Solution-processable small molecules as efficient universal bipolar host for blue, green and red phosphorescent inverted OLEDs

Inverted organic light-emitting diodes (IOLEDs) with a bottom cathode are of great interest for large-size active-matrix displays due to their easy integration with n-type thin film transistors (TFTs) based on low-cost and highly-uniform amorphous silicon and oxide. In this work, a solution-processable electron transporting material 2,7-bis(diphenylphosphoryl)-9,9'-spirobi[fluorene] (SPPO13) is employed to blend with a solution-processable hole transporting material 4,4',4 ''-tri(9-carbazoyl) triphenylamine (TCTA) to be used as a universal bipolar co-host for blue, green and red phosphors, and for the first time, phosphorescent IOLEDs are fabricated by solution-processing small molecules. High efficiency and reduced efficiency roll-off are achieved in the solution-processed IOLEDs, which mainly contribute to the high quality of the solution-processed small molecule films as well as the balanced charge injection in the co-host system. Importantly, the solution process is advantageous over vacuum evaporation to deposit multi-component small molecule films, and can be expected to reduce manufacturing costs. Our results demonstrate a promising approach to fabricate low-cost and high-performance IOLEDs for n-type TFT-based displays.