RSM2013 Proc. 2013, Langkawi, Malaysia

Electroluminescence Behavior of MEH-PPV based Organic Light Emitting Diode

Nurul Hafizah A. Rahman1,*, Azrif Manut1, Shafinaz Sobihana Shariffudin1, Aimi Bazilah Rosli1, Mohd Hannas1,

Mohamad Rusop1,2 1NANO-ElecTronic Centre (NET), Faculty of Electrical Engineering, Universiti Teknologi MARA (UiTM),

Selangor, MALAYSIA 2NANO-SciTech Centre (NST), Institutes of Applied Sciences, Universiti Teknologi MARA (UiTM),

Selangor, MALAYSIA E-mail*: fizahrahman88@gmail.com

Abstract—Electroluminescence behavior of Poly [2-methoxy-5-(2-ethylhexyloxy) -1,4-phenylenevinylene] (MEH-PPV) as the emissive layer has been investigated in organic light emitting diode (OLED). The typical structure of polymer light emitting diode (PLED) is gold (Au) / MEH-PPV / ITO (Indium Tin Oxide). In this study, we investigated the EL behavior of single layer MEH-PPV at various applied voltages from 1 V to 5 V. The absorbance of MEH-PPV thin film and the EL spectra from 400 nm to 800 nm wavelength of the MEH-PPV device were discussed. The turn-on voltage is almost 1 V and the result showed that higher intensity with 5 V turn-on voltages. Keywords—OLED; MEH-PPV; J-V characteristics; electroluminescence.

I. INTRODUCTION Organic light emitting diodes (OLEDs) is the new

technology for light sources [1]. OLEDs as semiconducting organic materials were being developed for many applications [2-4]. The phenomenon of OLEDs from conjugated polymer thin films is one of the significant interest to both fundamental science and practical applications [5,6]. Electroluminescence (EL) in conjugated polymer was first discovered by Burroughes et al [7].

The simplest polymer electroluminescent device contains a thin polymer film sandwiched between two electrodes which are transparent anode and metallic cathode [2,8]. The use of MEH-PPV as the emissive layer in this work is an attractive conjugated polymer for display technology [2,8]. The thin films that form OLEDs has various specific goals same as anode and cathode where cathode works as electrons transport layer (ETL), anode usually indium tin oxide (ITO) as holes transport layer (HTL), and MEH-PPV as emitting layer (EML).

In this work, Poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) with an average molecular weight of 40 000 to 70 000 was used in the fabrication of the device. The basic principle of MEH-PPV is 2.7 eV for its lowest unoccupied molecular orbital (LUMO) and 5.1 eV for its highest occupied molecular orbital (HOMO) [9]. MEH-PPV is the one of the conjugated polymers that soluble in common organic solvent. It has the ability to produce good optical quality thin film by spin coating technique [8,10]. Spin coating technique usually applied in the depositing polymer

film which can produce films on a large area [10,11]. The molecular structure of MEH-PPV can be seen as shown in Fig. 1.

Fig. 1. Molecular structure of MEH-PPV

The objective of this study is to investigate the EL behavior

of single layer MEH-PPV device at various applied voltages from 1 V to 5 V. The investigation was divided into two which are electrical properties and optical properties.

II. EXPERIMENTAL DETAILS

A. Solution Preparation MEH-PPV with an average molecular weight from 40,000

to 70,000 was used without further purification. The MEH-PPV was dissolved in toluene with concentration of 5mgml-1. As reported before, photoluminescence (PL) spectra of toluene gives higher peak within range from 550 nm to 600 nm [10]. The solution then was stirred for 48 hours at room temperature to ensure the MEH-PPV solution was completely dissolved in toluene.

B. Thin Film Formation The thin films of MEH-PPV were formed by using spin

coating on pre-cleaned indium tin oxide (ITO) glass substrate at room temperature. The ITO coated glass will be used for anode of OLED fabrication. Usually the ITO glass was cleaned consecutively with ethanol in ultrasonic bath for 10 minutes at 50 C. Lastly, the ITO glass was de-ionize water (DI) and then blown dry with nitrogen gas. The spin speed and spin time were fixed at 2000 rpm and 60 seconds respectively. Then, each coating was dried at 50 C. This process was repeated 3 times in order to get the thickness as desired.

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RSM2013 Proc. 2013, Langkawi, Malaysia

C. Thin Film Characterization The characterization of MEH-PPV thin films was carried

out for electrical and optical properties with gold (Au) as top electrode or as the cathode. The electrical properties were characterized specifically by using two point probes solar simulator (Bukoh Keiki CEP-2000) current-voltage (I-V) measurements of the MEH-PPV films within 0 to 10 V and surface profiler (Veeco Dektak 750) for film thickness measurements. For the optical properties, EL spectra were measured using EL spectrometer (PL FluoroMax-3) and transmittance spectra were measured using UV-Vis spectrophotometer (JASKO UV-Vis).

