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18.4%-Efficient Heterojunction Si Solar Cells Using Optimized ITO/Top...

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18.4%-Efficient Heterojunction Si Solar Cells Using Optimized ITO/Top Electrode Namwoo Kim, Han-Don Um, Inwoo Choi, Ka-Hyun Kim, and Kwanyong Seo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00981 • Publication Date (Web): 19 Apr 2016 Downloaded from http://pubs.acs.org on April 25, 2016

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18.4%-Efficient Heterojunction Si Solar Cells Using Optimized ITO/Top Electrode Namwoo Kim1‡, Han-Don Um1‡, Inwoo Choi2, Ka-Hyun Kim2* and Kwanyong Seo1* 1

2

Department of Energy Engineering, UNIST, Ulsan 44919, Republic of Korea.

KIER-UNIST, Advanced Center for Energy, Korea Institute for Energy Research, Ulsan 44919, Republic of Korea.

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ABSTRACT We optimize the thickness of a transparent conducting oxide (TCO) layer, and apply a micro-scale mesh-pattern metal electrode for high-efficiency a-Si/c-Si heterojunction solar cells. A solar cell equipped with the proposed micro-grid metal electrode demonstrates a high shortcircuit current density (JSC) of 40.1 mA/cm2, and achieves a high efficiency of 18.4% with an open-circuit voltage (VOC) of 618 mV and a fill factor (FF) of 74.1% as result of the shortened carrier path length and the decreased electrode area of the micro-grid metal electrode. Furthermore, by optimizing the process sequence for electrode formation, we are able to effectively restore the reduction in VOC that occurs during the micro-grid metal electrode formation process. This work is expected to become a fundamental study that can effectively improve current loss in a-Si/c-Si heterojunction solar cells through the optimization of transparent and metal electrodes.

KEYWORDS: heterojunction solar cell, current loss, indium tin oxide, top electrode, micro-grid

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Introduction Many studies have focused on crystalline Si (c-Si) solar cells, owing to their high efficiency and stability. These cells make up the majority of the solar cell market.1,2 A p-n junction is formed via a high temperature doping process on the surface of conventional c-Si solar cells. However, because this process requires a long thermal processing period, it will not be helpful in realizing low-cost production. Moreover, high-temperature thermal processes degrade the quality of c-Si substrates by allowing the diffusion of undesired impurities; this can cause efficiency degradation in solar cells by decreasing the effective minority carrier lifetime.3 This high-temperature doping process can be avoided by using heterojunction silicon solar cells. Previous studies on heterojunction silicon solar cells replaced conventional p-n junctions by depositing a film – such as organic material,4–8 metal oxide material,9–11 carbon nanotube (CNT),12,13 or amorphous Si (a-Si)14,15 – on the c-Si surface. Moreover, these films are deposited by simple and convenient spin-coating processes, thermal evaporation, and plasma enhancedchemical vapor deposition (PE-CVD). They can all be processed at low temperatures (i.e., below 200 °C). Among the various films that are deposited onto c-Si, a-Si/c-Si heterojunction solar cells form relatively stable junctions, because the deposited a-Si film can passivate the dangling bonds on the c-Si surface, which prevents recombination at the a-Si/c-Si interface.14 Despite the aforementioned benefits, the power conversion efficiency (PCE) of a-Si/c-Si heterojunction solar cells is relatively low compared to passivated emitters and rear locally diffused (PERL) solar cells (24%).16 One of the primary reasons for this is that the short-circuit current density (JSC) of a heterojunction solar cell is lower than that of the PERL solar cell. This is due to current loss caused by the transparent conducting oxide (TCO) used on its front surface. Because the conductivity of a-Si (which is an emitter for a-Si/c-Si heterojunction solar cells) is

