Organic Light-Emitting Field-Effect Transistors Based upon Pentacene

Feb 14, 2013 - Citation data is made available by participants in Crossref's Cited-by Linking service. For a more comprehensive list .... Dae-Kyu Kim ...
0 downloads 10 Views 3MB Size
Download PDF
Article pubs.acs.org/JPCC

Organic Light-Emitting Field-Effect Transistors Based upon Pentacene and Perylene Hoon-Seok Seo, Dae-Kyu Kim, Jeong-Do Oh, Eun-Sol Shin, and Jong-Ho Choi* Department of Chemistry, Research Institute for Natural Sciences Korea University, 1, Anam-dong, Seoul 136-701, Korea ABSTRACT: Air-stable, ambipolar heterojunction-based organic light-emitting field-effect transistors (OLEFETs) with a top-contact, multidigitated, long-channel geometry were produced, and the current−voltage−light emission (I−V−L) characteristics were systematically examined. Two active layers of p-type pentacene and n-type N,N′-ditridecylperylene3,4,9,10-tetra carboxylic diimide (P13) as well as a protecting layer of 2,5-bis(4-biphenyl) thiophene (BP1T) were successively deposited using the neutral cluster beam deposition method. On the basis of the growth of high-quality, well-packed crystalline thin films, the OLEFETs demonstrated good fieldeffect characteristics, well-balanced ambipolarity, operational stability, and electroluminescence (EL) under ambient conditions. The operating conduction and EL mechanisms responsible for the observed recombination zone are discussed with the aid of light-emission images obtained using a charge-coupled device.

■

INTRODUCTION Optoelectronic devices based on π-conjugated organic and polymeric compounds have attracted extensive attention due to their many potential advantages, which include low cost, ease of synthesis and fabrication processing, and mechanical flexibility, as well as providing new prospects for basic investigations. Promising applications are well-illustrated by organic lightemitting diodes (OLEDs) and organic field-effect transistors (OFETs).1−4 For example, the performance of select OFETs is competitive with that of hydrogenated amorphous silicon transistors, and they are being developed and included as switching devices in the fabrication of commercial flat-panel, active matrix displays.5 Because unipolar light-emitting transistors were produced for the first time using tetracene thin films, novel combinations of both electrical switching and light-emission properties in single organic devices have become a new class of functional optoelectronic devices known as organic light-emitting fieldeffect transistors (OLEFETs). The electrical switching operation is achieved by modulation of the current flow between the source and drain electrodes by tuning the gate voltage, whereas electroluminescence (EL) takes place by electron−hole recombination in the active channel.6−10 Such an OLEFET arrangement provides not only an opportunity to simplify the circuit design but also new potential applications of organic semiconductors, such as highly integrated optoelectronics, electrically pumped organic lasers, and so on.11,12 In addition, OLEFETs are considered to be excellent systems to investigate physical processes such as charge-carrier injection, transport, and EL in organic-based devices.13−15 High-performance OLEFETs can be constructed on the basis of both good balance in hole−electron concentrations and control of the exciton formation location within the active channel. However, in unipolar OLEFETs, carrier imbalance and © 2013 American Chemical Society

inevitable exciton formation occurring close to the drain electrode result in strong exciton quenching at the metal contact and inefficient emission. Such limitations might be overcome by utilizing single ambipolar materials16−20 or combining two unipolar materials through coevaporated21−23 or bilayer heterojunction-based structures.23−25 However, in most cases of single-component and blend-based OLEFETs, efficient light emission has not been observed due to the difficulties in achieving balanced injection and carrier transport. In contrast, whereas there is a physical separation between hole and electron transport layers, heterojunction-based OLEFETs have demonstrated well-balanced ambipolarity and improved EL. In this study, air-stable, ambipolar heterojunction-based OLEFETs are described, with a top-contact, multidigitated, long-channel geometry using p-type pentacene and n-type N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (P13). 2,5-Bis(4-biphenyl)thiophene (BP1T) was used as a protecting layer. Three organic layers were successively deposited using the neutral cluster beam deposition (NCBD) method. Molecular structures, the schematic diagram of the device, and the relative positions of the highest occupied and lowest unoccupied molecular orbitals (HOMO, LUMO) of the pentacene, P13, and BP1T are shown in Figure 1. The energy levels of pentacene and P13 are well-matched to form singlet excitons for efficient EL. Here the BP1T, with a wide band gap, is transparent to the light emission. We first focus on characterization of the morphological, structural, and photoluminescence properties of the organic active layers using atomic force microscopy (AFM), X-ray Received: December 1, 2012 Revised: February 12, 2013 Published: February 14, 2013 4764

