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Thermally Stable Silver Nanowires-Embedding Metal Oxide for Schottky Junction Solar Cells Hong-Sik Kim,†,⊥ Malkeshkumar Patel,†,⊥ Hyeong-Ho Park,‡ Abhijit Ray,§ Chaehwan Jeong,# and Joondong Kim*,† †

Photoelectric and Energy Device Application Lab (PEDAL) and Department of Electrical Engineering, Incheon National University, 119 Academy Road Yeonsu, Incheon 406772, Republic of Korea ‡ Applied Device and Material Lab., Device Technology Division, Korea Advanced Nano Fab Center (KANC), Suwon 443270, Republic of Korea § Solar Research and Development Center, Pandit Deendayal Petroleum University, Gandhinagar 382007, Gujarat, India # Applied Optics and Energy Research Group, Korea Institute of Industrial Technology, Gwangju 500480, Republic of Korea S Supporting Information *

ABSTRACT: Thermally stable silver nanowires (AgNWs)embedding metal oxide was applied for Schottky junction solar cells without an intentional doping process in Si. A large scale (100 mm2) Schottky solar cell showed a power conversion efficiency of 6.1% under standard illumination, and 8.3% under diffused illumination conditions which is the highest efficiency for AgNWs-involved Schottky junction Si solar cells. Indium− tin−oxide (ITO)-capped AgNWs showed excellent thermal stability with no deformation at 500 °C. The top ITO layer grew in a cylindrical shape along the AgNWs, forming a teardrop shape. The design of ITO/AgNWs/ITO layers is optically beneficial because the AgNWs generate plasmonic photons, due to the AgNWs. Electrical investigations were performed by Mott−Schottky and impedance spectroscopy to reveal the formation of a single space charge region at the interface between Si and AgNWs-embedding ITO layer. We propose a route to design the thermally stable AgNWs for photoelectric device applications with investigation of the optical and electrical aspects. KEYWORDS: transparent conductor, thermal stable AgNWs, metal oxide, Schottky junction, solar cells, surface plasmon, impedance spectroscopy

1. INTRODUCTION Photoelectric devices convert electric energy to photon energy or vice versa. Most photoelectric devices require transparent conductors (TCs) to transmit light to generate electric energy of solar cells and to pass the emission of LEDs, displays, and lighting devices. TCs possess high ability for use in efficient energy conversion devices.1−4 Metal oxides (AZO, FTO, ITO, graphene oxide) and metal nanoscaled networks (graphene, silver nanowires (AgNWs), CNT) have been explored as TCs for use in functional semiconductor applications and are envisioned to have a significant impact on the realization of TC-employed Schottky junction devices for solar energy conversion.1,4−12 Inarguably, the formation of a Schottky junction using TC will offer deep insight on making solar energy conversion technology affordable. Tremendous interest has been attracted by graphene, AgNWs, and their hybrids due to the high optical transmittance and the excellent electrical conductivity of these materials.13−16 Meanwhile, the difficulties of large-scale processes and thermal instability have retarded practical applications of these materials. To resolve these © 2016 American Chemical Society

concerns, AgNW networks have been used in various metal oxides such as AZO,15−19 ITO,20,21 graphene oxide,22−24 and polymers,14,15,25−28 which has led to interesting results in this field. However, these approaches have been investigated only for p−n junction devices, which are inherently intensive compared to Schottky junction devices.14−19,21,23 Although graphene/Si Schottky devices (or solar cells) have shown relatively high efficiency, the degradation of graphene-based solar cell has been a cause of profound concern.13,29,30 Recently, light trapping via AgNWs networks with AZO and ITO materials has been reported to show crucial enhancement of photoelectric devices, including solar cells.31−34 AgNWs networks would provide advantages of light manipulation, in the aspects of (1) scattering from NWs to increase effective optical path, (2) excitation of localized surface plasmon for Received: December 28, 2015 Accepted: March 14, 2016 Published: March 14, 2016 8662

DOI: 10.1021/acsami.5b12732 ACS Appl. Mater. Interfaces 2016, 8, 8662−8669

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Fabrication process flow. (b) Cross-sectional view of the FIB processed sample. (c) Cross-sectional TEM image. (d) Cross-sectional geometry of teardrop shaped ITO cylinder along the AgNWs. (e) High resolution/low resolution TEM images. (f) XRD patterns of AgNWs and AgNWs-embedding ITO film.

