Diketopyrrolopyrrole-containing polyfluorenes: Facile method to tune emission color and improve electron affinity

A series of novel diketopyrrolopyrrole (DPP)-containing polyfluorenes, coded as PF-DPP01-50, were synthesized through palladium-catalyzed Suzuki polycondensation with different feed ratios between fluorene dibromide and DPP dibromide. Their chemical structures and compositions were verified by H-1 NMR and elemental analysis. DSC and TGA results show that they have good to excellent thermal stabilities. Absorption and photoluminescence (PL) properties of PF-DPP01-50, determined in both CHCl3 solutions and thin films, exhibit regular changes with increasing of DPP contents in copolymers, with absolute PL efficiencies being in the range 13.8-26.9%. Electroluminescence (EL) properties of all the copolymers were investigated with device configurations of ITO/PEDOT/copolymer/Ba/Al and ITO/PEDOT/copolymer/Al. A very low content of DPP units (1%) is needed to achieve full energy transfer from fluorene segments to DPP units; hence, exclusive emission of the latter occurs. EL colors of these copolymers vary from orange to red, corresponding to CIE coordinates from (0.52, 0.46) to (0.62, 0.37). The best performance was achieved by orange-emitting PF-DPP01 in device configuration of ITO/PEDOT/copolymer/Ba/Al, with maximum EQE of 0.45% and maximum brightness of 520 cd/m(2). Devices with configuration of ITO/PEDOT/copolymer/Al prove that DPP units can effectively improve the electron affinity of these copolymers. One of these devices (ITO/PEDOT/PF-DPP25/Al) can realize maximum EQE of 0.14% and maximum brightness of 127 cd/m(2). Therefore, color-tuning (red-shift) and improvement of electron affinity can be achieved at the same time through incorporation of DPP units.