Highly efficient green TADF organic light-emitting diodes by simultaneously manipulating hole and electron transport

This work demonstrates effective performance improvement by simultaneous manipulating of the hole injection and electron transport layers for (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile (4CzIPN) based green thermally activated delayed fluorescent (TADF) organic light-emitting diodes (OLEDs). A 3 wt% sorbitol doped PEDOT:PSS layer results in the highest maximum current efficiency (CE_max) of 28.28 cd A^-1 and external quantum efficiency (EQE) of 17.04%. Single carrier devices denote that hole mobility gradually rises with the sorbitol ratio. The electroluminescence mainly originates from the emission of 4CzIPN. Atomic force microscopy images imply that 3 wt% sorbitol doped PEDOT:PSS film includes the largest PEDOT aggregate, which contributes to a higher electric conductivity thus the better performance of 3 wt% sorbitol doped device. Also the 4CzIPN ratio in the emissive layer was optimized, and 4 wt%-4CzIPN in CBP achieves the highest EQE of 20.99% and CE_max of 34.99 cd A^-1. The EL spectrum is independent of the luminous angle at a low 4CzIPN ratio but becomes more sensitive to the luminous angle at a high 4CzIPN ratio. Finally, we find out that the TADF OLED performance is very sensitive to TPBi thickness ranging from 20 nm to 65 nm, and 40 nm of TPBi achieves a CE_max up to 64.10 cd A^-1 and an excellent EQE of 25.14%, ascribing from its more balanced carrier transport.