Controlled enhancement in hole injection at gold ... - ACS Publications

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Organic Electronic Devices

Controlled enhancement in hole injection at gold nanoparticle-on-organic electrical contacts fabricated by spark-discharge aerosol technique Jongcheon Lee, Hyungchae Kim, Kyuhee Han, Yongmoon Lee, Mansoo Choi, and Changsoon Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16303 • Publication Date (Web): 14 Jan 2019 Downloaded from http://pubs.acs.org on January 15, 2019

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ACS Applied Materials & Interfaces

Controlled Enhancement in Hole Injection at Gold Nanoparticle-on-Organic Electrical Contacts Fabricated by Spark-Discharge Aerosol Technique Jongcheon Lee,† Hyungchae Kim,† Kyuhee Han,‡,¶ Yongmoon Lee,† Mansoo Choi,‡ and Changsoon Kim∗,† †Graduate School of Convergence Science and Technology, and Inter-University Semiconductor Research Center, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea ‡School of Mechanical and Aerospace Engineering, and Global Frontier Center for Multiscale Energy Systems, Seoul National University, 1 Gwanak-ro, Gwanak-gu, Seoul 08826, Republic of Korea ¶Present address: Samsung Electronics, 1-1 Samsungjeonja-ro, Hwaseong-si, Gyeonggi-do 18848, Republic of Korea E-mail: [email protected]

Abstract We demonstrate that hole injection from a top electrode composed of Au nanoparticles (AuNPs) capped with a thick Au layer into an underlying organic semiconductor, N,N’diphenyl-N,N’-bis-[4-(phenyl-m-tolyl-amino)-phenyl]-biphenyl-4,4’-diamine (DNTPD), is significantly enhanced compared to that in a control device whose top electrode is composed entirely of a thick Au layer. The fabrication of this organic hole-only device with the AuNP electrode is made possible by dry, room-temperature distribution

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of AuNPs onto DNTPD using a spark-discharge aerosol (SDA) technique capable of ¯ of the AuNPs. The enhancement in hole injection varying the average diameter (D) ¯ with the current density of a device with D ¯ is found to increase with decreasing D, = 1.1 nm being more than three orders of magnitude larger than that of the control device. Intensity-modulated photocurrent measurements show that the built-in potentials of the devices with the AuNP electrode are smaller than that of the control device by as much as 0.68 V, indicating that the enhanced hole injection originates from the increased work functions of these devices, which in turn decreases the hole injection barrier heights. X-ray photoelectron spectroscopy reveals that the increased work functions of the AuNP electrodes are due to surface oxidation of the AuNPs resulting in ¯ AuN and Au3 N. The degree of oxidation of the AuNPs increases with decreasing D, ¯ consistent with the D-dependencies of the hole injection enhancement and the built-in potential reduction.

Keywords metal nanoparticles, spark discharge, charge injection, aerosol deposition, organic electronic devices

1

Introduction

Because metal nanoparticles (MNPs) exhibit unique electrical, optical and chemical properties that are not observed in bulk metal, 1 research has been intensely carried out to exploit MNPs for improving the performance of optoelectronic and electronic devices, and overcoming their existing limitations. For example, surface plasmon resonance supported by noble MNPs, with its frequency tunable by varying the composition, size, and shape of the MNPs, can enhance the absorption, 2 scattering, 3 and emission of light, 4,5 and therefore has been utilized for solar cells 6–8 and light-emitting devices. 9,10 In memory devices, MNPs act as charge