dx.doi.org/10.1021/jp311816t | J. Phys. Chem. C 2013, 117, 4764−4770

The Journal of Physical Chemistry C

Article

decompose into individual molecules, which energetically migrate to form a smooth, uniform, thin film. Improvements of surface morphology, crystallinity, and room-temperature deposition have been observed, which cannot be achieved using traditional vapor deposition or solution-processing. In recent years, a series of prototypical OFET and OLEFET devices have been successfully produced utilizing the NCBD method.29−35 Each organic material was placed inside an enclosed evaporation crucible with a 1.0 mm diameter and 1.0 mm long nozzle and sublimated by resistive heating. Highly directional, weakly bound neutral cluster beams were generated at the throat of the nozzle and directly deposited onto the substrates. The optimized temperatures and deposition rates were 500−520 K and 0.5 to 1.0 Å/s for pentacene, 530−570 K and 1.0 to 1.5 Å/s for P13, and 550−570 K, and 0.7 to 1.0 Å/s for BP1T, respectively. Electron-beam evaporation with a properly shaped shadow mask was utilized to produce a 500 Å thick Au source and drain electrodes at a deposition rate of 6−8 Å/s. The electrode had a channel width (W) of 181 mm and channel length (L) of 150 μm. Characterization of the morphological and structural properties of active layers was carried out using AFM (PSI Co.) and XRD (Rigaku Co.). The current−voltage−light emission (I−V−L) characteristics of the OLEFETs were measured under ambient conditions using an optical probe station connected to an HP4140B and 818-UV Si photodiode with a 1830-C power meter (Newport Co.). Lightemission imaging was obtained using a CCD (Q Imaging Co.) camera mounted on an optical microscope.

Figure 1. Molecular structures of (a) pentacene, (b) P13, and (c) BP1T. (d) Energy level diagram for the pentacene/P13/BP1T/Au electrode device (units in eV). (e) Schematic view of the OLEFET with a top-contact, multidigitated, long channel-width geometry.

diffraction (XRD), and a charge-coupled device (CCD). 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 EL 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 EL 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.

■

RESULTS AND DISCUSSION Morphological and Structural Properties. Comparative characterization of the surface morphological and structural properties for various thin films deposited on SiO2 substrates at room temperature was carried out using AFM and XRD. Figure 2 shows the 2-D AFM micrographs obtained by conducting section analyses over 5 × 5 μm2 in a noncontact mode and the corresponding XRD diffractograms obtained using Cu Kα radiation in symmetric reflection, coupled θ−2θ mode. All films exhibited complete coverage with highly packed grain crystallites. In the cases of bilayer and trilayer films, the lower layer did not disturb the grain growth in the upper layer. The root-mean-square roughness (Rrms) values for the pentacene, P13, pentacene (bottom)/P13(top), and pentacene(bottom)/ P13(middle)/BP1T(top) films were measured to be 45.4 ± 2.1, 19.6 ± 2.8, 54.3 ± 14.5, and 49.1 ± 5.3 Å, respectively. The low Rrms values observed showed that the weakly bound cluster beams undergo efficient fragmentation into energetic individual molecules, leading to uniform, highly packed thin films without thermal-annealing processes. The XRD diffractograms in Figure 2 showed that all singlelayer and bilayer films showed ordered structure with strong reflection peaks, which was in good agreement with the previous crystallographic investigations: both pentacene and P13 single crystals are known to have a triclinic structure. The sharp first-order peaks as well as distinctive higher-order multiple peaks in Figure 2a,b can be fitted to a series of (00l) reflection lines with multiple d spacings based on the crystallographic parameters of pentacene and P13. For pentacene films, the four reflection peaks located at 2θ = 5.7, 11.4, 17.2, and 22.9° corresponded to the d spacings of 15.5, 7.8, 5.2, and 3.9 Å, respectively. The three peaks of P13 films located at 2θ = 3.3, 6.6, and 9.9° were assigned to d spacings of 26.8, 13.4, and 8.9 Å, respectively. In the case of the bilayer