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low, the TCO layer is required on the surface of a-Si in order to achieve an effective carrier collection in large areas.17,18 Indium tin oxide (ITO), a well-known TCO, is a wide bandgap material that reduces the light absorption of solar cells. ITO film has low transmittance in the visible light spectrum and intrinsically absorbs wavelengths below 400 nm.19,20 As a consequence, the number of photo-generated carriers in silicon is reduced, leading to JSC degradation. A representative study on improving JSC reduced the thickness of ITO films and adjusted the properties of ITO films.17,19 Typically, the transmittance of ITO films is improved by decreasing the dopant concentration. However, the resistivity of ITO films is increased while the transmittance is improved. This result could be explained by the fact that ITO conductivity decreases because of a reduction in free carriers. As a result, there exists a trade-off relationship between ITO transmittance and conductivity; this directly affects JSC and the fill factor (FF) and makes it difficult to improve the efficiency of solar cell. Here, we optimized the ITO thickness by analyzing changes to its photovoltaic and optical properties caused by changes in the ITO thickness in order to maximize the JSC of a-Si/c-Si heterojunction solar cells. Moreover, we applied a micro-scale metal electrode as a means to effectively collect carriers from the optimized ITO film and increase light absorption. A micro-scale mesh-pattern metal electrode occupies only 3% of a solar cell’s area (12% of the conventional finger-type electrode),21–26 thus decreasing shading loss and improving JSC by approximately 10%. In addition, owing to fine metal lines (with a width of 4–7 µm) that evenly cover an area of 1 cm2, photocarriers were effectively collected, thus improving the FF from 71.0% to 74.1%. The results included an opencircuit voltage (VOC) value of 618 mV, a FF value of 74.1%, and a JSC value of 40.1 mA/cm2; further, we constructed a high-efficiency solar cell that demonstrated a PCE of 18.4%. In this

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study, we were able to maximize the efficiency of the a-Si/c-Si heterojunction solar cell by optimizing the ITO thickness and the metal electrode’s design, on the basis of a fundamental study that aimed to improve JSC.

Experimental section Fabrication of a-Si/c-Si heterojunction solar cell Pyramidal-structured Si substrates ( CZ, 2−10 Ω⋅cm, 175 µm) were cleaned in a diluted HF solution (volume ratio HF:H2O = 1:9) in order to remove the native oxides. After cleaning, the p-type and n-type a-Si:H films were deposited via a PE-CVD cluster system made by SNTEK onto the front and back surfaces of the Si substrate, respectively. All processes were performed at a substrate temperature of 200 °C. The p-type a-Si:H film with a thickness of 15 nm was deposited at a radio frequency (RF) power of 35 W under a pressure of 1800 mTorr using a gaseous mixture of 3.5% silane (SiH4), 0.5% diborane (B2H6), and 96% hydrogen (H2). For the deposition of the n-type a-Si:H film, a gaseous mixture of 7% SiH4, 0.7% phosphine (PH3), and 92.3% H2 was used at an RF power 50 W under a pressure of 2000 mTorr. ITO layers were deposited onto both sides of the Si substrate via RF sputtering (MS200-413G, ITS) at room temperature with an RF power of 300 W and a pressure of 5 mTorr. To form the back electrode, a 1 µm thick Ag film was deposited on the ITO film along the entire rear surface of the Si substrate. The front metal electrode with bus and fingers was formed by depositing a 1 µm thick Ag film through a shadow mask. For the micro-grid electrode, the front surfaces of the heterojunction solar cells were covered with photoresist (AZ4330, AZ electronic materials, thickness of ~4 µm) before the metal deposition. The micro-grid pattern was fabricated using

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photolithography, which was followed by the deposition of the 1 µm thick Ag film. The area of the heterojunction solar cells was 1 cm2.

Device characterization The photovoltaic properties of our solar cells were obtained using a solar simulator (Class AAA, Oriel Sol3A, Newport) under AM 1.5G illumination. Incident flux was measured using a calibrated power meter, and double-checked using a NREL-calibrated solar cell (PV Measurements, Inc.). The external quantum efficiency (EQE) was measured using a quantum efficiency measurement system, which was equipped with a Xe lamp and a monochromator to produce wavelengths in the range of 400–1100 nm. Optical reflectance measurements were performed over the wavelength range of 400–1100 nm using a UV-vis-NIR spectrophotometer (Cary 5000, Agilent) equipped with a 110 mm integrating sphere to account for the total light (diffuse and specular) reflected from the samples.