layer was demonstrated with commercial p-Si wafer without causing deformation of AgNWs or the use of chemical etching. Hence, to make solar cells affordable and efficient, we are proposing an inline Schottky junction fabrication process flow combining sputtering and spin coating. In addition, the present study may have profound potential for deploying the proposed devices for all classes of solar cells.13,15,16,37,38

effective electron−hole generation, and (3) plasma polariton at an interface to propagate in the semiconductor layer.31,32,35,36 In this article, we report a fascinating TC design realized by combining solution-processed AgNWs and large-scale applicable ITO layers. Using the presence of functional TC layers on a Si substrate, an excellent Schottky junction solar cell was realized. The ITO capping spontaneously ensured the thermal stability of the AgNWs and signified that this was a functional TC design for Schottky junction solar cells. Systematic investigation has revealed that these devices are high-temperature processable and stable, and that they have optical and electrical functional capabilities attributed to the embedding of AgNWs in the conventional metal oxide. Moreover, Schottky junction formation using a design of AgNWs-embedding ITO

2. EXPERIMENTAL SECTION 2.1. Preparation of AgNWs. AgNWs were deposited according to the reported procedure.9 Material was dispensed onto spinning ITO coated p-Si wafer using micropipet at room temperature and thereafter annealed at 150 °C for 10 min to form Ohmic contact. AgNWs were purchased from PlasmaChem GmbH (Berlin, Germany); AgNWs had 8663

DOI: 10.1021/acsami.5b12732 ACS Appl. Mater. Interfaces 2016, 8, 8662−8669

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ACS Applied Materials & Interfaces average diameter of 50 ± 40 nm and length 0.5−50 μm (Cat. No. PLAgNW50-IP-25m). AgNW suspension (10 mg/mL in ethanol as purchased) was diluted with isopropyl alcohol (IPA) to vary the concentration. 2.2. Deposition of ITO Films. ITO bottom and top layers of AgNW networks were deposited at room temperature by 4-in. DC magnetron sputtering system. During the sputtering, a DC power source (3.7 W/cm2) was applied to a 4-in. ITO target (In2O2 containing 10 wt% SnO2) at room temperature under an Ar flow condition of 50 sccm.4 Rapid thermal processing (RTP) treatment was applied to AgNWs-embedding ITO films.39,40 2.3. Device Fabrication Steps. The solar cell had a structure of ITO/AgNWs/ITO/p-Si/Al. The proposed low temperature and inline device fabrication process flows is shown in Figure 1a; we used a Czochralski (CZ) grown 4-in. p-type Si wafer (resistivity, ρ = 1−10 Ω cm; orientation (100) ± 0.5°; thickness = 525 ± 20 μm; front side polished) as a light-absorbing semiconductor material. Al metal was coated by DC sputtering on the back side of Si. A bottom ITO layer was deposited by DC sputtering on the Si substrate. AgNWs containing ink were dispersed onto the (bottom) ITO-coated Si wafer using spin coating at a speed of 1000 rpm for 30 s; this was followed by baking at 150 °C to obtain the transparent conducting AgNW network. The top ITO layer was deposited to cap the AgNWs from air exposure, which also provides a high thermal stability. Thereafter, RTP treatment was performed at 500 °C for 10 min in order to improve the optical and electrical properties. During this RTP process, the back Al metal diffuses into Si to form a back surface field, resulting in an Ohmic contact. Then a front Al grid was formed onto the structure of top-ITO/AgNWs/bottom-ITO/p-Si. The solar cells were tailored to 100 mm2 size by diamond scribing to isolate electrical shunt along to the side. 2.4. Characterization. The planar morphologies were analyzed using a field emission scanning electron microscope (FESEM, JEOL, JSM_7800F) with 15 kV of field voltage, using an SE2 secondary detector. The crystal structures of the AgNWs-embedding ITO film were characterized by X-ray diffraction (XRD, Rigaku, SmartLab) with Cu Kα radiation (λKα = 1.540598 Å) in grazing mode with a glancing angle of 0.5°. In order to observe the crystal structures of the AgNWs, the ITO, and the interfaces, a field-emission transmission electron microscope (FETEM, JEOL, JEM-2100F) was used. Cross-sectional TEM samples were prepared using a focused ion beam system (FIB, FEI, Quanta 3D FEG). The elemental compositions, as line profiles of the cross-sectional layers, were determined using an energy dispersive spectroscopy (EDS) attachment to the FETEM. The thickness and average surface roughness of the deposited films were determined using a surface profiler (Vecco, Dektak XT-E). Optical characterization was carried out using a UV−visible spectrophotometer (Shimadzu, UV-1800); characterization was performed by recording the transmission and absorbance spectra of the thin films in the range 300− 1100 nm. The sheet resistivity values were measured using the fourpoint probe method (CMT-100S, Advanced Instrument Technology) at room temperature. A simulator system (McScience-K3000, Korea) was employed to measure solar cell performances. A photovoltaic power meter (McScience-K101) was used to monitor the I−V characteristics under one sun (100 mW cm−2) illumination. Carrier collection efficiencies of solar cells were profiled using a quantum efficiency measurement system (McScience-K3100, Korea) coupled with a monochromator (Oriel Cornerstone 130 1/8 m), source measurement unit (2440, Keithley), and lock in amplifier (K102, McScience). Mott−Schottky analyses (C−V characteristics) and impedance spectra of the Schottky junction devices were obtained using the potentiostat/ galvanostat (ZIVE SP1, WonA Tech, Korea). The high-temperature stability was studied by vacuum-compatible probe station under the monochromatic illumination coupled with temperature controller (TC-200P, Misung Scientific), digital oscilloscope (TBS 1102B-EDU, Tektronix), and function generator (MFG-3013A, MCH Instruments).