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trapping centers, contributing to the increase of the current on/off ratio. 11 Furthermore, because MNPs can be sintered at low temperatures, inks composed of MNPs dispersed in solvents can be printed on substrates to form electrodes and electrical connections in printable electronics. 12,13 To fully exploit the characteristics of MNPs for these applications, not only their composition, size, and shape, but also the distribution density and position must be carefully engineered. In most previous demonstrations, MNPs were distributed using wet processes such as spin coating and dip coating where the MNP distribution were controlled by varying the coating condition and the concentration of the MNPs in solutions. 10,14–16 For organic devices typically composed of small molecules or polymers that are easily degraded, if not dissolved, by exposure to aqueous or organic solvents, the use of a wet process for distributing MNPs poses a restriction on the device structure: MNPs must be distributed on a substrate prior to the deposition of an active layer 7,9,16 or be randomly dispersed in a solution-processed active layer, 8,15 disallowing a precise control of the distance between the MNPs and the active material required to fully utilize the optical and/or electrical interaction between them. This limitation can be overcome by using a dry aerosol technique. 17–19 For example, an aerosol technique where MNPs generated by a spark discharge process are directed toward a substrate by electrostatic attraction is a method particularly suitable for optimizing MNPenhanced organic devices, because it can systemically control the location, size and density (number of particles per area) of MNPs. 17–19 We previously applied this spark-discharge aerosol (SDA) technique to embed Au nanoparticles (AuNPs) on an optimal plane in an organic light-emitting device (OLED), thereby maximizing its quantum efficiency. 19 The physical and chemical properties of metal deposition in the SDA process are significantly different from those in the conventional vacuum thermal evaporation (VTE) technique. First, spark-dischage-generated MNPs are rapidly cooled down by a nitrogen carrier gas and impinge on a substrate with a kinetic energy that is significantly smaller (e.g., < ∼5 meV for a MNP with a diameter < 5 nm) than that of metal vapors in VTE. 19,20 As a result, unlike in

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VTE, the SDA technique was shown to create a sharp MNP–organic interface without MNP penetration into the organic layer. 19 Second, immediately after being generated, MNPs in the SDA technique have a high thermal energy, and at the same time, are surrounded by N2 molecules and ions generated by dielectric breakdown, whereas metal vapors in VTE are in an inert environment. Therefore, when viewed as an electrode deposition technique for organic electronic devices, the SDA method is expected to create metal–organic interfaces with charge injection characteristics different from those obtained using the conventional VTE process. Here, we investigate the hole injection properties of SDA-deposited AuNPs by characterizing organic hole-only devices whose hole-transport layer and top electrode are, respectively, N,N’-diphenyl-N,N’-bis-[4-(phenyl-m-tolyl-amino)-phenyl]-biphenyl-4,4’-diamine (DNTPD) and ¯ capped with a thick Au layer SDA-deposited AuNPs with a different average diameter (D) deposited by VTE. The current density versus voltage (J–V ) characteristics show that the hole injection from the SDA-deposited AuNPs are significantly improved compared to that in a control device whose top electrode is composed entirely of a VTE-deposited Au layer. Fur¯ with J of a device thermore, the enhancement in hole injection increases with decreasing D, ¯ = 1.1 nm being more than three orders of magnitude larger than that of the control with D device. Intensity-modulated photocurrent and X-ray photoelectron spectroscopy measurements indicate that the increase in hole injection of the devices with the AuNP electrode is attributed to a decreased hole injection barrier originating from oxidation at the AuNP surfaces resulting in AuN and Au3 N. The improvement of hole injection by SDA-deposited ¯ may complement the conMNPs, with the degree of improvement controllable by varying D, ventional methods used to form electrodes with efficient charge injection, such as interfacial doping, 21,22 surface modification, 23–25 and insertion of a charge-injection layer. 26,27

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2

Results and Discussion

Figure 1a is a schematic illustration of an apparatus (pin-to-plate type spark discharge generator 18 ) used to generate an aerosol of AuNPs and deposit them onto an organic layer, in order to form AuNP top electrodes of organic electronic devices. 19 A pin and a plate

(a)

(b)

Au N2 carrier gas

DNTPD ITO

Au DNTPD

AuNP Electron N2 cation

ITO

Au DNTPD

Organic layer Substrate

ITO

(c)

(d)

(e)

20 nm

20 nm

20 nm

Figure 1: (a) Illustration of the generation and deposition of AuNPs by the SDA technique. (b) Schematic of the fabrication process of a hole-only device whose top electrode consists of AuNPs capped with a thick Au layer. (c–e) High-resolution TEM images and size distributions of the AuNPs deposited by the SDA technique, with C varied from (c) 50 pF, (d) 200 pF to (e) 2.0 nF. with an exit hole at the center, both made of Au, are connected to a dc high voltage source

