Enhancing efficiency of organic light-emitting diodes using a

carbon nanotube-doped hole injection layer

Shui-Hsiang Su*, Wang-Ta Chiang, Chung-Ting Kuo, Yu-Che Liu, Meiso Yokoyama

Department of Electronic Engineering, I-Shou University

1, Section 1, Hsueh-Cheng RD, Ta-Hsu Hsiang, Kaohsiung County, Taiwan

*E-mail: shsu@isu.edu.tw

Abstract Nanocomposite layers consisting of multi-walled carbon nanotubes

(MWCNTs)-doped poly(3,4,-ethylene dioxythiphene):polystyrene sulfonic

acid (PEDOT:PSS) were employed as a hole injection layer (HIL) in organic

light-emitting diodes (OLEDs). The structure of OLED is glass/indium-tin

oxide (ITO)/MWCNTs-doped PEDOT:PSS/PEDOT:PSS/

tris(8-hydroxyquinolinato)aluminium (Alq3)/LiF/Al. The luminous efficiency

of the OLED is as high as 2.1 cd/A, which is 70% higher than that of a

conventional device without a MWCNTs-doped HIL. A device with a

‘‘hole-only” structure, ITO/MWCNTs-doped PEDOT:PSS/Al, was constructed to

elucidate the mechanism of carrier injection and transporting. The MWCNTs-doped

PEDOT:PSS layer exhibits a low carrier transporting ability.

Keywords: Multi-wall carbon nanotubes; PEDOT:PSS; organic light-emitting

diodes

Introduction

Efficient organic light-emitting devices (OLEDs), that based on small

molecular organic materials or polymers, have attracted extensive research

interest because of their potential application to full-color flat panel displays.

Significant progress has recently been made in improving the characteristics

of device’s structure for emissive display applications [1,2]. Fluorescent

emission in OLEDs is associated with the radiative recombination of singlet

excitons. One of them is an imbalance of electron and hole mobilities [3,4],

which results in a shift of the recombination zone toward an electrode,

lowering the device efficiency due to exciton quenching by the metal electrode.

Polymer/carbon nanotube (CNT) nanocomposite has been regarded as a

solution to low electron mobility [5-8], and the polymer/CNT junctions act as

exciton quenching centers, improving charge transport and quenching

photoluminescence [9-12]. More balanced charge injection is associated with

higher external quantum efficiency of the OLED. A key discovery of recent

years is that doping improves emission efficiency and intensity. The

predominant electronic interaction in the nanocomposite s is known to be

polymer-to-CNT photoinduced charge (electron) transfer [13-15], while

photoinduced energy (exciton) transfer has been suggested to occur in the

polymer/CNT bilayer [16]. However, while various potential applications of

CNTs have been suggested, few practical uses have been realized. The main

difficulties concern the poor purity and process ability of MWCNTs that

contain powder.

In this investigation, the prepared MWCNTs were purified and then doped

as a very small fraction of up to 2.5 wt% into extensively used PEDOT:PSS.

Double-layer and hole-only devices were fabricated, and the device

characteristics were studied to examine the role of MWCNTs in OLEDs.

Experimental Details

The MWCNTs in this investigation were synthesized using chemical

vapor deposition (CVD) and purified following a standard procedure. 100mg

of MWCNTs powder was added to 50ml of a mixture of sulfuric acid and nitric

acid in a 3:1 ratio, and the mixture was stirred for 15min on a hot plate at 100°

C. The suspension was then diluted to 200ml. Finally, the MWCNTs were

collected by membrane filtration (0.45 μm pore size), and washed with

sufficient deionized water to remove residual acids. The PEDOT:PSS solution

was filtered through a 0.45 μm polyvinyl difluoride (PVDF) syringe filter.

