Summary of the electronic properties of the FeCl3-doped SLG used.
The inherent complexity of the structures of active-matrix (AM) organic light-emitting diode (OLED) displays severely limits not only their size but also device performance. Surface-emitting organic light-emitting transistors (OLETs) may offer an attractive alternative to AM displays. We report some characteristics of vertical-type OLETs (VOLETs) composed of a source electrode of low-dimensional materials and an emissive channel layer. With a functionalized graphene source, it is shown that the full-surface electroluminescent emission of a VOLET can be effectively controlled by the gate voltage with a high luminance on/off ratio (104). The current efficiency and effective aperture ratios were observed to be more than 150% of those of a control OLED, even at high luminances exceeding 500 cd m−2. Moreover, high device performance of micro-VOLET pixels has been also successfully demonstrated using inkjet-patterned emissive channel layers. These significant improvements in the device performance were attributed to the effective gate-voltage-induced modulation of the hole tunneling injection at the source electrode.
- organic light-emitting diode (OLED)
- organic light-emitting transistor (OLET)
- vertical-type OLETs
- on/off ratio
- aperture ratio
- inkjet printing
- tunneling injection
In recent years, researchers of state-of-the-art electronics utilizing organic semiconducting materials have succeeded in advancing various devices, such as organic light-emitting diodes (OLEDs), photovoltaic cells, organic thin-film transistors (OTFTs), and sensors, among others [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]. Among these, intensive efforts in OLEDs have led to high brightness, efficiency, and full-color electroluminescent (EL) emissions for various light-emitting optoelectronic devices [7, 8, 9, 10]. The advantages of such OLEDs over conventional liquid crystal displays (LCDs) are well known, especially for high-quality displays in terms of their viewing angle, response time, thickness, and contrast ratio . For instance, small OLED displays are constructed on an array of thin-film transistor (TFT) switches, allowing precise control of the states of the pixels [12, 13, 14]. In such active-matrix OLEDs (AM-OLEDs), the OLED is driven in the current mode; thus, at least two TFTs, in this case a switching TFT to select a pixel and a driving TFT to operate the OLED, are required, as shown in Figure 1(a) [12, 13]. Perhaps unexpectedly, however, the complexity of such pixel circuit designs with their sophisticated procedures has led to a significantly limited light-emitting area and aperture ratio (the light-emitting area as a fraction of the total area of the device, typical aperture ratios: 25–34%) [13, 14, 15], introducing severe problems associated with limited device performance and limited display sizes for AM-OLEDs. Besides these issues, fundamental factors related to the architecture of the OLED itself, such as exciton quenching and photon loss, also still limit the efficiency and brightness of these devices.
To overcome some of the limitations of (AM-)OLEDs, research on different structures and materials is currently yielding new developments [15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30]. Among these, organic light-emitting transistors (OLETs), such as static-induction-transistor OLETs (SIT-OLETs) [17, 18], metal-insulator-semiconductor OLETs (MIS-OLETs) , lateral-type OLETs [20, 21, 22, 23, 24, 25, 26, 27, 28, 29], and vertical-type OLETs (VOLETs) , have been devised by integrating the capability of the OLED to generate EL light with the switching functionality of a field-effect transistor (FET) into a single device structure. In these OLETs, the current that flows through emissive semiconductor channel layers can be controlled by the gate voltage, which can also change the EL emission brightness state from the dark off- to the bright on-state. The on-state implies that holes and electrons injected into the channel layer form excitons that recombine radiatively to generate EL light [17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30]. These OLETs are of key interest; not only do they provide a novel device architecture to investigate fundamental optoelectronic properties related to charge carrier injection, transport, and radiative exciton recombination processes in organic semiconducting materials, at the same time OLETs can also be used to develop highly integrated organic optoelectronic devices such as highly bright and efficient light sources, optical communication systems, and electrically driven organic lasers [21, 22, 23, 24, 25, 26, 27, 28, 29, 30].
