Effects of Doping and Electrode Contacts on Performance of Organic

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Effects of Doping and Electrode Contacts on Performance of Organic Light-Emitting Transistors Based on Pentacene and Tris(8-Hydroxyquinoline) Aluminum Dae-Kyu Kim, Jeong-Do Oh, Jang-Woon Kim, Han-Young Lee, and Jong-Ho Choi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02964 • Publication Date (Web): 13 Jun 2016 Downloaded from http://pubs.acs.org on June 14, 2016

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The Journal of Physical Chemistry

Effects of Doping and Electrode Contacts on Performance of Organic Light-Emitting Transistors Based on Pentacene and Tris(8-Hydroxyquinoline) Aluminum Dae-Kyu Kim, Jeong-Do Oh, Jang-Woon Kim, Han-Young Lee, and Jong-Ho Choi* Department of Chemistry, Research Institute for Natural Sciences, Korea University, Anam-Dong, Seoul 136-701, Korea

ABSTRACT In this study, heterojunction-based organic light-emitting field-effect transistors (OLEFETs) with a top-contact, long-channel geometry were fabricated and comparatively characterized. The neutral cluster beam deposition (NCBD) method was used to successively deposit two layers of p-type pentacene and n-type tris(8-hydroxyquinoline) aluminum (Alq3). For doped OLEFETs, 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) was used as a highly fluorescent dye dopant to enhance the light-emission efficiency and change *

Corresponding author. Tel.: +82 2 3290 3135; Fax: +82 2 3290 3121.

E-mail address: [email protected] (J.-H. Choi).

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the emission color. OLEFETs revealed fine device characteristics based on deposition of highly crystalline active layers. The combination of the highly fluorescent DCM-doping and asymmetric electrode configuration (Au and Li:Al or LiF/Al) exhibited efficient energy transfer, enhanced electroluminescence (EL) emission. The operating light-emission mechanisms were discussed based on EL photos acquired using a charge-coupled device (CCD) camera.

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INTRODUCTION In comparison to traditional silicon-based devices, optoelectronic devices using extended π-conjugated organic compounds have many distinctive advantages owing to their low-cost processing, mechanical flexibility, and provision of new opportunities for basic studies.1-5 Recent advances in organic thin-film devices have led to organic field-effect transistors (OFETs), lightemitting diodes, memories, and solar cells.6-12 For example, OFETs are being established as switching components for commercial flat-panel OLED displays. An electrical switching operation in the OFETs is realized by modulating the current flow by tuning the gate voltage, while the light-emission phenomenon, i.e., electroluminescence (EL) in the OLEDs results from electron-hole recombination in the active channel. The performance of select OFET devices is in a stage of competition with hydrogenated amorphous silicon transistors. In producing complex organic-based integrated circuits (ICs), simplifying the circuit design and reducing the number of circuit components are critical. Fabrication of π-conjugated tetracene-based light-emitting transistors first reported in 2003 offered a novel combination of both switching and EL characteristics in an optoelectronic device, leading to a new category of functional devices known as organic light-emitting field-effect transistors (OLEFETs).1 The OLEFET array as one of the simplest IC devices considerably increases the scope of prospective applications of organic-based devices such as highly integrated electronics and lasers. Most OLEFETs, including the initial tetracene-based devices, use single organic and polymeric compounds showing either p- or n-type unipolar behaviors. As a result, typical unipolar

