Saturated and Stable White Electroluminescence from Linear Single Polymer Systems Based on Polyfluorene and Mono-Substituted Dibenzofulvene Derivatives

Abstract

The development of new electroluminescent copolymer based materials for white polymer light emitting diodes (WPLED) with special emphasis on the conceptual novelty of utilizing aggregation-induced emission enhancement (AIEE) active or dual state emitting, monosubstituted dibenzofulvene (M-DBF) derivatives 4-(5-fluoren-9-ylidenemethyl-thiophen-2-yl)-7-(4-methoxy-phenyl)-benzo[1,2,5]thiadiazole (DT) or 4-[5-(4-fluoren-9-ylidenemethyl-phenyl)-thiophen-2-yl]-7-(4-methoxy-phenyl)-benzo[1,2,5]thiadiazole (DP) as orange/red fluorophores into polyfluorene (PF) main chain is demonstrated. The mol % of these M-DBF monomer based special small π-system has been optimized as low as 0.0003, 0.0006, and 0.001% in the poly[2,7-(9,9′-dioctylfluorene)-co-4-(5-fluoren-9-ylidenemethyl-thiophen-2-yl)-7-(4-methoxy-phenyl)-benzo[1,2,5]thiadiazole (WDT) and poly[2,7-(9,9′-dioctylfluorene)-co-4-[5-(4-fluoren-9-ylidenemethyl-phenyl)-thiophen-2-yl]-7-(4-methoxy-phenyl)-benzo[1,2,5] thiadiazole (WDP) copolymers, respectively. We report the utilization of “dual emission” peaks from PFO and M-DBFs in the blue and orange region that enabled the fabrication of bias-independent WPLEDs which is fundamentally very important and highly challenging in the case of linear polymers. These copolymers showed excellent solubility in organic solvents viz. CHCl3, THF, toluene, p-xylene, etc. The synthesized copolymers were characterized by UV–vis and photoluminescence (PL) spectroscopy, and WPLEDs with ITO/PEDOT:PSS/WDP or WDT/TPBi/LiF/Al configurations were fabricated to study their electroluminescence (EL) properties. Partial energy transfer has been achieved by adjusting the PFO and DT or DP concentrations in the single polymer main chain leading to the white emission in a facile manner. The copolymers WDP-1 and WDT-1 gave saturated white emission with CIE coordinates of (0.31, 0.33) and (0.35, 0.34) at 8–14 voltages and exhibiting excellent voltage independent emission. The highest luminous efficiencies of 7.82 and 4.57 cd A–1 were achieved for WDP and WDT polymers with the highest brightness values of 9753 and 7436 cd m–2, respectively.