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 (Tsub) 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 Tsub and becomes weaker as Tsub 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 Tsub 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 Tsub and becomes the highest when Alq3 layers are vacuum-deposited at Tsub = 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.
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