Results and Discussion We fabricated a-Si/c-Si heterojunction solar cells to analyze the correlation between the PCE and the thickness of ITO film, which was used as the TCO. A schematic diagram of the fabricated a-Si/c-Si heterojunction solar cell structure is shown in the inset of Fig. 1a. The substrate thickness of conventional c-Si solar cells – considering light trapping, wafer handling, and unit cost – is typically between 165 and 240 µm.27 In this study, we used c-Si with a thickness of 175 µm, which falls within that range. Pyramidal structures that exist on both sides of c-Si increase light absorption by reducing the reflectance of the silicon surface. Because the atomic density of the (111) direction is relatively higher than that of the (100) direction, the

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growth of the non-uniform c-Si layer was limited when a-Si was deposited.28,29 As a result, the growth of crystal structures was prevented and defect sites at the a-Si/c-Si interface were minimized; thus, high VOC and JSC values should be expected. A p-type a-Si emitter was formed on the front surface of the solar cell where the light was illuminated, and an n-type a-Si layer was deposited on the back surface of the solar cell to form a back surface field (BSF) layer. In order to efficiently collect photocarriers, ITO films were deposited on the entire surface of the back and front sides. As a back electrode, an Ag film was deposited on the entire surface of the

Figure 1. (a) EQE spectra and (b) J−V curves of a-Si/c-Si heterojunction solar cells with front ITO films with different thicknesses: 70 nm (black solid line and rectangular symbols), 90 nm (red solid line and circular symbols), 110 nm (blue solid line and triangular symbols), and 130 nm (green solid line and inverted triangular symbols)

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back side. At the front side of solar cell, a bus/finger-shaped Ag electrode, deposited with thermal evaporation using a shadow mask, is composed of a central bus bar and finger-shaped bars. The ITO thickness was optimized in a-Si/c-Si heterojunction solar cells with the bus/finger-shaped metal electrode. The optical properties related to the ITO thickness can be confirmed by measuring the external quantum efficiency (EQE), which is a useful tool for measuring the collection probability of the photo-generated carriers at each wavelength. For high-energy photons in the ultraviolet (UV) region, the EQE response is mainly affected by front surface reflection, parasitic absorption, and front surface passivation. In typical a-Si/c-Si heterojunction solar cells, reflection and parasitic absorption in the ITO film constitute a major source of JSC loss in the front surface. Because UV light is absorbed near the surface, high front surface recombination can also reduce EQE in the UV region. On the other hand, because the near infrared (NIR) region can reach the back surface of the solar cells, the EQE of the NIR region can be influenced by optical losses at the back surface. Free-carrier absorption of ITO and internal reflection can be prominent sources of loss in the NIR region. Optimization of the front ITO thickness is critical; that is because the front ITO works as both an electrode and an anti-reflection layer. There exists a trade-off relationship between conductivity and front surface reflection, because the film thickness that provides optimal antireflection often differs from the desired film thickness of the electrodes. When considering conductivity alone, using very thick ITO films constitutes the best approach for solar cells. However, thick ITO films can lead to higher parasitic absorption in both the UV and NIR regions. Increasing the front ITO thickness from 70 nm to 130 nm causes the upper baseline of EQE, shifting the highest EQE position to a longer wavelength (Fig. 1a). This can be attributed

