Performance improvement of blue thermally activated delayed fluorescence organic light emitting diodes via in-situ fabricated honeycomb porous polystyrene pattern

"Breath figure" (BF) method is an efficient strategy for preparing regular, porous optical array. In the BF process, the condensed water droplets are uniformly distributed on the solution surface because of the capillary force. Under the surface tension the water droplets are aggregated into larger droplets. After they are evaporated, the self-organized close-packed hexagonal porous array is formed. To inhibit the light loss of substrate-mode in organic light emitting diodes (OLEDs), we have in-situ fabricated honeycomb patterned (HCP) porous polystyrene (PS) external light extraction film via BF method outside the blue thermally activated delayed fluorescence (TADF) OLEDs. The performance of the corresponding OLED devices was investigated in detail. Different morphological structures of the HCP PS films were obtained by adjusting the PS concentration and relative humidity. The pore diameter fluctuates from 3.0 mu m to 10.0 mu m, and the pore height varies from 2.0 mu m to 7.0 mu m. The HCP PS film achieves the height/diameter (H/D) ratio near to 1.0 under relative humidity (HR) of 75% and 85%. The corresponding blue TADF OLED device depicts the highest current efficiency of 31.05 cd/A and power efficiency of 16.31 lm/W by the porous PS light-scattering film, with an increase by 53% and 21%, respectively. The finite-difference time-domain (FDTD) simulation exhibits that the H/D ratio near to 1.0 for the HCP PS film is the optimal tradeoff point for the light extraction in OLED device. Higher duty ratio of the pore distribution contributes to better light out-coupling effect. Furthermore, the appropriate refractive index (n) and the n difference between the film and the pores also help the light-extraction. The largest enhancement of external quantum efficiency for the TADF OLEDs employing the in-situ fabricated HCP porous PS is 17.11%.