Characterization of Perylene and Tetracene-Based Ambipolar Light

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J. Phys. Chem. C 2010, 114, 6141–6147

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Characterization of Perylene and Tetracene-Based Ambipolar Light-Emitting Field-Effect Transistors Hoon-Seok Seo, Min-Jun An, Ying Zhang, and Jong-Ho Choi* Department of Chemistry and Center for Electro- and Photo-ResponsiVe Molecules, Korea UniVersity, Anam-Dong, Seoul 136-701, Korea ReceiVed: December 3, 2009; ReVised Manuscript ReceiVed: February 10, 2010

Herein is presented systematic analysis of air-stable, ambipolar heterojunction-based organic light-emitting field-effect transistors (OLEFETs). Top-contact OLEFETs with multidigitated, long channel-width geometry were produced by the successive deposition of electron-transporting N,N′-ditridecylperylene-3,4,9,10tetracarboxylic diimide (P13) and hole-transporting tetracene layers, using the neutral cluster beam deposition (NCBD) method. The morphological, structural, and photoluminescence properties of the untreated and thermally post-treated P13/tetracene active layers were examined by atomic force microscopy, X-ray diffraction, and laser scanning confocal microscopy. From the comparative analysis of the NCBD thin films, the neutral cluster beams led to the preparation of smooth, uniform bilayer films consisting of well-packed grain crystallites. The OLEFETs demonstrated good field-effect characteristics, stress-free operational stability, and electroluminescence under ambient conditions. The operating conduction mechanism that accounts for the observed light emission is also discussed. Introduction Optoelectronic thin-film devices based on extended π-conjugated organic and polymeric compounds offer many unique and potential advantages in comparison to well-established, traditional silicon-based devices, including ease of synthesis and fabrication, low cost, mechanical flexibility, and compatibility with active-matrix, flat-panel displays, as well as providing new opportunities for fundamental investigations. This is exemplified by organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and photovoltaic cells.1-4 The performances of select OFETs are being developed as switching devices for commercial active-matrix OLED displays and are competitive with hydrogenated amorphous silicon devices. Significant technological progress has been made in a novel combination of both electrical switching and luminescence functionalities in a single organic device. Since the fabrication of tetracene-based light-emitting transistors, a new class of functional optoelectronic devices known as organic lightemitting field-effect transistors (OLEFETs) has attracted particular attention due to the wide range of potential applications, including highly integrated optoelectronics and electrically pumped lasers.5-11 An electrical switching operation in transistors is achieved by modulation of the current flow between the source and drain electrodes by applying a gate voltage. Electroluminescence occurs by formation of a singlet exciton via electron-hole recombination in the active channel. In most OLEFETs, including the tetracene-based devices, however, either holes or electrons are preferably transported as the majority charge carriers, as such, p- or n-type unipolar transistors experience significant unbalanced carrier conduction. Therefore, inevitable exciton quenching leading to inefficient light emission occurs at the drain metal contact in unipolar transistors. Good balance in electron-hole concentrations and control of the exciton formation location within the active channel are * To whom correspondence should be addressed. Phone: +82 2 3290 3135. Fax: +82 2 3290 3121. E-mail: [email protected].

critical in producing high-performance OLEFETs. Such requirements can be efficiently realized by utilizing single ambipolar materials12-16 or combining two unipolar materials through coevaporated17-19 or bilayered structures.19-21 Ambipolar OLEFETs allow the carrier balance, as well as the controlled positioning of the recombination zone between source and drain electrodes, to be tuned by the gate voltage. In most cases of single component- and blend-based OLEFETs, however, good ambipolarity was not obtained because of the unbalanced injection of charge carriers and transport. In contrast, although a physical separation and growth compatibility between the pand n-type layers exist, heterojunction-based OLEFETs are more likely to display balanced ambipolarity with high carrier mobilities and efficient light emission. Two significant investigations of heterojunction-based OLEFETs using thiophene oligomers and perylene derivatives have been reported in recent years.20,22 The present paper describes the fabrication and systematic analysis of air-stable, ambipolar heterojunction-based OLEFETs produced by the successive deposition of organic N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (P13) and tetracene layers. Both tetracene and P13 are hole- and electron-transporting materials with high mobilities. In addition, the relative positions of the highest occupied and lowest unoccupied molecular orbitals (HOMOs, LUMOs) of the tetracene and P13 are estimated to be (-5.3 and -2.9 eV) and (-5.4 and -3.4 eV), respectively (Figure 1), which are quite well matched to form singlet excitons for efficient electroluminescence. The deposition scheme employed herein is the neutral cluster beam deposition (NCBD) method. The presented homemade NCBD apparatus has demonstrated significant improvements in surface morphology, crystallinity, packing density, and room temperature deposition in producing a series of optoelectronic devices.22-33 Such unique advantages cannot be achieved by using traditional vapor deposition and/or solution-processing techniques.

