Thermally annealed nano-crystalline Alq₃ thin films for yellow and green OLED solid-state lighting application

In this work, we investigate the effect of thermal annealing under ambient conditions on the structural, optical, electrical and morphological properties of nano-crystalline tris(8-hydroxyquinolinato)aluminum (Alq₃) thin films and their impact on the performance of flexible yellow and green organic light-emitting diodes (OLEDs) and solid-state lighting devices. Nano-crystalline Alq₃ thin films under ambient conditions were deposited on ITO-based flexible substrates using spin coating technique and subsequently annealed at various temperatures(room temperature-200 °C) to optimize their crystallinity and emission characteristics. X-ray diffraction (XRD) and scanning electron microscopy analyses reveal enhanced crystallinity and controlled surface morphology with increasing annealing temperature particularly at 150 °C, leading to improved charge transport and photoluminescence properties and formation of emissive polymer Al(8-Hq-N-O)₃[tris(8-hydroxyquinoline-N-oxide)aluminum(III)]. Optical studies show a significant enhancement in the emission intensity and a desirable shift in the photoluminescence spectrum suitable for white light generation. Prototype flexible green and yellow OLEDs incorporating the optimized Alq₃ thin films demonstrate low turn on voltage of 2.8 V and improved luminance efficiency of 6338.6 cd/m², color stability with CRI 98 and EQE of 1.48%, indicating their promising potential for next-generation flexible displays and energy-efficient solid-state lighting. These findings highlight the crucial role of thermal annealing in tailoring the microstructure of organic emissive layers to achieve high-performance, flexible optoelectronic devices.