3. RESULTS AND DISCUSSION Figure 1a showed the fabrication processes of the AgNWsembedding Si (ITO/AgNWs/ITO/p-Si) Schottky devices. A bottom ITO layer was previously formed before the solutionprocessed AgNWs were coated above it. A top ITO layer was coated over the AgNWs. This process does not utilize any toxic chemicals and does not require doping. The various device structures, employing combinations of transparent conductors (ITO, AgNWs, and AgNWs-embedding ITO), are shown in Figure S1; all samples have active areas of 100 mm2. Microstructural aspects of the cross sections were studied using a dual beam instrument, combining transmission electron microscopy (TEM) and focused ion beam (FIB) technologies. The cross-sectional specimen, fabricated using FIB technology, is shown in Figure 1b. Figure 1b shows the functional structures of the ITO/AgNWs/ITO on a Si substrate. We observed for the first time a distinct morphological arrangement of bottom and top ITO layers after identical sputtering conditions. Teardrop-shaped compact ITO cylinders were found to have formed, along with the top ITO layer growth above AgNWs (Figure 1c). The formed ITO cylinder has an equal, geometrical distribution above the 100 nm-thick ITO layer (Figure 1d). The interface between the bottom ITO and the pSi wafer was found to be very smooth and defect free. Meanwhile, a noticeably different growth preference was observed from the top ITO layer. The top ITO growth above the AgNWs is about 2 times faster than that above the bottom ITO layer (Figure 1c−e); this preferential growth direction induces the teardrop-shaped ITO cylinders surrounding AgNWs. The geometrical parameters, estimated using HRTEM analysis, revealed that the teardrop cylinder has a diameter of ∼125 nm and a length ∼150 nm; AgNWs (Figure 1c−e) with a diameter of 25−30 nm were found. This geometrical arrangement of the ITO material may have the capability of offering better transmittance of incoming light for generation of free carriers in the absorbing materials. This material has shown the great potential of the sputtering deposition method, in which the AgNW surfaces facilitate the growth of incoming sputtered In2O3 particles, which have profound hosting ability compared to that of the ITO aperture. This increased hosting ability is probably attributable to the electrical conductivity of AgNWs, which is higher than that of the ITO surface in the present case of the DC magnetron sputtering deposition method. HRTEM analysis focused on the center location at which the AgNWs were embedded has revealed that one-dimensional AgNWs are surrounded by the ITO film, as shown in Figure 1e. We can see two different orientations of the ITO atoms at the top and bottom surfaces of the AgNWs. The top ITO has a higher order of crystallinity than that of the bottom ITO. No void formation was found in the contacts of the AgNWs and the ITO layers. It is desirable for there to be minimum recombination of free carriers, which can eventually lead to better probability of their collection, which is preferable for the long-term stability of AgNWs. As voids may cause the oxidation of AgNWs, there is higher probability for deformation of AgNWs and degradation of carrier collection.41−43 Furthermore, in order to understand the crystalline properties of the observed one-dimensional AgNWs and the AgNWsembedding ITO film on Si, glancing angle high-resolution XRD analysis was carried out (Figure 1f). The AgNWs on the ITO film are found to be preferentially oriented in the (111) 8664