10.1021/jp9114699  2010 American Chemical Society Published on Web 03/12/2010

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Figure 1. Molecular structures of (a) electron-transporting P13 and (b) hole-transporting tetracene. (c) Energy level diagram for the Au source electrode/P13(bottom)/tetracene (top)/Au drain electrode device (units in eV). (d) Schematic view of the OLEFET with top-contact, multidigitated, long channel-width geometry and its bias condition. (e) Electrode image of the OLEFET device with a multidigitated, long channel-width geometry taken with a CCD camera mounted on an optical microscope.

In this article, the focus is initially upon characterization of the morphological, structural, and photoluminescence properties of the organic active layers with use of atomic force microscopy, X-ray diffraction, and laser scanning confocal microscopy. Afterward, the top-contact OLEFETs with multidigitated, long channel-width geometry were fabricated (Figure 1). Various device parameters such as hole- and electron-carrier mobilities, threshold voltages, and electroluminescence were derived from the fits of the observed current- and light emission-voltage characteristics of thermally untreated and post-treated OLEFETs. The heterojunction-based OLEFETs herein demonstrated good ambipolar characteristics, stress-free operational stability, and electroluminescence under ambient conditions. Device performance strongly correlated with surface morphology and structural properties of the organic active layers is discussed, together with the operating conduction mechanism. Experimental Section A schematic diagram and an electrode image of the OLEFETs with a multidigitated, long channel-width geometry employed

Seo et al. in this study are shown in Figure 1. The substrates consisted of a highly doped, n-type Si wafer coated with an Al layer as the gate electrode and a thermally grown, 2000 Å-thick SiO2 layer as the gate dielectric. To improve OLEFET performance, a rigorous cleaning procedure was necessary, including a series of sequential ultrasonic treatments in acetone, hot trichloroethylene, acetone, HNO3, methanol, and deionized water, blown dry with dry N2.34,35 The substrates were finally exposed to UV (254 nm) for 15 min. Electron-beam evaporation with 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 geometry of the OLEFETs had a longer channel width of 181 mm at a channel length of 150 µm. A sequential deposition of the organic active layers of P13 and tetracene (Aldrich Co.) on the substrates was carried out with the NCBD apparatus. The homemade deposition apparatus has been described in detail elsewhere, and only a relevant account is presented here.23 The system consisted of two cylindrical graphite crucibles for the P13 and tetracene, a drift region, and the substrate. Each as-received organic material was placed inside the enclosed evaporation crucibles with a 1.0mm diameter and 1.0 mm-long nozzle, and sequentially sublimated by separate resistive heating between 530 and 570 K for P13, and 480 and 510 K for tetracene. Each organic vapor then underwent adiabatic supersonic expansion into the highvacuum drift region at a working pressure of approximately 6.0 × 10-6 Torr. Highly directional, weakly bound neutral cluster beams were formed at the throat of the nozzle and directly deposited onto the substrates. The optimized thickness and deposition rate were determined to be 150 Å at 1.0-2.0 Å/s for the bottom P13 layer and 300 Å at 4.0-5.0 Å/s for the tetracene top layer, respectively. For thermal post-treatment, the as-deposited P13 thin films were heated to 373 K in a vacuum oven for 1 h and then slowly cooled to room temperature.25 Characterization of the morphological, structural, and luminescence properties of the films was performed by atomic force microscopy (AFM: PSI Co.), X-ray diffraction (XRD: Rigaku Co.), and laser scanning confocal microscopy (LSCM: Olympus Co.). The current-voltage characteristics of the OLEFETs and their light emission intensities were measured simultaneously in air, using an optical probe station attached to an HP4140B pA meter-dc voltage source unit and an 818-UV Si photodiode with an 1830-C power meter (Newport Co.). The charged coupled device (CCD: Photometrics Co.) camera mounted on an optical microscope was also used for imaging the photoluminescence (PL) of the organic active layers. Results and Discussion Comparative Surface Analysis of the NCBD Thin Films. Various active layers deposited on SiO2 substrates at room temperature were examined with use of AFM, XRD, and LSCM. The 2-dimensional AFM micrographs obtained by conducting section analyses over 5.0 × 5.0 µm2 in a noncontact mode are shown in Figure 2. All films exhibited complete coverage with the highly packed grain crystallites. The four root-mean-square roughness (Rrms) average values for the P13, tetracene, and untreated and thermally post-treated P13/tetracene films were measured near 56.2 ( 4.5, 36.6 ( 3.1, 47.5 ( 6.1, and 35.3 ( 1.3 Å, respectively (Figure 2). The low Rrms values observed in this study clearly exhibited that the weakly bound cluster beams undergo efficient fragmentation into energetic individual molecules, leading to uniform, highly packed thin films. In the case of the bilayer films, the thermal annealing process appears to favor formation of more uniform, highly packed thin films with larger grain crystallites.