■

EXPERIMENTAL SECTION Two types of OLEFETs, with and without BP1T as a protecting layer, were produced. A schematic view and image of the OLEFETs with a top-contact, multidigitated, longchannel-width geometry are shown in Figures 1 and 5, respectively. The substrates were made up of a highly doped, n-type Si wafer coated with a 1000 Å thick Al layer as the gate electrode and thermally grown 2000 Å thick SiO2 as the gate dielectric. The substrates were rigorously cleaned to improve the device performance using a series of sequential ultrasonic treatments in acetone, 20% HNO3, hot trichloroethylene, methanol, and deionized water, and then blown dry with N2 gas. Finally, the substrates were exposed to 254 nm UV for 15 min.26,27 The active layers of pentacene (40 Å), P13 (200 Å), and BP1T (50 Å) were sequentially deposited on top of the substrates using the NCBD method. The custom-built deposition apparatus has been described in detail elsewhere, and only a relevant account is presented here.28 Neutral cluster beams of weakly bound organic molecules were produced when sample molecules were evaporated by resistive heating, undergoing adiabatic expansion in high vacuum. Cluster beams have high translational kinetic energy and directionality, allowing precise deposition of clusters through collision with room-temperature substrates. Upon collision, the clusters 4765

dx.doi.org/10.1021/jp311816t | J. Phys. Chem. C 2013, 117, 4764−4770

The Journal of Physical Chemistry C

Article

Figure 2. Comparison of 2-D AFM micrographs (5 × 5 μm2) and X-ray diffractograms for (a) pentacene, (b) P13, (c) pentacene(bottom)/P13 (top), and (d) pentacene (bottom)/P13 (middle)/BP1T (top) thin films deposited on the SiO2 substrates at room temperature.

films in Figure 2c, the diffractogram displayed a combined feature composed of the distinct peaks from both the bottom pentacene and upper P13 films, implying that the pentacene layer does not disturb the deposition of the P13 layer at the interface in the early stage of additional crystalline growth. However, the feature was slightly less obvious in the trilayer films due to the amorphous BP1T top layer in Figure 2d. Output and Transfer Characteristics of OLEFETs. The output and transfer characteristics of the bilayer- and trilayerbased OLEFETs were obtained under ambient conditions.

Figure 3 displays typical characteristics of trilayer pentacene/ P13/BP1T-based transistors with the corresponding EL curves. Bilayer devices also showed similar output and transfer characteristics with EL curves (not shown in Figure 3). The plot in Figure 3a exhibits the characteristic dependence of IDS on VDS and VGS expected for ambipolar devices, where IDS is the drain-source current, VDS is the drain-source voltage, and VGS is the gate-source voltage. The crossover phenomena from holeto electron-dominated currents and vice versa were clearly observed in both forward and reverse drain modes. 4766

dx.doi.org/10.1021/jp311816t | J. Phys. Chem. C 2013, 117, 4764−4770

The Journal of Physical Chemistry C

Article

around VGS ≅ 30 V. In the region of VGS ≥ 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 in the high VDS regime, which is behavior typical of n-type transistors working in the accumulation mode. The average field-effect hole and electron mobilities (μeffh,avg, μeffe,avg) can be directly deduced from the transfer curves of the OLEFETs in Figure 3b. Each transfer scan was run at a constant VDS in the saturation regime. IDS satisfies the following relationship in this regime: IDS =

