Full-surface emission of graphene-based vertical-type organic light-emitting transistors with high on/off contrast ratios and enhanced efficiencies

Surface-emitting organic light-emitting transistors (OLETs) could well be a core element in the next generation of active-matrix (AM) displays. We report some of the key characteristics of graphene-based vertical-type organic light-emitting transistors (Gr-VOLETs) composed of a single-layer graphene source and an emissive channel layer. It is shown that FeCl3 doping of the graphene source results in a significant improvement in the device performance of Gr-VOLETs. Using the FeCl3-doped graphene source, it is demonstrated that the full-surface electroluminescent emission of the Gr-VOLET can be effectively modulated by gate voltages with high luminance on/off ratios (~104). Current efficiencies are also observed to be much higher than those of control organic light-emitting diodes (OLEDs), even at high luminance levels exceeding 500 cd/m2. Moreover, we propose an operating mechanism to explain the improvements in the device performance i.e., the effective gate-bias-induced modulation of the hole tunnelling injection at the doped graphene source electrode. Despite its inherently simple structure, our study highlights the significant improvement in the device performance of OLETs offered by the FeCl3-doped graphene source electrode.

For the SLGs studied, Raman spectroscopy was carried out using a confocal Raman system (LabRam Aramis, Horiba Jobin-Yvon) with a laser source operating at 514.5 nm (~1 mW on sample). As shown in Supplementary Fig. S2a, the Raman spectra of the SLGs studied have two strong characteristic peaks, a G band at around ~1580~1600 cm −1 , which is due to the E 2g vibrational mode of sp 2 -bonded carbon atoms, and a 2D band at around ~2644~2665 cm −1 , which is a second-order type of vibration caused by the scattering of phonons at the zone boundary 1,2 . There are very small disorder-induced D bands around ~1340~1350 cm −1 , indicating the sparse formation of sp 3 bonds due to fewer defects in the SLGs studied.
From the Raman peak intensities, it was found that the ratios of the integrated intensities of the G band to the 2D band for the FeCl 3 -doped SLG (SLG 1 ) and the cleaned SLG (SLG 2 ) sources were in the approximate range of 1.7~1.8, indicating that the SLGs studied here are high-quality monolayer graphenes 2 . Moreover, from the peak positions, it was found that while the G and 2D peaks of SLG 2 are at ~1579 cm −1 and ~2669 cm −1 , respectively, the G and 2D peak positions of SLG 1 are correspondingly upshifted to ~1585 cm −1 and ~2677 cm −1 . Similar to SLG 1 , it was found that the PEDOT:PSS HIL-coated SLG (SLG 3 ) has a G peak at ~1585 cm −1 and a 2D peak at ~2674 cm −1 . By comparison of these with other examples in an earlier report on the relationship between the G and 2D peak positions of graphenes 2 , it was verified that SLG 2 is a type of pristine graphene, whereas SLG 1 and SLG 3 are p-type doped graphenes.
The densities of defects, distance between defects, and porosities of nano-defects for both SLG 1 and SLG 2 were estimated from the ratios of the intensities of the G bands to the D bands, I D /I G , as shown in the above Raman spectra. The density 6 of defects (n D ) and distance between defects (L D ) for SLG 1 , estimated by the carbon amorphization trajectory (I D /I G ~ 0.117) 3,4 , were n D ~ 3.0 × 10 10 /cm 2 and L D ~ 32.8 nm, respectively, corresponding to a porosity of 9.4 × 10 -2 %. Similar to SLG 1 , it was found that the SLG 2 has I D /I G ~ 0.113 and thus n D of 2.9 × 10 10 cm -2 and L D ~ 33.4 nm, corresponding to a porosity of 9.1 × 10 -2 %. This result clearly indicates that the SLGs studied here are nonporous high-quality graphenes with a negligible number of porous defects introduced during synthesis, transfer, and EC cleaning treatment.
