Solution-processed organic light-emitting devices with two polymer light-emitting units connected in series by a charge-generation layer

We present a solution-based process to fabricate stacked OLEDs consisting of two polymer light-emitting units (LEUs), connected in series by a charge generation layer (CGL). We used Cs2CO3-doped ZnO nanoparticles as an EIL on the LE-polymer to improve the electron injection from the cathode. The surface morphology of a spin-coated metal oxide nanoparticle appears to be rough, with many gaps due to agglutination of nanoparticles. We chose poly(4-vinyl pyridine) (PVPy) as a binder to improve the film morphology of the ZnO:Cs2CO3 mixture and facilitate the formation of a uniform and dense film to prevent the solvent from soaking into the 1st LEU. The efficient solution-based processing of EILs in the 1st CGL containing MoO3/poly-TPD bilayers was employed for the construction of an MPE device. The device exhibited a sum current efficiency of 10 cd A−1, with 4 cd A−1 contributed by the 1st unit and 6 cd A−1 by the 2nd unit.