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OLEFETs suffer an inevitable hole-electron imbalance and exciton quenching at the metal electrode.3-4 Carrier balance in electron-hole concentrations and an increased formation efficiency of singlet excitons are important in fabricating high-performance OLEFETs. Unlike unipolar transistors, such requirements can be fulfilled by utilizing ambipolar materials13-16 or combining two unipolar compounds through blended (co-evaporated)17-19 or bi-layered structures.19-23 However, most ambipolar single-component- and blend-based OLEFETs do not achieve efficient EL owing to the complications in attaining a balanced carrier transport. In contrast, despite physical separation and growth incompatibility between the p- and n-type layers, bi-layered, heterojunction-based OLEFETs with high carrier mobilities show good balanced ambipolarity and EL efficiency.22-23 Recently, Hu and co-workers reported a high hole mobility and green light emission in a unipolar OLEFET with vertical heterojunctions using pentacene and tris-(8hydroxyquinolinato) aluminum (Alq3).24 Preparation of organic, crystalline thin films is critical to producing high-performance OLEFETs. The neutral cluster beam deposition (NCBD) scheme used herein is simple and useful for obtaining a variety of organic active layers.25 Unlike conventional solution-processing methods and physical vapor deposition, the NCBD technique utilizes the neutral cluster beams of weakly bound molecules when the evaporated organic molecules experience adiabatic supersonic expansion in a high vacuum. The distinctive traits of a neutral cluster beam are its high translational kinetic energy and directionality. The collision of the clusters with a substrate at room temperature induces facile fragmentation into individual molecules and the succeeding 4


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energetic migration leads to the formation of uniform, crystalline thin films. In recent years, a series of NCBD-based optoelectronic devices were successfully produced and substantial improvements in the crystallinity and packing density of the thin films were demonstrated.20-22,2527

In the present paper, we described the fabrication and comparative characterization of heterojunction-based OLEFETs with a top-contact, long-channel geometry. The p-type pentacene and n-type Alq3 were used as hole- and electron-transporting materials, respectively. For doped devices 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) was used as a highly fluorescent dye dopant for enhancing the device EL efficiency and tuning the emission color.25 Organic layers were successively deposited using the NCBD method. Molecular structures, the schematic diagram of the device and the relative positions of HOMO and LUMO (highest occupied and lowest unoccupied molecular orbitals) of the pentacene, Alq3 and DCM were shown in Figure 1. The energy levels of pentacene and Alq3 were matched to form singlet excitons for effective light emission. Since the LUMO and HOMO levels of the DCM dopant (5.6, 3.5 eV) are located inside those of Alq3 (5.7, 3.2 eV), the DCM dopant acts as good energy acceptors from the Alq3 matrix and as efficient trapping sites for electrons and holes. Furthermore, the effect of the drain electrode contacts on the OLEFET performance was also examined by tuning the electron injection: high work-function (WF) Au (5.1 eV) vs. low WF Li:Al (2.9 eV) or LiF/Al (2.9 eV).20,28-29 In this study, the surface and luminescence properties of the organic films were compared using X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence (PL) 5



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spectroscopy and laser scanning confocal microscopy (LSCM). Various device parameters such as the hole and electron carrier mobilities and current on/off ratios were derived from the current-voltage (I-V) and current-voltage-luminescence (I-V-L) plots of the heterojunction-based OLEFETs. The OLEFETs demonstrated good EL characteristics under ambient conditions and the performance strongly correlated with the structural properties of the organic films. The energetics under bias conditions was compared and discussed, together with the operating conduction mechanisms.

EXPERIMENTAL A schematic view of the OLEFET device with a top-contact, long-channel geometry was shown in Figure 1 with the molecular structures of pentacene, Alq3, (Sigma-Aldrich Co.) and DCM (Alfa Aesar Co.). Heavily n-doped Si substrates and thermally grown, 2000-Å-thick SiO2 atop the substrates were used as common gate electrodes and dielectrics, respectively. A series of rigorous cleaning procedures were applied to the gate dielectrics to enhance the device performance.30 The organic films were prepared using our own laboratory-made NCBD apparatus. The presented procedure has been previously described in detail.24 The apparatus consists of two enclosed cylindrical crucibles with a nozzle (1.0-mm diameter and a 1.0 mm-long) for pentacene and Alq3, a drift region, and a substrate. The pentacene was first evaporated by resistive heating between 500 and 520 K and went through adiabatic supersonic expansion at 6 × 10-6 Torr. 6


