Single Conjugated Polymer with Four Stepwise HOMO Levels for Effective Hole Injection Across Large Barrier 1.4 eV to Core–Shell Quantum Dot Layer for Electroluminescence in Inverted QLED

Abstract A novel graded‐HOMO‐level hole transport polymer (g‐HTP) is proposed for the first time with spiro‐polyfluorene (sPF) as the main chain and the three hole‐transporting moieties triphenyl amine (TPA), carbazole (Cz), and N , N ′‐dicarbazolyl‐3,5‐benzene (mCP) as side chains for effective hole injection across the large barrier 1.4 eV into the green‐emission core–shell quantum dot (QD) CdSe/ZnS emission layer in inverted QD light‐emitting diode (i‐QLED): ITO/ZnO/QD/HTP/MoO 3 /Al, in which both oleophilic ligands (OA and TOP) in the QD are partially removed from QD surface by annealing at 270 °C for preventing dissolution by solvent in subsequent coating of hole transport material (HTM) solutions atop and also improving electron and hole mobilities, especially the former. The proposed g‐HTP with various m : n mole ratios of mCP to TPA/Cz moieties along with various reported HTMs (PVK, Poly‐TPD and TFB) are investigated. Among them, the proposed g‐HTP with the comonomer mole ratio 1:1 gives the best performance η max 36.1 cd A −1 and B max 208 600 cd m −2 , which is the highest performance among the reported i‐QLEDs with single hole‐transport layer ever documented. In addition, its efficiency roll off is low from 35.8 cd A −1 at 10 000 cd m −2 to 31.1 cd A −1 at 100 000 cd m −2 .