06088367

7/29/2019 06088367

1/4

Abstract The potential of organic semiconductor based

devices for light generation is demonstrated by the

commercialisation of display technologies based on organic

light emitting diode (OLED). In OLED, organic materials plays

the role of emitting light once the current is passed through.

However OLED have drawbacks whereby it suffers from

photon loss and exciton quenching. Organic light emitting

transistor (OLET) emerged as a new technology to compensate

the efficiency and brightness loss encountered in OLED. The

structure has combinational capability to switch the electronic

signal such as the field effect transistor (FET) as well as to

generate light. Different colours of light could be generated byusing different types of organic material. The light emission

could also be tuned and scanned in OLET. The studies carried

out fabricated and analysed the current voltage and

luminescence characteristics of both organic light emitting

diode (OLED) and also organic light emitting transistor

(OLET). The proposed light emitting layer in this study is poly

[2 methoxy 5 - ( 2- ethyl hexyloxy ) 1 , 4 - phenylene

vinylene] (MEH-PPV).

Keywords: organic polymer, OLED, OLET, MEH-PPV

I. INTRODUCTIONrganic electronics are seen to be gearing up to the

second phase as a new and emerging technology.

H.J.Round was the first individual to discover

electroluminescence (EL) phenomenon in a piece of

carborundum (SiC) crystal.

Tang and Steven Van Slyke were the first individuals to

invent the first organic light emitting diode (OLED) at

Eastman Kodak [1]. The proposed structured gave

surprisingly high light output and low operating voltage.

Organic light emitting diodes (OLEDs) can be explained as

thin-film devices which could actively emit light with the

presence of conjugated polymer semiconductor or an

organic small molecule [2].

Basically, OLED consist of two charged electrode:

transparent and reflecting electrode which are placed insandwiched position on top of some light emitting materials

that are made of organic components. The major limitation

of OLED is that it suffers from photon loss and exciton

quenching. Recently a team comprising researchers from

Italy and United States came up with organic light emitting

transistors (OLET) [3]. The proposed organic light emitting

transistor (OLET) has combinational capability to switch the

electronic signal such as the field effect transistor (FET) as

well as generate light. Due to different driving structures, the

new OLET promises better efficiency and lifetime of the

used organic light emitting materials [3].

A. Working Principle of OLEDThe working principles of OLED can be explained from

the electron-hole behaviour. The electrons are injected from

the cathode whereas the holes are injected from the anode

when a specified voltage driven in between the anode and

cathode. Electrons and holes transport layers acts as a

medium for the electrons and holes to travel. The difference

between highest occupied molecular orbitals (HOMO) and

lowest unoccupied molecular orbitals (LUMO) and

electrode work function initialise the injection process.Electrons and holes recombine in the emissive layer whereby

the excited molecules or exciton are created. The

recombination zone can be altered to achieve balance in

electronhole pair. The exciton diffuses from high to low

concentration as it recombines to give light. The propagation

of photons do have two possibilities whereby a part of the

photons do move towards the cathode and reflect back while

the remaining photons will move out of the device through

the glass substrate [4].

B. Working Principle of OLETThe principle of charge carrier transport and

electroluminescence properties of OLET is similar toorganic light emitting field effect transistor (OLEFET). The

organic active layer is placed in contact with only the source

and drain electrode. The gate dielectric isolates the gate

electrode from the organic layer. The holes and electrons are

injected into the channel from source and drain electrode as

a result of appropriate gate electrode bias, VG. The

movement of holes and electrons is also influenced by drain

source bias, VDS. The amount of current that flows between

the source and drain electrode is controlled by the gate

electrode bias. The device is changed from off to on state by

using this amount of current [4].

The electroluminescence properties of OLETs are mainly

influenced by the type of organic materials used. The holes

and electrons will form exciton which in contrast willrecombine radiatively to generate light in the transistor

channel. The location of light emission within the channel

could be altered by changing the value of gate bias between

hole injecting and electron injecting electrodes [4].

