Fabrication and characterization of air-stable, ambipolar heterojunction-based organic light-emitting field-effect transistors

Organic Electronics 10 (2009) 1293–1299

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Fabrication and characterization of air-stable, ambipolar heterojunction-based organic light-emitting field-effect transistors Hoon-Seok Seo, Ying Zhang, Min-Jun An, Jong-Ho Choi * Department of Chemistry and Center for Electro- and Photo-Responsive Molecules, Korea University, Anam-Dong, Seoul 136-701, Republic of Korea

a r t i c l e

i n f o

Article history: Received 21 March 2009 Received in revised form 30 May 2009 Accepted 12 July 2009 Available online 18 July 2009 PACS: 73.40.c 73.61.Ph

a b s t r a c t We present our first application of the neutral cluster beam deposition (NCBD) method to fabricate bilayer heterojunction-based organic light-emitting field-effect transistors (OLEFETs) by superimposing two layers of a,x-dihexylsexithiophene (DH6T) and N,N0 -ditridecylperylene-3,4,9,10-tetracarboxylic diimide (P13) successively. Based upon well-balanced ambipolarity (hole and electron field-effect mobilities of 2.22  102 and 2.78  102 cm2/Vs), the air-stable OLEFETs have demonstrated good field-effect characteristics, stress-free operational stability and electroluminescence under ambient condition. Ó 2009 Elsevier B.V. All rights reserved.

Keywords: Neutral cluster beam deposition (NCBD) Air-stable Ambipolar organic light-emitting fieldeffect transistors (OLEFETs) a,x-Dihexylsexithiophene (DH6T) N,N0 -Ditridecylperylene-3,4,9,10tetracarboxylic diimide (P13) Bilayer heterojunction

1. Introduction Organic-based optoelectronic devices have been extensively investigated due to their many potential advantages that include low cost, ease of fabrication processing, mechanical compatibility with flexible active-matrix displays, and promising applications in organic light-emitting diodes (OLEDs) and organic field-effect transistors (OFETs) [1–4]. Select OLEDs have been included in the fabrication of commercial flat-panel displays while several OFETs, comparable to hydrogenated amorphous silicon-based transistors, are being developed as switching devices for active-matrix OLED displays [5]. Since the recent fabrication of unipolar light-emitting transistors based on tetracene thin films, the combination * Corresponding author. Tel.: +82 2 3290 3135; fax: +82 2 3290 3121. E-mail address: [email protected] (J.-H. Choi). 1566-1199/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.orgel.2009.07.009

of both electrical switching and luminescence functionalities in a single organic device has attracted much attention and become a new class of optoelectronic devices known as organic light-emitting field-effect transistors (OLEFETs) [6–12]. OLEFETs significantly increase the range of potential applications of organic semiconductors, including highly integrated optoelectronics and electrically pumped lasers [13,14]. Organic transistors utilize p-conjugated organic and polymeric compounds and typically exhibit either p- or n-type unipolar behaviors; the majority of carriers are holes or electrons. In the unipolar OLEFETs, however, carrier injection with an electron–hole imbalance undergoes inevitable exciton formation, occurring close to the drain electrode and not within the active channel, resulting in strong exciton quenching at the metal contact and inefficient luminescence. Such drawbacks within unipolar transistors might be overcome by inducing exciton recombination in the active

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channel through ambipolar transport. In fabricating efficient ambipolar OLEFETs, balanced carrier conduction and controlled positioning of the recombination region are crucial. So far, either utilizing single ambipolar materials or combining two unipolar materials through co-evaporated or bilayered structures have been applied to realize ambipolar carrier conduction [15–23]. In most cases of single component- and co-evaporated blend-based OLEFETs, however, efficient light emission has not been observed due to the difficulties in achieving a balanced injection and carrier transport. While there is a physical separation between hole and electron transports, bilayer heterojunction-based OLEFETs have demonstrated well-balanced ambipolarity with high carrier mobilities and have consequently improved luminescence by tuning the gate voltage. Another critical factor in fabricating high-performance OLEFETs is the preparation of good thin films. Unlike traditional vapor deposition and/or solution-processing methods, the neutral cluster beam deposition (NCBD) method employed in this study is a less popular, but promising deposition approach [24]. Neutral cluster beams of weakly bound organic molecules are generated at the throat of the nozzle when the vapor-phase molecules evaporated by resistive heating undergo adiabatic supersonic expansion in a high-vacuum. The unique characteristics of a neutral

