Photophysical Properties and Device Characteristics of Novel Organic Light-Emitting Materials

Abstract

In this thesis, we study the photophysical and electroluminescent (EL) properties of various novel organic light-emitting materials. First, we show that the photophysical and EL properties of phosphole-based oligomers can be tailored by variation of the 2,5-substitution pattern of phospholes and chemical modification of their P atoms. Phosphole-based gold complexes have broad dual-emitting emission spectra, which may be useful in applications of white organic light-emitting diodes. Next, we study the photophysical properties and the light-emitting electrochemical cells (LECs) of two diazaspirobifluorene-based cationic iridium complexes. Their superior steric hindrance is confirmed by their highly retained photoluminescence quantum yields (PLQYs) in neat films (> 50 % of the PLQYs for both complexes dispersed in a high-gap solid-state host matrix). The LECs based on neat films of these complexes possess high external quantum efficiencies (EQEs) over 7 %. To further reduce self-quenching of excited-states in neat-film emissive layers, which deteriorates the device efficiency, the complex with a lower bandgap (guest) is doped in that with a higher bandgap (host) to form a host-guest system. The host-guest films are shown to exhibit 1.5X higher PLQYs than neat films of host or guest, indicating suppressed self-quenching. The EQEs of the LECs based on optimized host-guest diazaspirobifluorene-based cationic iridium complexes are enhanced to exceed 10 %. Finally, white LECs based on host-guest cationic iridium complexes (blue-green emitting complex as the host and red-emitting one as the guest) are demonstrated. These white LECs exhibit power efficiency of 7.8 lm/W and EL spectra with high color rendering indices up to 80, suggesting their potential applications in solid-state lighting.