Tuning wavelength: Synthesis and characterization of spiro-DPVF-containing polyfluorenes and applications in organic light-emitting diodes

Abstract

We have synthesized polyfluorene copolymers containing bis(2,2-diphenylvinyl)fluorene pendent groups attached orthogonally to the C-9 positions of fluorene units. These polymers possess high glass transition temperatures and good thermal stability. The results from PL studies indicate that most excitons formed in the polyfluorene backbone by direct photoexcitation are likely to migrate to lower-energy pendent groups, from which emission occurs. An organic light-emitting device using the copolymer PF4-DPVF as the emitting layer exhibits a voltage-independent and stable blue emission with color coordinates (0.15, 0.17) at 11 V, with a maximum brightness of 3137 cd/m(2) at 9 V (262 mA/cm(2)) and a maximum external quantum efficiency of 1.06%. In addition, we blended PF4-DPVF as the host material with 0.5 wt % of MEH-PPV to realize a white electroluminescence having CIE coordinates of (0.29,0.34) and a maximum brightness of 3258 cd/m(2) (119 mA/cm(2)). We demonstrate that both Forster energy transfer and direct charge trapping/recombination on the MEH-PPV guest are responsible for the observed EL.