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Synthetic Metals 160 (2010) 2417–2421

Contents lists available at ScienceDirect

Synthetic Metals

journa l homepage: www.e lsev ier .com/ locate /synmet

ield effect transistor behavior of organic light-emitting diodes with a modifiedonfiguration of ITO anode

in Lua, Fangfang Yua, Li Longa, Jianning Yua, Bin Weia,∗, Jianhua Zhanga, Musubu Ichikawab

Key Laboratory of Advanced Display and System Application, Ministry of Education, Shanghai University, 149 Yangchang Road, Shanghai 200072, PR ChinaDepartment of Functional Polymer Science, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan

r t i c l e i n f o

rticle history:eceived 15 April 2010

a b s t r a c t

We have developed the organic light-emitting diodes (OLEDs) with a modified configuration of ITO anodein which a thin channel was etched to form a bottom-contact field effect transistor (FET) using ITO and

eceived in revised form 31 August 2010ccepted 17 September 2010vailable online 23 October 2010

eywords:ield effect transistor

MgAg as a source/drain electrode and a gate electrode, respectively. The hole injection layer in OLEDsfunctioned as an active layer of FET and the other organic layers as insulator-like layer. The devices werefound to exhibit a behavior of FET due to horizontal charge migration between source and drain, andan electro-optical transfer characteristic due to vertical charge transport and recombination. We haveinvestigated the dependence of drain current on the channel length from 5 to 30 �m and found that the

could

rganic light emitting diodeshannel length

modified channel length

. Introduction

With the demonstrations of organic light-emitting diodesOLEDs) [1,2], organic field-effect transistors (OFETs) [3–5] andrganic lasing action [6–8], the merits of organic electronicsver III–V inorganic semiconductor, such as mechanically flexi-ility, light weight, durability, and easiness to print rapidly over

arge areas, have spread its broader potential for optoelectronicspplications in recent years. Furthermore, organic light-emittingransistors (OLETs) have received much attention due to the behav-or of field effect transistor (FET) and electro-optical transferharacteristics of OLEDs [9–13].

Since OFETs based on organic thin film or single crystals (such asetracene, pentacene or rubrene) [14–17] can control the amountnd the species (hole or electron) of injection carriers by the threelectrodes, OLETs are expected to become promising high per-ormance light-emitting devices making drive circuits of displaysimpler than using OLEDs.

To date, some studies focused on the fundamental understand-

ng of the physics of OLETs, which involves the emitting and thearrier injection and transport mechanism. In addition, some effortsave been made to find new organic materials with high-mobilitynd good light-emitting characteristics, such as the use of single-

∗ Corresponding author at: Key Laboratory of Advanced Display and System Appli-ation, Ministry of Education, Shanghai University, P.O. Box 143, 149 Yanchang Road,hanghai University, Shanghai 200072, PR China. Tel.: +86 21 56334331;ax: +86 21 56334331.

E-mail address: bwei@shu.edu.cn (B. Wei).

379-6779/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.synthmet.2010.09.020

change drain current directionally and quantitatively.© 2010 Elsevier B.V. All rights reserved.

crystal BP3T or to develop new-type OLET structure based onp-n-heterojunction bilayer [18,19] and p-light emitting layer-n-heterojunction trilayer [20] by combining the characteristics ofOLEDs and OFETs [21].

In this paper, we have used one modified OLEDs configurationwith a very thin channel in ITO anode whose structure was simi-lar to bottom-contact FET. The channel length was changed from5 to 30 �m. The electroluminescent intensity and drain currentoutput characteristics have been investigated. While the channellength was 15 �m, the devices were found to exhibit a behavior ofFET due to horizontal charge migration between source and drain,and an electro-optical transfer characteristic due to vertical chargetransport and recombination.

