Introduction

Organic light-emitting transistors (OLETs) have been developed by integrating the ability of organic light-emitting diodes (OLEDs)1,2,3,4,5,6 to generate light with the electrical-switching functionality of organic field-effect transistors (FETs)7,8 into a single device structure9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29. In OLETs, the current flowing through emissive semiconductor channel layers can be modulated by the gate voltage, which can also change the electroluminescent (EL) emission brightness from the dark off-state to the bright on-state9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29. In addition to such EL switching functionality, OLETs have other advantages over OLEDs, such as relatively high luminance efficiency and external quantum efficiency13,14. Thus, OLETs are of key interest, as they can provide a unique type of device architecture for investigating fundamental opto-electronic properties related to the charge carrier injection, transport, and radiative exciton recombinations in organic semiconducting materials. Moreover, due to their compact architecture, they can be used to develop integrated organic opto-electronic devices such as highly efficient light sources, optical communication systems, and/or electrically driven organic lasers11,12,13,14,15,16,17,18.

For these principal reasons, the luminance from an OLET can be modulated without any additional driving devices, and displays using OLETs thus offer the additional advantage of reducing the number of high mobility-driving thin film transistors (TFTs) and simplifying the inherent complexity of the circuits of conventional active-matrix (AM) OLEDs14,17,20. In this sense, OLETs can be an effective means of increasing the aperture ratio (the light-emitting area as a proportion of the total area of the device), making it higher than that (~34%) of a typical AM-OLED17,20. Hence, surface-emitting OLETs may offer an attractive alternative to conventional AM displays as a key element in the development of next-generation AM display technology17,20.

Most OLETs have lateral source-drain geometries but exhibit line-, band-, or circular-type emission characteristics in a limited zone between the source and drain electrodes9,10,11,12,13,14,15,16,17,18,19,20. Thus, much effort has been made to improve the device performance and to extend the light emission area; multilayer structures12,14,16 and/or modified electrodes18,19,20 have been suggested to control the charge injection, charge transport, and charge carrier recombinations in the emissive channel layers of these devices.

To obtain highly increased source drain current flows even at low gate voltages, vertical OLETs (VOLETs) were also constructed with short channel lengths by coupling a static induction transistor structure21,22, constructing a metal insulator semiconductor structure23,24, or using micro-networks with periodic vacancies in a vertical configuration25,26. These devices have shown stripe-type or quasi-surface emission patterns. In these cases, a reliable and high-resolution patterning method for electrodes and/or an insulating charge-restriction layer would be crucial to control the leakage currents and switching characteristics.

In another effort, a novel type of VOLET and a related fully functional display were developed using a source electrode consisting of randomly distributed carbon nanotubes (CNTs)27,28,29. The CNT-based VOLET (CNT-VOLET) has shown a number of remarkable improvements, such as highly bright and efficient full-surface emissions with high on/off ratios27,28. This good switching ability was mainly attributed to the gate-voltage-induced modulation of the lateral (or horizontal) Schottky barrier height for the dilute network of the CNT source electrode in the CNT-VOLET27,28. Nevertheless, the CNT-VOLET may experience non-uniformity as well as poor connectivity of the CNTs in the source electrode8,25,26. In addition, the irregular interface between the CNT source electrode and its adjacent functional layer may act as traps to hinder the charge transport25,26. Moreover, the CNT electrode typically forms a very rough surface, thus requiring a fairly thick adjacent functional layer8,28. Thus, the ultimate goal of simple and reliable OLETs capable of high device performance with good switching ability remains unmet.

