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/MoO₃/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.