Host-Free Blue Phosphorescent Dendrimer Organic Light-Emitting Field-Effect Transistors and Equivalent Light-Emitting Diodes: A Comparative Study

Light-emitting field-effect transistors (LEFETs) integrate functions of an organic light-emitting diode (OLED) and a field-effect transistor, and hence offer significant advantages in simplifying the device architecture for next-generation active matrix full-color displays as well as having potential in communications and electrically pumped lasers. There is little work on phosphorescent light-emitting field-effect transistors, particularly those based on host-free phosphorescent light-emitting materials. In this report, we demonstrate solution-processed host-free LEFETs and OLEDs based on a blue phosphorescent dendrimer as the active emitter and compare their performance. The LEFETs exhibit excellent electrical and optical characteristics with a luminance of 650 cd m^-2 and an EQE of 2.1%, which is comparable to the equivalent OLEDs based on the same dendrimer. The LEFETs were fabricated in a heterostructure bilayer configuration using a p-type charge transport material, poly(2,5-bis(3-n-hexadecylthiophene-2-yl)thieno[3,2-b]thiophene) (PBTTT), underneath the emissive layer. The equivalent OLEDs were fabricated using PBTTT and MoO_x as the hole-injecting and -transporting layers. Remarkably, negligible EQE roll-off in the LEFETs at high current density and brightness was observed. These results indicate that for phosphorescent materials there could be an advantage in using LEFETs over OLEDs at high current density or brightness. In addition, we discuss the operating mechanism and effects of solvent used to deposit the dendrimer layer on the interface formed with the PBTTT layer. X-ray reflectometry showed that the interface between the PBTTT and dendrimer layer changed in terms of the lamellar ordering of the PBTTT layer upon solution deposition of the dendrimer layer, resulting in a lower charge carrier mobility in the devices.