Electron injection in inverted organic light-emitting diodes with poly(ethyleneimine) electron injection layers

Accepted Manuscript Electron injection in inverted organic light-emitting diodes with poly(ethyleneimine) electron injection layers Makoto Takada, Takashi Nagase, Takashi Kobayashi, Hiroyoshi Naito PII:

S1566-1199(17)30382-8

DOI:

10.1016/j.orgel.2017.07.049

Reference:

ORGELE 4241

To appear in:

Organic Electronics

Received Date: 6 June 2017 Revised Date:

28 July 2017

Accepted Date: 30 July 2017

Please cite this article as: M. Takada, T. Nagase, T. Kobayashi, H. Naito, Electron injection in inverted organic light-emitting diodes with poly(ethyleneimine) electron injection layers, Organic Electronics (2017), doi: 10.1016/j.orgel.2017.07.049. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Electron injection in inverted organic light-emitting diodes with poly(ethyleneimine)

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electron injection layers

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Makoto Takada a, Takashi Nagase a,b, Takashi Kobayashi a,b, and Hiroyoshi Naito a,b,*

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Japan

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Sakai 599-8531, Japan

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Department of Physics and Electronics, Osaka Prefecture University, Sakai 599-8531,

The Research Institute for Molecular Electronic Devices, Osaka Prefecture University,

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*Corresponding author. Tel.: +81 72 254 9266; fax: +81 72 254 9266.

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E-mail address: [email protected] (H. Naito)

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Abstract

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Electron-injection

mechanisms

from

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light-emitting polymer layer are studied. The device configuration is aluminum (Al)

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doped

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(PEI)/poly(9,9-dioctylfluore-ne-alt-benzothiadiazole)

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trioxide/Al, known as an inverted organic light-emitting diode (iOLED). PEI reduces

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the electron injection barrier between AZO and F8BT by 0.4 eV, and blocks holes at

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AZO(PEI)/F8BT interface in iOLEDs. The accumulation of holes at the interface

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greatly enhances the electron injection because of the Fowler-Nordheim type tunneling

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injection, leading to high current efficiency of iOLEDs.

air-stable

metal-oxide

cathode

oxide

(AZO)/poly(ethyleneimine) (F8BT)/

molybdenum

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Keywords

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inverted organic light-emitting diode, electron injection, poly(ethylene imine)

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1. Introduction

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the

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Organic light emitting diodes (OLEDs) have the advantage of flexibility, high

contrast, surface emission, and light weight in contrast to inorganic LEDs, and represent

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promising alternatives in optoelectronics applications, lightings and flexible displays.

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Typical

bottom-emission

OLEDs

have

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a

transparent

metal-oxide

electrode,

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indium-tin-oxide (ITO) with work function of 5.0 eV, deposited on glass for anode and

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have an opaque metal for cathode. In the device structure of such a conventional OLED,

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it is essential to deposit low work function and air-sensitive cathode materials such as

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magnesium:silver (Mg:Ag) [1], lithium fluoride (LiF)/ aluminum (Al) [2], and calcium

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(Ca)/Al [3], and to encapsulate rigorously because of protection from moisture and

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oxygen.

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Inverted OLEDs (iOLEDs) [4, 5] with an air stabile bottom metal-oxide

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cathode have recently been attracted considerable attention as substitutes for

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conventional OLEDs. The air-stable iOLEDs are suitable for flexible light-emitting

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devices because of allowing the preparation of devices without rigorous encapsulation

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and using flexible substrates such as a plastic film. The flexible substrates suffer from

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poor gas and moisture barrier performance, and the water vapor transmission rate

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(WVTR) of commercially available flexible films for organic devices is 10-5 - 10-2 g m-2

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day-1 (The WVTR of conventional glass substrates is 10-8 g m-2 day-1). It is widely

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known that conventional OLEDs require barrier films with a WVTR of less than 10−6 g m-2 day-1 [6]. The configuration of iOLEDs is inverted with respect to that of

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conventional OLEDs: the representative device structure of iOLEDs is substrate/metal

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oxide (cathode)/a light-emitting polymer/molybdenum trioxide (MoO3)/gold (Au)

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(anode). The metal-oxide cathodes of iOLEDs are titanium dioxide (TiO2) [4], zinc

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oxide (ZnO) [7, 8], zirconium dioxide (ZrO2) [9], and ITO [10] reported in literature.

