Spectroscopic Analysis of Radical Molecules Using Charge Induced Spectroscopy

Enhancing the lifetime of organic devices is one of the essential considerations for achieving stable and high-performance devices. Small molecules in organic electronics are chemically or electrically stressed in the radical state, when they are oxidized or reduced for charge transfer through layers or for electrical doping to form charge transfer complexes. The lifetime research of organic light emitting diodes (OLEDs) primarily focuses on the understanding of the degradation characteristics that are prevalent in OLED devices during electrical operation. These characteristics; thermal or chemical instability of small molecules in OLEDs have been pointed out to be the reason behind the degradation of OLEDs which causes their short operational lifetime. Analyzing these degradation characteristics in organic materials prior to the fabrication of OLED devices is an unsophisticated method to achieve a better understanding of the stability of OLEDs.<br/>In this research, we investigated the stability of various emitting molecules includeing bis(2-phenylpyridine)(acetylacetonate)iridium(III) (Ir(ppy)₂(acac)), which is widely used as a green phosphorescent dopant in OLED devices, through matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) and charge-induced spectroscopy (CIS). In the MALDI-TOF investigation, the thermally degraded (aged) Ir(ppy)₂(acac) in a vacuum chamber was analyzed to have an m/z peak of 622.9 compared to its intrinsic m/z peak of 599.8. In the case of the CIS investigation, different absorption spectra were observed between the radical states of intrinsic and the degraded Ir(ppy)₂(acac). The Gaussian fitting technique was subsequently employed to fit the absorbance spectra which distinguished the additional radical absorption peaks after degradation and elucidated the thermal degradation mechanism. We therefore propose that the solution state analysis of thermal or electrical stability of organic materials using CIS can be a considerable approach to investigate degradation mechanism of OLEDs.