Plasmonic Periodic Nanodot Arrays via Laser ... - ACS Publications

Nov 3, 2016 - One of the most efficient plasmonic organic photovoltaic cells reported to date was established by integrating Ag ND arrays incorporatin...
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Plasmonic Periodic Nanodot Arrays via Laser Interference Lithography for Organic Photovoltaic Cells with >10% Efficiency Yulin Oh,†,⊥ Ju Won Lim,‡,⊥ Jae Geun Kim,† Huan Wang,‡ Byung-Hyun Kang,† Young Wook Park,†,§ Heejun Kim,‡ Yu Jin Jang,‡ Jihyeon Kim,‡ Dong Ha Kim,*,‡ and Byeong-Kwon Ju*,† †

Display and Nanosystem Laboratory, College of Engineering, and §The Institute of High Technology Materials and Devices, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic of Korea ‡ Department of Chemistry and Nano Science, College of Natural Sciences, Ewha Womans University, 52 Ewhayeodae-gil, Seodaemun-gu, Seoul 03760, Republic of Korea S Supporting Information *

ABSTRACT: In this study, we demonstrate a viable and promising optical engineering technique enabling the development of high-performance plasmonic organic photovoltaic devices. Laser interference lithography was explored to fabricate metal nanodot (MND) arrays with elaborately controlled dot size as well as periodicity, allowing spectral overlap between the absorption range of the active layers and the surface plasmon band of MND arrays. MND arrays with ∼91 nm dot size and ∼202 nm periodicity embedded in a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) hole transport layer remarkably enhanced the average power conversion efficiency (PCE) from 7.52% up to 10.11%, representing one of the highest PCE and degree of enhancement (∼34.4%) levels compared to the pristine device among plasmonic organic photovoltaics reported to date. The plasmonic enhancement mechanism was investigated by both optical and electrical analyses using finite difference time domain simulation and conductive atomic force microscopy studies. KEYWORDS: surface plasmon, metal nanodot array, laser interference lithography, organic photovoltaics

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voltaics (OPVs) based on the current optimum thickness level of ∼100−200 nm of the active layer. With a thicker active layer, however, it is inevitable that the short exciton diffusion length and low carrier mobility can lead to recombination of excitons, resulting in a lower efficiency.8,9 The trade-off between the thick active layer and charge transport in OPV devices motivates the development of a new strategy that can increase light absorption without employing thicker active layers.10 In this respect, the surface plasmon resonance (SPR) effect exhibited by metal nanoparticles (MNPs) or core−shell nanostructures has emerged as one of the most promising approaches.11−14 In most studies, the improved light absorption of OPVs was achieved based on near-field enhancement or the scattering effect by controlling the size, shape, and composition of metal nanostructures. Moreover, the spectral overlap between the SPR band of MNPs and the absorption range of photoactive polymers induced an increase in the photocurrent

uring the last decades, extensive attention has been paid to organic photovoltaic devices owing to their advantageous features such as low cost for device fabrication, flexibility, and lightweight. For practical applications, it has been highly required to satisfy the following conditions in organic photovoltaics: (1) the incorporation of semiconductor polymers with a higher light absorption coefficient to develop an indoor photovoltaic system; (2) utilization of lightweight materials to enhance the portability; (3) increased cell area to obtain a higher efficiency; and (4) decreased cost for device production.1−5 The development of efficient active layers, in particular, the p-type donor polymer, has been the central issue because they primarily determine the overall configuration and performance of the device.6 Recently, low-band-gap photoactive materials such as poly[4,8-bis[(2ethylhexyl)oxy]benzo[1,2-b:4,5-bA]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl] (PTB7) have gained significant attention owing to their ability to absorb solar light in a wider range, resulting in a higher photovoltaic performance.7 It was not facile to achieve a breakthrough in enhancing the efficiency of organic photo© 2016 American Chemical Society

Received: August 6, 2016 Accepted: November 3, 2016 Published: November 3, 2016 10143