WCiμeff 2L

(VGS − VT)2

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 transfer characteristics for more than 10 OLEFETs at a channel width (W) of 181 mm and a channel length (L) of 150 μm, the values for μeffh,avg and μeffe,avg were derived with the standard deviation (σ) and are listed in Table 1, together with other device parameters. The measurements presented were carried out under ambient conditions, unlike most of the previous investigations of organic devices, which were conducted either in an inert atmosphere or in vacuum.36,37 The μeff values of the bilayer-based OLEFETs in Table 1 were similar to those of trilayer-based OLEFETs. Also, the values were comparable to or somewhat less than those obtained from the NCBD-based, single-layer transistors, which were among the best to date for polycrystalline pentacene- and P13-based transistors using SiO2 dielectric layers. In a sense, such reduction, particularly in the μeffh values of the bottom pentacene layer, appears to be inevitable in the multilayered, heterojunction-based OLEFETs. In the present OLEFET configuration, when the gate electrode is negatively biased, the upper P13 layer acts as a blocking layer with a high energy barrier for hole injection (Figure 1). The resultant unfavorable current flow leads to the inefficient accumulation of hole carriers at the bottom pentacene layer, and might explain in part why the extent of reduction in the μeffh values was quite pronounced compared to the single-layer transistors. For the μeffe values, as negative gate voltages are applied, the bottom pentacene layer acts as an insulator and increases the effective thickness of the capacitor. Therefore, under the same bias condition, the density of the electrons accumulated at the upper P13 layer is expected to be somewhat lower. A similar decrease was also reported for P13/DH6T-based OLEFETs.31 However, it should be noted that in the present OLEFET configuration, the close hole and electron mobilities derived indicate that wellbalanced ambipolar transport was achieved. Such conduction was considered one of the critical prerequisites for producing high-performance EL devices.22 Although the extent of carrier transport was well-balanced, in the present configuration of the heterojunction-based devices, the air-sensitive P13 layer was deposited on top of the

Figure 3. Typical (a) output and (b) transfer characteristics of trilayer pentacene/P13/BP1T-based OLEFETs and the corresponding EL curves obtained under ambient conditions. In panel a, the curves in the first quadrant were measured at positive VGS, and the curves in the third quadrant were measured at negative VGS, respectively.

As can be seen in Figure 3a, in the region of VDS = 0 ∼ −10 V, the IDS induced by electron injection from the drain electrode appeared to contribute substantially. Around VGS ≅ −20 V, the crossover phenomenon from electron- to holedominated currents occurred. In the region of VGS ≤ −20 V, the IDS induced by hole injection from the grounded source electrode contributed substantially and indicated a typical ptype transistor working in accumulation mode: at a fixed −VGS, IDS initially decreased linearly with decreasing VDS, with IDS tending to saturate due to a pinch off in the accumulation layer. In contrast, in the reverse drain mode, a totally inverse phenomenon occurred. In the region of VGS = 0−20 V, the IDS induced by the hole injection from the drain electrode increased quadratically with increasing VDS. There was a crossover point from hole- to electron-dominated currents

Table 1. Device Parameters Deduced from the Characteristics of Bilayer- And Trilayer-Based OLEFETs classification (thickness) pentacene/P13 (40 Å/200 Å) pentacene/P13/BP1T (40 Å/200 Å/50 Å)

μeffh,avg ± (cm2/(V s)) −2

−2

5.7 × 10 ± 3.5 × 10 6.9 × 10−2 ± 4.6 × 10−2 4767

VTp (V) −17.7 0.0

μeffe,avg ± (cm2/(V s)) −2

−2

7.0 × 10 ± 2.3 × 10 4.3 × 10−2 ± 2.4 × 10−2

VTn (V)

VDS (V)

31.5 35.0

±60 ±60

dx.doi.org/10.1021/jp311816t | J. Phys. Chem. C 2013, 117, 4764−4770

The Journal of Physical Chemistry C

Article

pentacene layer. In general, most n-type organic-based devices, including P13 devices, are known to be sensitive to environmental contaminants such as moisture and oxygen, which can penetrate the channel region.38 As a result, μeffe tends to deteriorate with time and does not show reproducible characteristics or operational stability. The μeffe values were monitored as a function of time. Figure 4 shows that the μeffe of

Figure 5. (a) Electrode image of the trilayer-based OLEFET device with multidigitated, long channel-width geometry taken with a CCD camera mounted on an optical microscope. The red and blue lines indicate the source and drain electrodes, respectively. Dependence of the CCD images of OLEFETs on various VDS at fixed VGS = 0 and −60 V. (b) VGS = 0 V, VDS = 60 V. (c) VGS = 0 V, VDS = 50 V. (d) VGS = 0 V, VDS = 40 V. (e) VGS = −60 V, VDS = −60 V. (f) VGS = −60 V, VDS = −50 V. (g) VGS = −60 V, VDS = −40 V. (h) VGS = −60 V, VDS = −30 V. The upper and bottom sections of the electrode in panels b−h were colored to clearly identify positions of EL emission. In panels b and c, the EL emission occurred around the drain electrodes (blue), and in panels e−h the EL emission occurred around the source electrodes (red).