DOI: 10.1021/acsami.5b12732 ACS Appl. Mater. Interfaces 2016, 8, 8662−8669

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Figure 2. Observations of AgNWs after RTP processes: (a) deformation of bare AgNWs, and (b) shape-maintained AgNWs of ITO/AgNWs/ITO sample. Optical properties of various transparent electrodes: (c) transmittance spectra of AgNWs, ITO, and AgNWs-embedding ITO films, (d) reflectance spectra of AgNWs, ITO, and AgNWs-embedding ITO films, and (e) effect of top ITO layer thickness on the reflectance caused by AgNW embedded ITO films on Si substrate. (f) Room temperature J−V characteristics under dark and one sun illumination conditions. (g) Solar cell performance parameters. (h) Quantum efficiency spectra for Schottky junction solar cell.

retained their morphologies. Moreover, the ITO cylinder was found to become compact under the influence of the RTP treatment, likely due to the improved packing density. To examine the transmittance properties, we prepared several samples of the AgNWs, single ITO-film, and AgNWsembedding ITO film on quartz wafers (Figure 2c). In order to evaluate the optoelectronic performance of the transparent conductors, the ratio of direct current σdc to optical conductivity σop can be used as a figure of merit (FOM),44 which can be expressed as

direction along with lattice plans (002), (022), (113), and (222). In contrast, the AgNWs-embedding ITO layers showed strong preferential growth in the (222) lattice directions. The magnitude of the X-ray diffraction intensity of the ITO material was found to be very close to the value estimated for AgNWs; this confirms good crystalline nature of the ITO cylinders, showing it to be similar to that of the AgNWs. The selected area electron diffraction patterns of the one-dimensional AgNWs and of the ITO cylinders are shown in Figure S4; distinct diffraction spots confirm the crystalline nature of this material. In addition, FESEM analysis of the topography of the samples reveals the uniform dispersion of the AgNWs network on the p-Si and ITO coated p-Si surfaces (Figure S2). The top ITO layer deposition provided better integration of the AgNWs network and offered isolation from an ambient condition, eventually preventing any possible deformation processes such as corrosion, which is one of the critical issues in the area of nanostructured metal electrodes.41,42 As a result, integrated and highly compact AgNWs-embedding ITO cylinders can be observed on the ITO coated p-Si, shown in Figure S3. In order to assess their thermal feasibility and stability, ITOcapped AgNW (ITO/AgNWs/ITO) TCs were thermally treated at 500 °C using RTP for 10 min. As a comparator, a bare AgNWs sample was also treated in the identical RTP condition. Interesting results were observed, as the AgNWs networks deformed and agglomerated oblong Ag particles (Figure 2a). However, the AgNWs-embedding ITO film

σopt(λ) σdc(λ)

=

⎞ 2R sh ⎛ 1 1 − ⎜ ⎟ 273 ⎝ T (λ)2 ⎠

(1)

where T and Rsh are the transmittance (quoted normally at λ = 550 nm) and the sheet resistance, respectively. The σop/σdc values of the transparent conductors can be obtained using eq 1, and are shown in the inset of Figure 2c. The estimated FOM550 nm (at 550 nm) values of the AgNW networks and of the ITO films were found to be 28.2 and 35.4, respectively. On the other hand, AgNWs-embedding ITO layer led to an improved FOM550 nm value of more than 120, which is important for the application of such a material in highperforming photoelectric devices such as solar cells, photodetectors, light emitting diodes, and display devices. The FOM values, which correspond to the values of maximum transmittance in visible region for the AgNWs, ITO, and AgNWembedding ITO films, were estimated at 35, 122, and 250, 8665

