Thermal Cross-linked Electron Transport Polymers for Suppressing Efficiency Roll-off in Green Solution-Processed Inverted OLEDs

Solution-processed phosphorescent inverted organic light-emitting diodes (s-IOLEDs) have garnered significant attention due to their excellent stability and high performance. However, frequently used inorganic electron transport layers usually cause exciton dissociation at the emitting layer interface, leading to low device efficiency and severe efficiency roll-off. In this work, we designed a cross-linkable triazine-grafted electron transport copolymer (PPDPT-co-PBCB) with a high triplet energy (3.11 eV) to suppress this exciton dissociation. Balance between electron transport ability and cross-linkability was controlled by varying the ratios between PPDPT and PBCB (9:1 and 8:2), resulting in significantly improved device efficiency. Our s-IOLED incorporating PPDPT-co-PBCB (8:2) achieved a current efficiency of 59.85 cd A-1 and an external quantum efficiency (EQE) of 16.17% with a minimal efficiency roll-off (1.05%) at a luminance of 5000 cd/m2. Furthermore, estimations of the recombination zone width and interfacial mixing width revealed that the device with PPDPT-co-PBCB (8:2) featured a relatively broad recombination zone, a weak triplet-triplet annihilation, and a narrow interfacial mixing width, contributing to its enhanced efficiency and reduced roll-off.