† Corresponding author. E-mail:
Project supported by the National Natural Science Foundation of China (Grant No. 31872901) and the National Key Research and Development Program of China (Grant No. 2016YFA0501602).
The luminescence intensity regulation of organic light-emitting transistor (OLED) device can be achieved effectively by the combination of graphene vertical field effect transistor (GVFET) and OLED. In this paper, we fabricate and characterize the graphene vertical field-effect transistor with gate dielectric of ion–gel film, confirming that its current switching ratio reaches up to 102. Because of the property of high light transmittance in ion–gel film, the OLED device prepared with graphene/PEDOT:PSS as composite anode exhibits good optical properties. We also prepare the graphene vertical organic light-emitting field effect transistor (GVOLEFET) by the combination of GVFET and graphene OLED, analyzing its electrical and optical properties, and confirming that the luminescence intensity can be significantly changed by regulating the gate voltage.
Recently, organic light-emitting transistor (OLED), as a kind of light-emitting diode, has great development potential and broad application prospects because of its wide viewing angle, high color saturation and contrast, short response time, large light-emitting area, and low energy consumption.[1–3] Owing to the advantages of electrical conductivity, light transmission, stability, and mechanical properties,[4–7] graphene has been successfully used in OLED device preparation.[8–10] Vertical field effect transistor (VFET) is a kind of field effect transistor, whose channel direction is perpendicular to the surface of substrate, forming heterojunction structure between material layers.[11] Vertical electric field channel length can be shortened down to nanometer level that facilitates carriers’ recombination.[12–14] Carrier accumulation in graphene and semiconductor layers can be regulated by the gate electrode due to the adjustability of graphene work function, so that Fermi levels can be rearranged to change the interface resistance between graphene and semiconductor, which significantly influences the switching ratio of VFET. In addition, by combining VFET structure with OLED device, a new method of regulating the luminescence intensity can be achieved by changing gate voltage.[15]
In this paper we use ion–gel to prepare an ITO/ion gel film/graphene OLED composite structure. The ion–gel is a mixture of organic polymer and electrolyzable salt electrolyte material with good thermal stability and high dielectric value[16,17] and widely used in flexible devices.[16,18] Under the action of an external electric field, anion and cation in polymer will migrate and diffuse to form electric-double-layer distribution with charge layer on the surface of insulating layer, which is similar to a capacitive effect.[19] In previous study,[20] we used an ion–gel film as a dielectric layer which significantly improved the performance of graphene field effect transistor (GFET). In the present experiment we achieve the effectively regulation of luminescence intensity of OLED devices due to GVFET.
Single-layered graphene was synthesized on copper foil with CVD process; 6-wt % PMMA was dissolved in anisole; ion–gel solution was prepared by dissolving 10-wt % PVDF and 2.5-wt % [EMIM]TF2N in N,N-Dimethylformamide (DMF) at 60 °C by water bath for 7 h until a clear solution was obtained; poly(3,4-ethylenedioxythiophene)-poly (styrenesulfonate) (PEDOT:PSS) aqueous solution was purchased from J & K. Corp; organic luminescent material:1,1-Bis[(di-4-tolylamino)phenyl] cyclohexane (TAPC), 1,3-bis-(carbazol-9-yl)benzene (mCP), Bis(4,6-difluorophenylpyridine-N,C2’) pyridinecarboxamide (FIrpic) and 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB) were purchased from BanHe Tech. Corp.
Ion–gel film was used as gate dielectric in VFET. The prepared ion–gel solution was spin-coated (3000 rpm, time 15 s) on the surface of ITO glass, ion–gel was transparent after being dried at temperature 160 °C for 30 min to form a 2-μm-thick film tested by step meter, graphene was characterized after being trasferred to the surface of ion–gel film by Raman spectrometer and light transmittance tester. The 5-wt % dimethylformamide was added into PEDOT:PSS solution to make PEDOT:PSS film smoother on the surface of graphene, then the solution was spin-coated (1500 rpm, time 30 s) and dried at temperature 120 °C for 30 min. A 100-nm-thick aluminum layer was deposited on PEDOT:PSS film, serving as a top electrode and the device structure was prepared to be ITO/ion–gel/graphene/PEDOT:PSS/Al (100 nm).