Figure 4. Comparison of the electron mobilities (μeffe) of bilayer- and trilayer-based OLEFETs monitored as a function of time for 30 days. The BP1T layer deposited atop as a protective passivation layer presented a more reliable scheme in producing air-stable ambipolar OLEFETs.

the bilayer devices decreased by two orders for 30 days. To prevent the direct exposure of P13 to air, the BP1T passivation layer was superimposed on top of the P13 layer in producing the OLEFET devices. As expected, the top BP1T layer appeared to act as a protecting layer in the case of trilayer OLEFETs. The extent of degradation of μeffe was significantly reduced by more than one order compared with that of bilayer devices without any encapsulation process in ambient air. Thus, in this study, the air-stable trilayer-based geometry was adopted to examine the EL characteristics and image the light emission. Electroluminescence Mechanism. The EL characteristics were observed during the I−V measurements. The gate dependence of the EL phenomena is shown in Figure 3b with the corresponding transfer curves. In general, for singlelayer OLEFETs operating in ambipolar mode, there is a prerequisite condition for the light emission that the VGS value must have a value between VDS and VS (= 0 V). The maximum emission occurs at VGS = 0.5 VDS, where an equal voltage drop for hole and electron carriers exists. In contrast, in multilayer heterojunction-based OLEFETs, as shown in Figure 3b, strong light emission was observed in the regions of near VGS = 0 V and VGS ≤ −50 V, and the intensity increased with increasing | VDS| at fixed VGS. Therefore, the conventional model does not explain the observed light emission and carrier conduction adequately. During the EL measurements, a series of light-emission images were simultaneously captured using a CCD camera mounted on an optical microscope to identify the emitting electrodes clearly, and the dependence of EL intensity (IEL) on VDS voltages was analyzed as well. Figure 5 displays the dependence of CCD images on various VDS recorded at fixed VGS = 0 and −60 V. The locations for emitting electrodes and the dependence of EL intensities on VDS can be rationalized on the basis of the spatial distributions of carrier charges under proper gate biases and the energy levels. When the drain electrode is positively biased (VDS > 0 V) at VGS = 0 V, the

electric field between the drain and gate electrodes induces the formation of a positively charged accumulation layer due to the hole carriers with a high mobility in the pentacene around the drain electrode (Figure 6a). At the same time, upon application of the proper VDS ≥ 50 V, electrons injected from the source electrode flow into the P13 layer. Subsequent carrier recombination responsible for the observed EL occurs around the drain electrode, as in Figure 5b,c. In contrast, when the

Figure 6. Operating conduction and recombination mechanisms with the positions of EL emission identified. The EL emission occurred (a) around the drain electrode at VGS = 0 V, VDS = 60 V as in Figure 5b and (b) around the source electrode at VGS = −60 V, VDS = −60 V as in Figure 5e, respectively. 4768

dx.doi.org/10.1021/jp311816t | J. Phys. Chem. C 2013, 117, 4764−4770

The Journal of Physical Chemistry C drain and gate electrodes are negatively biased, the positively charged accumulation layer in the pentacene is formed around the source electrode (Figure 6b). Under VDS ≤ −30 V, electrons injected from the drain electrode flow into the P13 layer, and the EL emission occurs around the source electrode, as in Figure 5e−h. The EL intensities in Figures 3 and 5 increased with increasing |VDS|, which induces higher carrier densities. In principle, the EL intensity can be given by the relation IEL ∝ pn(μeffh + μeffe), expressed in terms of the mobility sum and the pn product (p and n: hole and electron densities). When balanced carrier conduction was achieved, as observed, the injected carrier densities increased with increasing |VDS|, leading to substantial light emission. Our observation of the EL processes herein stands in contrast with the investigation of the α,ωdihexylquarterthiophene(DH4T)/P13-based ambipolar OLEFETs reported by Dinelli et al. and previous work on α,ωdihexylsexithiophene(DH6T)/P13-based devices.24,31 In both cases, the VGS dependence of the emission intensity was similar to that in this study. However, in the former case, the EL was reported to occur under vacuum conditions only when the DH4T layer was placed at the bottom in direct contact with the dielectric. In the latter case, the light emission was observed in the region of the proper negative VGS, and the charge transport and light emission were consistent with the recent simulation results conducted by Kwok et al.39 It is clear that all EL phenomena occurring in the heterojunction-based OLEFETs appear to depend significantly on the organic materials, deposition sequence, and operation conditions employed. Further combined theoretical and experimental studies are required to deduce the transport and EL mechanisms and structure−performance relationships at the molecular level. Several OLEFETs using various π-conjugated organic molecules through the NCBD method are under way.