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The recent important approach of iOLEDs has been using ZnO thin films because

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iOLEDs with ZnO exhibit relatively high current efficiency [7], resulting from the

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favorable conduction band energy level, high transparency and high electron mobility of

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ZnO thin films.

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iOLEDs, however, have a high electron-injection barrier between ZnO cathode

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and an emissive layer: the bottom of the conduction band of ZnO is located 3.8-4.2 eV

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from the vacuum level. Materials for lowering electron-injection barrier between ZnO

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and a light-emitting polymer have been studied: for instance, the coating of the cesium

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carbonate (Cs2CO3) interlayer onto ZnO cathode gives high device efficiency, as it acts

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as

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ITO/ZnO/Cs2CO3/super yellow/MoO3/Au, where super yellow is a poly(p-phenylene

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vinylene) copolymer [11]. Cs2CO3, however, may be doped in the emission polymer

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the

device

configuration

of

the

iOLED

is

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hole-blocking

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because of cesium (Cs) ion diffusion, which causes quenching of photoluminescence at the interface [12]. Recently, it has been reported that surface modifiers based on

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polymers containing simple aliphatic amine groups, poly(ethylene imine) (PEI) and

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poly(ethylene imine) ethoxylated (PEIE) substantially reduce the work function of

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conductors including metals, transparent conductive metal oxides, conducting polymers,

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and graphene [13]. Coating PEI or PEIE on ZnO as the electron-injection layer (EIL)

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reduces the electron injection barrier from ZnO to a light-emissive layer, and highly

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efficient iOLEDs with PEI or PEIE EIL based on fluorescent polymers [14, 15] and

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phosphorescent molecules [16, 17], white emitting tandem iOLEDs [18], and inverted

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quantum dot LEDs [19] have been reported. However, the electron-injection

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mechanisms from metal-oxide cathodes with PEI have not been investigated except for

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reduction of work function of metal-oxide cathodes [13-15].

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In this paper, we study efficient electron injection mechanisms from air-stable

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metal-oxide cathodes with PEI to an emissive layer in working iOLEDs. These

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mechanisms are valuable information for development of efficient electron-injection

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materials and thus enable us to realize highly efficient iOLEDs. We fabricated

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poly(9,9-dioctylfluorene-alt-benzothiadiazole) (F8BT) based iOLED with or without

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PEI and examined the electronic properties of the metal oxide/PEI interface in the

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iOLEDs. We showed that the electron injection into the emissive polymer layer is greatly enhanced by the accumulation of holes at the metal oxide/emissive polymer interface.

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2. Experiment The device configuration of the iOLEDs was AZO/PEI/F8BT/MoO3/Al, where

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AZO is Al doped ZnO. The device configuration of electron-only devices (EODs) to

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characterize electron-injection properties was AZO/PEI/F8BT/Ca/Al. A patterned AZO

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glass (Geomatec) as a cathode [10] was cleaned using acetone, 2-propanol and

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UV-ozone. Subsequently, a thin layer of PEI, an electron-injection layer, was spun onto

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the surface of the AZO glass from an ethanol solution (0.3 wt%, 2000 rpm, 30 s). The

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substrate was then annealed in ambient atmosphere (5 min, 150 °C). A layer of 200-nm

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F8BT, a green light-emitting polymer, was spun onto the PEI layer from a

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chlorobenzene solution as an emissive layer (1 wt%, 1000 rpm, 60 s). After the F8BT

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emissive layer deposition, the substrates were dried at 85 °C for 20 min. In the iOLEDs,

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10-nm MoO3 and 50-nm Al layers were successively thermally evaporated onto the

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F8BT emissive layer in a vacuum chamber at a base pressure of 10−4 Pa. In the EODs,

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15-nm Ca and 50-nm Al layers were successively thermally evaporated onto the F8BT

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emissive layer. Finally, the iOLEDs and the EODs were encapsulated with epoxy. The processes mentioned above were carried out in a nitrogen-filled glove box (dew point:

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-80 °C). iOLEDs without PEI were fabricated as well to investigate the effect of the PEI

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layer. The active area of the devices was 2 mm-square area.