In principle, the luminance from OLETs can be modulated by the gate voltage without any additional driving devices; thus, displays using OLETs have the advantage of greatly reducing both the number of TFTs and the circuit complexity (Figure 1(b)), thereby providing an effective means of increasing the aperture ratio . Hence, OLETs could be a key part of the development of next-generation AM display technology . Indeed, a proof-of-principle device was recently developed using carbon nanotubes (CNTs, Figure 2), delivering a CNT-based vertical-type OLET (CNT-VOLET) [31, 32, 33, 34]. In the CNT-VOLET, a dilute network of CNTs is used as a source electrode, leading to several improvements, such as a high on/off ratio, attributed to the gate-bias-induced modulation of the lateral (or horizontal) Schottky barrier height [31, 32]. Nevertheless, the improvement of the effective aperture ratio (
In this chapter, for the VOLET, the use of a nonporous, homogeneous, smooth, and easily processable graphene layer is described as the source contact, together with an emissive channel layer. Here, the graphene is a two-dimensional (2D) material in the form of a single atomic layer of carbon with a hexagonal lattice structure bonded in the sp2 configuration (Figure 2) [36, 37, 38]. Despite the similar low dimensionality of graphene to CNTs [36, 37], the optoelectric properties of a VOLET based on graphene have not yet been fully characterized. Herein, the fabrication and characterization are described for a simple VOLET with a single-layer graphene (SLG) source contact (Gr-VOLET), capable of efficiently modulating device performance levels with high luminance on/off ratios (~104) upon the application of a gate voltage. The Gr-VOLETs with doped SLG sources with FeCl3 are demonstrated to exhibit greatly improved device performance, especially in their enhanced current efficiency and
2. 2D material electrode: doped CVD graphene
2.1 Preparation of SLG source electrodes
2.2 Characterization of SLGs
In this study, a transferred SLG was investigated as a source contact, where the FeCl3 doping is processed spontaneously during the graphene transfer process . The basic properties of the three SLG sources are shown in Figures 3 and 4 and are summarized in Table 1.
|Work function [eV]||Dirac point energy [eV]||Hole mobility [cm2V−1 s−1]||Sheet resistance, [kΩ square−1]|
In order to identify the SLG used, the surface composition of the SLG on the SiO2/Si substrates was analyzed by X-ray photoelectron spectroscopy (XPS). Figure 3(a) presents the wide-scan XPS spectra, showing strong photoelectron lines at binding energies of ~104, ~285, and ~531 eV, which are attributed to Si2p, C1s and O1s, respectively. Note that there is no Cu peak in the range of 932–935 eV (Cu2p and Cu2+), implying the complete etching of the Cu foil. In addition, the XPS spectra also revealed small but measurable amounts of Cl and Fe. These are likely residues of the etchant (FeCl3) used during the etching process. When such FeCl3 residues adsorb onto the SLG, the transfer of electrons to the Cl from the SLG (chlorination)  induces unintentional p-doping of the SLG.
Figure 3(b) shows the surface morphology of the SLG on the VOLET substrate as measured by a noncontact atomic force microscope (AFM). As indicated by the AFM morphology, the SLG samples exhibit a fairly smooth surface; the SLG presented an AFM morphology that was nearly identical at different positions on the investigated SLG samples with low RMS roughness levels of 1.4–2.0 nm.
The surface-contact potential difference (
Next, the transport characteristics of the SLG used were observed by assessing a liquid-gated lateral FET with SLG channels, a Gr-FET, as shown in Figure 3(d). The lateral FET substrate was prepared using the VOLET substrate or a heavily doped n-type Si wafer substrate (0.05-ohm cm) with a thermally grown SiO2 layer (300-nm-thick) as the gate dielectric for the OTFT, together with a laterally patterned metal source and drain electrodes consisting of a Cr layer (5.5-nm-thick) and a Au layer (50-nm-thick) formed on the substrate via a vacuum deposition process with a mask. The channel length (
For the SLG used here, the Gr-FET showed a clear asymmetrical V-shaped
Here, 4.8 eV is the absolute energy level of the ferrocene and ferrocenium (Fc/Fc+) redox couple below the vacuum energy level, and
For the SLG studied here, Raman spectroscopy was also carried out using a confocal Raman system with a laser source operating at 514.5 nm (~1 mW on sample surface). As shown in Figure 4(a), the Raman spectra of the SLGs studied here have two strong characteristic peaks, a G band at around ~1580–1600 cm−1, due to the E2g vibration of sp2-bonded carbon atoms, and a 2D band at around ~2644–2665 cm−1, which is a second-order type of vibrational mode caused by the scattering of phonons at the zone boundary [54, 55]. It can be observed that there are very small disorder-induced D bands around ~1340–1350 cm−1, indicating the sparse formation of sp3 bonds due to the relatively few defects in the SLGs studied.