Next, for the SLG 1 studied, polarised optical microscopy was also carried out using SLG 1 covered with commercial nematic liquid crystals (NLCs, Merck LC ZLI-2293) under a crossed polarisation state 5 . As shown in Supplementary Fig. S2b, the polarised optical microscopic image of a spin-coated NLC layer on SLG 1 shows large graphene domains (with an average radius of the domains > 100 µm) in the form of highly uniform optical retardation, beside small domains of several hundred nanometers 6,7 , clearly indicating that the SLGs studied here are high-quality graphenes with large-area graphene domains. We also investigated the AFM morphologies of the SLGs (insets in Supplementary Fig.   S3a). As shown by the AFM morphologies, the SLGs exhibit fairly smooth surfaces on the VOLET substrates; the SLGs present AFM morphologies that are nearly identical, with a low RMS roughness of 1.4~2.0 nm.
Next, the transport characteristics of the SLGs used were observed by assessing liquid-gated lateral Gr-FETs with SLG channels (channel length 50 µm, Supplementary   Fig. S4), as shown in Supplementary Fig. S3b. For the SLG 1 Supplementary Fig. S3b), confirming that SLG 2 is undoped and intrinsic SLG. From the V G,Dirac value of SLG 2 , the estimated E D is approximately ~4.44 eV. Note that the E D value of 4.44 eV is in good agreement with that (~4.49 eV) of epitaxial monolayer graphene 13 . which again confirms that the SLG 2 source used here is certainly undoped and intrinsic SLG. Similar to SLG 1 , the SLG 3 channel also shows a clear, asymmetrical V-shaped curve with a V Dirac value of ~0.63 V/V Ag/AgCl (lower panel in Supplementary Fig. S3b). With this V Dirac , the estimated value of E D for SLG 3 is approximately ~4.98 eV. Thus it is noted that the work function and Dirac point energy of SLG 3 are similar to those of SLG 1 , indicating that the PEDOT:PSS HIL may have a p-type doping effect on SLG 11 . From the transfer characteristics, the carrier mobilities μ of the SLGs were also estimated using the relationship μ = (L/WC g V DS )(ΔI D /ΔV GS ) 14 , where L, W, and C g are respectively the channel length (50 µm), the width (1600 µm), and the top-gate capacitance of graphene (~1.9 µF/cm 2 ) 15 . The estimated hole mobilities for SLG 1 and SLG 3 are approximately ~410 cm 2 /(V s) and ~530 cm 2 /(V s) , respectively, while the hole and electron mobilities for SLG 2 are approximately ~580 cm 2 /(V s) and ~530 cm 2 /(V s), respectively ( Table 1).
The above observations allow us to deduce the energy band diagrams of the studied SLGs on VOLET substrates at V GS = 0 V (Supplementary Fig. S3c). In the diagrams, ∆E FD represents the Fermi level with respect to its Dirac point energy E D . For 11 SLG 1 , ∆E FD is approximately 0.32 eV, which is slightly higher than those of both SLG 2 (0.26 eV) and SLG 3 (0.23 eV). The potential difference (Δ) with regard to the ITO/Al 2 O 3 /SLG interfaces was also determined; the Δ values are -0.01 eV for SLG 1 , 0.50 eV for SLG 2 , and -0.01 eV for SLG 3  Regarding the transport characteristics of the SLGs studied, a liquid-gated lateral graphene FET (Gr-FET) was prepared using an FET substrate with the ACN electrolyte, 12 which was identical to that used in the EC-cleaning treatment (see Methods section).
The channel of the studied SLG of the Gr-FET was gated through the ACN electrolyte with the Ag/AgCl reference electrode by sweeping the gate voltages from -0.8 to 0 V and then to +0.8 V with a sweep rate of 30 mV/s at V DS = 100 mV. In general, the liquid-gate Gr-FET has better transfer characteristics than conventional back-gate Gr-FETs, because the liquid gate exhibits higher capacitance than the back gate 11,15 . The electrical characteristics of the Gr-FETs were measured using a source meter (Keithley 2400). For bottom-contact OTFT devices, we used a SiO 2 /Si FET substrate, as described in the Methods Section. Prior to the deposition of the active layer, the substrates were cleaned using UV ozone. A self-assembled monolayer of pentafluorobenzenethiol (Aldrich) was formed on the Au electrodes to improve the metal/organic contact.
Hexamethyldisilazane (Aldrich) was then spin-coated on the substrate at 4000 rpm and baked at 125 °C for 10 min. The active channel layer was then solution-coated on the substrates from a blended solution using a simple solution-coating method 16