Monochromatic Red Electroluminescence Based on Copolymer of N-Vinylcarbazole and Eu-Complexed Acrylic Acid

Abstract

Europium complexes exhibited intense red fluorescence with a narrow spectral bandwidth (5-10 nm). But non-uniform blending or dispersion of complex resulted in phase separation, decomposition and inefficient energy transfer to reduce emission efficiency. The complexes were covalently attached to the main chain of polymers, which improved film quality and solubility. Eu(TTA)2(AA)phen as europium complex monomer (ECM) was prepared by Eu3+ coordinating with 1,10-phenanthroline (phen), acrylic acid (AA) and 2-thenoyltrifluoroacetone (TTA). Fluorescent polymer marked as p1-30 (molar ratio for ECM/N-vinylcarbazole (NVK), 1/30) was synthesized. The emissions of ECM and p1-30 in the films peaked at ca. 610 nm. The fluorescence lifetime of p1-30 in solid was measured to be 384.58 (τ1) and 802.44 μs (τ2). The highest occupied molecular orbit (HOMO) and the lowest unoccupied molecular orbit (LUMO) energy levels were -5.36 and -1.99 eV. The red organic light-emitting devices (OLED) A ITO/PEDOT: PSS/PBD: p1-30/AlQ3/LiF/Al and B ITO/PEDOT: PSS/PBD: p1-30/BCP/AlQ3/LiF/Al were fabricated. Devices A and B mainly emitted at 614 nm with full width at half maximum (FWHM) of 12.6 nm and exhibited luminance of 375 and 136 cd/m2 with Commission International de L’Eclairage (CIE) coordinates of (0.50, 0.42) and (0.63, 0.35). The copolymer p1-30 had good film quality to avoid phase separation. The emission from the carbazole moieties was suppressed, electroluminescent (EL) spectra revealed that efficient intramolecular energy transfer from carbazole unit to europium complex occurred and that emission efficiency was greatly improved. Device B showed monochromatic red emission.