(a) C6H13

(c)

S

S

S

S

S

S

C6H13

(b)

O

O

C13H27

N

N C13H27

O

O

(d)

vacuum level

0

cluster beam are its high translational kinetic energy and directionality. The collision of such directional, energetic neutral clusters with a substrate of interest induces their facile decomposition into individual molecules, where their subsequent migration leads to the formation of smooth, uniform thin films. The novel scheme allows for significant improvement in surface morphology, crystallinity, packing density, and room temperature substrate deposition, whose unique advantages cannot be easily achieved through traditional vapor deposition techniques. In recent years a series of optoelectronic devices have been successfully prepared and characterized utilizing the NCBD method [25–33]. In this article, we describe our application of the NCBD method to fabricate bilayer heterojunction-based OLEFETs by successively superimposing two layers of a,x-dihexylsexithiophene (DH6T) and N,N0 -ditridecylperylene3,4,9,10-tetracarboxylic diimide (P13). DH6T and P13 are hole- and electron-transporting materials with high mobilities and the relative positions of their highest occupied and lowest unoccupied molecular orbitals (HOMOs and LUMOs) are well-matched to form singlet excitons for high luminescence; the (HOMO, LUMO) levels of DH6T and P13 are estimated to be (5.2 and 2.9 eV) and (5.4 and 3.4 eV), respectively (Fig. 1). For the first time, our air-stable, heterojunction-based OLEFETs successfully

-2.9

E (eV)

-3

-3.4

DH6T

-4 -5 -6

-5.1

Au

-5.2

P13 -5.4

(e)

-5.1

Au

14.25 mm S

(Au)

3.85 mm

D

(Au)

L = 150 μm Fig. 1. Molecular structures of (a) DH6T and (b) P13. (c) Energy level diagram (units in eV) for the Au source electrode/P13 (bottom)/DH6T (top)/Au drain electrode device. (d) Schematic cross-sectional view of the top-contact transistor with its bias condition. (e) Electrode configuration of the OLEFET device with a multi-digitated, long channel-width geometry.

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demonstrated good field-effect characteristics, stress-free operational stability, and electroluminescence based on well-balanced ambipolarity under ambient conditions.

480 and 510 K for DH6T. Each sample vapor then underwent adiabatic supersonic expansion into the high-vacuum drift region at a working pressure of approximately 6  106 Torr. Highly directional, weakly bound neutral cluster beams were formed at the throat of the nozzle and directly deposited onto the substrates. The optimum thickness and deposition rate were 300 Å at 1.0–2.0 Å/s for P13 and 150 Å at 0.5–1.0 Å/s for DH6T, respectively. The OLEFET devices with a multi-digitated, long channel-width geometry were fabricated in the top-contact configuration as shown in Fig. 1. The substrates consisted of a highly doped, n-type Si wafer coated with an Al layer as the gate electrode and thermally grown 2000 Å-thick

2. Experiment A homemade NCBD apparatus was employed to prepare the P13 and DH6T active layers [24]. The apparatus consisted of a pair of evaporation crucibles, a drift region, and the substrate. The as-received samples were placed inside the enclosed cylindrical crucibles (1.0 mm diameter, a 1.0 mm-long nozzle) and sequentially sublimated by separate resistive heating between 530 and 570 K for P13 and

(a)

(b)

3.3O / 26.8 Å

O

Intensity (a. u.)

Intensity (a. u.)

3.2 / 27.6 Å

O

6.6 / 13.4 Å

0

10

20

30

40

50

6.7 O / 13.2 Å O

10.0 / 8.8 Å 13.3 O / 6.7 Å

60

0

4

8

2 θ (degree)

12

16

20

2 θ (degree) O

3.3 / 26.8 Å O

Intensity (a. u.)

Intensity (a. u.)

6.6 / 13.4 Å

O

9.9 / 8.9 Å O

13.3 / 6.7 Å

4

6

8

10

12

14

16

2θ (degree)

0

5

10

15

20

25

30

2 θ (degree) Fig. 2. (a) Comparison of two-dimensional AFM micrographs (10  10 lm2 area) for the DH6T and P13/DH6T thin films. (b) Comparison of X-ray diffractograms for the DH6T, P13, and P13/DH6T thin films deposited on the SiO2 substrates at room temperature.