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Current density–voltage (J–V) characteristics of iOLEDs were recorded with a

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source measure unit (Keithley 2411), and luminance was measured with a luminance

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meter (Konica Minolta CS-200). The photovoltaic measurement to determine built-in

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potential Vbi was carried out using a solar simulator (Asahi Spectra, HAL-320) as a light

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source. Capacitance–voltage (C–V) characteristics were measured using a Solartron

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1260 impedance analyzer with a 1296 dielectric interface at 1 Hz. Photoemission yield

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spectroscopy in air was made using a photoemission yield spectrometer (AC-2, Riken

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Keiki). All measurements were performed in laboratory atmosphere.

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3. Results and discussion

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The J–V and the luminescence-voltage (L-V) characteristics of the iOLEDs

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with or without PEI are shown in Fig. 1. The luminescences at 6 V are 11000 cd m-2 (at

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240 mA cm-2) and 3900 cd m-2 (at 370 mA cm-2) for the iOLEDs with and without PEI, respectively. Below the current turn-on voltages (< 2.0 V), the current density of the

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iOLEDs without PEI is about 104-105 times higher than that of the iOLEDs with PEI.

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One possible origin of the increase in the current density below 2 V is the recombination

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current via AZO/F8BT interface states. The recombination current is induced by the

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recombination of electrons and holes at interface states [20, 21]. PEI passivates the AZO

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surface states, and reduces the current density below the current turn-on voltages. Above

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3.0 V, the current density of the iOLEDs with PEI is almost the same as that of the

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iOLEDs without PEI, while the luminance of the iOLEDs with PEI is higher than that

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of the iOLEDs without PEI. Lower luminance of the iOLEDs without PEI is due to the

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quenching of excitons at the AZO/F8BT interface (The PEI blocks excitons at AZO

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(PEI)/F8BT interface in the iOLEDs with PEI [15]).

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The current efficiency-current density characteristics of the iOLEDs obtained

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from Fig. 1 are shown in Fig. 2. The current efficiencies at 100 mA cm-2 are 5.5 cd A-1

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and 0.88 cd A-1 in the iOLEDs with and without PEI, respectively.

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We study the changes in the electron-injection barrier between the AZO (PEI)

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cathode and the F8BT emissive layer caused by the deposition of PEI. A

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carrier-injection barrier is defined as the difference between the work function of an

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electrode and lowest unoccupied molecular orbital (LUMO) (or highest occupied molecular orbital (HOMO)) level of an organic semiconducting material, where the

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work function of the electrode is measured by ultra-violet photoemission spectroscopy

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(UPS) or x-ray photoelectron spectroscopy (XPS) [22, 23]. Although many studies on

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organic metal interfaces were conducted on clean metal surfaces in ultrahigh vacuum

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(<10-7 Pa), it is markedly different from the circumstances in organic device fabrication.

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The polymer semiconductors deposited by a wet process on the electrode surfaces

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exhibit an energy-level alignment that is different from that obtained on the clean

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electrode surfaces in ultrahigh vacuum [22-24].

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Instead of UPS and XPS measurements, we estimate the electron-injection

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barrier between the AZO cathode and the F8BT emissive layer by measuring Vbi of

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working iOLEDs. Measurements of Vbi can yield information concerning the difference

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in the work functions between the anode and the cathode in working devices and have

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been carried out using electroabsorption [25] or photovoltaic measurements [26]. The

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Vbi of the iOLEDs was determined by the photovoltaic measurements in this paper

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because the photovoltaic measurements are much simpler and easier than the

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electroabsorption measurements [25, 27], and because the Vbi obtained from both

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measurements are essentially the same [26].