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Figure 3. Typical LSCM images of the P13/tetracene bilayer films by scanning section analyses over 31 × 31 µm2. (a) LSCM image with the excitation wavelength λex of 540 nm, showing the green emission from the top tetracene layer (band gap Eg ) 2.4 eV). (b) LSCM image with the longer excitation wavelength λex of 600 nm, showing only the red emission from the buried P13 bottom layer (Eg ) 2.0 eV).

Figure 2. Comparison of 2-dimensional AFM micrographs (5.0 × 5.0 µm2) and X-ray diffractograms for (a) P13, (b) tetracene, (c) untreated P13/tetracene, and (d) thermally post-treated P13/tetracene thin films deposited on the SiO2 substrates at room temperature.

The effects of the thermal post-treatment of the P13 bottom layer on sequential deposition of the tetracene active layer are also clearly displayed in the XRD measurements below. Figure 2 shows the XRD diffractograms for four different types of thin films examined by X-ray diffraction, using Cu KR radiation in a symmetric reflection, coupled θ-2θ mode. All thin films showed a highly ordered structure. According to the previous crystallographic investigations, both P13 and tetracene single crystals are known to have a triclinic structure. The sharp firstorder peaks, as well as distinctive higher order multiple peaks in Figure 2, can be fitted to a series of (00l) reflection lines with multiple d spacing based on the crystallographic parameters of P13 and tetracene. In the case of P13 thin films, the four reflection peaks located at 2θ ) 3.3°, 6.7°, 10.0°, and 13.3° were assigned to be the d spacings of 26.4, 13.2, 8.8, and 6.6 Å, respectively. For tetracene films, the three peaks located at 2θ ) 7.3°, 14.5°, and 21.9° correspond to d spacings of 12.1, 6.1, and 4.1 Å, respectively. All of the spacings observed in the single-layered thin films were in good agreement with previous XRD reports. For the bilayer films, the XRD diffractograms displayed combined features composed of the distinctive peaks from both the P13 and tetracene films. The locations of those peaks did not show any noticeable displacement from those in the respective single layer films, suggesting that the

P13 bottom layer does not disturb the deposition of the tetracene top layer at the interface at the early stage of additional crystalline growth. It is clearly demonstrated in the diffractograms that the post-thermal treatment applied to the P13 thin films led to a higher signal-to-noise and more pronounced tetracene peaks in comparison with those of the untreated bilayer films. The high-temperature annealing seemed to promote formation of closely packed tetracene grains with better crystallinity through more favorable self-assembling processes. The XRD results are also consistent with the aforementioned AFM measurements, in which larger grain crystallites show more distinct, sharper reflection peaks with lower Rrms values. Information of the buried bottom layer as well as that of the top layer can also be provided by the LSCM technique, together with the XRD measurement. The LSCM scans the thin sample layers using laser radiation and images both the top and bottom layers by collecting photoluminescence (PL). Variations in PL images are known to be due to nonuniform films and/or unaligned orientation of grain crytstallites.20 Thus, the morphology of as-deposited bilayer films can be easily examined by the LSCM films. Figure 3 shows typical LSCM images of the P13/tetracene bilayer films by scanning section analyses over 31 × 31 µm2. The LSCM image with an excitation wavelength λex of 540 nm shows the green emission that is dominated by the top tetracene layer (band gap Eg ) 2.4 eV). Conversely, the LSCM image with the longer excitation wavelength λex of 600 nm displays only a red emission from the buried P13 bottom layer (Eg ) 2.0 eV). Since the 600 nm excitation is not energetic enough to induce an optical transition in the tetracene layer with the larger Eg, only the PL emission from the P13 bottom layer was observed. Both images showed uniform PL intensities, suggesting that the bottom P13 layer was well covered with the top tetracene layer and that both layers were well grown to form continuous thin films with little variation in film thickness. From the systematic, comparative surface analysis of the NCBD thin films, it can be concluded that the neutral cluster beams employed in this study lead to the preparation of smooth, uniform thin bilayer films with the well-packed grain crystallites without growth incompatibility, which is a critical factor in fabricating high-performance, heterojunction-based OLEFETs. Device Characteristics. The output characteristics of the OLEFETs obtained under ambient conditions are displayed in Figure 4. The plot of drain-source current (IDS) as a function of the drain-source voltage (VDS) for various gate-source voltages (VGS) clearly exhibits the characteristic IDS ) IDS (VDS, VGS) dependence expected for typical ambipolar devices. In both forward (VDS > 0) and reverse (VDS < 0) drain modes, the