Purified MWCNTs was doped into PEDOT:PSS solution with concentrations

of 0, 0.7, 1.5 and 2.5 wt%. The mingle solution in an ultrasonic bath for

approximately 24 hr. OLEDs were fabricated as follows. ITO-coated glass

substrates (with a sheet resistance of 7Ω/) were cleaned by ultrasonic in a

detergent solution and deionized water. A HIL of MWCNTs-doped

PEDOT:PSS nanocomposite was spin-coated on the optically transparent ITO

glass substrates at 3000 rpm, which were then backed for 1 hr at 120°C in a

vacuum oven. The other layers (organic layer and Al cathode) of the devices

were fabricated by conventional thermal evaporation in a high vacuum

chamber at a base pressure of 5×10-6 Torr without breaking the vacuum. The

deposition rate of the organic layer was 0.05 nm/s and that of the Al metal

cathode was 0.5 nm/s. Figure 1 shows the structure and energy-level diagrams

of the OLEDs. The device consists of a 120 nm-thick anode layer of ITO, a 60

nm HIL of MWCNTs-doped PEDOT:PSS, a 60 nm emissive layer and

electron-transporting layer (ETL) of Alq3, a 0.7 nm electron injection layer of

lithium fluoride (LiF) and a 150 nm cathode layer of aluminum (Al). The

transmission electron micrographs of MWCNTs-doped PEDOT:PSS

nanocomposite s film were obtained using an FEI Tecnai transmission electron

microscope (TEM) with a point-to-point resolution of 0.14 nm. The surfaces

of the films were analyzed using atomic force microscopy (AFM). The

current-voltage characteristics were measured using a Keithley 2400 source

meter, and the luminance of OLEDs was measured using a PR-650

spectrometer. All the measurements were made in the dark at room

temperature.

Results and discussion

Untreated pristine MWCNTs were doped into the PEDOT:PSS to act as an

HIL in an OLED. However, poor optoelectronic characteristics of OLED have

been observed- even an absence of light emission at a very high driving

current. Several investigations have suggested that acid treatment can cut

MWCNTs and shorten them, while reducing the diameter of MWCNTs [17-20].

It can also remove amorphous carbon and contaminating metals. Figure 2

shows TEM images of MWCNTs after acid treatment. As shown in Fig. 2(a),

acid treatment removed contaminating metallic catalyst from the top of the

graphite tube wall. Figure 2(b) demonstrates that treatment with 1:3 HNO3:H2SO4

acidic solution for 48 hr produces a sharp graphite tube wall, indicating that the

MWCNTs had been cut off and thereby shortened. Figure 3 plots the J-V

characteristics of OLEDs with PEDOT:PSS doped with various concentrations of

MWCNTs. As the fraction of MWCNTs increases, the current density declines

and the turn-on voltage increases. The purification of the prepared MWCNTs

is critical to making the MWCNTs an effective material in the fabrication of

OLEDs. Impurities such as amorphous and metallic catalytes in solution

remarkably degrade the physical and chromatic characteristics of the device -

especially at the interface and inside the bulk.

Figure 4 plots the luminance-current density characteristics of the OLEDs.

OLEDs with MWCNTs-doped PEDOT:PSS HIL exhibit higher luminance. The

OLEDs with 1.5 wt% MWCNTs-doped PEDOT:PSS HIL had a luminance of

1650 cd/m2 at a current density of 100 mA/cm2. Figure 5 presents the

calculated luminous efficiency. The overall trend in which associated property

in the observed luminance of the undoped devices equals that of the doped

ones. The luminous efficiency of MWCNTs-doped devices increases with the

concentration of MWCNTs. One plausible cause is the retard of

hole-transporting and polymer-nanotube interactions, which rigidify the

polymer chains. The device with 1.5 wt% doping has higher luminous efficiency.

However, the luminous efficiency of the 2.5 wt% MWCNTs-doped device falls

sharply as the current density rises above 30 mA/cm2. Doping PEDOT:PSS with

excess MWCNTs may destroy the polymer chain, reducing luminous efficiency at

high current density.

The variation of the surface roughness caused by MWCNTs-doping in

PEDOT:PSS might affect the performance of OLEDs. An OLED employing an

MWCNTs-doped PEDOT:PSS HIL covered by PEDOT:PSS had been prepared.