2. Experimental details

We have developed one FET-type OLEDs with bottom-contactconfiguration, as shown in Fig. 1(a). The MgAg (150 nm) was servedas a gate electrode, and ITO (150 nm) was served as a drain/sourceelectrode, respectively. The hole injection layer of high mobilityorganic material in OLEDs functioned as an active layer of FET andthe other organic layers as insulator-like layer. The organic layersin this device are tris[2-naphthyl(phenyl)amino] triphenylamine(2-TNATA, 220 nm)/4,4′-bis[N-(1-napthyl)-N-phenyl-amino]-

biphenyl (NPB, 60 nm)/tris(8-hydroxyquinoline)aluminum (Alq3,30 nm)/bathocuproine (BCP, 10 nm)/2,5-bis(6′-(2′,2′′-bipyridyl))-1,1-dimethyl-3,4-diphenylsilole (PyPySPyPy, 80 nm). The 2T-NATA,NPB, Alq3 were used as hole injection layer (HIL), hole transportlayer (HTL), and emissive layer (EML), respectively. The BCP was

2418 L. Lu et al. / Synthetic Metals 160 (2010) 2417–2421

N

N

SiN

N

N N

BCP

PyPySPyPy 2T-NATA

ITO (Drain)

2T-NATA

ITO (Source)

NPB

Alq3

BCP

PyPySPyPy

SiO2

(Gate)Mg:Ag N

N

N

N

(b)(a)

l

F ectrice study

eeEodletm

slwo

3eacapswmtaniwok

3

gttoWw

ig. 1. The OLETs structure with a modified ITO configuration as channel of field-ellectrode, respectively (a); the molecular structure of organic materials used in this

mployed as hole block layer to prevent hole from leaking intolectron transport layer (ETL). The PyPySPyPy was employed asTL due to the fast electron mobility. The molecular structure ofrganic materials used in this study is shown in Fig. 1(b). Theevice with total thickness of 400 nm could be operated under

ow voltage without degradation. The organic layers and MgAglectrode (9:1 mass ratio) were thermally deposited. The deposi-ion rates were typically 0.1 and 0.5 nm/s for organic and metal

aterials, respectively.The electric characteristics of devices were measured with two

ource electrometers (Advantest, R6245) in vacuum (10−3 Pa oress) environment at room temperature. Two Au wires as probes

ere placed onto gate and drain contacts carefully by observingptical microscope (Keyence digital HF microscope VH-8000).

A very thin channel on ITO with a changed length from 5 to0 �m was formed by the following procedures. First, we usedlectron beam (EB) lithography (Tokyo technology Co. Ltd.) to formnarrow pattern with an order of �m on a cleaned nonlumines-

ent glass substrate. Next, EB resist (ZEP520A) was developed andthick LiF was thermally deposited on the glass substrate of EB

attern. Subsequently, opaque metal Cr was deposited onto theubstrate and then LiF was liftoffed by a treatment in hot waterith ultrasonic wave for 30 min. Finally, using the completed Crask onto an ITO substrate which has been spincoated with pho-

oresist agent under UV exposure, we etched the substrate in mixedcid for 35 min and obtained an ITO pattern with a very thin chan-el. At the case of channel length of 30 �m, the channel was located

n the middle of ITO electrode. Then the length of source electrodeas improved 15 �m and 25 �m towards drain electrode region for

btaining a channel length of 15 �m and 5 �m, respectively, whileeeping drain electrode area fixed.

. Results and discussion

Light emission could be detected due to the leakage current fromate to source or drain while the MgAg gate electrode was nega-

ive biased. The horizontal current in the source–drain channel wasuned as the drain voltage varied from 0 V to −20 V. Here, the effectf channel length l was important to change the horizontal current.hile the l increased from 5 �m to 30 �m, the horizontal currentas decreased in the source–drain channel.

transistor. The l is channel length; ITO and MgAg are used as source/drain and gate(b).

The channel current was extremely low while the l was 30 �m.However, when the l was decreased to 15 �m, the channel currentcould be tuned by the MgAg gate electrode. In addition, it was clearto observe that Ids from source electrode to drain electrode wouldincrease with the applied electric field due to the decreased l. FETbehavior was contributed to the hole transport towards the drainfor the active layer of 2T-NATA, and the mobility of 2T-NATA ismuch higher than that of electron transport material.

The characteristics of current–voltage, luminance–voltage, elec-troluminescence (EL) spectral and current efficiency–currentdensity for the devices with l of 30 �m under the operation volt-age (Vg) of 0 V and −20 V between source and drain are shown inFig. 2. It was found from Fig. 2(a) that the current towards gatedefined as “Igd + Igs” and named as the gate current began to increaserapidly after |Vg| of 10 V. The gate current reached 3.78 and 1.85 mAat the |Vg| of 20 V for the Vd of 0 V and −20 V, respectively, whichwas attributed to the reason that the right OLED did not work. Thecharacteristics of luminance against Vg showed that the turn-onvoltage was about 10 V, and the luminance (the size of OLED was2 mm × 2 mm, so the channel width in the devices used in the studywas 2 mm) was improved to be 189 and 95 cd/m2 at the |Vg| of 20 V,as shown in Fig. 2(b). The EL spectral at 100 cd/m2 was shown inFig. 2(c). It is interesting to note that two peaks were produced,which might be contributed to the microcavity effect, due to the useof very thick organic layers. The current efficiency reached about2.0 cd/A at 10 mA/cm2, as shown in Fig. 2(d).