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Figure 4. Output characteristics of thermally post-treated P13/ tetracene-based OLEFETs and corresponding electroluminescence characteristics obtained under ambient conditions.

Figure 5. Transfer curves of thermally post-treated P13/tetracene-based OLEFETs in the saturation regime. Each transfer scan was run at a constant VDS.

crossover points from hole- to electron-dominated currents and vice versa were observed. In the region of VGS ) 0-20 V, the IDS induced by the hole injection from the drain electrode increased quadratically with increasing VDS. Around VGS = 30 V, the crossover phenomenon from hole- to electron-dominated currents occurred. In the region of VGS g 30 V, the IDS due to electron injection from the grounded source electrode increased linearly within the low VDS regime, and then grew susceptible to saturation due to a pinch-off in the high VDS regime, behavior typical of n-type transistors working in the accumulation mode. On the contrary, in the reverse drain mode a totally inverse phenomenon occurred. In the region of VDS ) 0 to -20 V, the IDS induced by electron injection from the drain electrode appeared to contribute substantially. There was a crossover point from electron- to hole-dominated currents around VGS = -30 V. In the region of VGS e -30 V, the IDS induced by hole injection from the grounded source electrode contributed substantially and showed a typical p-type transistor working in the accumulation mode. The charge-carrier mobilities and the extent of transport balance in the electron and hole concentrations can be directly deduced from the transfer curves. Figure 5 shows the typical transfer characteristics of the OLEFETs in the saturation regime; each transfer scan was run at a constant VDS. In the saturation regime, IDS satisfies the following relationship:

IDS )

WCiµeff (VGS - VT)2 2L

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. From the fits of the observed I-V characteristics for more