The OLED structure prepared in this study was graphene/PEDOT:PSS/TAPC (50 nm)/mCP (10 % FIrpic) (30 nm)/TmPyPB (50 nm)/LiF (0.5 nm)/Al (100 nm) (Fig.
After preparation, the voltage–luminance–current density characteristic curve of GOLED was detected at room temperature by using an electroluminescence test system consisting of Keithley Model 2400 current source and PR 655 spectrometer. The electrical characteristics of GVFET were measured by using Keithley 2612a dual channel source meter.
Transmittance of dielectric layer material has an effect on the luminescence intensity of graphene OLED, showing that the transmittance of visible light (wavelength 390 nm–760 nm) in grapheme/ion–gel/ITO/glass composite stracture reaches up to 90 % (Fig.
In addition, we also measure the specific capacitance of 2-μm-thick ion–gel film sandwiched between gold electrode and ITO as a function of voltage frequency (c = –1/2π fZ, where Z denotes the impedance and f the frequency), and the results are shown in Fig.
PEDOT:PSS is often used as OLED hole injection material because of its high work function and high transmittance.[24] In this experiment, the PEDOT:PSS is spin-coated on the surface of graphene to form a heterojunction structure.[8] In addition, the ion–gel gate structure is introduced to form G-VFET, and the energy band of graphene can be opened by gate regulation, so that the Fermi levels of graphene and barrier height between graphene and PEDOT: PSS are changed, and thus affecting the interface resistance and current output. In the transfer characteristic curve, the source–drain voltage (Vsd) is constant (at 5 V), and the gate voltage varies from 0 V to 10 V. According to the circuit superposition principle,[25] equivalent value of gate voltage (Veg) is the difference value between gate bias and source–drain voltage, i.e., (Vg–Vsd), the actual gate voltage variation ranges from –5 V to 5 V. In the output characteristic curve, source–drain voltage varies from 0 V to 2 V, gate voltage variation is in a range from 1 V to 9 V (step 2 V). Switching ratio (Jon/Joff) can be obtained from transfer characteristic curve to be about 102 (Fig.
Graphene needs to be hydrophilized by UV treatment for 30 min after being transferred to make PEDOT:PSS solution dispersed on the grapheme surface smoothly. In addition, UV treatment can improve graphene work function[26] and PEDOT:PSS film can reduce surface roughness of graphene, which are beneficial to the improvement of the luminescent properties of OLED device.[8,27,28] In order to confirm the luminescence properties of OLED device with graphene/PEDOT:PSS as the composite anode, we prepare an OLED structure as shown in Fig.
According to the GVFET structure, we add OLED structure between graphene and aluminum layer, and design the structure of GVOLEFET.[15,29] When the source–drain voltage is 10 V, the OLED luminous intensity decreases as the gate voltage increases (from 0 V to 25 V). The inset in Fig.
In addition, we also test the repeatability of device. We can see that the short-term (within 10 min) repeatability is better (Fig.
In this research, we fabricated and characterized the graphene/PEDOT:PSS vertical field effect transistor with ion–gel film gate dielectric, which shows current switching ratio up to 102. Heterojunction structure formed with graphene and PEDOT:PSS is affected by the gate voltage to regulate its interface resistance and current output. The graphene OLED prepared by using the graphene/PEDOT:PSS serving as a composite anode exhibits good luminescence performance. The GVOLVEFT is prepared by combining GVFET and graphene OLED, and the luminescence intensity is significantly changed by regulating the gate voltage. In recent years, graphene have been widely used as transparent electrodes in OLED research and development. Combined with the high light transmittance of gate dielectric materials, GVFET will be used as an important device platform for developing various photoelectric sensors.
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