In this article, we report on the first use of a VOLET with a homogeneous, smooth, and easily processable graphene layer as the source electrode, together with an emissive channel layer. As a two-dimensional material in the form of a single layer with a carbon-based hexagonal lattice structure bonded in the sp2 configuration30,31,32, graphene has been used as a transparent electrode material in small OLEDs, as a proof of concept33,34,35. Despite the low dimensionality of graphene, similar to that of CNTs31,32, the successful operation of full-surface EL light emission from a VOLET based on graphene has not yet been reported, as the work function (~4.6 eV) of pristine graphene is too low for the hole injection33,34,35. We describe the fabrication and characterization of a simple graphene-based VOLET (Gr-VOLET) with a FeCl3-doped single-layer graphene (SLG) source electrode. We find that the EL properties of the Gr-VOLET can be efficiently modulated with high luminance on/off ratios (~104) through the application of gate voltage. More interestingly, our Gr-VOLETs with doped SLG sources are shown to exhibit greatly improved device performance, especially for their higher current efficiencies as compared to those of control OLEDs, even at high luminance levels exceeding 500 cd/m2, making them all the more attractive. We discuss the operating mechanism that explains these significant improvements in the device performance; i.e., the effective modulation of the hole tunnelling injection from the FeCl3-doped graphene source.

Results and Discussion

Operating characteristics of Gr-VOLETs

Our first challenge relates to the structure and operating characteristics of our Gr-VOLET, including the bottom indium tin oxide (ITO) gate, the Al2O3 gate dielectric layer, the SLG source, the functional channel layers including the organic light-emitting layer (EML) of the Super Yellow (SY) conjugated copolymer, and the Al metal drain, in sequence (Fig. 1a, see also Methods and Supplementary Fig. S1a for details of the structure and layer thicknesses). During the operation of the Gr-VOLET, the electron injection occurs from the Al drain into the channel layer, and the hole injection from the SLG source can be modulated by adjusting the gate voltage VGS, as discussed below. Figure 1b shows the EL light emissions of a sample Gr-VOLET operating under different VGS levels with a fixed source-drain voltage VSD of 3.8 V. As shown in the figure, the EL light emission is uniformly bright (in the fully on-state), grey, and dark (off-state) over the entire surface of the active area for negative, zero, and positive VGS values, respectively. (See also Supplementary Information Video 1 and Supplementary Fig. S1b for images of the device in the dark). Hence, VGS essentially influences the hole injection and current flow through the emissive channel layer and thus may contribute to the charge balance for efficient hole-electron recombinations in the channel layers. The EL emission spectra observed are nearly identical to those obtained from a conventional ITO-OLED (Fig. 1c). Furthermore, the temporal responses of the Gr-VOLET with respect to step changes in VG show rapid rising and falling times of 4.7 ms and 2.8 ms, respectively (Fig. 1d), which are also comparable to those of conventional OLEDs and much faster than those of traditional liquid crystal displays (LCDs)5. The above characteristics highlight several interesting features of our Gr-VOLET. To investigate its distinct characteristics further, we tested three types of SLG materials as source electrodes; (i) p-doped SLG with FeCl3 (hereafter SLG1 for fabricating Gr-VOLET1), where FeCl3 doping is done spontaneously during the graphene transfer process, as shown previously35; (ii) pristine (intrinsic) SLG, cleaned by an electrochemical process35, as a comparative reference (SLG2 for Gr-VOLET2); and (iii) SLG coated with a conventional hole-injection layer (HIL) of (3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)34 as a second comparative reference (SLG3 for Gr-VOLET3). The basic properties of the three SLG sources (with low porosities below 0.1%) are shown in Supplementary Fig. S2,S3 and S4 and are summarised in Table 1.

Figure 1
figure 1

Structure and operation of graphene-based vertical organic light-emitting transistors (Gr-VOLETs). (a) Schematic illustration of the structure and a SEM image of a cross-sectional slice of the Gr-VOLET with a single-layer graphene (SLG) source, stacked layers of organic functional channel layers, an Al drain, and an ITO gate separated with an Al2O3 gate dielectric (scale bar: 200 nm). (b) Light emission from a Gr-VOLET (4 mm × 2 mm, white squares) for three different gate voltages of VGS for a fixed source-drain voltage, VSD, of 3.8 V (see also Supplementary Video S1 and Supplementary Fig. S1b). (c) Relative electroluminescent spectra of a Gr-VOLET (solid curves) and of a corresponding control ITO-OLED (dotted curve). The molecular structure of Super Yellow (SY) is shown in the inset. (d) Temporal response of a Gr-VOLET with respect to the step gate voltages for a fixed VSD of 3.8 V.

