Blue-emissive polymer light-emitting diodes through anode/cathode interfacial modification

We report highly efficient blue polymer light-emitting diodes (PLEDs) achieved by introducing two nanoscale interfacial layer, made of poly(fluorine-co-triphrnylamine) [PFO-TPA] and cesium carbonate (Cs ₂CO₃), between (1) the PEDOT:PSS and blue poly[9,9-diarylfluorene-co-2,5-Bisphenyl-1,3,4- oxadiazole] (P1)and (2) the aluminum cathode and the P1 emitter, individually. PFO-TPA with highest occupied molecular orbital level (-5.36 eV) lies between those of PEDOT:PSS (∼5.0 ∼ 5.2 eV) and P1 emitter (∼5.54 eV), forming a stepwise energy ladder to facilitate the hole injection. For Cs₂CO₃, firstly, it enhances the injection of electrons by providing an lower electron injection barrier. Secondly, applied Cs₂CO₃ buffer decreases the PL intensity slowly down to ∼96 % of the pristine P1 film, located at 422 nm, is less efficiency quenched than the Calcium (Ca). Therefore the overall electron injection attributed by Cs₂CO₃ buffer is higher. Thirdly, the device with Cs₂CO₃ buffer did not show the low-energy emission band originated from the fluorenone defects which are often introduced by Ca, thus stabilized blue emission from devices with high brightness can be demonstrated. Based on the hole-transporting PFO-TPA and the Cs₂CO₃/Al cathode, we obtained device efficiency and brightness as high as 13.99 cd A-1 and 35054 cd m-2, which is an improvement by two orders of magnitude higher over devices using Ca/Al as cathode and without hole-transporting PFO-TPA.