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through an RC circuit shown in Figure S1. When the voltage across the capacitor reaches a threshold value (Vth ) at which the electric field between the pin and the plate exceeds the dielectric strength of the surrounding N2 gas, a burst of an electrical discharge occurs, which vaporizes Au and ionizes the N2 molecules. AuNPs are then formed via aggregation of the Au vapors, and become charged by adsorption of the N2 molecular ions or electrons produced during the electrical discharge. Subsequently, the AuNPs are transported by a N2 carrier gas to a target substrate that is electrically biased to attract them. After each discharge, the dc high voltage source recharges the capacitor up to Vth at which another discharge occurs, thereby creating a series of repetitive discharges. The size distribution of the AuNPs is controlled by varying the capacitance (C) value of the RC circuit: as C increases, the electrical energy stored in the capacitor at Vth and, therefore, the amount of Au vaporized ¯ of the AuNPs. 17 This was confirmed during each discharge increases, thereby increasing D by transmission electron microscopy (TEM) images of AuNPs deposited by the SDA process ¯ on TEM grids, with C varied from 50 pF, 200 pF to 2.0 nF (Figures 1c–e). The value of D was 1.1, 1.5 and 1.8 nm, when C is 50 pF, 200 pF, and 2.0 nF, respectively, with a standard deviation of the diameter also increasing with C. In addition, as C decreases, the relative area covered by small AuNPs increases, as shown in Figure S2. To investigate the hole injection property of the SDA-deposited AuNP top electrode, a hole-only device was fabricated, as shown in Figure 1b. After depositing a 100-nm-thick layer of DNTPD on a glass substrate precoated with indium tin oxide (ITO), AuNPs were distributed on top of the DNTPD layer by the SDA process, which was then capped with a 100-nm-thick Au layer deposited by VTE. Also, a device whose top electrode is composed entirely of a 100-nm-thick Au layer deposited by VTE was fabricated as a control device. Hereafter, we refer to the former and latter hole-only devices as NP and VTE devices, respectively. Figure 2 shows the representative J–V characteristics of the NP devices fabricated with different C (50 pF, 200 pF, and 2.0 nF), compared with those of the VTE device. For each type of the NP devices and the VTE device, ten devices were measured and their J–V

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characteristics are shown in Figure S3. In preparing the NP devices, to minimize the depo10 3

Current Density (mA/cm2)

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10 2 10 1 10 0 10 −1 10 −2

10

−

NP device, (D = 1.1 nm) − NP device, (D = 1.5 nm) − NP device, (D = 1.8 nm) VTE device

−3

10 −4 0

5

10

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Voltage (V) Figure 2: Current density versus voltage characteristics of the AuNP devices with different ¯ values and the VTE device. D sition time (t) of the AuNPs and, at the same time, ensure that the hole-injection at the top electrode is governed by the AuNP–DNTPD interface, we chose values of t, beyond which the increases in J with t show saturation behaviors. It was found that, at all V , all NP devices have the J values that are significantly larger than that of the VTE device, indicating that the hole-injection from the AuNPs is much superior to that of the VTE Au. Furthermore, ¯ the improvement the hole injection property of the AuNPs has a strong dependence on D: ¯ As a result, at V = 5 V, the NP device in J of the NP devices increases with decreasing D. ¯ = 1.1 nm has J = 111 mA cm−2 , which is more than three orders of magnitude larger with D than J (= 17.7 µA cm−2 ) of the VTE device and is even larger than J (= 57.2 mA cm−2 ) of a device with a hole-injection layer composed of MoO3 , the most common choice for this purpose (Figure S4). The dramatic increase in J of the NP devices indicates that the hole injection at the AuNP–DNTPD interface is much more efficient than that at the interface between DNTPD and Au deposited by VTE. To identify the origin of the difference in hole injection, we 7



first performed x-ray photoelectron spectroscopy (XPS) on films of AuNPs and VTE Au, both deposited on Si wafers. Figure 3 shows XPS spectra, in the Au 4f region, of the VTE ¯ = 1.1, 1.5 and 1.8 nm (Figures 3b– Au (Figure 3a) and the SDA-deposited AuNPs with D d, respectively). For the VTE Au, the spin–orbit split Au 4f5/2 and Au 4f7/2 peaks, with

(a)

(b)

(c)

(d)