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In the photovoltaic measurement, the current density under illumination JL is

given by

qd  Vbi − V  (µ e g ec + µ h g ha ) − (µ e g ea + µ h g hc )e J L = J D + Ie  , (1) e qd − 1  d 

where JD is the current density in the dark, I is the illumination intensity, e is the 9

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electron charge, V is the applied voltage, d is the device thickness, µe and µh are the

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electron and the hole mobility, gea and gec are the density of photogenerated electrons at

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the anode and the cathode, and gha and ghc are the density of photogenerated holes at the

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anode and the cathode, respectively. The voltage at which the net photocurrent density

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∆J = JL - JD is zero is regarded as the Vbi. The net photocurrent density-voltage (∆J-V)

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characteristics of the iOLEDs with or without PEI are shown in Fig. 3. The Vbi are

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determined to be 1.0 V and 0.64 V in the iOLEDs with and without PEI, respectively.

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The band diagram of the iOLEDs determined by the Vbi is shown in Fig. 4, where the

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work function of the AZO cathode has been adopted as the standard because the work

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function of the AZO cathode is much more stably determined than the bottom of the

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conduction band of MoO3 (measured by optical absorption [28] or inverse

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photoemission spectroscopy [29]). This is due to the fact that the bottom of the

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conduction band of MoO3 becomes smaller over time [28] and is induced significant

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changes by exposing to ambient atmosphere [29, 30] (measured to be 5.2 – 6.7 eV in

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literature [28-30]). The electron-injection barrier between the AZO cathode and the F8BT emissive layer is lowered from 1.4 eV to 1.0 eV by the deposition of PEI, where

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the work function of the AZO cathode is 4.9 eV, determined by photoemission yield

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spectroscopy measurements in air. The lowering of the injection barrier is due to the

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molecular dipole moments associated with neutral amine groups in PEI [13]. Although

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electron injection from such a high electron-injection barrier (~1 eV) is expected to be

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inefficient [2], the electron injection occurs efficiently from the AZO cathode to the

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F8BT emissive layer in the iOLEDs with PEI. The efficient electron injection from PEI

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cannot be understood in terms of the electron-injection barrier lowering only.

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Electron-injection characteristics from the AZO/PEI cathode to the F8BT

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emissive layer are studied by examining the J-V characteristics of the iOLEDs and the

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EODs. The J-V characteristics of the iOLEDs and the EODs with PEI are shown in Fig.

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5, where the AZO electrode works as the cathode in both the iOLEDs and the EODs.

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The current density of the iOLEDs with PEI is almost comparable to that of the EODs

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with PEI below 2.0 V, indicating that the current density of the iOLEDs below 2.0 V is

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due mainly to the electron injection from the AZO cathode.

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As shown in Fig. 5, the current density at 6.0 V are 10-1 A cm-2 and 10-7 A cm-2

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for the iOLED and the EOD, respectively, indicating that electrons are not injected

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efficiently from the AZO cathode in the EODs above 2 V. The current density of the iOLEDs increases drastically above 2 V, which is due to the increase in the electron

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current because the electron mobility of the F8BT µn is much higher than the hole

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mobility of the F8BT µp: µn and µp are 10-3 cm2V-1s-1 and 10-5 cm2V-1s-1, respectively

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[31, 32]. The results in Fig 5 show that the efficient electron injection observed in the

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iOLEDs can be induced by hole injected from the MoO3/Al anode.

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The influence of the hole transport properties on the performance of F8BT

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based iOLEDs has been discussed in Ref. [33]. In Ref. [33], different amounts of

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N,N’-diphenyl-N,N’-bis(3-methyl-phenyl)-(1,1’-biphenyl)-4,4’-diamine

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added to a F8BT light-emitting layer to form hole traps and hence to reduce the hole

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mobility of F8BT (the HOMO energy level of TPD is 0.4 eV above the HOMO of the

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F8BT and thereby TPD can act as hole traps in F8BT). The current density and

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luminance of the iOLEDs whose device structure was ITO/TiO2/TPD doped

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F8BT/MoO3/Au were decreased with increasing amounts of TPD. In Ref. [33], it has

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been suggested that the accumulation of holes at the F8BT/TiO2 interface is the key

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factor required to achieve electron injection into the F8BT layer. Thus, the increase in

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the hole trap (TPD) density results in a redistribution of the free carriers through the

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F8BT film modifying the spatial field across the TiO2/F8BT interface. The results

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were

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clearly show that the reduction in hole mobility decreases the electron injection in the iOLEDs, and hence deteriorates the performance of the F8BT iOLEDs.