ACKNOWLEDGMENTS

■

REFERENCES

This work was supported by a grant from the National Research Foundation of Korea, funded by the Korean Government (2010-0014418), LG Yonam Foundation, and the Priority Research Centers Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science, and Technology (NRF20100020209).

(1) Garnier, F.; Horowitz, G.; Peng, X.; Fichou, D. An all-organic “soft” thin film transistor with very high carrier mobility. Adv. Mater. 1990, 2, 592−594. (2) Dodabalapur, A.; Torsi, L.; Katz, H. E. Organic Transistors: TwoDimensional Transport and Improved Electrical Characteristics. Science 1995, 268, 270−271. (3) Tang, C. W.; VanSlyke, S. A. Organic Electroluminescent Diodes. Appl. Phys. Lett. 1987, 51, 913−915. (4) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Photoinduced Electron Transfer from a Conducting Polymer to Buckminsterfullerene. Science 1992, 258, 1474−1476. (5) He, Y.; Hattori, R.; Kanicki, J. Four-Thin Film Transistor Pixel Electrode Circuits for Active-Matrix Organic Light-Emitting Displays. Jpn. J. Appl. Phys. 2001, 40, 1199−1208. (6) 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. (7) Santato, C.; Capelli, R.; Loi, M. A.; Murgia, M.; Cicoira, F.; Roy, V. A. L.; Stallinga, P.; Zamboni, R.; Rost, C.; Karg, S. F.; Muccini, M. Tetracene-Based Organic Light-Emitting Transistors: Optoelectronic Properties and Electron Injection Mechanism. Synth. Met. 2004, 146, 329−334. (8) Santato, C.; Manunza, I.; Bonfiglio, A.; Cicoira, F.; Cosseddu, P.; Zamboni, R.; Muccini, M. Tetracene Light-Emitting Transistors on Flexible Plastic Substrates. Appl. Phys. Lett. 2005, 86, 141106. (9) Cicoira, F.; Santato, C.; Dinelli, F.; Murgia, M.; Loi, M. A.; Biscarini, F.; Zamboni, R.; Heremans, P.; Muccini, M. Morphology and Field-Effect-Transistor Mobility in Tetracene Thin Films. Adv. Func. Mater 2005, 15, 375−380. (10) Santato, C.; Cicoira, F.; Cosseddu, P.; Bonfiglio, A.; Bellutti, P.; Muccini, M.; Zamboni, R.; Rosei, F.; Mantoux, A.; Doppelt, P. Organic Light-Emitting Transistors Using Concentric Source/Drain Electrodes on a Molecular Adhesion Layer. Appl. Phys. Lett. 2006, 88, 163511. (11) Takenobu, T.; Bisri, S. Z.; Takahashi, T.; Yahiro, M.; Adachi, C.; Iwasa, Y. High Current Density in Light-Emitting Transistors of Organic Single Crystals. Phys. Rev. Lett. 2008, 100, 066601. (12) Cicoira, F.; Santato, C.; Dadvand, A.; Harnagea, C.; Pignolet, A.; Bellutti, P.; Xiang, Z.; Rosei, F.; Meng, H.; Perepichka, F. Environmentally Stable Light Emitting Field Effect Transistors Based on 2-(4-Pentylstyryl)tetracene. J. Mater. Chem. 2008, 18, 158−161. (13) Cornil, J.; Brédas, J. L.; Zaumseil, J.; Sirringhaus, H. Ambipolar Transport in Organic Conjugated Materials. Adv. Mater. 2007, 19, 1791−1799. (14) Cicoira, F.; Santato, C. Organic Light Emitting Field Effect Transistors: Advances and Perspectives. Adv. Funct. Mater. 2007, 17, 3421−3434. (15) Muccini, M.; Koopman, W.; Toffanin, S. The Photonic Perspective of Organic Light-Emitting Transistors. Laser Photonics Rev. 2012, 6, 258−275. (16) Rost, C.; Karg, S.; Riess, W.; Loi, M. A.; Murgia, M.; Muccini, M. Light-Emitting Ambipolar Organic Heterostructure Field-Effect Transistor. Synth. Met. 2004, 146, 237−241. (17) Swensen, J. S.; Soci, C.; Heeger, A. J. Light Emission from an Ambipolar Semiconducting Polymer Field-Effect Transistor. Appl. Phys. Lett. 2005, 87, 253511. (18) Zaumseil, J.; Friend, R. H.; Sirringhaus, H. Spatial Control of the Recombination Zone in an Ambipolar Light-Emitting Organic Transistor. Nat. Mater. 2006, 5, 69−74.