The output characteristics of the devices with varied l, Id − Vd,are shown in Fig. 3. Here, the drain current of Id was defined as“Ids − Igd”. While the applied Vd was 0 V, the Id value profile is dis-cussed in Fig. 3(a), which showed the Id − Vd profile at Vg = −16 Vfor the OLEDs of bottom-contact configuration with l of 5, 15 and30 �m. It was observed that the Id current profile switched from theplus exponential drop (30 �m) to the negative exponential increase(5 �m) with decreasing the Vd. The modified l could change draincurrent directionally and quantitatively. At Vg of −8 V and less, verysmall Id was observed for the devices (l was 30 �m) and it wasindependent of Vd because the FET channel did not conduct. For

the devices with l of 5 and 15 �m, there existed a potential dif-ference between source and drain electrode at 0 V of Vd, resultingfrom the different covering area of source and drain. Furthermore,there also exited the current flow between source and gate. Thesewould produce holes drift from the source electrode to the drain

L. Lu et al. / Synthetic Metals 160 (2010) 2417–2421 2419

201510500

1

2

3

4

|Vg| (V)

Cur

rent

(m

A)

unfilled square Vd=0 V

filled square Vd=-20 V

201510500

50

100

150

200

250unfilled square Vd=0 V

filled square Vd=-20 V

Lum

inan

ce (

cd/m

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|Vg| (V)

700600500400

0.0

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EL

Int

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ty (

a.u.

)

Wavelength (nm)

at 100 cd/m2

10864201.6

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1.8

1.9

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2.1

Cur

rent

eff

icie

ncy

(cd/

A)

Current Density (mA/cm2)

(a) (b)

(c) (d)

Fig. 2. The characteristics of current–voltage (a), luminance–voltage (b), EL spectral (c) and current efficiency–current density (d) for the device with l of 30 �m.

0-5-10-15-20

-0.8

-0.4

0.0

0.4

0.8

1.2

×1/100

I d (m

A)

Vd (V)

5 µm

15 µm

30 µm

×100

0-5-10-15-20

0.0

5.0x10-5

1.0x10-4

1.5x10-4

2.0x10-4

I d(A

)

Vd (V)

Vg=-20 V

Vg=-16 V

Vg=-12 V

Vg=-8 V

0-5-10-15-20-2.0x10-9

-1.5x10-9

-1.0x10-9

-5.0x10-10

I d(A

)

Vd (V)

Vg=-20 V

Vg=-16 V

Vg=-12 V

Vg=-8 V

0-5-10-15-20-9.0x10-7

-6.0x10-7

-3.0x10-7

0.0

I d(A

)

Vd (V)

V =-20 Vg

V =-16 Vg

V =-12 Vg

V =-8 Vg

(a) (b)

(c)(d)

Fig. 3. The different evolution of Id − Vd profile at Vg = −16 V for the devices with different l (a), and the Id − Vd characteristics of the devices with bottom-contact configurationfor the different l: 30 �m (b), 15 �m (c) and 5 �m (d).

2420 L. Lu et al. / Synthetic Metals 160 (2010) 2417–2421

0-5-10-15-20

-2.0x10-9

0.0

2.0x10-9

4.0x10-9

6.0x10-9

Vg (V)

I d (

A)

Vd= -8 V

Vd= -14 V

Vd= -20 V

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ig. 4. The Id − Vg characteristics of the devices with bottom-contact configurationor l of 15 �m.

lectrode, leading to the different initial value for three devices atV of Vd.

When the l was 30 �m, as shown in Fig. 3(b), the device exhibitedSchottky effect, which the current increased rapidly with appliedoltage between source and gate. The currents at the Vd = 0 Veached 1.83 × 10−4, 5.73 × 10−5, 1.13 × 10−5, and 5.85 × 10−7 Ahen the Vg was at −20, −16, −12 and −8 V, respectively. More-

ver, we found that the value of Vd shifted from 0 to −14 V with Vg

ncreasing from −8 V to −20 V. At high Vg of −20 V, when Vd wasmall, all the leakage current was measured by the drain amme-er because drain electrode was at the same potential as sourcelectrode.