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Figure 3. (a) Mott−Schottky characteristics and estimated energy band diagram of AgNWs-embedding ITO films at p-Si interface. (b) Estimated band edges of the AgNWs-embedding ITO film at the Si interface. (c) Schottky junction parameters estimated from equivalent circuit fitting of impedance spectra. (d) Photograph of custom designed homemade vacuum compatible high-temperature characterization setup for solar cell, and (e) high temperature J−V characteristics under monochromatic (λ = 850 nm, 20 mW cm−2) illumination condition.

diode properties. The estimated diode parameters of the devices before and after RTP are summarized in Table S1, where F (1.26) and Jo (8.5 × 10−5 mA cm−2) can be seen to have improved. These improvements indicate the formation of a high quality junction using an AgNWs-embedding ITO film with p-Si, which confirms the formation of a Schottky junction. In order to investigate the AgNWs-embedding ITO/p-Si device for use as a solar cell, the prepared devices were characterized using a simulator system under one sun (AM1.5, 100 mW cm−2) illumination in connection with a power meter. The AgNWs-embedding ITO/p-Si device provided the conversion efficiency (η) of about 5%, with a short circuit current density (JSC) value of 24 mA cm−2 and an open circuit voltage (VOC) value of 550 mV. The estimated fill factor (FF) value of 37.3% shows the upper bound of the efficiency. After the RTP treatment, this device showed an improvement in the conversion efficiency of 6.1% due to the enhanced FF value of 50%. Figure 2g shows the J−V characteristics under AM1.5 illumination; solar cell parameters are summarized in the inset. The improved η is mainly attributed to the enhanced FOM property and the diode characteristics, which were influenced by the RTP treatment,39,40 and elevated the FF value without compromising the values of JSC or VOC. This important observation demonstrates the retention of junction quality under high-temperature RTP conditions. To examine the carrier collection performance, external (EQE) and internal (IQE) quantum efficiencies were measured for all samples (Figure 2h). The AgNWs-embedding ITO/p-Si junction showed an EQE on the order of 60% for the entire visible and near-infrared (NIR) region. This clearly shows that the AgNWs-embedding ITO network on p-Si is very effective at collecting the photogenerated carriers. Furthermore, the IQE spectra were recorded to assess the conceivable surface plasmon effect resulting from the presence of the AgNW-

respectively; these values are much larger than those of monolayer graphene (26), graphene-AgNW hybrid (53− 168),13 or composite AgNW-resin (71).25 Moreover, the transmittance values in the visible region are not much changed compared to that of single ITO film. The obtained state of the art FOM values using the AgNW and ITO combined transparent electrodes make these materials attractive and affordable for application in photovoltaic devices with high-temperature processing stability. In addition, reflectance profiles of the various transparent conductors on p-Si are shown in Figure 2d. AgNWs-embedding ITO film has the possibility of reducing the reflectance of incoming photo flux in comparison to those of AgNW networks or a conventional metal oxide layer. Even more, the design of AgNWs-embedding ITO film can be modulated in an effective manner. When the top ITO layer thickness was varied from 25 to 100 nm on the AgNWs/100 nm-thick bottom ITO layer, we found that the thickness of the top ITO layer and of the cylinders has profound impacts on the ability to tailor the reflectance (Figure 2e). Among all the samples, the reflectance caused by the 100 nm-thick top ITO film on a 100 nm-thick bottom ITO film on the p-Si wafer was found to be the minimum value over a spectrum range from 300 to 1100 nm. Hence, we have chosen this combination of transparent conductors for further device level investigation. To realize the Schottky devices, a rectifying current flow should be confirmed.39,40,45−47 In order to investigate the diode properties, current−voltage (J−V) characteristics were obtained at room temperature, as shown in Figure 2f (please refer to Figure S5 for diode analysis). An AgNWs-embedding ITO layer on p-Si device provided a diode-ideality factor (F) of 1.3, with a low saturation current density (Jo) value of 4.2 × 10−4 mA cm−2, and a significantly high rectification ratio (RR) of about 1000. RTP treatment was found to be effective to improve the 8666