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to the anti-reflection effect of d = λ/(4n), where d is the film thickness, n is the refractive index of the film, and λ is the wavelength of the light inside the film. However, increasing the ITO thickness also leads to a decrease in EQE in the UV and NIR regions. There are two reasons for this. First, by increasing the ITO layer thickness, the next interference fringe appears from the sub-300 nm region and shifts into the UV region, causing a strong decrease in the EQE in the UV region. The second reason concerns the parasitic absorption of the ITO film, and in particular the band-to-band transitions for regions over the ITO bandgaps and free carrier absorption in NIR regions. As a result, the front ITO film thickness of 110 nm provides the optimal JSC value of 37.9 mA/cm2. Despite the fact that the front ITO film thickness is optically optimized at approximately 110 nm, the desired film thickness for electrodes can be different. Table 1 shows the photovoltaic parameters of a-Si/c-Si heterojunction solar cells with different front ITO film thicknesses, ranging from 70 nm to 130 nm. Note that an increase in ITO film thickness leads to a higher FF. As shown in Fig. 1b, current density−voltage (J−V) curves of a-Si/c-Si heterojunction solar cells with thinner front ITO films show limited FF values, which results from the high series resistance. These results

Table 1. Photovoltaic properties of a-Si/c-Si heterojunction solar cells with front ITO films of different thicknesses. ITO thickness

VOC (mV)

JSC FF 2 (mA/cm ) (%)

PCE (%)

70 nm

627

36.4

65.4

14.9

90 nm

629

37.0

68.7

16.0

110 nm

627

37.9

67.1

16.0

130 nm

627

37.4

71.0

16.7

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suggest that even if the best JSC was produced with a front ITO thickness of 110 nm, an optimal PCE would be produced with a front ITO thickness of 130 nm, owing to the high FF. Despite the optimization of the ITO film, the JSC of the bus/finger bar metal electrode produced a lower value than the other conventional solar cells (Table 1). In order to fundamentally solve this problem, the intrinsic shading loss that occurs in 8–12% of the total

Figure 2. (a) Schematic diagram of a-Si/c-Si heterojunction solar cell with the micro-grid electrode. (b) Reflectance spectra of a-Si heterojunction solar cells with bus/finger bar (Finger6: black solid line and rectangular symbols; Finger9: red solid line and circular symbols) and micro-grid electrodes (blue solid line and triangular symbols). Insets of Fig. 2b show from left to right, show the optical images of a-Si/c-Si heterojunction solar cells with six bars, nine bars, and micro-grid electrodes.

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metal electrode area must be reduced. Therefore, we applied a micro-scale metal mesh electrode to replace the bus/finger bar electrode. Fig. 2a shows a schematic diagram of a solar cell equipped with a micro-grid pattern electrode. The micro-grid electrode was applied to a heterojunction structure, on which the ITO film thickness was optimized. The micro-grid electrode is a mesh-pattern metal electrode composed of fine metal lines, with a width of 5 µm and a spacing of 400 µm. Photocarriers are generated by the light absorption of the c-Si exposed between the mesh lines of the micro-grid electrode; electrons that are separated from the emitter are ultimately collected through the mesh lines. The largest difference between the micro-grid and the conventional bus/finger bar electrodes is the difference in the metal electrode area that covers the solar cell surface. While the bus/finger bar electrode occupies 8–12% of the area, that of the micro-grid occupies less than 3%. This significantly reduces the shading loss of incident light resulting from the top electrode. The reflectance spectrum was measured to clarify the difference in light absorption caused by changes in the electrode shape. The incident light on the solar cells should be absorbed or reflected, as the light cannot be transmitted to the back metal electrode. Thus, we can assume that the non-reflected light is all absorbed by the solar cell. The optical images in the insets of Fig. 2b, show (from left to right) a solar cell with six bars (Finger6), a solar cell with nine bars (Finger9), and a micro-grid solar cell. First, when we compared the reflectance of Finger6 and Finger9, the latter (with more bars) demonstrated higher reflectance. This is because the total area of the light-reflecting metal increases as the number of bars increases. The metal is a material without a band gap that absorbs light and immediately emits it as photons, ultimately preventing the light absorption of c-Si and causing shading loss. On the other hand, the microgrid solar cell demonstrates much lower reflectance, even when compared to Finger6 (with fewer