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The influence of the hole injection on the J-V characteristics of iOLEDs has

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been also discussed in Ref. [33]. The current density of iOLEDs with a hole-injection

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layer (HIL) (ITO/TiO2/F8BT/MoO3/Au) was greatly enhanced by more than five orders

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of magnitude in comparison with that of iOLEDs without HIL (ITO/TiO2/F8BT/Au).

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The increase in hole injection improves the performance of the iOLEDs.

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The C–V characteristic of the iOLEDs with PEI is shown in Fig. 6. A

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geometrical capacitance of the iOLEDs is 17 nF cm-2 observed in the applied voltage

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range from -1.0 V to 1.0 V. The capacitance is increased rapidly from 2.0 V, and the

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peak value of the capacitance is 650 nF cm-2 at 2.4 V. The layer thickness estimated

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from the peak value of the capacitance is about 5.0 nm, which can be corresponding to

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PEI thickness. Thus the hole accumulation at AZO (PEI)/F8BT interface leads to the

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increase in the capacitance and is likely to greatly enhance the electron injection in the

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iOLEDs. In addition, the capacitance is decreased rapidly from 2.4 V, and the negative

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capacitance

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electroluminescence threshold voltage. The negative capacitance is observed only under

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double-injection condition and a recombination coefficient lower than the Langevin

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recombination coefficient [34]. Carrier injection from an electrode to an organic semiconductor with energy

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barrier is described by the Richardson-Schottky (RS) model for thermionic emission or

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the Fowler-Nordheim (FN) model for tunneling injection [35]. The RS model assumes

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that an electron from a contact can be injected once it has acquired a thermal energy

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sufficient to cross the potential maximum resulting from the superposition of the

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external and the image-charge potential. Tunneling through the barrier is ignored. It has

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been shown in organic semiconductor materials [36, 37] that the J-V characteristics can

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be described well by the RS model. For high carrier-injection barriers (> 0.6 eV) the RS

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model is generally accurate (low carrier-injection barriers (< 0.6 eV) do not obey the RS

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model, which is pinned at about 0.6 eV) [37], and J-V characteristics in single-layer

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OLEDs with high carrier-injection barriers follow the RS behavior [38]. Electron

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injection of the iOLEDs may be understood from the high electron-injection barrier

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between the AZO cathode and the F8BT emissive layer (about 1 eV) in terms of the RS

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model. The current density for the RS model JRS is given by

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(2)

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J RS

 φ − e 3 4πε ε F  r 0  = A T exp − B  , k BT   ∗

where A* is the Richardson constant, T is the temperature, φB is the energy barrier, εr is

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the dielectric constant of the semiconductor, ε0 is the permittivity of vacuum, F is the electric field, and kB is the Boltzmann constant.

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Plots of JRS versus V1/2 for the iOLEDs with PEI are shown in Fig. 7a. A

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linear relationship between current density and V1/2 in Fig. 7a is observed from 0 V to

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2.0 V, which indicates that electron injection by RS thermionic emission is dominant 14

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below the luminescence turn-on voltages (Fig. 8a). The values of φB for the electron

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injection from the AZO cathode to the F8BT emissive layer calculated from the slope in

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Fig. 7a with Eq. 2 is 0.87 eV, where we assume that m* is the mass of free electron

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[39-43] (A* = 120 A cm-2 K-2). The φB estimated by RS model is almost comparable to

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the electron-injection barrier determined by the photovoltaic measurement.

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Above the current turn-on voltage, the electric field crowding is caused by hole

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accumulation at the AZO (PEI)/F8BT interface in the iOLEDs in Fig. 6. The FN model

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has been used to explain such high field J-V characteristics [44, 45].