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SiO2 layers as the gate dielectric. A rigorous cleaning procedure and UV treatment were necessary to improve OLEFET performance, involving a series of sequential ultrasonic treatments in acetone, hot trichloroethylene, acetone, HNO3, methanol, deionized water, and blown dry with dry N2. The substrates were finally exposed to UV (254 nm) for 15 min [34,35]. The multi-digitated transistors had a significantly longer channel-width (W) of 180.95 mm at a channel length (L) of 150 lm. Electronbeam evaporation using a properly shaped shadow mask was utilized to produce 500 Å-thick Au source and drain electrodes at a deposition rate of 6–8 Å/s. The current– voltage characteristics of OLEFETs and their light emission intensities were measured simultaneously in air, using an optical probe attached to an HP4140B pA meter-dc voltage source unit and an 818-UV Si photodiode with an 1830-C power meter (Newport Co.).

dominated current. At low VGS = 0 to 20 V, the drain current due to electron injection at the drain electrode decreases quadratically with decreasing VDS (increasing |VDS|). In the region of VGS 6 30 V, the drain current induced by hole injection from the grounded source electrode appeared to contribute substantially and showed a typical p-type transistor working in the accumulation mode. At a fixed VGS, IDS initially decreased linearly with decreasing VDS with IDS tending to saturate due to a pinch

-5

4.0x10

VGS ±10 ±20 ±30 ±40 ±50 ±60

-5

3.0x10

-5

2.0x10

IDS (A)

1296

-5

1.0x10

=0 V V V V V V V

0

60

40

20

0.0 -20 -5

-1.0x10

3. Results and discussion

-40

-5

-2.0x10 -3.0x10

0

-60

-5

-4.0x10

-60

-40

-20

0

20

40

60

VDS (V) Fig. 3. Typical output characteristics of P13/DH6T-based OLEFETs obtained under ambient conditions.

-4

10

-5

(A)

10

-6

IDS

10

-7

10

-8

10

-9

10

3.0 VDS = ±30 V

EL intensity (a.u.)

Surface analysis using an AFM apparatus was carried out to perform the comparative characterization of surface morphologies for the DH6T and P13/DH6T films deposited on SiO2 substrates at room temperatures. The root-meansquare (Rrms) values for the DH6T and P13/DH6T films were also measured as about 11 and 12 Å, respectively. Both films exhibited complete coverage with the highly packed grain crystallites, suggesting that the P13 bottom layer did not disturb the deposition conditions at the early stages and subsequent grain growth. The effect of the P13 bottom layer on the sequential deposition of DH6T active layer was also examined by the X-ray diffraction measurements using Cu Ka radiation in a symmetric reflection, coupled h–2h scanning mode. Fig. 2b shows the XRD diffractograms for three different kinds of DH6T (300 Å), P13 (500 Å), and P13 (300 Å)/DH6T (150 Å) films. All thin films show a highly ordered structure. The sharp first-order peaks, as well as distinctive higher-order multiple peaks can be fitted to a series of (0 0 l) reflection lines with multiple d spacing. The XRD peaks of DH6T films located at 2h = 3.2° and 6.6° correspond to d-spacings of 27.6 and 13.4 Å. The four reflection peaks of P13 films located at 2h = 3.3°, 6.7°, 10.0°, and 13.3° were assigned to be the dspacings of 26.8, 13.2, 8.8, and 6.7 Å, respectively. In the case of the P13/DH6T bilayer films, strong and narrow first-order peaks were observed. Although the higher-order multiple peaks were much weaker compared to those in the DH6T films, the reflection peaks clearly showed the feature of the DH6T layer, indicating that the P13 layer placed at the bottom supports the crystalline growth of the DH6T top layer without causing the structural mismatch at the interface. Fig. 3 displays the typical output characteristics of P13 (bottom)/DH6T (top)-based OLEFETs obtained under ambient conditions. The plot clearly exhibits the characteristic IDS = IDS (VDS, VDS) dependence expected for ambipolar devices, where IDS is the drain-source current, VDS the drain-source voltage, and VGS the gate-source voltage. In the case of the reverse drain mode (VDS < 0), there is a crossover point from electron-dominated current to hole-

-5

±40 V ±50 V ±60 V

2.5 2.0 1.5 1.0 0.5 0.0 -60

-40

-20

0

20

40

60

VGS (V) Fig. 4. Typical transfer curves of P13/DH6T-based OLEFETs in the saturation regime together with the gate-dependence of the electroluminescence characteristics. Each transfer scan was run at a constant VDS.