¯ = Figure 3: XPS spectra (black) of Au 4f peaks of (a) VTE Au and AuNPs with (b) D ¯ = 1.5 nm and (d) D ¯ = 1.1 nm, and fits (red) obtained with three pairs (Au0 : 1.8 nm, (c) D green, Au1+ : magenta, Au3+ : blue) of the spin-orbit split Au 4f5/2 and Au 4f7/2 peaks, whose positions are shown in the parentheses. The cyan lines are Shirley backgrounds. binding energies of 88.0 and 84.3 eV, respectively, are observed as shown in Figure 3(a), in good agreement with a typical XPS spectrum of pure Au (Au0 ). 28,29 In contrast, the XPS spectra of the AuNPs (black solid lines in Figures 3b–d) exhibit deviations from the spectrum ¯ By fitting these of the VTE Au, where the degree of deviation increases with decreasing D. XPS spectra with three pairs of spin–orbit split Au 4f5/2 and Au 4f7/2 peaks having a peak separation of 3.7 eV, identical full widths at half maximum, and a 4f5/2 :4f7/2 intensity ratio of 3:4, it is shown that the spectrum of each AuNP film is composed of the Au0 spectrum (green 8


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dashed line) and two spectra shifted respectively by (0.7 ± 0.2) and (2.3 ± 0.2) eV (magenta and blue dashed lines, respectively) from that of Au0 . The locations of the peaks in each case are shown in the legends of Figure 3. The magnitudes of the shift in binding energy indicate that the shifted spectra are the spin–orbit split Au 4f5/2 and Au 4f7/2 peaks of Au1+ and Au3+ , 28,29 with the spectrum of Au1+ (Au3+ ) corresponding to the magenta (blue) dashed line. In the XPS spectra in the N 1s region (Figure S5), a peak was detected only from the AuNPs films at (399.3 ± 0.2) eV, which are relatively far from the binding energy of the N 1s electrons (402–404 eV) in N2 molecules physisorbed on a surface, 30,31 but close to that of the N 1s electrons (399.7 eV) in AuN films where N atoms are in a reduced state of N3− . 31 Hence, the result of our XPS analysis suggests that Au atoms on the AuNP surfaces are oxidized to form AuN and Au3 N, with the number of AuN much larger than that of Au3 N. This is quite plausible considering that AuNPs with diameters less than 3 nm are known to be easily oxidized even in ambient atmosphere, 32,33 and, furthermore, the AuNPs generated by the SDA process have high thermal energies while being surrounded by N2 molecules as well as N2 ions produced during the spark discharge. To estimate the degree of surface oxidation of the AuNPs in each film, atomic ratio of (Au1+ +Au3+ )/(Au1+ +Au3+ +Au0 ) was calculated from the XPS spectra using the elemental relative sensitivity factors, as shown ¯ meaning that in Figure 4a. The atomic ratio monotonically increases with decreasing D, ¯ decreases. In the case of the AuNPs with the the degree of surface oxidation increases as D ¯ (= 1.1 nm), as much as ∼60 % of the Au atoms in the surface region are oxidized. smallest D The oxidation of the SDA-deposited AuNPs suggests that their work function and the vacuum level shift at the AuNP–DNTPD interface are likely to be different from the corresponding values for the case of the VTE Au. 34–36 Since the AuNPs in the NP devices are deposited on top of DNTPD, a direct investigation by photoelectron spectroscopy 36,37 of the energy level alignment at the AuNP–DNTPD interface is very difficult. We therefore measured the built-in potential (Vbi ) of each device, which varies as a result of the combined change of the work function and the vacuum level shift at the interface. The measure-

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Atomic Ratio (%)

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80 60 40 20 0 VTE Au

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1.1

Average Diameter (nm)

(b) Built-in Potential (V)

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1 DN DN TPD ITO TPD VTE Au ITO

0.8 0.6

AuNP

qVbi = 0.86 eV qVbi = 0.18 eV

0.4 0.2 0 VTE Au

1.8

1.5

1.1


Figure 4: (a) Atomic composition of VTE Au and AuNPs estimated from the XPS measurement. The surface oxidation of AuNPs are schematically shown above the top axis, where yellow and light blue regions correspond to Au and Aux N, respectively. (b) Built-in potential of the VTE and NP devices determined from the intensity-modulated photocurrent measurement. Inset: schematic energy level diagrams of the VTE device and the NP device ¯ = 1.1 nm under zero bias. with D

