| Abstract [eng] |
Increasing demand of organic light emitting diodes (OLED) in lighting and display markets are associated with excellent electroluminescence (EL) spectra quality (absent emission in UV/infrared spectral region, smooth EL spectra without sharp spikes, high colour purity, etc.) and flexibility/optical transparency of commercial products offered by this technology. Due to huge interest of consumers, many scientists are working on further improvements of electricity-light conversion efficiency, operation stability, EL spectra quality of OLEDs. Therefore, input of this study is aimed towards fabricating human eyes friendly white colour OLEDs with excellent electroluminescent properties, which is one of the top priorities in the OLED community. In this work, white hybrid OLEDs were developed using two newly synthesized iridium (III) complexes with phosphorescence emission in orange or deep-red colour regions. Before OLED development, the chosen complexes were precisely investigated by photophysical, electrochemical and photoelectrical techniques in order to verify the potential of them to be used as a phosphorescent emitters in optoelectronic applications. The analysed compounds exhibited photoluminescence (PL) emission in orange and deep red regions with PL decays in microseconds range and PL quantum yield extending up to 60%. The solid-state layers based on the investigated molecules were characterized by ionization potentials falling in range of 5.17–5.4 eV and identical electron affinity value of 2.87eV. In the proposed device structure of white OLEDs, blue fluorescent, green thermally activated delayed fluorescent (TADF), and phosphorescent emitter were mixed in the light-emitting layer for obtaining white electroluminescence. For OLED fabrication, phosphorescent (triplet) and TADF emitters were selected due to their light-emissive singlet and triplet exciton recombination probability (theoretically 100%). In device structures, the blue fluorescent emitter additionally acted as the host for green and red emitters in the light-emitting layer. Ultralow concentrations of these emitters were optimized for preventing full energy transfer from the host to green TADF and red phosphorescent emitters. In case of usage of deep-red phosphorescent emitter, high-quality white electroluminescence with colour rendering index reaching 85 was observed from the hybrid, partially solution-processed, OLEDs exhibiting maximum brightness exceeding 10000 cd/m2 and high external quantum efficiency of 6.3%. In case of using orange phosphorescent emitter, white hybrid devices were characterized by high quality (human-eyes-friendly) electroluminescence with CIE1931 coordinates of (0.34, 0.39), colour temperature of 2910 K and colour rendering index of 72 as well as high maximum external quantum efficiency of 8.7%. |