Luminous efficiency can be increased to 2.1 cd/A at a current density of 60 mA/m2. It

is 70% higher than that of the conventional device without a MWCNTs-doped

PEDOT:PSS HIL. The surface roughness of MWCNTs-doped PEDOT:PSS on

ITO was investigated by the tapping mode AFM and shown in Figs. 6 (a) and

(b). The root-mean-square (RMS) surface roughness of MWCNTs-doped

PEDOT:PSS was approximately 10.86 nm, whereas bestrew smooth layer the

RMS of the PEDOT:PSS was approximately 1.74 nm. Obviously, the smooth

layer can provide the better surface contact between HIL and EML interface.

It was reported that the HIL with smaller roughness led to the enhanced device

performance.

It is well known that the driving voltage depends on the carrier injection

and transporting, while current efficiency depends not only on carrier

injection, but also on the balance of electrons and holes. To verify the

hole-transporting ability of the MWCNTs-doped PEDOT:PSS HIL, the devices with

‘‘hole-only” structures were fabricated by sandwiching the MWCNTs-doped

PEDOT:PSS or pristine PEDOT:PSS films between two electrodes (ITO and

Al) and their electrical properties were examined. The device structure was

ITO/MWCNTs-doped PEDOT:PSS/Al. The concentrations of MWCNTs were 0 and

2.5 wt%. Figure 7 plots the current density–voltage curves of the hole-only devices.

At the driving voltage of 5 V, 2.5 wt% MWCNTs-doped PEDOT:PSS device

produced a current density of 318 mA, which was much lower than 645 mA for the

undoped device. On the other hand, under the current density of 200 mA/cm2, the

voltage drop of 2.5 wt% MWCNTs-doped PEDOT:PSS device is 3.3 V, which is

much larger than 1.7 V for the undoped device. This indicates that the

MWCNTs-doped PEDOT:PSS layer exhibits a low carrier transporting ability.

Conclusion

This study investigated the feasibility of using MWCNTs-doped

PEDOT:PSS as an HIL in OLEDs. The structure of OLED was

glass/ITO/MWCNTs-doped PEDOT:PSS/PEDOT:PSS/Alq3/LiF/Al. Research

into a nanocomposite of PEDOT:PSS and MWCNTs has clearly demonstrated

that the MWCNTs can be easily purified, processed and used as an effective

material in OLEDs. The luminous efficiency of the OLED is 2.1 cd/A, which

is 70% higher than that of a conventional device without a MWCNTs-doped

PEDOT:PSS HIL. A smooth layer covering on the MWCNTs-doped

PEDOT:PSS improves the surface roughness and further enhances the

luminous efficiency. The hole-only device reveals that the MWCNTs-doped

PEDOT:PSS layer exhibits a low carrier transporting ability. MWCNTs can be a

candidate in such a form in organic solar cells or other organic optoelectronic

devices.

Acknowledgements

The authors would like to thank the National Science Council of the

Republic of China, Taiwan, for financially supporting this research under

Contract No. NSC 96-2221-E-214-015 and NSC 96-2221-E-214-016. The

authors would also like to thank the MANALAB at ISU, Taiwan.

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Figure captions

Fig. 1 (a) Structure and (b) energy-level diagram of OLEDs.

Fig. 2 TEM images of MWCNTs treated with acidic H2SO4/HNO3

mixture.

Fig. 3 J-V characteristics of OLEDs with various weight percentages of

MWCNTs doped in PEDOT:PSS films.

Fig. 4 Luminance-current density characteristics of OLEDs with various weight

percentages of MWCNTs doped in PEDOT:PSS films.

Fig. 5 Luminous efficiency-current density characteristics of OLEDs with

various weight percentages of MWCNTs doped in PEDOT:PSS films.

Fig. 6 AFM images of (a) ITO/MWCNTs-doped PEDOT:PSS and (b) ITO/MWCNTs-doped PEDOT:PSS/PEDOT:PSS.

Fig. 7 Current-voltage characteristics of hole-only devices. The inset is structure of the hole-only device.

ITO4.7ev

PEDOT

:PSS

Alq3

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ITO Glass MWCNTs-doped PEDOT:PSS

Alq3

LiF/Al

Figure 1 Su et. al.

(a) (b)

Figure 2 Su et. al.

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Figure 3 Su et. al.

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Figure 4 Su et. al.

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Figure 5 Su et. al.

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Figure 7 Su et. al.