It is now considered that organic light emitting diodes (OLEDs) are the most appropriate technological solution for high quality displays [1].
One of the most investigated compounds applicable for wet processing is Ir(ppy)3 (tris(2-phenylpyridine)iridium) [2] due to its exceptional thermal and chemical stability and high photoluminescence quantum yield (PPL) of near 1.0. If it is vacuum deposited, OPL retains near a unity, in contrast, if it is solution processed, aggregation of Ir(ppy)3 occurs, which leads to phase separation between emitter and host material [3], as a consequence – efficiency of OLED decreases. To overcome this condition it has been determined that adding to Ir(ppy)3 bulky groups with purely isolating or charge transport functionality provides physical barrier that suppress the tendency of aggregate formation and helps to keep a large distance between emitter molecules.
In this study Ir(ppy)3 and its structural analogues with gradually increasing number of attached passive isolating groups were investigated. The molecular composition of these compounds consists of Ir(ppy)3 core and one (1TPY), two (2TPY) or three (3TPY) attached bulky triphenylmethane groups (TR). Absorption and emission spectra of all compounds did not show significant deviations among compounds, which means that adding TR groups does not greatly affect the electronic configuration of Ir(ppy)3 core.
Onwards series of corresponding OLEDs with solution-processed emissive layer containing electron deficient, hole deficient or balanced host material were created and analyzed.
The best outcome in terms of turn on voltage, current and power efficiencies and maximal brightness were achieved using balanced host material.
Compounds investigated in this study can be used as emitters in solution-processible OLEDs, but further studies are required.