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Table 1 Summary of the basic electronic properties of the SLGs studied here.
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We now describe the detailed output current and luminance characteristics of the three prototype Gr-VOLETs mentioned above. For comparative purposes, we also observed the diode characteristics of the Gr-VOLETs with the gate electrodes disconnected from the external circuits (Gr-OLEDs) (Fig. 2a and Supplementary Figs S5a and S6a). As shown, the current density-voltage (JD-VSD) and luminance-voltage (L-VSD) characteristics of the Gr-VOLETs present four key characteristics: (1) the JD-VSD characteristics are similar to those of a diode without current saturation, which were generally observed in vertical-type organic field-effect transistors due to the short vertical channel lengths8, (2) similar behaviours to the JD-VSD curves were observed in the L-VSD characteristics, (3) both JD and L for a given VSD increase with a decrease in the negative VGS, even at a low VSD, which shows that current modulation by VGS can change EL emission brightness. Hence, the VGS-dependent turn-on voltage (Vonset) can be reduced to well below Vonset of the Gr-OLED, and (4) both JD and L also depend on the direction of the change of VGS, i.e., increasing (upward)/decreasing (downward), implying hysteretic behaviour. Among the Gr-VOLETs, interestingly, the Gr-VOLET device with the doped SLG1 source (Gr-VOLET1, Fig. 2a) exhibits highly improved device performance, superior to that of Gr-OLED1. For example, at VGS = −40 V, the JD value is higher than that of Gr-OLED1 and the luminance reaches L ~ 2,000 cd/m2 at VSD = 6.0 V (Vonset = 2.3 V), which is more than twice that (L ~ 740 cd/m2 and Vonset = 2.5 V) of Gr-OLED1. These outcomes indicate improved and balanced charge (hole) injections from the SLG1 source in the case of a negative VGS. Conversely, at VGS = +40 V, JD and L of Gr-VOLET1 are much lower, possibly due to the switching off of the hole injection from the SLG source. The highest values observed for the peak on/off ratios of JD and L were approximately 102 and 104, respectively, at VGS = ± 40 V. This gate-bias-induced modulation effect of Gr-VOLET1 is shown to be more efficient than those of the other Gr-VOLETs tested here with the pristine (pure) SLG2 source (Gr-VOLET2), or even with the PEDOT:PSS HIL-coated SLG3 source (Gr-VOLET3).

Figure 2
figure 2

Output characteristics of Gr-VOLET1 and comparison with the control ITO-OLED. Gate-voltage (VGS)-dependent current density-voltage (JD-VSD) (a) and luminance-voltage (L-VSD) (b) characteristics of Gr-VOLET1 with a FeCl3-doped SLG1 source for upward (left) and downward (right) changes in VGS. For comparison, the characteristics of a gate-disconnected Gr-VOLET1 (i.e., Gr-OLED1) are also shown (dotted curves). J-L-V (c) and ηC-L (d) comparisons of Gr-VOLET1 in the bright on-state (VGS = −40 V) with its respective ITO-based control OLED (ITO-OLED1, ITO/SY/CsF/Al).

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We then estimated the device performance capabilities of the Gr-VOLETs in the on-state (VGS = −40 V), comparing these with control OLEDs fabricated using the same batch process on ITO anodes (ITO-OLEDs) (Fig. 2b and Supplementary Figs S5b and S6b). As shown, only Gr-VOLET1 exhibits luminance higher than that of the control ITO-OLED (ITO/SY/CsF/Al) in the source-drain voltage region VSD < 4.0 V (Fig. 2b). For example, when VSD = 3.8 V, Gr-VOLET1 emitted luminance of 490 cd/m2, while the control ITO-OLED emitted luminance of 455 cd/m2 at V = 3.8 V. Moreover, Gr-VOLET1 was found to be more efficient than the control ITO-OLED, in contrast to the other Gr-VOLETs (Fig. 2c and Supplementary Figs S5c and S6c). For instance, at a luminance level of 500 cd/m2, Gr-VOLET1 emitted EL light with current efficiency ηC of 7.13 cd/A, which is approximately 1.54 times higher than that (4.64 cd/A) of the control ITO-OLED (ηC Gr-OLET/ηC OLED = 1.54). Even at luminance levels exceeding 2,000 cd/m2, Gr-VOLET1 maintains an enhanced ηC Gr-OLET/ηC OLED ratio of ~1.62. It is therefore clear that Gr-VOLET1 has highly enhanced current efficiency compared to other devices investigated (Table 2). This offers another important advantage: given this level of enhanced current efficiency, the brightness of the device can be maintained with a lower JD, promising a longer device lifetime6. It is noteworthy that ηC (7.13 cd/A) for Gr-VOLET1 was approximately 1.38 times higher than that (5.17 cd/A) of ITO-OLED3, the latter possessing the optimised HIL of PEDOT:PSS (Supplementary Fig. S6c). Thus, it is clear that the SLG1 source in Gr-VOLET1 provides amplification of both the emission and current efficiency, although further optimization of the drain electrodes is still possible.