II. EXPERIMENTAL DETAILSThe OLET have structures similar to OLED and the

schematic illustrations are shown in Figure 1 a) and b). All

fabrication steps are performed in normal laboratory

condition. ITO- coated glass from Sigma Aldrich is used as

anode material. Prior to film deposition, the substrate was

cleaned using acetone, isopropanol, deionised (DI) water

Fabrication of MEH-PPV Based Organic Light

Emitting Diode and Transistor

S Suppiah1,*

, M Mohamad Shahimin1

and N Juhari1

1School of Microelectronic Engineering, Universiti Malaysia Perlis (UniMAP),

P.O Box 77, d/a Pejabat Pos Besar, 01000 Kangar, Perlis, Malaysia*Corresponding email: [email protected]

O

RSM2011 Proc., 2011, Kota Kinabalu, Malaysia

388 978-1-61284-846-4/11/$26.00 2011 IEEE

7/29/2019 06088367

2/4

and ultrasonic-bathed in isopropanol. The ITO glass was

first spin coated with PEDOT:PSS at 3000rpm for

20seconds. The sample is placed on hot plate for 15 minutes

at 100OC to ensure the solvent totally evaporated. 4 mg/mL

of MEH-PPV is diluted using chloroform and stirred at

75OC on a hot plate for 10 minutes until the polymer is fully

dissolved. The MEH-PPV solution is then spin coated with

three different spin speeds; (spin speed:1000rpm, 2000rpm ,

3000rpm time:20s). After that, the device is placed on a hot

plate for 15 minutes at 100O

C. Finally aluminium (Al) as acathode contact is deposited on top of the MEH-PPV layer.

The device is then applied voltage in the range of -5V to 5V

to measure its I-V characteristics. Electrical data is obtained

using Keithley 4200-SCS semiconductor parameter analyser

(SPA) tool. Atomic force microscope (AFM) is used to

analyze the surface morphology. OLET structures have

additional LiF layer deposition after the deposition of active

layer.

III RESULTS AND DISCUSSION

III. RESULTS AND DISCUSSIONA. I-V characteristics of OLED

The thickness of MEH-PPV which acts as an active layer

is varied in order to see the effect on the electrical

properties. Three different spin speeds are used in this study:

1000rpm, 2000rpm and 3000rpm while the time is constant

at 20 seconds. The uneven surfaces of deposited materials

did affect the I-V graph. At some points, smooth graph of a

diode could be obtained. Hence the position of the probes on

both electrodes(ITO and Al) plays a vital part in obtainingsmoother and better graphs. The probe acts as a medium to

transfer the varied amount of voltage to the device. Resultant

current flow as a result to the voltage is then represented by

a graph.

Figure 2 summarizes the average current value for three

different thicknesses (using different spin speed) of active

layer as the voltage varied from -5V to 5V. The fabricated

OLED devices do exhibit similar I-V characteristics of

conventional diode. Error bars of standard deviation can be

observed in all the average current values. Error bars visual

device is generally used to convey uncertainty. Rather than

depicting actual errors, these error bars indicate how widelythe sepal lengths are spread around the mean. For 1000rpm

and 200rpm the sepal lengths are spread largely at the

beginning and ending of voltage values. From -3V to 2V the

sepal lengths are very narrow whereby the values are within

the average values. But in 3000 rpm spin speed the very

narrow compared to ending values of voltage. The widest

sepal lengths can be observed at -5V and 5V.

B. I-V characteristics of OLET

(a)

(c)

(b)

(a)

Figure 1: Schematic illustration of a) OLED and b) OLET

(a) (b)

Figure 2: Graph of average current value of 3 different points for a)

1000rpm b) 2000rpm c) 3000rpm spin speed of MEH-PPV layer


389

7/29/2019 06088367

3/4

Figure 3: I-V graph obtained for MEH-PPV at a) 1000rpm b)

2000rpm and c) 3000rpm

VGS(V) Maximum current (Imax)

1000rpm 2000rpm 3000rpm

1 -0.58 -0.65 -0.56

2 -0.57 -0.67 -0.51

3 -0.57 -0.58 -0.52

4 -0.55 -0.55 -0.52

5 -0.59 -0.58 -0.52

6 -0.57 -0.56 -0.53

Figure 3 shows I-V graphs for different thickness of

MEH-PPV layer. The fabricated transistor illustrates

function well according to the normal NMOS transistor. The

curve splits nicely as the step value of VGS is inserted. The

start and end value ofVDS is set. For OLET testing, it is set

to 0V and -1.00E+1V respectively. The maximum current

obtained for each value ofVGS is shown in Table 1. It can be

concluded the maximum current for all spin speeds are in therange of 0.51 to -0.67.

C. SURFACE MORPHOLOGY OF ITO LAYER

It can be observed that surface of ITO appeared to be

rougher before the cleaning process carried out as shown in

Figure 4 (a). At the same time particles could be present onthe surface as this study is conducted in laboratory condition

rather than clean room condition. Many peaks on the ITO

surface can be observed in Figure 4 (b) and (c). The height

of each peak should be reduced in order to enhance the

stability as well as efficiency of devices. As the cleaning

time increased, smoother surface of ITO can be observed as

shown in Figure 4 (d). Further increase in cleaning time

beyond 60 seconds could yield better surface morphology of

ITO layer.