III. RESULTS AND DISCUSSIONS

A. Electrical Properties The process for the top electrode (Au) is by using thermal

evaporation. Gold (Au) works as a cathode was deposited after depositing MEH-PPV thin films. Fig. 2 (a) shows the Au/MEH-PPV/ITO device structure. The cathode works as the electron transport layer (ETL) and anode works as the hole transport layer (HTL) [8]. The thickness of gold and MEH-PPV is in average 41.22 nm and 113.85 nm respectively. The current density-bias voltage (J-V) characteristics of MEH-PPV based organic light emitting diodes (OLEDs) with Au/MEH-PPV/ITO is presented in Fig. 2 (b). The J-V characteristics were measured and it is found that the turn-on voltage of the device is approximately 1 V. In observing the electrical response of the J-V characteristics, three ranges of voltage can be identified. At low voltage, space charge limited current and dominate the injected charge contribution. Increasing the forward bias fills the limited number of traps occasioning a rapid increase in the effective hole mobilities [12].

Fig. 2 (a). Electrical measurement of Au/MEH-PPV/ITO device structure.

0 1 2 3 4 5 6 7 8 9 10 110.0000

0.0005

0.0010

0.0015

0.0020

0.0025

0.0030

Cur

rent

Den

sity

, J (m

A/cm

2 )

Voltage(V)

Fig. 2 (b). Current density-bias voltage (J-V) characteristics of Au/MEH-PPV/ITO device.

B. Optical Properties 1) Absorbance The Fig. 3 shows the UV–vis absorption spectra of the

MEH-PPV thin films with concentration of 5mgml-1. The reason the concentration of 5 mgml-1 of MEH-PPV dissolved in toluene was used because it has the highest absorbance peak in a wavelength within range from 450 mm to 550 mm. The peak will be decreased as the concentration more than 5 mgml-1 when dissolved in toluene [10].

300 400 500 600 700

Abso

rban

ce (a

.u.)

wavelength (nm)

5mgml-1

Fig. 3. Absorbance peak of 5mgml-1 of MEH-PPV.

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RSM2013 Proc. 2013, Langkawi, Malaysia

500 600 700 8000

20

40

60

80

100

T=89 %

emitting region

Tran

smitt

ance

(%)

Wavelength (nm)

Fig. 4. Transmittance spectra of MEH-PPV.

Fig. 4 shows the transmittance of the MEH-PPV thin film was deposited onto ITO glass substrate. In general, the optical transmittance in the visible range increased. The result indicated that transmittance rate almost 90 % at 590 nm. When MEH-PPV was deposited onto ITO glass substrate, the optical transmittance increased in the range from 550 to 600 nm. The transmittance rate decreased gradually after reached 650 nm because of the reducing the dispersion and the extinction of light [13].

2) Electroluminescence (EL) Spectra When voltage is applied the charges start moving in the

device, holes are more likely to be transferred from the ITO electrode to the MEH-PPV and electrons will be moved from the Au electrode to the MEH-PPV [14,15]. Electrons and holes moved in the opposite direction. The recombination of these charges leads to the creation of a photon between the LUMO and HOMO levels of the MEH-PPV. The electrical power applied to the electrodes is transformed into light.

Fig. 5 illustrates the EL spectra of Au/MEH-PPV/ITO device. EL spectra of MEH-PPV polymer layer prepared from toluene for solution concentration of 5 mgml-1 and at constant spin speed of 2000 rpm. The device was investigated with various voltages. The range measurement is between 400 nm to 800 nm. From the EL spectra, it can be seen that when the device applied with 1, 2, and 3 V of bias voltages, it obtained approximately the same lowest intensity at peak of 590 nm. However when bias voltage increases to 4 V is applied to the device, the intensity of EL spectra becomes 2 times higher compared to 1, 2, and 3 V. Finally, the intensity increased 3 times when the applied voltage exceeded 5 V compared to 1, 2, and 3 V. The results clearly showed the same peak which is at 590 nm even if the device was applied with various voltages. The peak wavelength indicates that the range is in the red region [16]. The electron transporting material used in our study is itself a red color. The color of the emitted photon depends on the position of the elect ron-hole recombination. Due to the stronger field dependence of the electron mobility relative to the hole mobility, the recombination area is forced

400 500 600 700

Inte

nsity

(a.u

.)

Wavelength (nm)

1V 2V 3V 4V 5V

Fig. 5. Electroluminescence (EL) spectra of ITO/MEH-PPV/Au device at various bias voltages.

from the cathode and concentrated in the emissive layer as the voltage increases [17]. Emission from MEH-PPV layer is controlled by the ease with which charge is injected, which in turn depends on the applied voltage [16,17]. One explanation for this behavior is that there is the hole accumulation between anode and polymer interface due to lower mobility, compared with hole mobility located at the cathode interface. In other words, although the dilution effect may be important for performance, the interfaces may also play an important role, since charge concentration at the anode and polymer interfaces increases the probability of charge recombination and exciton formation is probably also increased [18,19].

IV. CONCLUSION As a conclusion, we have fabricated a single layer MEH-

PPV device. The PLED of Au/MEH-PPV/ITO device has been investigated. Electroluminescence (EL) behavior of MEH-PPV was characterized and the performances of EL spectra for polymer OLED were shown as well as its turn-on voltage. The current density-bias voltage (J-V) characteristics showed a very low turn-on voltage which is approximately 1 V. However, the device showed better performances for intensity at 5 V.

ACKNOWLEDGEMENT

Authors are thankful the supervisors, technicians and all that involve in the Nano-ElecTronic Centre, UiTM for helpful advice and discussions, and also Research Management Institute of Universiti Teknologi MARA for providing financial support under grant 600-RMI/ERGS 5/3 (41/2012).

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