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Pentacene cluster beams formed at the throat of the nozzle were directly deposited onto the substrates. For the undoped OLEFET devices, Alq3 was deposited on top of the pentacene layer at 610 K. For the DCM-doped devices, Alq3 and DCM placed in separate cells were heated to 610 K and co-deposited.25,29 The optimized thickness and deposition rate were determined as 500 Å at 0.7 – 1.0 Å/s for pentacene, and 500 Å at 0.1 – 0.3 Å for Alq3. A comparative study was conducted to inspect the effect of electrode contacts on OLEFET performance. In the devices with symmetric Au source and drain electrodes, Au was thermally evaporated onto the Alq3 using a shadow mask at a deposition rate of 2 – 4 Å/s with a thickness of 50 nm. In the asymmetric structure, Au and low-WF metals (Li:Al or LiF/Al) were successively deposited: for the Li:Al electrode at a deposition rate of 3 – 4 Å/s with a thickness of 70 nm, and for the LiF and Al electrodes at deposition rates of 0.1 – 0.2 and 1 – 2 Å/s with thicknesses of 0.7 and 70 nm, respectively. The OLEFETs had a geometry with a long channel width (W) of 10 mm at a channel length (L) of 200 µm. The morphology and crystallinity of the organic active layers grown on the SiO2 substrates were examined by AFM (PSI Co.) and XRD (Rigaku Co.), respectively. All the I-V and I-V-L characteristics of the OLEFET devices in this study were simultaneously measured in air using an optical probe attached to an HP4145B, and an 818-UV Si photodiode with an 1830C power meter (Newport, Co.), unlike most of the previous measurements that were carried out either under an inert atmosphere or in a vacuum.6,9,12-16 The luminescence images of the singleand bi-layered thin films were obtained using F7000 (Hitachi, Ltd) and LSM700 (LSCM: Carl Zeiss) laser scanning confocal microscopes. The EL images of the OLEFETs were recorded 7



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using a charge-coupled device (CCD; Q Imaging Co.) camera mounted on an optical microscope.

RESULTS AND DISCUSSION Comparative Analysis of Morphology and Crystallinity The surface morphology and structural properties of various organic thin films grown on the SiO2 substrates were carried out using AFM and XRD. Two-dimensional AFM topographic images were acquired by section analyses over 5.0 × 5.0 µm2 in a non-contact mode. The 2-D micrographs of the three pentacene, DCM-doped Alq3 (Alq3:DCM), and pentacene/Alq3:DCM films were shown in Figure 2. All films exhibited pin-hole free coverage with the closely packed grains on the substrates. The rootmean-square roughness (Rrms) average values for the three films were 56.4 ± 4.5, 10.2 ± 1.7 and 29.0 ± 2.7 Å, respectively. The low Rrms values clearly demonstrated that the organic cluster beams underwent efficient breakup into active individual organic molecules, resulting in smooth thin films without the need for any thermal annealing process. The XRD diffractograms of the pentacene and Alq3:DCM films, and the bi-layered pentacene/Alq3:DCM films were obtained and of the single-layered films, only the pentacene films exhibited ordered, strong reflection peaks, as shown in Fig. 2(a), indicating that the dielectric layer acted as a well-ordered template during the initial growth of the pentacene molecules. Previous crystallographic investigations indicate that pentacene single crystal has a triclinic structure and the four (00l) reflection peaks correspond to the d spacings of 15.0, 7.6, 5.1 and 3.8 Å, respectively. For Alq3 (not shown) and Alq3:DCM films in Figure 2(b), no distinct 8


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reflection peaks were observed, indicating that both undoped and DCM-doped Alq3 films were amorphous.31 The diffractogram of the bi-layered films shown in Figure 2(c) showed a simple aspect consisting of the individual peaks only from the bottom pentacene with the slightly lower signal-to-noise peaks due to the upper amorphous Alq3 layer.