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ACS Nano density as well as external quantum efficiency (EQE) values, enhancing the power conversion efficiency (PCE).15−19 The best efficiency of plasmonic OPV devices has been reported to be up to ∼10% based on PTB7:PC70BM photoactive layers, where an overall enhancement by less than ∼20% was obtained with respect to the plasmon-free control device.11,20 It is noted, however, that the performance of plasmonic cells reported so far is still lower without employing specifically designed polymer systems or tandem configurations.21−23 For plasmonic solar cells, MNP solutions were dispersed into charge transport layers or active layers in OPVs in many cases. However, the use of MNPs has a few technical problems to overcome: (1) aggregation between MNPs in the hole transport or active layer; (2) difficulty in synthesizing uniform nanoparticles; (3) uncontrollable spatial arrangement of MNPs; (4) the reduction of near field generated from MNPs in charge transport layers by hindering the contact between MNPs and active materials; and (5) the direct contact between MNPs and active materials, which can induce negative effects such as exciton quenching or charge recombination.24−26 Considering the aforementioned background and issues, increasing attention has been devoted to the development of periodic metal nanostructures (PMNs) with controlled size, shape, interdistance between particles, and spatial arrangement.27,28 PMNs not only can produce more intense plasmonic effects than randomly distributed MNPs because of the availability of hot spots in the nanogaps but also can scatter and confine more incoming light in the device.29−32 To fabricate PMNs, several techniques have been extensively studied including nanoimprinting, electron beam lithography, laser ablation, and laser interference lithography (LIL).33−36 Particularly, LIL is a well-known simple process that can yield perfectly ordered patterns over a large area and has advantages compared to other nanolithography technologies because of the following reasons: (1) low cost, but high throughput; (2) no contamination on the surface; (3) large-area fabrication up to hundreds of millimeters; (4) easy control of patterns with different period, size, and shape on the nanoscale.37,38 Therefore, combining the PMNs fabricated by the LIL process with OPVs can be a beneficial approach and ideal platform to understand the mechanism of the plasmonic effect and significantly improve the PCE based on the fine-tuned optical characteristics of PMNs. Herein, we demonstrate the establishment of the most efficient plasmonic OPV device integrated with LIL-assisted plasmonic MND patterns, where the SPR band matches with the absorption of photoactive layers. Notably, the plasmonic OPV cell showed the best PCE level reported to date and >30% increase in the PCE compared to the reference cell (from 7.52% to 10.11%). Also, we systematically investigated the enhancement mechanism with optical and electrical analysis using finite difference time domain (FDTD) simulation and conductive atomic force microscopy (c-AFM) studies. Thus, a well-designed protocol for a high-efficiency plasmonic OPV device was proposed with advanced optical engineering methods.

P=

λ 2 sin θ

(1)

where P is the periodicity, λ is the wavelength of the laser, and θ is the incident angle of Lloyd’s mirror to the specimen. The detailed illustration of the laser interference lithography mechanism is shown in Figure S2. In this study, λ of the laser source was fixed at 257 nm, and θ was adjusted to 5°, 10,° 15°, 20°, 25°, 30°, 35°, and 40°. As summarized in Table 1 and Table 1. Measured Hole, Pitch, and Dot Sizes as a Function of the Incident Laser Angle LIL angle (deg) pitch (nm) hole (nm) dot (nm)

5

10

15

20

25

30

35

40

1260 774 624

683 397 350

452 234 209

405 228 202

301 177 146

250 163 140

214 155 135

202 100 91

Figure 2a, the values of pitch and individual hole size decreased by increasing θ, indicating that the structural and optical properties of PMNs can be simply tuned in a facile manner by changing the incident angle of the laser beam. The representative scanning electron microscopy (SEM) images of the hole arrays before the lift-off process of the photoresist (PR) fabricated at a θ of 5°, 10°, 20°, and 40° are shown in Figure 2b−e. By deposition of silver and a lift-off process, periodic MNDs with different sizes were obtained. The size scale of the MNDs is plotted as a function of the angle (see Figure 3a), and the SEM images of a series of MNDs are shown in Figure 3b−e. The size of the MNDs as well as the periodicity was controlled in a systematic and uniform manner over the large surface area. The observed diameter of MNDs was slightly smaller than those of the holes, because the edge of holes was not completely filled by Ag during the thermal evaporation process. To analyze the optical characteristics, extinction spectra of MND arrays on an indium tin oxide (ITO)-coated glass substrate were obtained by UV−vis spectroscopy as shown in Figure 4a. The localized surface plasmon resonance (LSPR) extinction peaks are observed at 573, 729, 1001, and 1278 nm for 91, 202, 350, and 624 nm dot size, respectively. The plasmonic resonance of the smallest MND arrays was close to the absorption peak of the active layer, thereby suggesting increased light-absorption capacity. The corresponding plot of period versus LSPR λmax in Figure 4b shows that each ND array exhibits characteristic LSPR bands, and the measured λmax show that the red-shift behavior of the periods increased.40 The extinction λmax fits well with the linear equation with a high accuracy (R2 = 0.968). Figure 5a shows the schematic diagram of the device structure including Ag NDs between the ITO substrate and PEDOT:PSS layer. The control device was fabricated under the same conditions except for the inclusion of Ag NDs. Using the four types of Ag NDs and without Ag NDs (control device), the photocurrent density−voltage (J−V) characteristics of the OPV devices are shown in Figure 5b under AM 1.5 G illuminations. All the Ag ND-embedded devices showed better performance than the control device, and the best device performance was obtained for the device with smallest Ag NDs (91 nm). Intuitively, this result can be ascribed to the enhanced photocurrent density, where the LSPR extinction band matches optimally with the absorption band of the photoactive layer.