From the Raman peak intensities, it was found that the ratios of the integrated Raman intensities of the G band to the 2D band for the FeCl3-doped SLG were in the approximate range of 1.7–1.8, indicating that the SLGs studied here are high-quality monolayer graphene . Moreover, from the peak positions, it was found that while the G and 2D peaks of the intrinsic undoped SLG are positioned at ~1579 cm−1 and ~2669 cm−1, respectively, the G and 2D peak positions of the SLG used are correspondingly upshifted to ~1585 cm−1 and ~2677 cm−1. Through a comparison of these with other examples in an earlier report of the relationship between the G and 2D peak positions of graphenes , it was verified that the SLG used here is p-type doped SLG.
Subsequently, the densities of the defects, the distances between the defects, and the porosities of nano-defects for the SLG were estimated from the ratio of the Raman intensities of the G bands to the D bands, ID/IG, as shown in the Raman spectra above. The density of the defects (
Next, for this SLG, polarized optical microscopy was also carried out using SLG covered with commercial nematic liquid crystals (NLCs, Merck LC ZLI-2293) in a crossed polarization state . As shown in Figure 4(b), the polarized optical microscopic image of a spin-coated NLC layer on the SLG shows large graphene domains (with an average radius of the domains >100 μm) in the form of highly uniform optical retardation, in addition to small domains of several hundreds of nanometers in size [59, 60], clearly indicating that the SLG studied here is high-quality graphene with large-area graphene domains.
3. VOLETs with a doped CVD graphene source
3.1 Fabrication of SLG-based VOLETs
Figure 5 presents a schematic illustration of the structure used and the stages of the fabrication of the SLG-based VOLETs (Gr-VOLETs) with an ITO gate separated by an Al2O3 gate dielectric layer, a SLG source, organic channel layers, and an Al drain. The fabrication steps of the Gr-VOLET investigated are described below. To construct the Gr-VOLET, SLG (4 mm by 20 mm) was transferred onto a VOLET substrate, as mentioned above (Steps 1, 2). The source electrode used was FeCl3-doped SLG. Next, organic semiconducting materials were deposited over the SLG source electrode regions; a channel layer of poly(para-phenylene vinylene) copolymer (known as SY, 70-nm-thick) was coated as an emissive channel layer by spin coating (Step 3), after which a 2-nm-thick electron injection layer of CsF and a 80-nm-thick drain electrode of Al were deposited on the top of the SY channel layer in sequence via thermal deposition at a rate of 0.05 nm s−1 under a base pressure of less than 2.7 × 10−4 Pa (Step 4). Finally, the fabricated device was encapsulated with an epoxy resin in a glove box. The photograph in Figure 5 shows the microscopic morphology of the device cross section as observed by field emission scanning electron microscopy (SEM).
3.2 Operating characteristics of Gr-VOLETs
The operating characteristics of the Gr-VOLET were observed using a luminance meter in conjunction with two source meters. To operate the Gr-VOLETs, source-drain voltage
Figure 6 shows the EL light emissions of a Gr-VOLET operating under different
The output current and luminance characteristics of the Gr-VOLET were investigated as described below. For comparative purposes, the diode characteristics of the Gr-VOLET were also observed with the gate electrodes isolated from the external circuits (Gr-OLED). As shown in Figure 7, the current density-voltage (
Interestingly, as shown in Figure 7, at
Next, the device performance,
3.3 Charge injection process at SLG sources
At this point, our investigation turns to the hole injection mechanism at the interface between the SY channel layer and the SLG source. To be injected across the interface (SLG/SY), the holes must overcome the barrier height at the interface either via thermionic emission or tunneling processes [63, 64, 65, 66, 67, 68]. Figure 9(a) shows examples of Fowler-Nordheim (F-N) curves [63, 64, 65, 66, 67] for the Gr-VOLET at various
The observations above show the working principle of the Gr-VOLET, as illustrated in the energy-level diagrams in Figure 10. At a given
In addition, notable instances of hysteresis were clearly observed, as shown above. Thus, bistable-like switching operations of a Gr-VOLET can allow novel applications for simple and inexpensive driving schemes together with low power consumption. However, this hysteresis effect may become an issue when attempting to realize high-quality grayscale outcomes and should be carefully, therefore, controlled when preparing the dielectric layer.