Table 2 Comparison of the current efficiencies of Gr-VOLETs (ηC Gr-OLET) and ITO-based control OLEDs (ηC OLED) with their ηC Gr-OLET/ηC OLED ratios at a luminance level of 500 cd/m2.
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Charge injection process at SLG sources

Our investigation also focused on the hole injection mechanism from the SLG source into the SY channel layer. To be injected across the SLG/SY interface, the holes must overcome the potential barrier at the interface either via thermionic emission or tunnelling processes36,37,38,39,40,41. Figure 3a shows examples of Fowler-Nordheim (F-N) plots36,37,38,39,40,41, ln(JD/VSD2) vs 1/VSD, for the three Gr-VOLETs when VGS = 0 V during upward changes in VGS. All of the plots show two distinct hole injection processes with the transition voltage (VT), at which value the injection mechanism changes from Schottky thermionic emission to tunnelling39,40,41.

Figure 3
figure 3

Charge injection processes in Gr-VOLETs. Fowler-Nordheim plot, ln(JD/VSD2) vs 1/VSD, for Gr-VOLETs with different SLG sources at VGS = 0 V (a) and with the SLG1 source at various VGS levels (b) for upward VGS changes. The inset in (a) shows a schematic energy band diagram of the thermionic emission and tunnelling at the SLG/SY interface along the normal direction of the interface between the SLG and the SY channel layer. The inset in (b) shows an example of the theoretical fittings based on the tunnelling current model (solid curve). (c) Gate-bias-modulated hole tunnelling barrier height, Φ, extracted from the fittings in the hole-dominant regimes. ΔΦ: gate-bias-induced Φ modulation when VGS =  ± 40 V.

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Figure 3b presents F-N plots of a typical Gr-VOLET1 at various values of VGS during upward VGS changes. It is interesting to note that VGS affects both Schottky thermionic emission and tunnelling; thus, VT strongly depends on VGS. It is also noteworthy that because the EL emission from the Gr-VOLETs occurs when VSD > Vonset, higher than VT, the main hole injection process for light emission is tunnelling injection in the Gr-VOLETs. According to a modified tunnelling current model42, the tunnelling current density (J) for a single charge carrier through a triangular barrier at a metal/polymer junction is related to the potential barrier height Φ and the temperature T: ln(J/V2) = −P1/V + ln(P2/V) − ln[sin(P3/V)], with Φ = (3/2) πkBT (P1/P3), where kB is the Boltzmann constant, and Pi denotes parameters related to Φ42. This relationship allows the F-N curves to be analysed, and the potential barrier heights Φ at the SLG/SY interfaces between the Fermi level of the SLGs and the highest occupied molecular orbital (HOMO) level (~5.3 eV) of the SY channel layer35 to be obtained (Fig. 3c), all of which are in reasonable agreement with values in the literature43. Among the SLG/SY interfaces, the SLG1/SY interface exhibited the strongest gate-bias-induced Φ modulation (ΔΦ) along the direction normal to the interface; i.e., ΔΦ at VG = ±40 V was approximately 110 meV, which is much higher than the values of ΔΦ for SLG2/SY (~60 meV) and SLG3/SY (~20 meV). This strongest gate-bias-induced Φ modulation of the FeCl3-doped SLG1 source leads to the efficient modulation of the device performance of Gr-VOLET1 as tested here. Note that when VSD > Vonset, the theoretical predictions begin to deviate from the experimental data, mainly due to minority carrier (electron) injections into the SY channel layer from the Al drain. Nevertheless, tunnelling at the SLG/SY interface is the major hole injection process, being responsible for the radiative recombination of electron-hole pairs. This analysis is also supported by an inspection of the weak dependence on T of JD-VSD and VT for Gr-VOLET1 (Supplementary Fig. S7); the other injection process, Schottky thermionic injection, is in contrast strongly dependent on T36,39,40. Therefore, the tunnelling analysis provides clear evidence that our device operates via the modulation of the vertical barrier height along the direction normal to the source surface (i.e., parallel to the gate field direction), in contrast to the CNT-VOLET based on lateral (or horizontal) Schottky barrier height modulation along the horizontal direction on the source surface (i.e., perpendicular to the gate field direction)27,28, and different as well from conventional graphene-based barristors that operate via the modulation of the Schottky thermionic injection44. It is also noted that if one considers the further contribution of the gate field to the electric field inside the channel layer together with an appropriate dielectric constant of the channel layer, one may then obtain a more precise value of the barrier height from the F-N analysis for the injection characteristics. However, this is beyond the scope of this report, and further details about such an analysis and a related discussion will therefore be reported elsewhere.

