Active Control of Spontaneous Orientation Polarization of Tris(8-hydroxyquinolinato)aluminum (Alq₃) Films and Its Effect on Performance of Organic Light-Emitting Diodes

Vacuum-deposited amorphous organic films with an anisotropic amorphous structure are known to exhibit spontaneous orientation polarization (SOP). In this study, amorphous structures of tris(8-hydroxyquinolinato)aluminum (Alq₃) films are controlled by substrate temperature (T_sub) during vacuum deposition. The effect of density, molecular orientation, and SOP of Alq₃ films on the performance of organic light-emitting diodes (OLEDs) is comprehensively investigated. The SOP of Alq₃ films is significant at low T_sub and becomes weaker as T_sub increases. The presence of SOP certainly improves electron injection from a metal cathode and, therefore, increases the current in OLEDs. External quantum efficiencies (EQEs) of OLEDs are increased by ≈40% when T_sub for the Alq₃ deposition is increased from 260 to 328 K. The increased EQEs are attributed to several factors, which include not only increased photoluminescence quantum yields and light outcoupling efficiencies but also suppressed singlet-polaron annihilation originating from the reduced SOP. Operational stability of OLEDs is also affected by T_sub and becomes the highest when Alq₃ layers are vacuum-deposited at T_sub = 299 K. These findings indicate the importance of controlling organic amorphous structures and SOP in films to maximize OLED performance and contribute to a better understanding of working mechanisms of OLEDs.