Work-function-tunable metal-oxide mesh electrode and novel soluble bipolar host for high-performance solution-processed flexible TADF-OLED

Abstract

Substantial effort has been dedicated to the development of solution-processed flexible organic light-emitting diodes (sf-OLEDs). However, to simultaneously enhance device performance and ensure stable operation under severe mechanical deformation, highly flexible transparent electrodes and efficient soluble organic emitting materials must be optimized together. Here, highly efficient green-emitting thermally activated delayed fluorescence (TADF) sf-OLEDs were developed using a mesh-structured Ni-doped indium zinc oxide (mNIZO) flexible electrode and a novel soluble bipolar host. The mNIZO electrode had excellent optical, electrical, and mechanical properties. The work function of the mNIZO electrode was engineered through surface doping with Ni without optical and electrical losses. 5-(9 H-Carbazol-9-yl)− 3′-(3,6-di-tert-butyl-9 H-carbazol-9-yl)-[1,1′-biphenyl]− 3-carbonitrile (CzCN-tCz) was synthesized as a soluble host material by incorporating a tert-butyl group into 3,3′-di(carbazol-9-yl)− 5-cyano-1,1′-biphenyl (mCBP-CN). Consequently, the mNIZO-based TADF sf-OLED with CzCN-tCz had a maximum external quantum efficiency of 21.0 % on a poly(ethylene 2,6-naphthalate) substrate, and 82 % of the initial luminance was maintained under severe mechanical stress, which is attributable to the high deformability of the mNIZO electrode. The proposed framework is expected to facilitate the design of high-performance cost-effective flexible displays with various forms through a simple process.