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bars). This is because the area covered by the metal electrode is smaller than that of Finger6, thus reducing the reflection caused by the metal electrode. The reflectance spectra of the three solar cells with different top electrodes demonstrated similar profiles; only the absolute values showed differences. These results coincide with the differences in the metal electrode areas, as explained earlier. Accordingly, through the reflectance comparison between the three solar cells, we were able to confirm that the micro-grid metal electrode structure is more suitable for light absorption, compared to the a-Si/c-Si heterojunction structure. The J−V curves of solar cells with different top electrodes were measured as shown in Fig. 3a. The VOC, JSC, FF and PCE values properties are summarized in Table 2. The most significant difference between the bus/finger bar electrode solar cells and the micro-grid solar cell is JSC. In Fig. 3b, as the number of fingers increased from 6 to 9, JSC decreased from 36.0 to

Figure 3. (a) J−V curves (Finger6: black solid line and rectangular symbols, Finger9: red solid line and circular symbols, micro-grid: blue solid line and triangular symbols), (b) JSC (red) and FF (blue) values, (c) EQE spectra (Finger6: black solid line and rectangular symbols, Finger9: red solid line and circular symbols, micro-grid: blue solid line and triangular symbols), and (d) VOC (green) and PCE (black) values of a-Si/c-Si heterojunction solar cells with bus/finger bar and micro-grid electrodes.

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35.3 mA/cm2. On the other hand, along with the increase in the number of fingers, FF increased from 75.1 to 76.0%. This is because the series resistance was reduced by the decrease in the distance between the fingers, which is the maximum distance that an electron must move through the ITO film. In other words, in the case of the bus/finger bar electrode, the improvements in the ultimate efficiency are limited owing to the trade-off relationship that exists between JSC and FF, which depends on the number of fingers. However, in the case of the micro-grid solar cell, both JSC and FF improved significantly. A maximum increase of 4 mA/cm2 for JSC is clearer in the EQE spectra in Fig. 3c. The EQE improved across all light spectra from 400 nm to 1100 nm. Such increases in EQE coincide with the decreasing tendency in light reflectance according to the electrode area, as shown in Fig. 2b. In the case of the bus/finger bar electrode, there was some gain in JSC, but also an FF degradation. By way of contrast, in the case of the micro-grid electrode, there was no loss in FF despite the decrease in the area of the metal electrode. The

Table 2. The photovoltaic properties of a-Si/c-Si heterojunction solar cells with bus/finger bar and micro-grid electrode. Annealing VOC process (mV)

Electrode type Bus/finger bar

JSC FF 2 (mA/cm ) (%)

PCE (%)

Finger6

614

35.9

75.1

16.6

Finger9

620

35.3

76.0

16.7

Asfabricated

589

39.9

76.5

18.0

Two-step annealed

602

39.7

74.3

17.8

Postannealed

618

40.1

74.1

18.4

Micro-grid

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spacing between micro-grid patterns is 400 µm, and the maximum distance a carrier needs to move is approximately 200 µm. This is about 1/5 the size of Finger9 (1 mm). Because the microgrid electrode has a very short carrier path length and is uniformly distributed across an area of 1 cm2 (the size of an active cell), photocarriers can be collected effectively without JSC and FF degradations. While the heterojunction solar cell applied with a micro-grid electrode demonstrated an improvement in JSC and FF, VOC showed a different trend. As shown in Fig. 3d, in the case of the bus/finger bar electrode, VOC increased from 614 to 621 mV as the number of fingers increased. This is because recombination decreased owing to the short collection path of electrons. However, compared to the bus/finger bar electrode solar cell, the VOC of the micro-grid electrode solar cell decreased to 589 mV, despite its relatively short carrier path. This VOC degradation can be explained using the following equation:

where KB is the Boltzmann constant, T is the absolute temperature, q is the electric charge, IL is the photo-generated current, and I0 is the reverse saturation current. In the case of micro-grids, the decrease in VOC that occurred despite the significant increase in IL indicates a relative increase in I0. This increase in I0, which occurred despite the micro-grid electrode’s short collection path, was due to a degradation caused by adopting a new process different from the one used by the bus/finger bar electrode. If this degradation in VOC can be prevented, we can expect an improvement in efficiency that is higher than 18%, based on the high JSC and FF obtained through the micro-grid.