Seo et al. than 20 OLEFETs at a channel width (W) of 181 mm and a channel length (L) of 150 µm, the field-effect hole and electron mobilities (µeffh, µeffn) can be derived with the corresponding average mobilities (µeffavg) with the standard deviation (σ), and are listed in Table 1, together with several other device parameters. Herein, it should be noted that the measurements presented were carried out under ambient conditions, unlike most of the previous OLEFET investigations conducted under either an inert atmosphere or vacuum. The mobilities were determined to be µeffh ) 3.9 × 10-3 cm2/(V s) at VDS ) -60 V and µeffn ) 0.20 cm2/(V s) at VDS ) 60 V for untreated P13/tetracene devices, and µeffh ) 2.8 × 10-2 cm2/(V s) and µeffn ) 0.27 cm2/ (V s) for thermally post-treated devices, respectively. Select distinctive features related to the observed mobilities can be deduced in Table 1. First, the thermal post-treatment clearly enhanced both µeffh and µeffn values. This can be rationalized from the fact that thermal annealing improved the quality of active layers through the formation of closely packed tetracene and P13 grains with better crystallinity, induced by favorable self-assembling processes, as already manifested in the AFM and XRD measurements. The resulting higher structural organization led to an efficient carrier transport via face-to-face intermolecular interactions between the π-π stacks. Second, while the µeffn values were comparable to or higher than those obtained from the NCBD-based, single-layer OFET devices, the µeffh values decreased by one or 2 orders of magnitude. Here, those mobility values from the single layerbased transistors were among the best to date for polycrystalline tetracene- and P13-based transistors with use of SiO2 dielectric layers. Similar behavior was also reported in the P13/thiophene derivative-based organic devices. In a sense, such a decrease, particularly in the µeffh values, appeared to be inevitable in heterojunction-based OLEFETs due to the lattice mismatch occurring at the interface. In the case of P13/tetracene OLEFETs, although P13 and tetracene have the same triclinic structures, the corresponding unit cells with three nonequivalent nonperpendicular axes (a, b, c) do not match well at the interface (a ) 4.67 Å, b ) 8.59 Å, c ) 25.3 Å for P13; a ) 6.06 Å, b ) 7.84 Å, c ) 13.01 Å for tetracene). This mismatch might result in unfavorable growth at the early stage of a second layer growth on top of the bottom layer and eventually in the decrease of device performance to some extent. Third, as displayed in Figure 6a, the hole and electron mobilities monitored as a function of time did not change substantially, clearly demonstrating that the operational stabilities of the presented OLEFETs were well maintained without degradation. In general, most n-type organic-based devices, including P13 devices, are known to be sensitive to environmental contaminants such as moisture and oxygen that can penetrate the channel region. As a result, µeffn tends to deteriorate with time and does not show reproducible characteristics or operational stability. In the case of the bilayer OLEFETs with air-sensitive layer deposited atop, the measurements should be carried out under either inert atmosphere or vacuum. However, in the presented heterojunction-based OLEFETs, the air-stable tetracene layer was superimposed on top of the P13 and appeared to act as a protective passivation layer, preventing direct exposure of P13 to air. Therefore, any significant decrease in µeffn was not observed in the measurements conducted under ambient conditions. Fourth, the alleged stress phenomenon occurring when the devices were repeatedly operated was not found in our OLEFETs. Figure 6b shows µeffh and µeffn as a function of the number of measurements. The transistor characteristics were consistently

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TABLE 1: Device Parameters Deduced From Single-Layer OFETs and Bilayer Heterojunction-Based OLEFETs Characteristics classification (thickness) OFET

OLEFET

tetraceneb (500 Å) P13c (500 Å) P13/Tetracene (150 Å/300 Å) Untreated P13/Tetracene (150 Å/300 Å) thermally post-treated

µeffn (cm2/(V s))

µeffn,avg ( σa (cm2/(V s))

VTn (V)

µeffh (cm2/(V s)) 0.16

µeffh,avg ( σa (cm2/(V s)) 0.12 ( 0.03

VTh (V)

VDS (V)

-37.0

-60

0.16

0.11 ( 0.03

46.3

0.20

0.13 ( 5.7 × 10-2

40.7

3.9 × 10-3

2.5 × 10-3 ( 2.0 × 10-3

-20.0

(60

0.27

0.16 ( 7.9 × 10-2

38.5

2.8 × 10-2

9.6 × 10-3 ( 7.4 × 10-3

-24.7

(60

100

a The mobility data in the text represents the best values. Considering the distributions of the OFET and OLEFET characteristics, all µeff values lie within µeffavg ( 2.5σ (standard deviation). b Reference 24. c Reference 25.

Figure 6. Hole and electron mobilities of untreated and thermally posttreated P13/tetrancene-based OLEFETs monitored as a function of (a) time (days) and (b) the number of measurements.

reproducible during repetitive operations up to 50. Therefore, the bilayer heterojunction structure with an air-stable layer deposited atop as a protective passivation layer presents a promising, reliable scheme for producing air-stable, stress-free ambipolar OLEFETs. Electroluminescence and Conduction Mechanism. The drain dependence of the electroluminescence was examined during the measurements. The light emission characteristics are shown in Figure 4, together with the corresponding output curves. Typically, the light emission was observed in the region of VDS e -40 V, together with a weak emission in the region of VDS g 50 V. In cases of single-layer OLEFETs, the maximum emission was generally expected to occur at VGS ) 0.5 VDS, where an equal voltage drop for hole and electron carriers exists. To the contrary, the light emission in the bilayer heterojunctionbased OLEFETs was not required to satisfy the relationship. As displayed in Figure 4, the emission intensity increased with decreasing VDS (increasing |VDS |) and the maximum emission increased with increasing VGS (decreasing |VGS |). One possible operating mechanism to account for the observed drain dependence of the light emission can be described on the basis of the energy level diagram and the device structure in Figures 1 and 7a. When the drain electrode is negatively biased (VGS ) 0 V,