Luminescence Characteristics of Thin Films The luminescence, doping extent and dopant distribution for undoped and DCM-doped Alq3 films were examined by PL and LSCM techniques. The typical normalized PL spectra of the Alq3 and Alq3:DCM layers with an excitation wavelength (λex) of 400 nm were shown in Figure 3(a). The PL emission peak positioned at 511 nm corresponds to the Alq3 emission (Eg = 2.5 eV). While the DCM dye (Eg = 2.1 eV) in a diluted solution emits in the orange-red region, in the condensed medium both the fluorescence efficiency and wavelength depend strongly on the dopant concentration due to interdopant quenching.25,32-33 The optimum dopant concentration for DCM-doped thin films typically ranges from 0.25 to 1.0 mol %.24 In this study, the doping level was determined at 0.5 mol %. The concentration was examined by comparing the observed fluorescence with the wellknown maximum appearing in the region of 575 – 590 nm and further confirmed by the analysis of the N1S X-ray photoelectron spectra of the pristine and DCM-doped Alq3 films (molecular structures in Fig. 1). The observed peak at 585 nm in Figure 3(a) corresponds to the DCM emission, indicating near complete energy transfer from Alq3 to DCM. The LSCM scans the single- or bi-layered films using laser radiation to induce an optical transition and images the whole layers by recording the resulting PL. Non-uniform films and/or unaligned orientation of grains reportedly cause variations in PL images.23 Typical LSCM 9



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images of the Alq3, Alq3:DCM and pentacene/Alq3:DCM films on scanning section analyses over 212 × 212 µm2 with λex = 405 nm were shown in Figures 3(b)-3(d). The green emission in Figure 3(b) shows that the Alq3 cluster beam forms a continuous layer on the SiO2 dielectric substrates. In the cases of the DCM-doped films in Figures 3(c) and 3(d), the uniform red emission in the whole LSCM images was due to the DCM dopants, implying that the dopants were well distributed in the host Alq3 layers to form continuous films during the co-deposition process, independent of whether the deposition occurred on the dielectrics or on the pentacene layer. As a result, an efficient energy transfer from Alq3 to DCM occurred in all the doped layers, leading to detectable red DCM emission consistent with the PL spectra. Such lack of growth incompatibility in forming the bilayers is important in producing high-performance, heterojunction-based OLEFETs. Although energetic laser radiation can penetrate the buried pentacene bottom layer, pentacene exhibits no fluorescence in the condensed state.34

Effects of Electrode Contacts on Device Characteristics. The effects of symmetric (Au source and drain) and asymmetric (Au source and Li:Al or LiF/Al drain) electrode configurations on the transistor performance were examined to determine device characteristics. Figures 4 and 5 showed typical output (drain current vs. drain voltage, IDS vs. VDS) and transfer (drain current vs. gate voltage, IDS vs. VGS) curves of the pentacene/Alq3:DCM OFETs with the symmetric and the asymmetric contacts, respectively. The three plots in Fig. 4(a), 5(a) and 5(b) showed the characteristic dependence of IDS on VDS expected for unipolar p-type pentacene-based transistors working in the accumulation mode for a fixed VGS. In the low-VDS regime, IDS 10


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increased linearly with VDS; whereas, in the high-VDS regime, saturation occurred owing to the pinch-off phenomena. The average hole mobilities (µeff

h,avg

) can be directly deduced from the

transfer curves of the OFETs. Each transfer scan in Fig. 4(b), 5(c) and 5(d) satisfies the following relationship:

IDS =

WC iµeff (V GS −VT )2 2L

Here, Ci is the capacitance per unit area of the dielectrics, and VT is the threshold voltage. The Ci value for SiO2 was 17.25 nF/cm2. The µeff h,avg values were derived from the fits of the transfer characteristics observed for more than 10 devices with the standard deviation, as listed in Table 1. Similar hole mobility and current on/off ratio were determined irrespective of the doping and electrode contacts: µeff h,avg = 1.2 – 2.0 × 10-1 cm2/Vs at VDS = –90 V and Ion/Ioff = 0.5 – 1 × 105. The µeff h,avg values in Table 1 were lower than those deduced from the single-layered OFETs.35 Such a decrease in the µeff h,avg values seems to be unavoidable in the bi-layered OFETs. At VGS < 0, the top Alq3 with an energy barrier works as a blocking layer for hole injection in the present heterojunction-based OLEFET structure (Fig. 1). The resulting poor current flow leads to the inefficient hole accumulation in the pentacene layer, and might account in part for the reduction in the µeff h,avg, compared to those from the single-layered OFETs. In the case of electron mobilities, the drain current was below detectable level in the present multi-layered transistors. In fact, although Alq3 is one of the most universal n-type semiconductors, only a few experimental reports have examined the electron mobility µeff e,avg of the Alq3-based transistors. In order to derive the magnitude of the µeff e,avg value, Alq3-based 11