■

CONCLUSIONS The fabrication and systematic characterization of air-stable, ambipolar heterojunction-based OLEFETs with a top-contact, multidigitated, long-channel geometry were presented. The successive deposition of two active layers of hole-transporting pentacene and electron-transporting P13, as well as a BP1T protecting layer, was carried out using the NCBD method. From the observed I−V−L characteristics, the OLEFETs demonstrated well-balanced ambipolarity, operational stability, and EL under ambient conditions. The conduction mechanism and the dependence of EL intensities on bias voltages were discussed with the aid of light-emission images obtained using a CCD. Fabrication and characterization of several OFETs and OLEFETs using various π-conjugated molecules and oligomers through the NCBD method are underway. It is the hope of the authors that this investigation provides some insights into the operating mechanisms and the structure−performance relationships at the molecular level and that the heterojunction structure will be applied to the production of high-performance, organic devices that avoid direct exposure of their air-sensitive transistors to ambient conditions.

■

■

Article

AUTHOR INFORMATION

Corresponding Author

*Phone: +82-2-3290-3135. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 4769

dx.doi.org/10.1021/jp311816t | J. Phys. Chem. C 2013, 117, 4764−4770

The Journal of Physical Chemistry C

Article

Transistor Device with Heterostructure of C60 and Pentacene. Appl. Phys. Lett. 2004, 85, 4765−4767. (37) Nagano, T.; Kuwahara, E.; Takayanagi, T.; Kubozono, Y.; Fujiwara, A. Fabrication and Characterization of Field-Effect Transistor Device with C2v Isomer of Pr@C82. Chem. Phys. Lett. 2005, 409, 187−191. (38) Bao, Z. Materials and Fabrication Needs for Low-Cost Organic Transistor Circuits. Adv. Mater. 2000, 12, 227−230. (39) Li, W.; Kwok, H. L. Charge Transport and Light Emission in Bilayer Organic Field-Effect Transistors. Thin Solid Films 2012, 520, 3600−3604.