To gain a better understanding of the hole injection mechanism at the SLG sources, we also investigated the dependence of the Fermi levels (work functions) of the SLG sources on the gate bias using the KPFM method (see also Supplementary Fig. S3). We observe that a VG sweep clearly modulates the work functions of the SLGs. As shown in Fig. 4a, for the pristine SLG (SLG2), a large change in the work function can be observed, from 4.44 eV to 5.00 eV, by sweeping VGS. However, for the doped SLG (SLG1), a largely downwardly shifted modulation of the work function was observed, from 4.72 eV to 5.29 eV. This work function of SLG1 is closer to the HOMO level of the SY channel layer than that of SLG2 at a negative gate bias. In contrast, the PEDOT:PSS-coated SLG (SLG3) shows only a minor modulation of the work function. This may be due to the negatively charged PSS in the PEDOT:PSS HIL on the SLG3 source45, which can generate a strong electrostatic field and accordingly weaken the gate-field effect, as also shown in Supplementary Fig. S6a. In addition, considerable hysteretic behaviour is apparent in all of the plots, possibly due to charge trapping in the Al2O3 dielectric layer46. These hysteretic work functions of SLGs are major causes of the hysteresis of the output characteristics of the Gr-VOLETs for upward and downward VGS changes.

Figure 4
figure 4

Gate-bias-induced modulation of SLG work functions and operation mechanism of Gr-VOLETs. (a) Gate-bias-induced modulation of the work functions of SLG sources on the VOLET substrates. (b) Energy-level diagrams of Gr-VOLET1 for high, mid, and low Φs at three distinct values of VGS at a given VSD. Φ depicts the tunnelling barrier height for the hole injection. ED: Dirac point energy of the SLG source used.

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The above observations reveal the working principle of Gr-VOLET1, as illustrated in the energy-level diagrams in Fig. 4b. At a given VSD, a positive gate bias induces an upward shift of the Fermi level of the SLG1 source in a direction that increases the barrier height Φ, resulting in reduced tunnelling and fewer hole injections into the HOMO level of the SY channel layer. In contrast, a negative gate bias induces a downward shift of the Fermi level of the SLG1 source, decreasing Φ significantly (enhancing tunnelling) and hence allowing increased hole injection and improved EL performance. Thus, together with the band-bending effect27, the main operating mechanism of Gr-VOLET1 is energy band matching, and the charge balance is thus achieved even without any HIL through gate-bias-induced modulation of the hole tunnelling injection at the SLG source, as controlled by p-type doping with FeCl3. Although the work function of SLG3 including the PEDOT:PSS HIL is nearly identical to that of SLG1, the modulation performance of Gr-VOLET3 is considerably lower than that of Gr-VOLET1. This may arise when the PEDOT:PSS HIL impedes the effect of VG on the modulation of the barrier height Φ due to its strong electrostatic field, in conjunction with the gate field-screening effect in the highly conductive PEDOT:PSS layer. Note that the changes in the potential barrier height Φ induced by VGS are somewhat lower than the changes in the work functions of the SLGs caused by VGS, which may be due to the field effect of VSD applied to the SLG sources together with the electrostatic interaction effect via the charge transfer47 at the SLG/SY interface (further details will be reported elsewhere).