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single-layered OFETs with symmetric Au or LiF/Al source and drain contacts were fabricated and characterized. Their transfer curves were shown in Figures 4(c) and 5(e). As expected, IDS was quite low and the µeff e,avg values for the OFETs with Au and LiF/Al contacts were 9.7 × 10-8 and 1.1 × 10-6 cm2/Vs at VDS = –60 V, respectively, which correspond to the space-chargelimited cases. The derived µeff e,avg was similar to that from the single-layered OFETs reported by Adachi’s group.36 Here, the OFETs with LiF/Al electrodes showed higher mobilities due to the lower energy barrier for the electron injection from the LiF/Al electrode into the Alq3 active layer.

Electroluminescence (EL) mechanisms. The light-emission characteristics were simultaneously examined during the I-V measurements. The gate dependence of the luminescence intensity was shown in Fig. 5(c) and 5(d), together with the corresponding transfer curves. The EL was observed only in the OLEFETs with the asymmetric (Au source and Li:Al or LiF/Al drain) electrode configuration. Typical luminescence was observed in the region of VGS ≤ –60 V at VDS = –90 V, and the EL intensity increased with decreasing VGS (increasing VGS ). In general, for single-layered OLEFETs operating in ambipolar mode, the VGS must have a value between VDS and VS (= 0 V), leading to the relationship via which the maximum luminescence occurs at VGS = 0.5 VDS, where hole and electron carriers experience an equal voltage drop. In the present heterojunction-based OLEFETs, however, the relationship for the maximum luminescence does not adequately account for the observed EL emission and carrier conduction. The EL phenomena in the multi-layered OLEFETs, depends on organic materials, device 12


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structure and bias conditions. The EL mechanism operative in the observed VGS dependence can be rationalized on the basis of the energy levels and the spatial distributions of carrier charges under proper gate and drain biases shown in Figure 6(a). At VGS < 0 and VDS = 0 V, the HOMO and LUMO levels in both pentacene and Alq3 move up with respect to the Fermi levels, which lowers the barrier for the hole injection and raises the barrier for electron injection simultaneously, as shown in Figure 6(b). In the pentacene layer, a positively charged accumulation layer forms an active channel. On the other hand, such an active channel does not form in the Alq3 layer, because Alq3 is known as an electron-transporting layer with an insignificant hole mobility. Instead, due to the thin pentacene bottom layer (500 Å), the positive charges in the pentacene layer induce a favorable attraction to the negative charges in the electron-conducting Alq3 layer, leading to the formation of the pentacene/Alq3 dipolar doublelayer. Subsequently, upon application of the negative drain bias VDS < 0 V at VGS < 0 V, charge carriers begin to conduct and recombine to form the singlet excitons. The energy diagram in Figure 6(c) revealed the occurrence of facile hole injection from the Au source electrodes into the LUMO level of pentacene. In contrast, the electron injection depends on the work function of the drain electrodes. While electrons carriers are easily injected from the low-WF drain electrodes such as Li:Al or LiF/Al into the LUMO level of Alq3, the injection of electrons from high-WF Au electrodes is unfavorable due to the high energy barrier. In principle, the EL intensity is given by the relation IEL ∝ pn (µeffh + µeffe ). Here, p and n are hole and electron densities. Due to the blocked injection of electron carriers in the symmetric Au electrode configurations, unbalanced carrier conduction was achieved, which resulted in no light emission. EL was not observed in any of the undoped devices, even with the asymmetric electrode 13



