Efficiency optimization of organic light-emitting diodes [thesis]

Shuwei Liu
Interest in Organic light-emitting diode (OLEO) technology was extensive lately, because of the unique advantages compared to its inorganic counterpart and other technologies. With extensive research of more than two decades, OLEO has advanced from pure laboratory research and stepped into the commercial world, however, due to current limitation in efficiency, yield and cost, OLEO technology has still a long way to go to be a dominant force in display and lighting market. This thesis focuses on
more » ... the optimization of OLEO through improvement of the internal and external efficiencies. Firstly, high-efficiency green emtsston phosphorescent OLEO with much better roll-off of efficiency was demonstrated. Replacing N,N'-bis(naphthalen-1-yl)-N,N'-bis(phenyl)-benzidine (NPB) with 4,4',4"tris(N-carbazolyl)triphenylamine (TCTA) as hole transporting (HTL) and electron blocking layer (EBL) not only eliminates the exciton quenching by relocating the exciton formation zone from the EBLI emissive layer interface to the hole blocking layer (HBL)/ emissive layer interface, which avoid exciton quenching by the lower triplet NPB, it also broadens the exciton formation with much more uniform exciton distribution profile within emissive layer, which is very useful to supress triplet-triplet annihilation, reducing the efficiency roll-off. Secondly, a graded host approach was employed to achieve higher efficiency red phosphorescent OLEO with lower roll-off. The graded host is a combination of two hosts with opposite charge transport abilities (i.e. n-type and p-type respectively), whose concentration fades towards the other emissive layer/ blocking layer interface. Due to the unipolar transport nature of most host material, the exciton formation region was commonly located at the emissive layer/ blocking layer interface, the narrow ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library Efficiency Optimization of Organic Light-emitting Diodes exciton formation zone has detrimental effects on the efficiency at higher current injection level. The use of graded host layer allows self-balanced charge transport within emissive layer; the gradual reducing host composition relocates the recombination zone away from the interface, which helps reduce the exciton quenching such as triplet-triplet annihilation, where two triplet excitons interact and result in only one triplet. 1 Lastly, to further improve the efficiency of OLEO, an external outcoupling technique was employed to extract portion of the light trapped (wave-guided) in the glass substrate which is also known as substrate mode. The outcoupling layer was fabricated by imprinting a layer of ZnO nanopillars arrays with varying pitches at the back side of the glass substrate. By studying the outcoupling characteristics of different pitched ZnO nanopillars, we achieved an efficient non-lambertian emission with much improved extraction efficiency. The angular emission characteristic of an OLEO was changed significantly with the pitch of ZnO nanopillar, the optimized device shows a much wider angular distribution with peak luminous intensity observed at 40° viewing angle. Both the theoretical fitting and simulation confirm that the improved extraction was achieved by diffraction and higher extraction probability around the pillars due to light incident on the vertical walls. ii ATTENTION: The Singapore
doi:10.32657/10356/64800 fatcat:25uoxtvnmrf2xpe6ihyzvktyyu