Transfer characteristics and TFT-switching Gr-VOLETs

We now turn our attention to the transfer characteristics of Gr-VOLET1. As shown in Fig. 5a, at a given VSD (3.2 V), JD increases when VGS is biased towards higher negative values, and thus Gr-VOLET1 is normally in the “on-state,” while it is “switched off” at a positive gate bias. During the switching operation, the device exhibits a fairly low gate leakage current density (JG) (inset in the upper panel). Similar to the output curves, no region of saturation is observed in the transfer curves for the VG range applied. In addition, notable instances of hysteresis are clearly observed in the transfer curves, caused by the hysteretic loop of the work function of the SLG1 source, as shown above; bistable-like switching operations (or memory-like effects) of Gr-VOLET1 are thus verified at VGS = 0 V, as shown in the insets in the lower panel. This property of a Gr-VOLET may allow novel applications for inexpensive and simple driving schemes with low power consumption. However, this effect may become more significant when realising high-quality grayscale; moreover, it should be carefully controlled when preparing the dielectric layer. It is also noted, as shown in the figure, that the on/off ratios of the current and luminance modulations are somewhat reduced, mainly due to the increased leakage current between the source and drain via the deterioration of the device during the measurement process. Thus, to maintain the high on/off ratios of the devices, one should control the degradation and leakage current flows precisely.

Figure 5
figure 5

Transfer characteristics and Gr-VOLET switched by an OTFT. (a) Transfer JD-VGS (upper) and L-VGS (lower) curves of Gr-VOLET1 at a given VSD of 3.2 V. The insets in the upper and lower panels respectively show the JG-VGS characteristics of Gr-VOLET1 and two photographs of the Gr-VOLET at VGS = 0 V for a given VSD, revealing bistable-like operation. (b) A simple AM-OLET pixel circuit diagram of a Gr-VOLET combined with a switching TFT (upper), and two photographs of Gr-VOLET1 switched by an OTFT, showing the bright on-state (middle) and the dark off-state (lower).

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To investigate its switching capability, we now describe an application of our Gr-VOLET combined with only one switching device, excluding any other high mobility-driving TFTs or storage capacitors as commonly used in conventional AM-OLEDs (Fig. 5b)4. Here, the switching device used was an organic TFT (OTFT), produced using 6,13-bis(triisopropylsilylethynyl)pentacene blended with a polymer binder of poly(a-methylstyrene), with mobility of approximately 0.1 cm2/(V s) (see Supplementary Fig. S8). For its operation, a VDS value of ±80 V and a VGS value of −50 V were applied to the OTFT, resulting in VGS applied to the gate of the Gr-VOLET being as high as ±20 V with a fixed VSD of 3.2 V for the Gr-VOLET. Most interestingly, the EL light output of the Gr-VOLET1 was successfully switched from the bright on-state (middle panel) to the dark off-state (lower panel) by the OTFT operation, despite the fact that the mobility of the OTFT is much lower than that (0.6–1 cm2/(V s)) of a typical Si TFT4. From this application, we contend that our Gr-VOLET shows considerable promise for reliable and high-performance AM displays.

The foregoing results clearly demonstrate the remarkable device performance of the Gr-VOLET with a FeCl3-doped SLG source electrode, showing considerable promise with regard to the development of high-performance OLETs. To the best of our knowledge, this is the first demonstration of high-performance VOLETs fabricated with single-layer graphene source electrodes modified by FeCl3 doping, exhibiting high on/off contrast ratios and enhanced efficiency levels.

