In this study, we dissolved hole transport layer (HTL) material NPB in THF (tetrahydrofuran) solvant, and spin-coated
the N,N'-Bis(naphthalene-l-yl)-N,N'-bis(phenyl)-benzidine (NPB) solution on the surface of Indium Tin Oxide (ITO)
anode to enhance the luminance efficiency and lifetime of flexible phosphorescent organic light emitting diodes
(POLEDs), where the 2,2',2''-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) was employed as hole
blocking layer (HBL) and its thickness was optimized. Such an improvement in the device performance was attributed to
the improved hole-electron balance. Finally, we employed 2,9-Dime-thyl-4,7-dphenyl-1,10-phenanhroline (BCP) or
TPBi as hole blocking layer. The maximum luminance efficiency reaching 24.4 cd/A can be obtained.