VDS < 0 V), the electric field between the drain and gate electrodes induces formation of a negatively charged accumulation layer, due to the electron carriers bearing a high mobility in the P13 layer (Figure 7b). Upon application of the proper VDS e -40 V, holes injected from the Au electrodes flow into the tetracene layer and some subsequent carrier recombination occurs to form the singlet excitons responsible for the observed electroluminescence. Here, since P13 has a smaller energy gap, and the lower energy barrier for hole transport from tetracene to P13 exists compared to that for the electron transport from P13 to tetracene, most light emission observed is highly likely to occur in the P13 layer. Under such VDS < 0 V, the VGS dependence of the emission intensity can be rationalized with respect to the increased energy barrier for electron injection. As VGS decreases (increasing |VGS|), the HOMO and LUMO levels in both P13 and tetracene semiconductors are shifted up with respect to the Fermi levels of the Au electrodes, resulting in a lowering of the barrier for the hole injection and raising of the barrier for electron injection simultaneously (Figure 7c). In principle, the EL intensity (IEL) can be given by the relation IEL ∝ pn(µeffh + µeffn), expressed in terms of the mobility sum and the pn product (p and n: hole and electron densities). When unbalanced carrier conduction takes place, the decrease in n for the highly mobile electron carriers significantly affects the efficiency of the whole luminescence process and eventually reduces the maximum emission with increasing VGS, as shown in Figure 4. On the contrary, when the drain electrode is positively biased (VDS > 0 V), an unfavorable situation for injecting hole and particularly electron carriers exists, as shown in Figure 7d. Only some hole carriers contribute to the IDS and no EL emission is observed due to the absence of the electron carriers. Under such VDS > 0 V, however, as VGS increases, the HOMO and LUMO levels in both P13 and tetracene semiconductors are shifted down with respect to the Fermi levels of the Au electrodes. The resulting lowering of the barrier for the electron injection increases the contribution of electron carriers to IDS and leads to weak light emission susceptible to saturation at high VGS (Figure 7e). Similar bias dependence of the electroluminescence was also reported in the measurements of R-quinquethiophene (T5)/P13-based ambipolar OLEFETs reported by Rost et al. and Loi et al.17,18 The observation herein stands in contrast with the investigation of the R,ω-dihexylquarterthiophene (DH4T)/P13-based ambipolar OLEFETs conducted by Dinelli et al. and the authors’ earlier work on P13/R,ω-dihexylsexithiophene (DH6T) devices. In the former case the light emission was reported to occur under vacuum conditions 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.20 In contrast, in the

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Seo et al. Conclusions Fabrication and systematic characterization of air-stable, ambipolar heterojunction-based OLEFETs were presented. The successive deposition of electron-transporting P13 and holetransporting tetracene layers was carried out by using the NCBD method. From the comparative analysis of NCBD thin films, the neutral cluster beams lead to the preparation of smooth, uniform bilayer films consisting of well-packed grain crystallites. The OLEFETs with top-contact, multidigitated, long channelwidth geometry demonstrated good field-effect characteristics, stress-free operational stability, and electroluminescence under ambient conditions. The operating mechanism to account for the observed light emission was also discussed. It is the hope of the authors that these studies provide further insights into the operating mechanisms and structure-performance relationships at a molecular level. Acknowledgment. This work was supported by a Korea University grant and a Korea Science and Engineering Foundation (KOSEF) grant funded by the Korea government (MEST) (No. M10500000023-06J0000-02310). References and Notes

Figure 7. Operating conduction mechanism with the corresponding energy level diagrams under various bias conditions: (a) VGS ) VDS ) 0 V; (b) VGS ) 0 V, VDS < 0 V; (c) VGS < 0 V, VDS < 0 V; (d) VGS ) 0 V, VDS > 0 V; and (e) VGS > 0 V, VDS > 0 V.

latter case the light emission was observed only in the region of the proper negative VDS.22 It is obvious that all EL phenomenon occurring in the heterojunction-based OLEFETs appear significantly dependent on organic materials, deposition sequence, and operation condition. Accordingly, more theoretical and experimental work is required to understand transport phenomenon and various EL emissions at the molecular level. Several other OLEFET devices using various π-conjugated organic molecules through the NCBD method are underway to deduce the conduction and EL mechanisms and structureperformance relationships.

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