Finally, two points should be discussed with regard to our Gr-VOLET, the first of which relates to the improvement in its performance and lifetime. The light-emitting performance and lifetime can be improved further through the additional optimization of the materials used48. Specifically, in place of SY used as the light-emitting material, it would be possible to use other new materials3, including red, green, and blue light-emitting fluorescent or phosphorescent materials, which exhibit much higher brightness levels, efficiencies, and lifetimes than those of the Gr-VOLETs studied here. This could yield very bright and efficient Gr-VOLETs with long lifetimes. Second, it is possible to use a thin dielectric layer grown by other deposition methods, such as atomic layer deposition49, rather than the thick Al2O3 dielectric layer used here. This could yield efficient Gr-VOLETs operating at a low VGS levels, i.e., below 5 V, enabling the adoption of a-Si TFT backplanes.

The advances afforded by the Gr-VOLET with its reliable switching performance, shown here even at high luminance levels, clearly demonstrate its effective light-emitting transistor functionality and make it a viable candidate for use in new voltage-driving light-emitting devices and highly integrated organic opto-electronics. Further, the combination of these Gr-VOLETs with TFT backplanes will certainly presage the development of inexpensive, fast, large-area, and high-performance AM display devices.

Conclusions

In summary, we have herein explored the characteristics of a graphene-based VOLET consisting of a homogeneous SLG source, an emissive channel layer, and an Al drain, allowing efficient switching of the device performance with variations of the gate voltage. We have verified that low-drain-voltage operations and increased brightness levels with a high luminance on/off ratio of ~104 can be achieved even without a HIL using a p-doped SLG source with FeCl3. Moreover, the current efficiency of the Gr-VOLET was at least 1.5 times higher than that of a control ITO-based OLED at a given luminance level. These significant device performance improvements can be attributed to the efficient modulation of the hole tunnelling injection via gate-bias-induced Fermi-level control of the FeCl3-doped SLG source. Together with its simple structure and easy processability, this surface-emissive device using doped graphene provides a new platform for the development of advanced light-emitting devices and/or next-generation emissive display devices.

Methods

Preparation of substrates

The transparent VOLET substrate was prepared with a pre-patterned back gate electrode consisting of 80-nm-thick ITO (30 ohm/square sheet resistance) on a glass substrate, with a sputter-deposited aluminium oxide (Al2O3, 400 nm) top layer as a gate dielectric layer over the ITO gate electrode (glass/ITO/Al2O3). The VOLET substrate used was pre-cleaned with alcohol, followed by a UV treatment for 5 min immediately prior to the fabrication of the graphene-based devices.

The lateral FET substrate was prepared using a VOLET substrate or a 300-nm-thick layer of thermally grown SiO2 as the gate dielectric on a heavily doped n-type (100) Si wafer substrate (0.05 ohm cm) for the OTFT, together with laterally patterned metal source and drain electrodes of a 5.5-nm-thick Cr layer and a 50-nm-thick Au layer formed on the substrate via a conventional vacuum deposition process with a shield mask. The channel length (L) and width (W) of the FET were 50 μm and 1600 μm, respectively (see Supplementary Fig. S4).

Transfer of graphene

The procedure used for transferring the chemical-vapour-deposition (CVD)-grown graphene onto a target substrate35,50, in this case a VOLET substrate, a FET substrate, or a glass substrate, is described below. The first step involves CVD growth of monolayer graphene on a copper (Cu) foil35,50. A clean copper foil was placed in a quartz tube chamber and the temperature was increased to 1000 °C under Ar (10 sccm). For the growth of graphene, a gas mixture of CH4 (30 sccm) and H2 (10 sccm) was used at ~2.7 × 10−2 Pa. The second step involved spin-coating a poly(methyl methacrylate) (PMMA) solution (950PMMAC4, MicroChem) onto the CVD-grown graphene on the copper foil at 3000 rpm for 60 s. The graphene film grown on the back side of the copper foil was then removed by atmospheric-pressure oxygen plasma. Subsequently, a PMMA-coated Cu/Gr (Cu/Gr/PMMA) block with a width of 4 mm and length of 20 mm was floated on an aqueous FeCl3 solution (UN2582, Transene Co. Inc.) used to etch the copper foil entirely at 50 °C for 10 min. Subsequently, floating of the PMMA-coated Gr (Gr/PMMA) block on the FeCl3 solution was maintained for a further 10 min to dope the Gr film with FeCl3, as described previously35. Next, the Gr/PMMA block was rinsed with deionised (DI) water several times (10 min) and transferred onto the target substrate, after which the SLG-transferred substrate was dried under reduced pressure (~1 Pa) for 1 h and left in air for one day. The PMMA supporting layer was then removed by dissolving the PMMA in chloroform (60 min), monochlorobenzene (30 min) and chloroform again (30 min) in sequence.