RESULTS AND DISCUSSION In the LIL process, the incident angle and the wavelength of the laser beam determine the pitch and size of PMN arrays following eq 1.35,39 10144

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Figure 1. Schematic illustration to show the fabrication of PMNs: (a) spin-coating of PR on the ITO substrate, (b and c) fabrication of hole arrays on the PR by the LIL technique using the Lloyd’s mirror system, (d) removal of PR by washing in developer, and (e) thermal evaporation of Ag to fabricate a dot array, followed by lifting-off the PR.

Figure 2. (a) Pitch and hole sizes as a function of the incident angle between Lloyd’s mirror and substrate in the LIL system. Representative SEM images of the hole patterns fabricated at different angles of (b) 5°, (c) 10°, (d) 20°, and (e) 40°.

Figure 3. (a) Pitch and dot sizes as a function of the incident angle between Lloyd’s mirror and substrate in the LIL system. Representative SEM images of the PMNs fabricated at different angles (a) 5°, (b) 10°, (c) 20°, and (d) 40°. Average sizes of Ag NDs were 624, 350, 202, and 91 nm.

More detailed average values of photovoltaic parameters are summarized in Table 2. The parameters were calculated by averaging the values obtained from 16 devices (see Figure S4). When PMNs were embedded into the hole transport layer (HTL) of the OPV devices, a significant increase in the shortcircuit current density (JSC) values was observed, whereas the values of the fill factor (FF) and open-circuit voltage (VOC) were almost unchanged compared to the reference device with the pristine PEDOT:PSS HTL, reflecting that the enhancement of the PCE is intimately related to the increased JSC values. The highest efficiency was obtained from the device containing 91 nm size PMNs with ∼0.73 V VOC, ∼23.26 mA/cm2 JSC, ∼0.61

FF, and ∼10.11 (maximum 10.72%) PCE. The average JSC and PCE values increased by 33.4% and 34.4%, respectively, compared to those with the reference device without PMNs. These results represent one of the highest levels of the increase in PCE level compared with the control device, resulting from the plasmonic effect in organic photovoltaic devices based on a single-junction cell. Even though the transmittance of an ITO substrate with PMNs was lower than that of the pristine ITO substrate (see supplementary Figure S5a), the JSC value, light absorption, and scattering in the device were higher and thus contribute to EQE enhancement (Figure S5b). In that sense, a remarkable 10145

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Figure 4. (a) Extinction spectra of PMNs with different sizes. (b) Linear plot showing the relationship between the pitch and SPR band position of PMNs.

Figure 5. (a) Schematic diagram of an OPV device with Ag NDs. (b) J−V characteristics of OPV devices with and without Ag NDs under AM 1.5 G, 100 mW/cm2 illumination. (c) EQE spectra of OPV devices without and with 91 nm Ag NDs. (d) EQE enhancement after embedding Ag NDs in the OPV device and extinction spectra of 91 nm Ag NDs. The wavelength range of EQE enhancement corresponds with the extinction spectrum.

Table 2. Characteristic Parameters of Plasmonic OPVs Incorporated with Ag NDs Arrays of 91 nm Size in the PEDOT:PSS Layera ITO/PEDOT:PSS (ref.) ITO/Ag ND/PEDOT:PSS a

Voc (V)

JSC (mA/cm2)

FF

PCE (%)

0.75 ± 0.01 0.73 ± 0.01

17.43 ± 0.33 23.26 ± 0.60

0.60 ± 0.01 0.61 ± 0.01

7.52 ± 0.16 (7.70) 10.11 ± 0.42 (10.72)

Averaged values were calculated over several times of device fabrication. PCEs in parentheses indicate the highest values.

