Exploring the Origin of White Emission in TAPC: Electron Transport Layer Based Exciplex Devices

Abstract

This study investigated white organic light-emitting diodes (OLEDs) with a single emissive layer and comprising a hole transport material 1,1-Bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) and two electron transport materials 1,3,5-Tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi) and 1,3,5-tris(3-pyridyl-3-phenyl)benzene (TmPyPB). The devices exhibited unique emission characteristics, from sky blue and orange exciplex emissions at low voltages to yellow electromer emissions at higher voltages. The combination of these emissions results in a distinctive white light output. To understand the origin of the white emission, time-resolved photoluminescence and time-resolved area-normalized emission spectra analyses are employed. These techniques provided insights into the exciton dynamics in the OLED devices and facilitated the determination of mechanisms underlying the white light emission. The investigation highlights the significant effect of intermolecular bonding between materials, which enhances the understanding of the molecular dynamics associated with white emission. Notably, the findings indicate that TPBi facilitates the generation of tightly bound exciplexes owing to its rigid and planar structure, while TmPyPB promotes the formation of loosely bound exciplexes owing to its flexible and nonplanar structure. The observed temporal evolution of emission characteristics improves the understanding of white emission OLEDs with a simple structure and offers insights into the design and optimization of OLEDs.