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CSCR are the series resistance, shunt resistance, and space charge capacitance, respectively, under dark condition. IS analysis revealed a very low RS value of 37 Ω cm−2, which was caused by the front and back contact of the device. Figure 3c shows the dependence of the values of RSH and CSCR on the applied bias condition, which is governed by a single RS−RSH∥CSCR circuit representing the Schottky junction formed between the AgNWs-embedding ITO layer and p-Si. In addition, the minority carrier lifetime (τ), which is represented by the electron (τn) value of 0.155 ms, was estimated using exponential fitting of the open circuit voltage decay (OCVD) (Figure S9), giving a diffusion length (Ld) of 747 μm (considering the diffusion constants Dn = 36 cm2 s−1 and τ = 0.155 ms, where Ld = Dnτn indicates the average distance that photogenerated electrons travel before recombining).56 Meanwhile, in the p−n junction Si solar cells, the value of τn = 25 μs resulted from the surface recombination because of the inevitable surface texturing necessary to reduce the surface reflectance and to increase the junction surface area. Inarguably, conventional Si solar cells require expensive surface passivation treatment, which can be disregarded when using the proposed AgNWs-embedding ITO film with p-Si designed in the present study. The high-temperature stability and operational aspects of the developed AgNWs-embedding ITO transparent conductor were studied using a custom designed vacuum setup, as shown in Figure 3d. Further, the developed Schottky junction device was evaluated (from room temperature to 433 K) under monochromatic illumination conduction (λ = 850 nm, intensity = 20 mW cm−2) by recording the J−V characteristics (Figure 3e) and the OCVD value (Figure S10). The acquired data were systematic and consistent at high temperature, revealing the durability and stability of the AgNWs-embedding ITO with pSi, which showed no breakdown. These measurements demonstrate for the first time that the AgNWs-embedding metal oxide transparent conductor can be used to establish a Schottky junction with an active area 100 mm2. The obtained solar cell performance parameters, such as η, JSC, VOC, FF, and τ, are shown in Figures S10−S12. Furthermore, analysis revealed that the AgNWs-embedding ITO transparent conductor offers a high conversion efficiency value of more than 8.34% for a low level of injection, which makes these devices more attractive for diffused-illumination condition, such as cloudy days. We found that the efficiency drops down in a linear fashion from 8.34% to 0.5% because of the higher operational device temperature, which reduces the built in potential height and, consequently, reduces the VOC from 500 mV to 50 mV in a linear manner. In addition, recombination rate profoundly deteriorates the value of FF which dropped from 65% to 28%. Moreover, the value of τn was found to decrease exponentially from 0.225 to 0.05 ms as the device operational temperature was raised from 300 to 450 K, respectively.

embedding teardrop-shaped ITO cylinders. Interestingly, the short wavelength region (480−560 nm) of the visible spectrum showed a higher IQE than that of the other regions of the visible wavelengths. More interestingly, the IQE values of the visible photons in range of 500−550 nm are found to be less than the IQE values of the 650−900 nm region for the Si solar cell, due to the potential absorption at the surface and the presence of a serious Auger recombination effect resulting from the presence of a heavily doped emitter region (>10 20 cm−3).4,8,48 This observation provides us with two important facts about the developed device. First, AgNW-embedding ITO film can potentially be used to establish a Schottky junction with p-Si. Second, and more importantly, the AgNWsembedding ITO film induces a surface plasmonic effect due to the combination of the teardrop structure of the dielectric ITO and the metallic AgNWs networks on the Si substrate; this teardrop structure should potentially allow the concentration of light at a subwavelength volume beyond the optical diffraction limit.12,31,49−51 To study the junction properties at the interface of the AgNWs-embedding ITO film and p-Si, Mott−Schottky (MS) analyses were carried out by applying various frequencies with small AC bias (10 mV). The typical MS plot (1/C2 vs V) of the developed Schottky junction is shown in Figure 3a. The negative slope of the 1/C2 vs V relation shows the holes to be the majority carrier, with an acceptor carrier (NA) concentration on the order of 6 × 1015 cm−3. The obtained parameters (slope, built-in potential: ϕbi, and NA) are provided in the inset table in Figure 3a. Devices showed consistent ϕbi values of ∼0.7 V for the AgNW-embedding ITO film with p-Si. MS analysis confirmed the high quality of the Schottky junction properties, which were consistent during the application of various frequencies (1 kHz to 20 kHz). The procedure for estimating the band edges in the MS analysis is detailed elsewhere.52,53 Estimated energy band parameters of the AgNWs-embedding ITO film at the Si interface, determined using MS analysis, are shown in Figure 3b, where EO, EC, EF, EV, ϕbi, and SCR are the vacuum level, conduction band, Fermi level, valence band, built in potential, and space charge region, respectively. The present MS analyses reveal that the formation of a 950 nm wider SCR led to a value of ϕbi of 0.7 eV because of the 4.23 eV value of work function of the AgNWs-embedding ITO film on the p-Si; this work function value in turn influenced the polarization of the free charges. In order to examine surface states, MS analyses were employed to profile 1/C2 vs V for ITO/AgNWs/ITO (Figure 3a). The capacitance value is stable to frequency variations compared to that of the AgNWs/ITO (Figure S6). This indicates that the ITO-capping on AgNWs is effective to passivate the surface. Impedance spectroscopy (IS) was carried out to evaluate the interface aspect of the developed device; it is inarguable that this technique emphasizes simplifying the integrated junction capacitance and can bring out fine details of the space charge junction capacitance.54−56 Therefore, the obtained impedance spectra in forward and reverse bias conditions were determined and revealed the high quality of the junction, which was found to have considerably high shunt resistance (RSH) and space charge capacitance (CCSR) values of 14.23 kΩ cm−2 and 30.9 nF cm−2, respectively, giving a relaxation RC time constant of 0.4 ms for the zero bias condition. These parameters were estimated by considering transmittance line model (equivalent circuit (RS−RSH∥CSCR) is shown in Figure S7) fitting using the impedance spectra, as shown in Figure S8, where RS, RSH, and

