Hole Injection Characteristics and Annealing Temperature Dependence for Organic Light-Emitting Diodes Using Spontaneous Polarization

An efficient carrier injection at the electrode/organic semiconductor interface of organic light-emitting diodes (OLEDs) is challenging. It is well-known that an electric dipole at the electrode/semiconductor interface improves the carrier injection from the electrode because of a vacuum-level shift. In this study, poly (vinylidene fluoride-trifluoroethylene) [P(VDF/TrFE)] is used as a polar polymer with spontaneous polarization. P(VDF/TrFE) is spin-coated on an indium tin oxide (ITO) electrode surface as a hole injection layer for OLEDs. The P(VDF/TrFE) layer enhances the hole injection from the ITO electrode by causing a vacuum-level shift. As a result, the device driving voltage can be lowered. In addition, annealing temperature dependence is observed. To investigate the origin of the hole injection effect, the P(VDF/TrFE)-layered ITO surface and the thin film structure of the P(VDF/TrFE) layer are measured. The P(VDF/TrFE) layers at different annealing temperatures show no difference in surface properties but show significant differences in the thin film structure. The hole injection of OLEDs is significantly enhanced because of the structural change in ultrathin P(VDF/TrFE) layer.