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IDS ¼

environmental effects, the deposition sequence of two active layers has been found to be critical in the fabrication process of stable bilayer heterojunction-based OLEFETs. In this study, by inverting the deposition sequence, the transistors with the bilayered structure of P13 superimposed on top of DH6T were also fabricated and characterized. However, since most n-type organic-based devices, including P13 devices, are sensitive to environmental contaminants such as moisture and oxygen that penetrate the channel region, device parameters such as mobilities deteriorate with time and therefore the OLEFETs do not

(a) μeff h

Mobility (cm 2 /Vs)

off in the accumulation layer. On the contrary, in the forward drain mode (VDS > 0), a totally inverse phenomenon occurs. There is a crossover from the hole-dominated current to the electron-dominated current. At VGS = 0–20 V, the drain current due to the hole injection at the drain electrode increased quadratically with increasing VDS. In the region of VGS P 30 V, the drain current induced by electron injection from the grounded source electrode appeared to contribute substantially and showed a typical n-type transistor working in the accumulation mode. The extent of transport balance between the hole and electron carriers can be directly examined in the transfer characteristics. Fig. 4 shows the typical transfer curves in the saturation regime. Each transfer scan was run with a constant VDS. In principle, for well-balanced ambipolar transistors, a symmetric form of the transfer curve centered near the IDS minimum is expected. A good balance, especially in the positive VGS region, is demonstrated in Fig. 4. Using standard FET analysis, quantitative carrier mobilities (leff) can be calculated in the saturation regime by the following relationship:

WC i leff ðV GS  V T Þ2 2L

μeff n -2

10

-3

10

0

10

20

30

40

Days

(b) μeff

Mobility (cm2 /Vs)

where Ci is the capacitance per unit area of the SiO2 gate dielectric and VT is the threshold voltage. For thermally grown, 2000 Å-thick SiO2, the value for Ci is known to be 17.25 nF cm2. A hole mobility (leffh) of 2.22  102 cm2/ Vs at VDS = 60 V is estimated to be nearly equal to an electron mobility (leffn) of 2.78  102 cm2/Vs at VDS = 60 V, implying that a well-balanced ambipolar carrier transport is clearly achieved in our OLEFETs. Several other device parameters can be also derived from the fits of the observed I–V characteristics for more than 10 OLEFETs and are listed in Table 1. The maximum hole and electron mobilities are also listed with the corresponding average mobilities (leffavg) with the standard deviation (r). The leff values at room temperature are comparable to or somewhat less than those obtained from the NCBD-based single-layer OFET devices, which are among the best to date for polycrystalline DH6T- and P13-based transistors using SiO2 dielectric layers without any thermal post-treatment. Herein, it should be noted that our measurements were carried out under ambient conditions, unlike most of the previous OLEFET investigations conducted either under inert atmosphere or vacuum conditions. In addition, due to

h

μeff n -2

10

-3

10

0

10

20

30

40

50

Number of measurements (times) Fig. 5. Hole and electron mobilities of P13/DH6T-based OLEFETs monitored as a function of (a) time (days) and (b) number of measurements.

Table 1 Device parameters deduced from the characteristics of single-layer OFETs and bilayer heterojunction-based OLEFETs. Classification (thickness)

leffn (cm2/Vs)

leffn,avg ± ra (cm2/Vs)

VTn (V)

leffh (cm2/Vs)

leffh,avg ± ra (cm2/Vs)

VTh (V)

VDS (V)

DH6Tb (300 Å) P13c (500 Å) P13/DH6T (300 Å/150 Å)

– 0.16 2.44  10–2 2.56  102 2.63  102 2.78  102

– 0.11 ± 0.03 1.27  102 ± 1.00  102 1.21  102 ± 9.28  103 1.21  102 ± 8.51  103 1.21  102 ± 9.34  103

– 46.3 34.7 33.9 32.4 28.9

4.5  102 – 1.83  102 1.99  102 2.15  102 2.22  102

0.025 ± 0.016 – 1.23  102 ± 1.02  102 1.36  102 ± 1.14  102 1.37  102 ± 1.15  102 1.31  102 ± 1.14  102

9.3 – 12.7 11.5 10.0 8.5

60 100 ±30 ±40 ±50 ±60

a The mobility data in the text represents the best values. Considering the distributions of the OFET and OLEFET characteristics, the all leff values lie within leffavg ± 2r (standard deviation). b Ref. [26]. c Ref. [27].