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The FN model ignores image-charge effects and considers tunneling of

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electrons from a contact through a triangular barrier to conduction-band states. The

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current density for the FN model JFN is given by

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(3)

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J FN

 8π (2m* )1 / 2 φ B3 / 2  e3 F 2  , = exp − ehF 8πhφ B 3  

where h is the Planck constant, and m* is the effective mass of the carrier inside the

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semiconductor. JFN is insensitive to temperature. Plots of JV-2 versus V-1 for the iOLEDs with PEI are shown in Fig. 7b. A linear relationship between JV-2 and V-1 observed

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above 2.0 V in Fig. 7b indicates electron injection by FN tunneling (Fig. 8b). The values

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of tunneling barrier height φB between the AZO(PEI) cathode and the F8BT emissive

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layer calculated from the slope in Fig. 7b with Eq. 3 is 0.72 eV, which is almost 15

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comparable to the electron-injection barrier estimated by RS model. Thus electron

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injection above the luminescence turn-on voltage is greatly enhanced by the tunneling

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effect of electrons because of the electric field crowding due to the hole accumulation at

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the PEI/F8BT interface.

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4. Conclusions

We investigated the roles of PEI and the electron-injection mechanisms in the

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iOLEDs of the device structure of AZO/PEI/F8BT/MoO3/Al. The PEI layer can play a

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role in the electron-injection barrier lowering, the passivation of the surface states of

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AZO, the exciton blocking, and the hole accumulation at the AZO/F8BT interface.

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Electron injection of the iOLEDs is enhanced by the hole accumulation at the AZO

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(PEI)/F8BT interface rather than the electron-injection barrier lowering between the

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AZO cathode and the F8BT emissive layer. The enhanced electron injection is due to

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the electric field crowding at the AZO (PEI)/F8BT interface caused by hole accumulation, and such electric field crowding greatly enhances the FN tunneling

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electron injection from the AZO cathode to the F8BT emissive layer. We stress that the

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efficient electron-injection mechanism studied here is valuable information for the

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development of efficient electron-injection materials and hence for the fabrication of

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high performance iOLEDs.

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Acknowledgments

This work was partly supported by a Grant-in-Aid for Scientific Research on

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Innovative Areas “New Polymeric Materials Based on Element Blocks” (No. 2401)

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(JSPS KAKENHI Grand number JP24102011), by JSPS KAKENHI Grant Number

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JP17H01265, and by the Murata Science Foundation. The authors would like to thank

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Sumitomo Chemical Company, Limited for supply of F8BT, Nippon Shokubai

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Company, Limited for supply of PEI, and Mr. Minoru Yonekawa for the measurements

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of photoemission yield spectroscopy in air.

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Figure captions

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applied voltage for iOLEDs (circle: iOLEDs with PEI, square: iOLEDs without PEI).

Fig. 2. Plots of current efficiency versus current density for iOLEDs (circle: iOLEDs

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with PEI, square: iOLEDs without PEI).

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Fig. 1. Plots of current density (solid symbols) and luminance (open symbols) versus

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Fig. 3. Plots of net photocurrent density versus applied voltage for iOLEDs (circle: iOLEDs with PEI, square: iOLEDs without PEI). The voltages at ∆J = 0 are 1.1 V and 0.7 V in the iOLEDs with and without PEI, respectively.

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Fig. 4. The energy band diagram of iOLEDs with or without PEI. The work function of

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AZO with PEI and the bottom of the conduction band of MoO3 are estimated from the

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photovoltaic measurement in Fig. 3 and the work function of AZO determined by

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photoemission yield spectroscopy measurements in air.

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Fig.5. Current density-voltage characteristics of iOLEDs (open symbols) and EODs

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(solid symbols) with PEI. The AZO electrode works as a cathode in both the iOLEDs

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Fig.6. A capacitance-voltage characteristic of iOLEDs with PEI at a constant frequency

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of 1 Hz.

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Fig. 7. (a) Richardson-Schottky plot (below the luminescence turn-on voltage) and (b)

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Fowler-Nordheim plot (above the luminescence turn-on voltage) of iOLEDs with PEI,

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symbols: experimental data and lines: theories.

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Fig. 8. The electron-injection model of the iOLEDs in band diagrams, (a) RS schottky

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emission (below the luminescence turn-on voltage) and (b) FN tunneling (above the

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luminescence turn-on voltage).

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Highlights

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・The roles of poly(ethyleneimine) are shown in the iOLEDs.

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・Electron-injection mechanisms of iOLEDs are studied.

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・Electron injection is greatly enhanced by hole accumulation.