3.4 Inkjet-printing arrays of Gr-VOLET micro-pixels
Next, we turn our attention to a micro-pixel fabrication process for the Gr-VOLET using the inkjet-printing technique, as commonly used in solution-processable OLEDs [39, 71, 72]. Here, the inkjet technique used is based on the deposition of a small solvent drop onto an insulator layer, which can be easily redissolved and preferentially redeposited at the edge of the sessile drop (the contact line of the solvent drop), resulting in the formation of a via-hole with the shape of a crater, that is inkjet-etching .
To investigate the in situ formation of micro Gr-VOLET pixels created by means of inkjet-etching, an insulating polymer of poly(4-vinylpyridine) (P4VP) was introduced as a via-forming material, as P4VP is a hydrophilic polymer that dissolves in dimethyl formamide (DMF), toluene, chloroform, in lower alcohols, and in aqueous mineralic acids . To fabricate a via-hole forming layer, a solution of P4VP with isopropanol (IPA) was spin-coated on top of the light-emitting channel layer of SY pre-coated onto a Gr-VOLET substrate (VOLET substrate/SLG/SY/P4VP). For micro-patterning, an etching solvent of chloroform for P4VP was inkjet-printed on top of the SY/P4VP layers (Figure 11(a)). This inkjet-printed solvent drop of chloroform can dissolve the P4VP layer, and the capillary flow of the solvent pushes the dissolved P4VP from the center to the contact line of droplet due to the coffee ring effect [39, 72, 73, 74], resulting the formation of the via-hole through the P4VP layer. Thus, after the deposition of even a single solvent droplet (~150 pL per droplet) on a 30-nm-thick P4VP film, the P4VP polymers are removed from the printed position and completely etched, forming via-holes through the P4VP layer, of which the inner and outer diameters are ~90 μm and ~120 μm, respectively, and finally uncovering the surface of the underlying SY layer. These P4VP via-holes on the light-emitting SY layer act as micro-patterned pixel openings for the light-emitting active areas of the Gr-VOLETs. Then, to complete the device of an array of micro Gr-VOLETs, the CsF/Al/Ag cathode is deposited following the procedure described in Section 3.1.
Figure 11(a) also presents the switching behavior of EL light emissions from the array of micro Gr-VOLET pixels for two different gate voltages,
Next, the output characteristics of the inkjet-printed Gr-VOLET pixels were investigated. As shown in Figure 11(b), the
In summary, herein, graphene-based VOLETs have been explored, consisting of a nonporous, homogeneous, and p-doped SLG source with FeCl3, an Al drain, and an emissive channel layer for efficient switching of the device performance using the gate voltage. Initially, we investigated transferred CVD SLG, which was used as the source electrode. It was found that the SLG used here was unintended p-doped SLG, exhibiting a Dirac point energy of ~4.9 eV and a work function of 5.2 eV with a shift of the Fermi level from the Dirac point and high hole mobility. It is shown that the high device performance capabilities of SLG-based VOLETs were mainly due to the p-doping effects, which were estimated quantitatively and analyzed based on the energy levels of the SLGs. It is also shown that low-drain-voltage operations and increased brightness with a high luminance on/off ratio (~104) can be achieved even at high brightness for the Gr-VOLET without any HIL. Moreover, the current efficiency and effective aperture ratio of the Gr-VOLET were at least 150% higher than those of a control OLED, with low parasitic power consumption of 5%. These significant improvements of the device performance can be attributed to the gate-bias-induced modulation of the hole tunneling injection from the FeCl3-doped SLG source into the emissive channel layer. Further, the feasibility of the simple fabrication process of micro Gr-VOLET pixels, that is, the inkjet-printing technique, was also proven.
The foregoing results demonstrate the notable device performance of the Gr-VOLET with graphene source, indicating considerable promise with respect to the development of high-performance VOLETs. The advances afforded by the Gr-VOLET with reliable switching performance, even at high luminance levels, clearly show its effective light-emitting transistor functionality and make it a feasible candidate for development of new voltage-driving light-emitting devices and/or highly integrated organic optoelectronics. Finally, it will be possible to apply advanced material layers to these Gr-VOLETs, which could lead to more efficient devices that operate even at low voltage levels, enabling the development of inexpensive, large-area, fast, and high-performance AM displays. Further improvements and characterizations are in progress and will be published elsewhere.
This work was supported by a grant from the National Research Foundation of Korea (NRF) funded by the Korean Government (MEST) (2017R1A2A1A17069729).