Cleaning and de-doping of SLGs

For cleaning and de-doping of the SLGs on the substrate, a bubble-free electrochemical (EC) treatment was carried out in a non-aqueous electrolyte of acetonitrile (ACN, 99.8%, Aldrich) with 100 mM of tetrabutylammonium hexafluorophosphate (TBAPF6, >99.0%, Aldrich) using a potentiostat (DY2113, Digi-Ivy, Inc.)35. The SLG transferred onto the substrate was used as a working electrode with a platinum wire as a counter electrode and an Ag/AgCl electrode (3.5 M KCl) as a reference electrode35.

The EC-cleaning treatment was conducted using freshly prepared SLG under negative voltage ranges (0.0 ~ −0.7 V/VAg/AgCl) at a voltage-sweeping rate of 0.5 V/s for 10 min. After the cleaning process, the treated SLG was rinsed several times using pure ACN and DI water and then dried with N2 gas to remove the electrolyte entirely from the SLG surface. To calibrate the electrode potentials, ferrocene (98%, Sigma Aldrich) was used as a redox probe.

Fabrication of VOLETs

The fabrication steps of the Gr-VOLET used in this study are described below (see Supplementary Fig. S1a). To construct the Gr-VOLET, SLG with an area of 4 mm by 20 mm was transferred onto the VOLET substrate, as mentioned above. The SLG electrodes used were FeCl3-doped SLG (SLG1) or EC-cleaned (pristine) SLG (SLG2). Next, organic semiconducting materials were deposited over the source electrode regions; a 70-nm-thick channel layer of SY (poly (para-phenylene vinylene) copolymer, Merck OLED Materials GmbH) was deposited as an emissive channel layer by spin coating. Where necessary, a 20-nm-thick layer of PEDOT:PSS (CLEVIOSTM 4083, H. C. Starck Inc.) was also deposited as a HIL over the EC-cleaned SLG source by spin coating (SLG3) prior to the deposition of the SY layer. Subsequently, a 2-nm-thick electron injection layer (EIL) of CsF and a drain electrode of Al (80 nm thick) were deposited on the top of the SY layer in sequence via thermal deposition at a rate of 0.05 nm/s under a base pressure of less than 2.7 × 10−4 Pa. The fabricated device was finally encapsulated with an epoxy resin and a glass coverslip in a nitrogen-filled glove box.

Characterisations of the SLGs and SLG-based devices

The variations in the surface roughness and surface potential of the SLG on the substrate were monitored using non-contact AFM and simultaneous KPFM (FlexAFM, Nanosurf AG), respectively, by applying an AC voltage of 1 V at a frequency of 18 kHz to a Pt/Ir-coated silicon tip. To calibrate the work function of the SLG studied here, highly oriented pyrolytic graphite (HOPG, ZYB, Optigraph GmbH) was used as a reference surface. The microscopic morphology of the device was observed by field emission scanning electron microscopy (SEM, Model JSM-6700F, JEOL Co.).

The device performance of the Gr-VOLET was measured using a chroma meter (CS-2000, Konica Minolta) in conjunction with two source meters (2636 A, Keithley). The emission characteristics of the devices were also investigated using an LED measurement system (LCS-100, SphereOptics Inc.) with an integrating sphere. For the operation of the Gr-VOLETs, source-drain voltage VSD ( = −VDS) on the Al drain and gate voltage, VGS, were applied with respect to the SLG source electrode, held at ground potential.