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ment was carried out using the intensity-modulated photocurrent technique, 38,39 where the photocurrent of a device is measured as a function of dc bias while it is illuminated with intensity-modulated light, and Vbi is determined from the dc bias resulting in zero photocurrent. 39 The magnitude of the photocurrent and the relative phase between the photocurrent and the light intensity is shown for each device in Figure S6. It is found that all NP devices have Vbi lower than that of the VTE device: the Vbi values are 0.18, 0.26 and 0.36 V for ¯ = 1.1, 1.5 and 1.8 nm, respectively, whereas the VTE device has Vbi the NP devices with D ¯ plotted in Figure 4b indicates that Vbi decreases as of 0.85 V. The dependence of Vbi on D the degree of oxidation increases. The schematic energy level diagrams of the VTE device ¯ = 1.1 nm, whose Vbi values differ by 0.68 V under zero bias, are and the NP device with D compared in the inset of Figure 4b. Figure 5 shows the energy level alignment at the Au–DNTPD interface deduced from a photoelectron spectroscopy measurement previously performed for DNTPD thin films 40 and our Vbi measurement. As in the interface-limited charge injection model, 41,42 the following

(a)

(b)

∆NP,1

∆NP,2

𝐸VL

𝐸VL

∆NP (= |∆NP,2 | − |∆NP,1 |)

∆VTE 𝑊Au

𝑊Au

𝑊Au𝑥N

𝑊D,VTE

𝑊D,NP

𝐸F

𝐸F

ΦB,NP

ΦB,VTE

HOMO DOS

HOMO DOS 1st molecular 2nd molecular layer layer

Au

DNTPD

1st molecular 2nd molecular layer layer

Au

Au𝑥 N

DNTPD

Figure 5: Energy level diagrams at the metal–organic contact in (a) the NP device and (b) the VTE device. The thickness of the first and second molecular layers is one hopping distance. 41,42 The representative Gaussian density of states (DOS) in each layer is drawn. features of disordered organic semiconductors were considered in drawing these diagrams: 11



the HOMO levels of DNTPD are broadened by local disorder, with the degree of broadening of the first molecular layer being larger than that of the second molecular layer; the degree of electronic polarization at the metal–organic interface is greater than that in the organic bulk so that the distance between the HOMO level corresponding to the peak of the Gaussian density of states and the local vacuum level (EVL ) is smaller for the first layer than for the second layer. The EVL shifts at the interfaces are denoted by ∆ and a blue arrow: ∆ is positive and the arrow points upward when EVL on the left side of the interface is located higher than EVL on the right side. In the VTE device, the work function of Au (WAu ) is about 1 eV larger than that of the DNTPD film, leading to ∆VTE (> 0) at equlibrium due to electron transfer from DNTPD to Au, 35,40 as shown in Fig. 5 (a). In the NP device, electron transfer from Au to N due to the difference in electronegativity of Au and N atoms makes the the work function of Aux N (WAux N ) larger than WAu , 34,43 making the EVL shift at the Au–Aux N interface (∆NP,1 ) negative. Then, EVL at the DNTPD surface is lower than EVL at the Au surface by ∆NP (= |∆NP,2 | − |∆NP,1 |), where ∆NP,2 (> 0) is the EVL shift at the Aux N–DNTPD interface. It follows that ∆NP must be smaller than ∆VTE by the Vbi difference between the VTE and NP devices (e.g., ∆VTE − ∆NP = 0.68 V for the NP device ¯ = 1.1 nm) and that ∆NP decreases as the degree of oxidation of AuNPs increases. with D Consequently, the barrier for hole hopping from the first molecular layer to the second molecular layer, which is the current limiting step in the interface-limited charge injection model, 41,42 is smaller for the NP devices (ΦB,NP < ΦB,VTE ), explaining the enhancement in J of the NP devices.

