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

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 degrees 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 eta(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).