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Therefore, an annealing process was performed prior to metal deposition in order to prevent the metal from oxidizing; the plasma damage that occurred during ITO sputtering was reduced.14,30 In the case of micro-grid electrode solar cells, the J−V curve in Fig. 4a shows that VOC decreased as a result of UV damage that occurred during the photolithography process, although the annealing process had already been conducted to reduce the plasma damage that occurred prior to the electrode formation. Additional annealing was conducted to reduce this new damage (UV exposure during photolithography). As a result, VOC slightly increased from 589 to

Figure 4. (a) J−V curves of as-fabricated (black solid line and rectangular symbols), two-step-annealed (red solid line and circular symbols), and post-annealed a-Si/c-Si heterojunction solar cells with micro-grid electrode (blue solid line and triangular symbols). (b) VOC (red) and FF (blue) values of a-Si/c-Si heterojunction solar cells with micro-grid electrode depending on the annealing process.

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602 mV (Fig. 4). However, it was still lower than that of the bus/finger bar electrode (621 mV); this might be because a-Si was deformed by two annealing processes for plasma and UV damages. In order to perform the annealing process only once (for the bus/finger bar electrode solar cell), the annealing process was conducted during the final stage, after the ITO deposition and photolithography process was completed. As a result, the VOC of the micro-grid electrode solar cell increased to 618 mV, which is close to the value generated by the bus/finger bar electrode solar cell. Through the optimized annealing process, we were able to stably restore the UV damage that occurred while performing photolithography. However, because the annealing process was performed after the electrode was formed, FF slightly decreased to 74%. This is mainly due to the thermal oxidation of the metal electrode with a micro-scale. As a result, the PCE of the as-fabricated cell was 18.0%, whereas that of the two-step annealed cell was 17.8%, and that of the post-annealed cell was 18.4% (Table 2). However, through further studies, if we can prevent the decrease in FF due to electrode oxidization by replacing the Ag electrode with an oxidation-resistant metal, we can expect to develop a solar cell with a maximum PCE of about 19.0%. It is expected that the experimentally obtained VOC value of 618 mV, JSC value of 40.1 mA/cm2, and FF value of 76.5% can be produced by the non-oxidized metal electrode.

Conclusion We optimized the layer thickness of the ITO which was used as the TCO, and used a micro-grid electrode to improve JSC, which is the primary contributor to efficiency degradation in a-Si/c-Si heterojunction solar cells. We confirmed the effectiveness of an optimized ITO film with a thickness of 130 nm through a comparative analysis of the optical and electrical characteristics related to the ITO thickness. Moreover, by utilizing a micro-grid pattern, we

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minimized the shading loss caused by metal electrodes. By improving JSC to a maximum of 10% above that of the bus/finger bar metal electrode, we improved JSC to 40.1 mA/cm2. We were also able to restore VOC to 618 mV by analyzing the reason for its degradation while forming the micro-grid electrode and optimizing the annealing process. As a result, our solar cell with the micro-grid electrode achieved a 10% improvement, when compared to the conventional bus/finger bar electrode, by showing an efficiency of 18.4%. This result was demonstrated through optimization of the ITO and top electrode.

AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected] (Prof. Kwanyong Seo) * E-mail: [email protected] (Dr. Ka-Hyun Kim) Author Contributions ‡

These authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the 2013 research fund (1.130007.01) of UNIST (Ulsan National Institute of Science and Technology). It was also supported by NRF through the Basic Science

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Research Program (NRF-2014R1A1A1004885) from Ministry of Science, ICT & Future Planning. It was also supported by research and development program of the Korea Institute of Energy Research (B6-2432).

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