Controlling Charge Accumulation Properties of Organic Light‐Emitting Diodes using Dipolar Doping of Hole Transport Layers

Abstract It is demonstrated that dipolar doping of hole transport layers (HTLs) controls the density and polarity of the accumulated charge at the critical interface between the HTL and the emission layer (EML) in organic light‐emitting diodes (OLEDs). Dipolar doping enables spontaneous orientation polarization (SOP) even in nonpolar HTL, and consequently compensates for the negative interface charge originating from the SOP of the adjacent layer. This concept is applied to a phosphorescent OLED, where bis‐4(N‐carbazolyl)phenylphosphine oxide (BCPO) is employed as a polar dopant for the HTL. The net interface charge is completely compensated at ≈29.5% of doping and further doping even facilitates the positive interface charge. The luminescence loss due to triplet‐polaron quenching is observed for both hole and electron accumulations, and it is suppressed by reducing the net interface charge density. On the other hand, the carrier balance factor linearly decreases with increasing doping ratio of BCPO. The results suggest that besides the energy level offset, SOP and permanent dipole moment of the materials should also be taken into account for realizing efficient carrier blocking interfaces. Dipolar doping is a versatile tool to tune charge accumulation, and to study its influence on device performance as well as the role of SOP in OLEDs.