO2/FTO/glass; IPCE spectra of devices based on (PEA2PbI4)0.017(MAPbI3) prepared using precursor solutions of different concentrations; effects of precursor concentration on time-resolved photoluminescence (PL); scan rate and direction effects of devices based on precursor solutions of different concentrations; J−V curves of devices used for stable power output tests; and evolution of J−V curves as a function of storage time (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Donghwa Lee: 0000-0002-8956-3648 Nam-Gyu Park: 0000-0003-2368-6300 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Research Foundation of Korea (NRF) grants funded by the Ministry of Science, ICT and Future Planning (MSIP) of Korea under Contracts NRF2012M3A6A7054861 (Global Frontier R&D Program on Center for Multiscale Energy System), NRF2015M1A2A2053004 (Climate Change Management Program), and NRF-2012M3A7B4049986 (Nano Material Technology Development Program). This work was also supported in part by Grants NRF-2016M3D1A1027663 and NRF2016M3D1A1027664 (Future Materials Discovery Program). 36347


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