(19) Zaumseil, J.; Donley, C. L.; Kim, J.-S.; Friend, R. H.; Sirringhaus, H. Efficient Top-Gate, Ambipolar, Light-Emitting Field-Effect Transistors Based on a Green-Light-Emitting Polyfluorene. Adv. Mater. 2006, 18, 2708−2712. (20) Capelli, R.; Dinelli, F.; Toffanin, S.; Todescato, F.; Murgia, M.; Muccini, M.; Facchetti, A.; Marks, T. J. Investigation of the Optoelectronic Properties of Organic Light-Emitting Transistors Based on an Intrinsically Ambipolar Material. J. Phys. Chem. C 2008, 112, 12993−12999. (21) Rost, C.; Karg, S.; Riess, W.; Loi, M. A.; Murgia, M.; Muccini, M. Ambipolar Light-Emitting Organic Field-Effect Transistor. Appl. Phys. Lett. 2004, 85, 1613−1615. (22) Loi, M. A.; Rost-Bietsch, C.; Murgia, M.; Karg, S.; Riess, W.; Muccini, M. Tuning Optoelectronic Properties of Ambipolar Organic Light- Emitting Transistors Using a Bulk-Heterojunction Approach. Adv. Funct. Mater 2006, 16, 41−47. (23) Capelli, R.; Dinelli, F.; Loi, M. A.; Murgia, M.; Zamboni, R.; Muccini, M. Ambipolar Organic Light-Emitting Transistors Employing Heterojunctions of n-Type and p-Type Materials As the Active Layer. J. Phys.: Condens. Matter 2006, 18, S2127−S2138. (24) Dinelli, F.; Capelli, R.; Loi, M. A.; Muccini, M.; Facchetti, A.; Marks, T. J. High-Mobility Ambipolar Transport in Organic LightEmitting Transistors. Adv. Mater. 2006, 18, 1416−1420. (25) Di, C.; Yu, G.; Liu, Y.; Xu, X.; Wei, D.; Song, Y.; Sun, Y.; Wang, Y.; Zhu, D. Organic Light-Emitting Transistors Containing a Laterally Arranged Heterojunction. Adv. Funct. Mater 2007, 17, 1567−1573. (26) Guo, D.; Entani, S.; Ikeda, S.; Saiki, K. Effect of UV/Ozone Treatment of the Dielectric Layer on the Device Performance of Pentacene Thin Film Transistors. Chem. Phys. Lett. 2006, 429, 124− 128. (27) Kang, S. J.; Noh, M.; Park, D. S.; Kim, H. J.; Whang, C. N.; Chang, C. H. Influence of Postannealing on Polycrystalline Pentacene Thin Film Transistor. J. Appl. Phys. 2004, 95, 2293−2296. (28) Kim, J.-Y.; Kim, E.-S.; Choi, J.-H. Poly[2-(N-carbazolyl)-5-(2ethylhexyloxy)-1,4 phenylenevinylene]/tris (8-hydroxyquinoline) Aluminum Heterojunction Electroluminescent Devices Produced by Cluster Beam Deposition Methods. J. Appl. Phys. 2002, 91, 1944− 1951. (29) Seo, H.-S.; Jang, Y.-S.; Zhang, Y.; Abthagir, P. S.; Choi, J.-H. Fabrication and Characterization of Pentacene-Based Transistors with a Room-Temperature Mobility of 1.25 cm2/Vs. Org. Electron. 2008, 9, 432−438. (30) Seo, H.-S.; Zhang, Y.; Jang, Y.-S.; Choi, J.-H. Performance and Transport Characteristics of α,ω-Dihexylsexithiophene-Based Transistors with a High Room-Temperature Mobility of 0.16 cm2/Vs. Appl. Phys. Lett. 2008, 92, 223310. (31) Seo, H.-S.; Zhang, Y.; An, M.-J.; Choi, J.-H. Fabrication and Characterization of Air-Stable, Ambipolar Heterojunction-Based Organic Light-Emitting Field-Effect Transistors. Org. Electron. 2009, 10, 1293−1299. (32) Seo, H.-S.; An, M.-J.; Zhang, Y.; Choi, J.-H. Characterization of Perylene and Tetracene-Based Ambipolar Light-Emitting Field-Effect Transistors. J. Phys. Chem. C 2010, 114, 6141−6147. (33) An, M.-J.; Seo, H.-S.; Zhang, Y.; Oh, J.-D.; Choi, J.-H. Air Stable, Ambipolar Organic Transistors and Inverters Based upon a Heterojunction Structure of Pentacene on N,N′-Ditridecylperylene3,4,9,10-tetracarboxylic Di-imide. Appl. Phys. Lett. 2010, 97, 023506. (34) Zhang, Y.; Seo, H.-S.; An, M.-J.; Oh, J.-D.; Choi, J.-H. Influence of Gate Dielectrics on the Performance of Single-Layered Organic Transistors and Bi-Layered Organic Light-Emitting Transistors Prepared by the Neutral Cluster Beam Deposition Method. J. Appl. Phys. 2011, 109, 084503. (35) An, M.-J.; Seo, H.-S.; Zhang, Y.; Oh, J.-D.; Choi, J.-H. Air-Stable, Hysteresis-Free Organic Complementary Inverters Produced by the Neutral Cluster Beam Deposition Method. J. Phys. Chem. C 2011, 115, 11763−11767. (36) Kuwahara, E.; Kubozono, Y.; Hosokawa, T.; Nagano, T.; Masunari, K.; Fujiwara, A. Fabrication of Ambipolar Field-Effect 4770

dx.doi.org/10.1021/jp311816t | J. Phys. Chem. C 2013, 117, 4764−4770