While the l was 15 �m, an FET behavior of saturation regimef Id has been detected due to horizontal charge migrationetween source and drain in Fig. 3(c). The maximum output cur-ent improved from −1.61 × 10−9 to −1.80 × 10−9 A when the Vg

ecreased negatively from −20 to −8 V. The saturation regimeas observed for the presence of a pinch-off effect, resulting from

he transistor-like triple electrode arrangement. Meanwhile, anlectro-optical transfer characteristic has been observed, whichas produced from vertical charge transport and hole–electron

ecombination. Moreover, with decreasing the Vd, we found thathe Id of the device with l of 15 �m changed from positive to neg-tive quantity, indicating one balanced state achieved betweenertical and horizontal filed. The transfer characteristic of theevices (Id − Vg) with l of 15 �m is shown in Fig. 4. At the Vgs of

ess than −2 V, the characteristics exhibited a current crowdingue to high resistance between source and drain contacts. In addi-ion, the Id tended to reach saturated at the Vgs of bigger than −2 V,emonstrating an FET behavior. Both the output and the transferharacteristics of device with l of 15 �m have been demonstratedo exhibit an FET behavior.

Furthermore, when the l decreased to 5 �m, a completely dif-erent evolution pattern of Id − Vd from those of devices with l of5 and 30 �m was observed. It was found that the negative cur-ent increased exponentially, as shown in Fig. 3(d), revealing thathe hole transport towards drain showed Schottky characteristics,nd the carrier transport towards the gate could be negated. It wasorth to note that the increase in the gate voltage affected hardly

he evolution of Id − Vd. The current profiles at the Vg from −8 to20 V kept almost consistent due to a very low carrier densityetween the gate and drain contacts. One conceivable explana-ion is that this device suffered from some form of unintendedoping.

The charge transport and light-emitting mechanism waschematically explained from Fig. 5. The electric field betweenource and drain was named as E1 that was equal to Vs−d/l1, inhich the input current was named as Ids. The electric field between

Fig. 5. The schematic of charge transport and light-emitting mechanism in thedevices under the horizontal and vertical electric fields.

source and gate was named as E2 that was equal to Vs−g/l2. Whilethe source electrode was kept as zero voltage and gate was kept asnegative potential, the holes and electrons injected from sourceelectrode and gate electrode, respectively, and transported intoemitting layers under the electric field E2. At the same time, theholes migrated from source electrode to drain electrode by E1,while the Vd was applied, the output current from drain to gatewas named as Igd; therefore net value of Id could be estimated to beIds − Igd. The above different profiles in Fig. 3(b)–(d) might be con-tributed to the change in the direction of drain current from alongdrain-gate (outflow) Igd to along source–drain (inflow) Ids withthe decreased l. Accordingly, we assumed that hole could migratemainly from source contact to drain contact rather than from drainto gate contact with an increase in the applied external electric filedbetween the drain and source electrodes. The relative conductivityalong the channel with respect to vertical direction, depending onthe effective mobility and the channel length was mutually rivaledand would control either FET behavior or OLED characteristics. Theeffective hole mobility along the vertical direction, especially, theuse of hole block layer, was much lower than that of active layeralong the channel. At the case of optimized channel length, bothFET behavior and electro-optic transfer characteristics could beobserved.

4. Conclusion

In conclusion, we have investigated the characteristics of OLEDswith a modified ITO anode. The hole injection layer of high mobil-ity organic material in OLEDs functioned as an active layer of FETand the other organic layers as insulator-like layer. The deviceswere found to exhibit a behavior of FET due to horizontal chargemigration between source and drain, and an electro-optical transfercharacteristic due to vertical charge transport and recombina-tion. With changing the channel to be suitable length, i.e. 15 �min this study, the device exhibited both unipolar FET and light-emitting characteristics. This method might be a useful and simpleway to get a bottom-contact OLETs by integrating high mobil-ity of organic or inorganic material as an active layer of FET inOLEDs.

Acknowledgements

This work was supported by the financial support of theexcellent young teacher found of Shanghai Education Com-

etals

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