enhancement in the JSC value after the incorporation of plasmonic Ag NDs might be affected by the optical properties as well as electrical properties of plasmonic nanostructures, because the photocurrent improvement can also originate from the increased number or mobility of charge carriers. The EQE spectrum with the best performance of Ag ND-embedded plasmonic OPV is compared to that of the control device (without Ag NDs) in Figure 5c. A more increased EQE was obtained under the broadband wavelength range (460−730 nm) after incorporating the Ag NDs. On the other hand, decreased EQE in the relatively narrow wavelength range

(390−450 nm) might be attributed to the self-absorption of Ag NDs.11 The wavelength-dependent EQE enhancement in the 460−730 nm range in Figure 5d reflects that the EQE was enhanced in almost the same band region of the extinction spectrum of the Ag NDs. Ag NDs can exhibit both plasmonic scattering and near-field enhancement, leading to absorption enhancement in the active layer; the wavelength of absorption enhancement matches with the extinction spectrum. The broadband enhanced EQE can increase the current density according to the following equation: 10146

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Jsc = q

∫λmin

scattering factors, we measured scattered light with and without Ag NDs on an ITO substrate using an integrated sphere system. In the range of 460−730 nm, more scattered light is observed induced by Ag NDs, whereas in the less than 460 nm or higher than 730 nm region a similar tendency is observed compared with the neat ITO substrate (see Figure 7a). The wavelength-dependent scattering effect contributes to the increased optical path length by light trapping and JSC enhancement.43 To analyze the correlation between the scattering and absorption enhancement, we spin-coated a PTB7:PC70BM active layer on the substrate to calculate only the active layer’s absorption. In Figure 7b, the absorption of PTB7:PC70BM on the glass/ITO/Ag NDs substrate is calculated by subtracting the absorption of the glass/ITO/Ag NDs substrate from the glass/ITO/Ag NDs/PTB7:PC70BM substrate. For reference, we subtracted the absorption of the glass/ITO substrate from the glass/ITO/PTB7:PC70BM substrate by the same procedure. Considering the absorption of the active layer with and without Ag NDs, the Ag ND-based active layer showed more absorption in the range of 460−730 nm, which almost matches with the scattering spectrum of Ag NDs. Another enhancement mechanism is the excitation of the LSPR induced by Ag NDs at the resonance wavelength (573 nm), generating a strong electric field enhancement (see Figure 6b). To confirm the LSPR effect of Ag NDs, the FDTD simulation was carried out. Before the simulation, we confirmed the real structure of PEDOT:PSS coated on Ag NDs to reflect the exact model in the simulation. The real device configuration was confirmed by both SEM and AFM characterization as shown in supplementary Figures S6 and S7. The PEDOT:PSS layer, with a thickness of 50 nm, fully covers the Ag NDs, leading to a corrugated structure. On the basis of the real device configuration, the distribution of the electric field density was calculated for the device without (left) and with (right) PMNs, as shown in Figure 8a. It is clearly observed that a strong field enhancement was generated and scattered around the Ag NDs, which reflects the stronger near-field enhancement induced by the plasmonic structures. To further investigate the LSPR effect in the OPVs, we considered the ITO NDs instead of Ag NDs with the same configuration, as shown in supplementary Figure S8. When ITO NDs are embedded in the OPV, no LSPR is observed (see Figure S8a in the Supporting Information), indicating that the enhancement was mainly due to the plasmonic effect of Ag NDs. Figure 8b shows the light absorption density profiles of OPV devices without and with Ag NDs (left and right, respectively) at the 573 nm wavelength. In the PTB7:PCBM layer, more intense absorption was observed when Ag ND arrays were incorporated in the device compared to the Ag ND-free device, and this is in line with the large increase in the JSC and PCE values, as consistent with previous results. On the basis of the above results, the absorption spectrum at the PTB7:PCBM active layer was also calculated and plotted in Figure 8c. The simulated absorbance profile is consistent with the experimental result, clearly reflecting that the enhancement of JSC and PCE values is driven by the LSPR effect of Ag NDs in the device. In contrast, the device with ITO NDs shows less absorption enhancement (similar to the case of the reference device) because no plasmonic effects are induced by ITO NDs (see Figure S8b in the Supporting Information). Therefore, the excitation of LSPR induced by Ag NDs can induce strong electric field enhancement, which can increase the number of excitons generated in the active layer; a stronger