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contacts (Table 1). Co-deposition of highly fluorescent DCM dye in the Alq3 layer was necessary to increase the EL intensity and change the emission wavelength. The quantum efficiency of OLEFETs with doped emissive layers depends on the overlap of the emission of the Alq3 and the absorption of the DCM dopant. As already revealed in the PL spectrum in Figure 3(a), after the light absorption of Alq3, the energy transfer to DCM molecules with a smaller band gap by the Forster energy-transfer process occurs in the doped layers. In the DCM-doped OLEFET devices, the formation of the singlet exciton followed by the energy transfer to the DCM molecules led to the dominant EL, as shown in Figure 6(c). Therefore, the characteristic EL observed in Figure 5 was attributed only to the highly fluorescent DCM dye molecules, which work as efficient energy acceptors from the Alq3 host matrix responsible for the subsequent radiative process. Light-emission images were simultaneously taken during the EL measurements using a CCD camera to identify the emitting region clearly. Figure 7 showed the EL images from the pentacene/Alq3:DCM OLEFET with asymmetric electrodes [source electrode of Au and drain electrodes of (a) Li:Al or (b) LiF/Al] at VDS = –90V and VGS < 0 V. The recombination of holes and electrons responsible for the observed EL occurred only in the vicinity of the drain electrodes. The locations of light emission can be understood based upon the spatial distributions of carrier charges. Upon application of the proper VDS < 0 V, the holes and electrons injected from the electrodes flow into the pentacene and Alq3 layers, respectively. However, charge transport is largely dominated by the holes in the pentacene layer, because the hole mobility of pentacene is several orders of magnitude larger than the electron mobility of Alq3. Therefore, the carrier recombination and energy transfer leading to the observed emission occurs mostly in the electron-transporting Alq3:DCM layer around the drain electrode, as shown in Figs. 6(c) and 7. 14


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Furthermore, the EL intensities increased with increasing VDS , which induced higher carrier densities.

CONCLUSIONS Heterojunction-based OLEFETs were fabricated and comparatively characterized using two layers of p-type pentacene and n-type Alq3 (undoped and DCM-doped) deposited on the SiO2 dielectrics by the NCBD method. The morphological, structural, and light-emission properties of the organic films were characterized using AFM, XRD, PL spectroscopy and LSCM. Various device parameters such as the carrier mobilities and current on/off ratios were derived from the observed device characteristic curves. The OLEFETs demonstrated good EL characteristics under ambient conditions. Unlike the OLEFETs with symmetric Au electrodes, the DCM-doped OLEFETs with asymmetric electrodes exhibited EL and the operating EL mechanisms were compared and discussed.

ACKNOWLEDGMENT This work was supported by a grant from the National Research Foundation (NRF) of Korea

funded

by

the

Ministry

of

Science,

ICT

and

Future

Planning

(NRF2014R1A2A2A01005719), the Basic Science Research Program through the NRF funded by the Ministry of Education (NRF20100020209).

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REFERENCES (1) Hepp, A.; Heil, H.; Weise, W.; Ahles, M.; Schmechel, R.; Seggern, H. V. Light-Emitting Field-Effect Transistor Based on a Tetracene Thin Film. Phys. Rev. Lett. 2003, 91, 157406. (2) Muccini, M. A Bright Future for Organic Field-Effect Transistors. Nat. Mater. 2006, 5, 605613. (3) Capelli, R.; Toffanin, S.; Generali, G.; Usta, H.; Facchetti, A.; Muccini, M. Organic LightEmitting Transistors with an Efficiency that Outperforms the Equivalent Light-Emitting Diodes. Nat. Mater. 2010, 9, 496-503. (4) Di, C.; Yu, G.; Liu, Y.; Xu, X.; Wei, D.; Song, Y.; Sun, Y.; Wang, Y.; Zhu, D. Organic LightEmitting Transistors Containing a Laterally Arranged Heterojunction. Adv. Funct. Mater. 2007, 17, 1567−1573. (5) Ooi, Z.-E.; Foong, T. R. B.; Singh, S. P.; Chan, K. L.; Dodabalapur, A. A Light Emitting Transistor Based on a Hybrid Metal Oxide-Organic Semiconductor Lateral Heterostructure. Appl. Phys. Lett. 2012, 100, 093302. (6) Muhieddine, K.; Ullah, M.; Pal, B. N.; Burn, P.; Namdas, E. B. All Solution-Processed, Hybrid Light Emitting Field-Effect Transistors. Adv. Mater. 2014, 26, 6410-6415. (7) Generali, G.; Capelli, R.; Toffanin, S.; Facchetti, A.; Muccini, M. Ambipolar Field-Effect Transistor Based on α,ω-Dihexylquaterthiophene and α,ω -Diperfluoroquaterthiophene Vertical Heterojunction. Adv. Funct. Mater. 2010, 50, 1861-1865. 16


