Chemical Stability of OLED Materials

The detection of chemical reaction products in device geometries has largely been limited to the observation of indirect effects. Electrochemical instability has been used to explain the differences in the lifetimes of OLEDs based on compounds of a homologous series when the expected correlation with the glass transition temperature (Tg) or the ionization potential was not observed.19 The oxidative 

FIGURE 3.1. Overall mechanism for Alq3 degradation in the presence of water. Water replaces one of the oxyquinolate ligands on the complex. The free Hq can then go on to form oxidized species which act as luminescence quenchers. 

A few other techniques have been effectively used in the chemical study of OLEDs, including infrared spectroscopy [for the generation of carbonyl impurities in poly(p-phenylenevinylene)],20 photoemission spectroscopy (capable of measuring valence and core electronic states at the surface of the material during deposition),13,30,32 39 and gas chromatography (which is well suited for Alq3 because some of the reactants and products are volatile).16,35 

FIGURE 3.2. Typical chromatogram for the reaction experiment showing Hq and water peaks. Inset is a schematic of the injection port and sample cell. 

The hydrolysis of Alq3 was characterized with a gas chromatography and mass spectroscopy (GC-MS) apparatus modified with a headspace accessory designed to study volatile products evolved from solid materials (illustrated in Fig. 3.2, inset).41 Powder or film (deposited on foil) samples of Alq3 were held in a constant-temperature injection port for a given time, and the eluents at the end of the heating period were fed to the GC column. The relative areas of water and Hq peaks were used to measure the conversion (a typical trace is shown in Fig. 3.2). The prepurified Alq3 samples were allowed to absorb an equilibrium amount of water from the atmosphere, which occurred in about 3 min for the samples used.16 

---