Φ(λ) EQE(λ) dλ

where the q is the charge of the electron and Φ(λ) is the photon flux.41 The equation indicates that the enhancement factor of EQE is directly correlated with increased current density. To analyze the reason for enhanced current density, we assumed that the Ag NDs can facilitate the increase in the current density with three possible mechanisms (see Figure 6). In case of the LSPR effect, we considered the light scattering effect as well as field enhancement as an optical effect.11,16,42 First, the forward light scattering induced by Ag NDs lengthened the optical path length at the active layer, resulting in enhanced absorption (see Figure 6a). To clarify the light

Figure 6. Schematic illustration of (a) light trapping induced by the forward scattering effect and (b) enhancement of the electromagnetic field induced by the LSPR effect. (c) Mechanism illustration of enhanced current through the Ag ND layer. 10147

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Figure 7. (a) Measured light scattering factor using an integrated sphere system. (b) Measured absorption of the PTB7:PC70BM layer coated on different substrates. Absorption of PTB7:PC70BM coated on the glass/ITO substrate is obtained by subtracting the absorption of the glass/ ITO substrate from the glass/ITO/PTB7:PC70BM substrate (black line). Absorption of PTB7:PC70BM coated on the glass/ITO/Ag ND substrate is obtained by subtracting the absorption of the glass/ITO/Ag NDs substrate from the glass/ITO/Ag NDs/PTB7:PC70BM substrate (blue line).

Figure 8. Electric field density distributions (a) and absorption density profiles (b) in OPV devices with and without Ag NDs at the resonance wavelength (λ = 573 nm). Left and right structures in (a) and (b) show the control device and device with Ag NDs, respectively. (c) Calculated absorbance profiles of the OPV devices with and without Ag NDs. 10148

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Figure 9. (a) AFM image of PMNs fabricated on the ITO/glass substrate and (b) topographical height profiles along the red lines. (c) Current map of the PMN fabricated on the ITO/glass substrate under 0.5 V bias. (d) Current level of PMNs fabricated on the ITO/glass substrate along the blue lines.

field can contribute to the generation of more excitons by dissipating higher energy.44,45 In addition to the LSPR effect induced by Ag NDs, we also suggest the electrical effect of Ag NDs for the supporting mechanism of the performance increase as illustrated in Figure 6c. To investigate the electrical performance of Ag NDs, conductive atomic force microscopy was carried out. Figure 9a and b show the AFM image of the PMN arrays fabricated on the ITO/glass substrate and the topographical height profile along the red line. The average height of the Ag NDs was measured to be ∼50 nm. Figure 9c shows the current flow profile at the Ag NDs and ITO under 0.5 V bias condition, where the overflowed currents through the Ag NDs are clearly observed. The current levels at the Ag NDs and ITO surface along the blue line in the current map are shown in Figure 9d, and a remarkably increased current level was observed at the Ag NDs. Moreover, the currents at the Ag NDs are rapidly increased when the bias voltage is slightly larger than 0 V, whereas ITO shows gradually increased current flow as shown in Figure S9. It is clearly expected that the charge transport and extraction process is facilitated through the Ag NDs and therefore improves the electrical performance in the device.46 The results obtained from the c-AFM studies clearly certify that the plasmonic MNDs play an important role by production of an electrical channel, thus facilitating charge transportation and enhanced current flow.

induced both light scattering and a strong LSPR effect in the OPV devices, thus enhancing the light absorption at the photoactive layer. In addition to the LSPR effect of Ag NDs, we also analyze the advantages in terms of electrical properties, facilitating the charge transport and extraction through the Ag NDs. As a result, the current density and EQE remarkably increased, leading to a high-performance optimized OPV device exhibiting >10% PCE and a significantly enhanced level of efficiency increase by ∼34% compared to the Ag-free control device. In summary, we propose that configuration-adjusted PMNs fabricated by the LIL process can be a viable platform for the development of addressable, advanced, and large-scale photovoltaic and optoelectronic devices.