D. SURFACE MORPHOLOGY OF MEH-PPV

As observed in Figure 5, the surface morphology of MEH-

PPV with 1000rpm spin speed appeared to be smooth

compared to other spin speed. However there is a huge

(c)

(b)

(a) (b)

(c) (d)

Figure 4 :AFM surface morphology images of ITO a)before cleaning and after cleaning b)10s c) 30s d) 60s

(c)

(a) (b)

Figure 5: AFM surface morphology images of MEH-PPVdeposited at a) 1000rpm b)2000rpm and c)3000rpm

Table 1: Values of maximum current for different spin speeds as

VGSvaried.


390

7/29/2019 06088367

4/4

valley which suggests that MEH-PPV layer has not been

coated evenly. At that particular region, the thickness of

MEH-PPV is very low compared to other region. Figure 5

(b) and (c) suggest that there are many particles that are

found on the surface which attributed to the round small

hollow on the surface. It can be concluded that the powdered

MEH-PPV could be prone to contamination during the

dilution with chloroform process. Furthermore, uneven

surface of ITO could be one of the reasons in the formation

of such small hollows.

Average roughness is the arithmetic mean of the surface

roughness profile from the middle line within the measuring

length. The bar graph in figure 8 shows average roughness

value with respect to the scan areas and spin speeds. It can

be concluded that 1000rpm do have larger values of average

roughness for the first three scans area. At 5000m scan

area, 2000rpm do have the highest average roughness value

which is 1.20E+01nm. Overall, the value of average

roughness decreases as the scan area reduced from 20000m

to 5000m. However the value of average roughness at

5000m for 2000rpm is slightly higher compared to

10000m.

Peak to valley roughness as shown in figure 9 can be

explained as the vertical distance between highest and lowest

points within the overall measuring length. The roughness

value for 1000rpm increased from 2.20E+02nm at 20000m

to 2.45E+02 at 10000m while at 5000m the value

decreases to 1.06E+02.As the scan areas reduced, the

roughness value also should decrease. The increase in peak

to valley roughness in 1000rpm can be attributed to the huge

valley as shown in 8(a). The value increases as a result of

increase in the distance between highest and lowest pointwithin the scanning area.

Root mean square roughness can be defined as root mean

square value of the surface roughness profile from the

middle line within the measuring length. From graph 10, it

can be concluded that the values of root mean square

roughness do decrease as the scan area reduced. In scan

areas of 2000m and 15000m, the value of roughness for

1000rpm is very large compared to the other spin speeds.

However at 5000m scan area, 2000rpm do have larger

value of root mean square roughness which is 1.50E+01.

IV. CONCLUSIONThe fabricated OLED and OLET devices can be regarded

as successful from the electrical characteristics perspective.

Better electrical characteristic can be observed in OLED

device that uses MEH-PPV layer that being deposited at

3000rpm of spin speed. The value of current obtained in

response to voltage input (-5V to 5V) is slightly lower

compared to devices that being deposited with 2000rpm and

3000rpm of MEH-PPV. Increase in spin speed does help to

spread the active layer evenly on the glass substrate. As a

result, the recombination process could take place

effectively as more holes and electrons will be transported to

the active area.

V. ACKNOWLEDGEMENTSThe authors thank all technicians and teaching engineers

in the MicroFab Cleanroom, UniMAP for helpful advice and

discussions, provision of training and support for the OLED

and OLET fabrication.

VI. REFERENCES[1] Tang, C. W., Vanslyke, S. A. "Organic electroluminescent

diodes". Applied Physics Letters 51 (12),pp 913, 1987

[2] S. B. Wolfgang Brutting, Anton G. Muckl, "Device physics of

organic light -emitting diodes based on molecular materials,"

Organic Electronics 2, vol. 1, 2001.

[3] S. T. Raffaella Capelli, Michele Muccini. (June 2010) Organic

light-emitting transistors with an efficiency that outperforms the

equivalent light -emitting diodes. Nature Materials.

[4] M.-K. Wei, et al., "Emission Characteristics of Organic Light-

Emitting Diodes and Organic Thin-Films with Planar andCorrugated Structures," International Journal of Molecular

Sciences, vol. 11, pp. 1527-1545, 2010.

[5] C. S. Fabio Cicoira, Organic Light Emitting Field Effect

Transistors:Advances and Perspectives. Advanced Functional

Materials 17, pp. 3421-3434, 2007.

Figure 11 :Root mean square roughness (Rrms) of different

spin speeds

Figure 9 : Average roughness (Ra) of different spin

speeds

Figure 10 : Peak to valley roughness (Rpv) of different spin

speeds


391

Documents

06088367