**4. Conclusions**

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.
