Evolution of optical properties of tris (8-hydroxyquinoline) aluminum (Alq 3 ) with atmosphere exposure

Abstract

Tris (8-hydroxyquinoline) aluminum (Alq₃) represents a material of significant interest for electron transport and/or light emitting layer applications in organic light emitting diodes (OLEDs). In spite of advances in Alq₃ based devices, the knowledge and understanding of the optical properties of Alq₃ and its chemical and environmental stability is still limited. With the reports of decreased turn-on voltage and increased efficiency of OLEDs, the issues of lifetime and stability of those devices are attracting increasing attention. The degradation of Alq₃ based OLEDs and dark spots formation and growth have been intensively studied. The studies on degradation of optical properties of Alq₃ itself remain scarce. We have investigated effects of atmosphere exposure to properties of tris (8-hydroxyquinoline) aluminum (Alq₃) thin films by photoluminescence (PL) and absorption measurements. Alq₃ films were evaportated on glass substrates at different temperatures. The influence of annealing to the environmental stability of the films has also been investigated. It has been found that deposition at higher substrate temperature and annealing of the samples deposited at room temperature yields improvement in environmental stability of the films, i.e. less decrease of the PL intensity over time with atmosphere exposure, as well as increased PL intensity. To investigate further effects of the air exposure, films deposited at room temprature were stored for four days in air, nitrogen, and oxygen. No decrease in PL intensity has been found for storage in nitrogen, while decrease for the film stored in oxygen was smaller than that for film stored in air, indicating that both humidity and oxygen play a role in PL intensity decrease in Alq₃ thin films.