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Conclusions

SDA-deposited AuNPs were employed as a top electrode of a DNTPD-based hole-only device. The hole injection from the AuNP electrode was found to increasingly improve with decreas¯ and J of the NP device with D ¯ = 1.1 nm is more than three orders of magnitude ing D,

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larger than that of the VTE device. From the XPS analysis and the Vbi values determined from the intensity-modulated photocurrent measurement, it was found that the hole injection enhancement is attributed to the increased work function of the AuNP electrodes resulting from oxidation of the AuNPs, the degree of which was systematically controlled by adjusting the size of the AuNPs during the SDA process. Although the focus of our current study was mainly on the device with a Au capping layer, the effects of the material composition of the capping layer on the hole injection enhancement by the SDA-deposited AuNPs is worthy of future investigation. Our preliminary experiments show that the enhancement is affected by the material composition of the capping layer; when the capping layer was composed of Ag, the hole injection enhancement was observed, albeit with a much smaller degree compared to the case of the Au capping layer (Figure S4), whereas no enhancement was found with an Al capping layer. The SDA technique is expected to enable the formation and investigation of various MNP-on-organic or metal alloy NP-on-organic interfaces because various MNPs with different surface oxidized states can be obtained by varying the carrier gas and the electrodes of the spark discharge generator. 17,18

4

Experimental Section

Device Fabrication ITO-coated glass substrates (15 Ω/sq, 25 mm × 25 mm) were sequentially cleaned with detergent, deionized water, acetone, and isopropyl alcohol in an ultrasonic bath. After being dried in a vacuum oven at 150 ◦C for 10 min, the substrates were exposed to ultraviolet-ozone for 15 min. Deposition of DNTPD (Nichem Fine Technology) was performed using a vacuum thermal evaporator with a base pressure of ∼10−7 Torr. To fabricate the NP devices, AuNPs were deposited onto DNTPD layers using a homemade SDA system consisting of a spark discharge generator and an electrostatic precipitator in a N2 -filled glovebox connected to the vacuum thermal evaporator. 19 The Au pin of the spark discharge generator was connected 13



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to a grounded Au plate through an external RC circuit composed of a high voltage source (5.0 kV), a resistor (20 MΩ) and a capacitor whose capacitance was chosen (50 pF, 200 pF, and 2.0 nF) to control the size of the AuNPs. The potential at the substrate was −3.0 kV and the flow rate of a N2 carrier gas was 3.0 lpm. After the deposition of the AuNPs, the substrates were transferred back to the vacuum thermal evaporator to deposit Au cap layers through a shadow mask with 2-mm-diameter holes. The growth rates of DNTPD and Au were 1 Å s−1 and monitored by a quartz crystal oscillator.

Characterization To obtain the size distributions of AuNPs, they were first deposited on TEM grids (Electron Microscopy Sciences, CF400-Cu-UL) for 3 min using the SDA technique. Next, from high resolution TEM (JEOL, JEM-ARM200F) images, the equivalent diameters of the individual AuNPs were calculated from their projected areas using the ImageJ software. The J–V characteristics were measured using a source meter (Keithley, 2400). For the intensity-modulated photocurrent measurement, the devices kept at 78 K in a liquid nitrogen cooled cryostat were illuminated with monochromatic light with a wavelength of 400 nm generated from a broadband laser-driven light source (Energetiq, EQ-99X) connected to a monochromator. The light was modulated by a chopper at a frequency of 20 Hz, and the photocurrent was measured using a current preamplifier (Stanford Research System, SR 570) and a lock-in amplifier (Stanford Research System, SR 830). The XPS analysis was carried out using a Theta Probe XPS system (Thermo Fisher Scientific) equipped with an Al Kα X-ray source (1486.7 eV). The XPS spectra were fitted with a Shirley background and Gaussian-Lorentzian functions.

Acknowledgement This work was supported by the Basic Science Research Program (NRF-2014R1A1A1006332) and the Global Frontier R&D Program on the Center for Multiscale Energy System (2012M3A6A7054855),

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both funded by the National Research Foundation under the Ministry of Science, ICT, and Future Planning, Korea. The authors thank Hyangki Sung for help with the initial SDA experiment.

Supporting Information Available Circuit diagram of the spark discharge generator; cumulative areal distribution function of the AuNPs; J–V characteristics of the DNTPD hole-only devices; XPS spectra of the AuNPs in the N 1s region; magnitude and phase of the modulated photocurrents

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Graphical TOC Entry

Au

DNTPD ITO

AuxN

103

1.0

Oxidized Au/Au

102

0.8 0.78

0.6

101

0.65

100

0.4 0.2

0

10‒1

0.13 Control

1.8

1.5

1.1

10‒2


22


Current Density at 5 V (mA/cm2)

Au

Built-in potential (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 22

Controlled enhancement in hole injection at gold ... - ACS Publications

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