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2009, 10, 1293–1299. (23) Dinelli, F.; Capelli, R.; A. Loi, M.; Murgia M.; Muccini, M.; Facchetti, A.; Marks, T. J. High-Mobility Ambipolar Transport in Organic Light-Emitting Transistors. Adv. Mater. 2006, 18, 1416-1420. (24) Cui, S.; Hu, Y.; Lou, Z.; Yi, R.; Hou, Y.; Teng, F. Light Emitting Field-Effect Transistors with Vertical Heterojunctions based on Pentacene and Tris-(8-hydroxyquinolinato) Aluminum. Org Electron. 2015, 22, 51–55. (25) Kim, J.-Y.; Kim, E.-S.; Choi, J.-H. Poly[2-(N-carbazolyl)-5-(2-ethylhexyloxy)-1,4phenylenevinylene/tris(8-hydroxyquinoline)

Aluminum

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Table.1 . Device parameters deduced from the characteristic curves of heterojunction-based OLEFETs.

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Figure Captions Figure 1. Molecular structures of (a) pentacene, (b) Alq3, and (c) DCM. (d) Energy level diagram for the Au electrode/pentacene (bottom)/Alq3:DCM (top)/Li:Al or LiF/Al electrode (units in eV). (e) Schematic views of the OLEFETs with a top-contact, long-channel geometry: symmetric (Au source and drain: left) and asymmetric (Au source and Li:Al or LiF/Al drain: right) electrode configurations. Figure 2.

Comparison of AFM micrographs (5 × 5 µm2) and X-ray diffractograms for (a)

pentacene, (b) Alq3:DCM, and (c) pentacene (bottom)/Alq3:DCM (top) thin-films deposited on the SiO2 substrates at room temperature. Figure 3.

(a) Normalized photoluminescence (PL) spectra of the undoped and DCM-doped

devices with an excitation wavelength of λex = 400 nm.

Confocal laser scanning microscopy

images (212 × 212 µm2) with an excitation wavelength of λex = 405 nm for (b) Alq3, (c) Alq3:DCM and (d) pentacene/Alq3:DCM films. Figure 4.

Typical (a) output and (b) transfer characteristics of bi-layered pentacene/Alq3:DCM

-based OLEFETs with a symmetric electrode configuration. (c) Typical transfer characteristics of Alq3-based OFETs with a symmetric electrode configuration. Figure 5.

Typical (a), (b) output and (c), (d) transfer characteristics of bi-layered

pentacene/Alq3:DCM-based OLEFETs with an asymmetric electrode configuration and the corresponding EL curves obtained under ambient conditions. (drain contacts: (a), (c) Li:Al, (b),

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(d) LiF/Al). (e) Typical transfer characteristics of single-layered Alq3-based OFETs with a LiF/Al drain electrode. Figure 6. Operating conduction mechanisms with the corresponding energy level diagrams under various bias conditions to explain the light emission: (a) VGS = VDS = 0 V, (b) VGS < 0 V and VDS = 0 V. and (c) VGS < 0 V and VDS < 0 V. Figure 7. EL images of the OLEFETs taken with a CCD camera mounted on an optical microscope. Drain contacts: (a) Li:Al, and (b) LiF/Al. The electrodes were colored to clearly identify the positions of EL emission. At VDS = –90V and VGS < 0 V, the recombination of holes and electrons responsible for the observed EL occurred only in the vicinity of the drain electrodes (see text).

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Figure 1 (a)

(b)

(c)

(d)

(e)

25



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Figure 2

(a)

(b)

(c)

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Figure 3 (a)

(b)

Alq3

(c)

(d) Alq3:DCM

Pentacene/Alq3:DCM

27



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Figure 4 (a)

(b)

(c)

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Figure 5 (a)

(b)

(c)

(d)

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(e)

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Figure 6 (a)

(b)

(c)

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Figure 7 (a)

(b)

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Graphiical Abstraact


Effects of Doping and Electrode Contacts on Performance of Organic

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