Improved efficiency in fluorescent blue organic light emitting diode with a carrier confining structure

A blue organic light emitting device (OLED) with improved efficiency and good color purity is reported. The highest occupied molecular orbital (HOMO) level of the hole transport layer (HTL) and that of the emissive layer (EML) differs by 0.3 eV. This energy level mismatch confines the carriers at the HTL/EML interface. Conventional devices have only one HTL/EML interface, with a current efficiency of 2.9 cd/A. Without adding a separate hole blocking layer, incorporating multi-layers of the same HTL and EML increases this efficiency to 5.8 cd/A, with only a small increase in operating voltage yielding increased power efficiency also. But, there are an optimum number of layers, beyond which efficiency loss results. Also, including the multilayer structure simultaneously improves the blue color co-ordinates. To gain insight into the role of multilayer structures in modifying charge transport and recombination zone a simulator was developed. The simulated results could qualitatively explain the experimental observations.