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show reproducible device characteristics and operational stability. Similar deterioration phenomena were also reported in the P13-based organic devices [19,36]. On the other hand, as described in the previous section, when DH6T was deposited on top of the P13 layer, the DH6T layer appeared to act as a protective passivation layer and significantly improved operational stability. As displayed in Fig. 5a, the hole and electron mobilities monitored as a function of time did not change substantially, even after 40 days. Furthermore, the alleged stress phenomenon in our OLEFETs was not observed. The degradation process is known to occur when the devices are repeatedly operated. Fig. 5b shows the hole and electron mobilities as a function of the number of measurements. Transistor characteristics were consistently reproducible during repetitive operations up to 50, clearly manifesting that the operational stabilities of our OLEFETs were well maintained without degradation. Therefore, the bilayer structure with an air-stable organic compound deposited atop

as a protective passivation layer presented a reliable scheme in producing air-stable, stress-free ambipolar OLEFETs. The gate-dependence of the electroluminescence characteristics corresponding to the transfer curves is also shown in Fig. 4. The light emission was observed in the region of VGS 6 25 V. In cases of single-layer OLEFETs, the maximum emission is generally expected to occur at VGS = 0.5 VDS, where an equal voltage drop for hole and electron carriers exists. To the contrary, emission in bilayer heterojunction-based OLEFETs are not required to satisfy the relationship. Instead, as revealed in Fig. 3, the position of the emission maximum shifts slightly with decreasing VGS (increasing |VGS|) and the intensity increases with decreasing VDS (increasing |VDS|). On the basis of the energy level diagram and the device structure in Figs. 1 and 6a, the operating mechanism to account for the observed light emission can be described as follows. When the gate electrode is negatively biased, the large electric field causes the

Fig. 6. Device operation and energy level diagram. (a) VGS = VDS = 0 V. (b) VGS < 0 V, VDS = 0 V. When the gate is negatively biased, the HOMO and LUMO levels shift up with respect to the Fermi levels of the Au metal electrodes and formation of the P13/DH6T dipole layer occurs. (c) VGS < 0 V, VDS < 0 V. At the proper negative VDS, some carrier recombination takes place in the P13 layer to form the exciton leading to the electroluminescence.

H.-S. Seo et al. / Organic Electronics 10 (2009) 1293–1299

HOMO and LUMO levels in both semiconductors to shift up with respect to the Fermi levels of the Au electrodes, inducing formation of a positively charged accumulation layer in the bottom of the DH6T layer (Fig. 6b); the holes injected from the Au electrodes are transferred into the DH6T layer with high hole mobility (Table 1) to form the active channel placed near the organic interface. Since P13 is an electron-transporting material with negligible hole mobility, the formation of such an active channel does not take place in the P13 layer. Instead, due to the positive charges placed at the interface, the favorable electrostatic interaction induces the attraction of the negative charges in the P13 layer with high electron mobility, leading to formation of the DH6T/P13 dipole layer. Afterwards, upon application of the proper negative drain voltage, the mobile holes can flow into the P13 layer and some carrier recombination takes place to form the exciton leading to the observed electroluminescence (Fig. 6c). Here, since P13 has a smaller energy gap and the higher energy barrier for hole transport from DH6T to P13 exists compared to that for the electron transport from P13 to DH6T, most light emission is highly likely to occur in the P13 layer. Our observation of the luminescence process stands in contrast with the measurements for the a,x-dihexylquarterthiophene (DH4T)/P13-based ambipolar OLEFETs conducted under vacuum conditions by Dinelli et al. [19]. While the VGS dependence of the emission intensity examined under vacuum conditions was similar to that in this study, the light emission due to the balanced transport was reported to occur only when the DH4T layer was placed at the bottom, in direct contact with the dielectric, irrespective of the deposition sequence of the two layers. Further experimental and theoretical investigations related to the conduction mechanisms of bilayer heterojunction-based OLEFETs are required. 4. Summary The air-stable, heterojunction-based OLEFETs with well-balanced ambipolarity were fabricated for the first time, and good field-effect characteristics, stress-free operational stability, and electroluminescence were demonstrated under ambient conditions. Fabrication and characterization of several OLEFETs using various p-conjugated molecules through the NCBD method are underway. We hope these studies to provide further insights into the operating mechanisms and the structure-performance relationships at the molecular level. Acknowledgments This work was supported by a Korea University grant and a Korea Science and Engineering Foundation (KOSEF)

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