METHODS PMN Fabrication. The ITO substrates were sequentially cleaned in acetone, methanol, and deionized (DI) water for 30 min by ultrasonication and then dried in an oven at 120 °C for 10 min. The overall fabrication process of ND patterning is schematically shown in Figure 1. An adhesion layer (HMDS:PGMEA = 1:4) was spin-coated at 4000 rpm for 40 s on an ITO substrate before drying at 180 °C for 90 s. After that, the photoresist (AR-N4240, mixed with Thinner AR 300-12 at a ratio of 1:1) was spin coated at 4000 rpm for 40 s on the adhesion layer and then dried at 100 °C for 90 s. For the LIL process, the films were placed on Lloyd’s mirror and exposed to the laser source coming through a spatial filter as shown in Figure S1 of the Supporting Information. A frequency-doubled argon-ion laser with a wavelength of 257 nm was used, and the exposure power to the sample was 0.12 mW/cm2. The power of the laser system was controlled using an optical power/energy meter (Newport 1936-C). The initial laser beam diameter and the focal length were ∼0.15 and 3.4 mm, respectively. The distance between the spatial filter and the samples was ∼1.2 m, and the exposure time was set at 133 s. The pitch and size of the hole patterns could be controlled by changing the incident angle of the laser beam from 5° to 40° on the PR films. Finally, a Ag layer with a thickness of 50 nm was thermally evaporated under vacuum conditions, followed by the lift-off step of PR using acetone. The dependence of the cell efficiency on the height of Ag NDs was investigated, in which 50 nm was the optimum height (Figure S3).

CONCLUSIONS One of the most efficient plasmonic organic photovoltaic cells reported to date was established by integrating Ag ND arrays incorporating the devices. To realize this, a facile yet powerful optical engineering methodology via LIL techniques was used to fabricate well-ordered periodic Ag ND arrays. Uniformly distributed Ag ND arrays with different sizes and pitches were obtained, allowing systematic control over the optical property. The Ag NDs located between the HTL and bottom electrode 10149

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ACS Nano OPV Cell Fabrication. First, a PEDOT:PSS solution was spincoated at 3000 rpm for 30 s on the as-prepared Ag NDs/ITO substrates, and then the films were annealed at 110 °C for 30 min in a glovebox. Next, for active layer deposition, PTB7:PC70BM at a weight ratio of 1:1.5 were mixed in 1 mL of chlorobenzene/1,8-diiodooctane (97:3 volume ratio) solvent and spin-coated at 1000 rpm for 40 s. The films were kept in the glovebox for 30 min without any annealing process. Finally, 1 nm LiF and 100 nm of Al were thermally evaporated as a top electrode. The reference device without Ag NDs was fabricated using the pristine ITO substrate following the same procedure mentioned above. Characterizations. The Ag hole and dot arrays were observed using a field emission scanning electron microscope (FE-SEM, S-4800, Hitachi Ltd.). The optical extinction spectrum and scattering factor were measured using a UV−visible−NIR spectrometer (Cary 5000, Varian Technology) for the optical properties. The photovoltaic cell performance was measured using a solar simulator (Polaronix K3000, McScience) under AM 1.5 conditions with 100 mW/cm2 intensity. The EQE was measured using an IPCE measurement system (PECS20, HS Technology) to analyze the incident photo-to-current efficiency. c-AFM measurement was carried out to verify the surface and electrical properties of Ag NDs (Nanoscope Multimode AFM, Bruker). FDTD Simulation. The plasmonic near-field distribution of Ag ND arrays embedded in the PEDOT:PSS layer was simulated by using the Lumerical Solution software. The simulation domain condition used periodic boundary conditions for the x-axis, y-axis, and perfectly matched layer condition for the z-axis. The mesh size was set up at 0.5 nm for the elaborate design. The optical constants of Ag were taken from Palik in the spectral range from 300 to 800 nm. We used the plane wave source as incident light, and the electric field and absorption density are calculated at the resonance wavelength (λ = 573 nm). We considered z and x to be the light incident direction and the polarization direction, respectively. The absorption profiles were estimated by frequency-domain field and power monitor.

funded by the Korean Government (2014R1A2A1A09005656, 2014043187, 2015M1A2A2058365).

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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/acsnano.6b05313. Illustration of LIL process, characteristics of different Ag ND arrays, and device performance as well as additional results and figures (J−V characteristic using different heights of Ag NDs, statistical histogram of PCE, specular transmittance and EQE spectra of Ag NDs, surface analysis measured by AFM, FDTD simulation, and cAFM analysis) (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] (D. H. Kim). *E-mail: [email protected] (B.-K. Ju). Author Contributions ⊥

Y. Oh and J. W. Lim contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This study was supported by the Industrial Strategic Technology Development Program (10045269), Development of Soluble TFT and Pixel Formation Materials/Process Technologies for AMOLED TV funded by MOTIE/KEIT. J.W.L., H.K., Y.J.J., J.K., and D.H.K. acknowledge the financial support by the National Research Foundation of Korea Grant 10150

DOI: 10.1021/acsnano.6b05313 ACS Nano 2016, 10, 10143−10151

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DOI: 10.1021/acsnano.6b05313 ACS Nano 2016, 10, 10143−10151