Semiconducting Cellulose Nanocrystal-Polyfluorene Emissive Materials in Organic Light-Emitting Diodes

Cellulose nanocrystals (CNCs) were functionalized with semiconducting polyfluorene grafts and employed in blue emissive organic light-emitting diode (OLED) devices. CNCs functionalized with surface bound aryl halides were grafted with conjugated poly (9,9-dihexyl fluorenes) through Ni(0) Yamamoto cross-coupling. The composite material, g-PF-CNC, exhibited polymer grafts that were isolated through an ester cleavage reaction and analyzed via MALDI-ToF, UV–vis, and TGA. Grafted polymer material properties were ascertained, demonstrating a weight average molecular weight of Mw ∼4000 Da, network-like CNCs interlinked with polyfluorenes, and a polymer loading above 10 wt %. OLEDs were fabricated on ITO/glass substrates using g-PF-CNC as the emissive layer coupled with a poly (4-n-hexyl triphenylamine) interlayer. I–V sweeps from 0–8 V revealed turn on voltages (Von) around 4.5 V followed by a steadily increasing current and blue emission characteristics of standard polyfluorene emitters. Emissive characteristics of the composite material OLED were compared with a standard polyfluorene OLED to highlight similarities in emission behaviors. Excitonic and excimeric emission peaks were found at similar wavelengths in both types of devices. Differences in overall emissive intensity were large but readily explainable by large differences in the amount of emitting material present. Differences in the relative intensities of the excitonic and excimeric emissions were observed, potentially manifesting due to low intensity readings, slight oxidations, or grafting behavior. The results of this work highlight synthetic and analytical methods used to produce a composite material which directly binds a semiconducting emitter polymer with a sustainable nanoscale substrate material.