4. CONCLUSIONS In summary, using a large scale sputtering method with spin coating inline solar cell production, we have for the first time demonstrated a high-temperature processable and operational AgNW-embedding metal oxide transparent conductor with the 6.1% efficiency (8.34% in low light injection, 20 mW cm−2) Schottky junction. In-depth systematic structural and physical analysis revealed that the solution-processed AgNWs enable an 8667

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interesting teardrop network of the ITO to form, which reduces the inevitable surface reflection of the polished Si wafer and simultaneously yields a high quality Schottky junction with superior diode properties. AgNWs, embedded at the bottom center of the ITO cylinder, enable the generation of surface plasmonic subwavelength photons; this process was confirmed by IQE measurement and offers a very high figure of merit value of 250. An all-inclusive, device level investigation estimated work function value of 4.23 eV for the AgNWsembedding ITO has the potential to yield a value of 0.7 eV for the built in potential, which should lead to a 747 μm diffusion length of the photogenerated electrons in the p-Si wafer. Mott− Schottky analysis combined with impedance spectroscopy revealed the formation of a single space charge region at the interface. The AgNWs-embedding ITO transparent conductor, which has the attributes of high temperature processing and operational functionality, was demonstrated and should bring widespread benefits to the field of transparent conductors for the improved efficiency of energy conversion devices.



REFERENCES

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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/acsami.5b12732. Photographs of developed Schottky junction solar cells, various surface morphology including AgNWs and AgNWs embedded ITO, HRTEM analysis including a selected area electron diffraction pattern of one-dimensional AgNW and teardrop ITO. Dark J−Vmeasurement. Impedance spectroscopy including transmission line equivalent circuit, and cole−cole plot for forward and reverse bias conditions. Minority carrier lifetime analysis including open circuit voltage decay measurement. Solar cell performance analysis and their temperature dependence. Device statistics in tables/plots as well as detailed experimental description (PDF)



Research Article

AUTHOR INFORMATION

Corresponding Author

* Email: [email protected] Author Contributions ⊥

J.K. conceived this research. H.K. performed the device fabrication. M.P. and H.K. investigated the photoelectric devices. H.P. performed TEM analyses. All the authors contributed to prepare this manuscript. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS The authors acknowledge the financial support of the Korea Institute of Energy Technology Evaluation and Planning, in a grant funded by the Ministry of Knowledge Economy (KETEP20133030011000) and Basic Science Research Program through the National Research Foundation (NRF) of Korea by the Ministry of Education (NRF-2015R1D1A1A01059165). Dr. Chaehwan Jeong is grateful for the support through the Korea Institute of Industrial Technology through Breeding of Hidden Champion program. 8668

DOI: 10.1021/acsami.5b12732 ACS Appl. Mater. Interfaces 2016, 8, 8662−8669

Research Article

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