Three kinds of red electroluminescent (EL) polymers based on polyfluorene as blue host and 2,1,3-benzothiadiazole derivatives with different emission wavelengths as red dopant units on the side chain are designed and synthesized. The influence of the photoluminescence (PL) efficiencies and emission wavelengths of red dopants on the EL efficiencies and color purities of the resulting polyfluorene copolymers of dopant/host system is investigated by adjusting the electron donating ability of the donor units in D-pi-A-D typed 2,1,3-benzothiadiazole derivatives. The devices of these red-emitting polymers realize remarkable EL efficiency/color purity trade-offs. The single-layer devices with the configuration of ITO/PEDOT:PSS/Polymer/Ca/Al show pure red emission at 624 nm with a luminous efficiency of 3.83 cd A(-1) and CIE of (0.63, 0.35) for PFR1, saturated red emission at 636 nm with a luminous efficiency of 2.29 cd A(-1) and CIE of (0.64, 0.33) for PFR2, respectively. By introduction of an additional electron injection layer PF-EP(Ethanol soluble phosphonate-functionalized polyfluorene), high performance pure and saturated red emission two-layer devices (ITO/PEDOT:PSS/Polymer/PF-EP/LiF/Al) were achieved with maximum luminous efficiencies of 5.50 cd A(-1) and CIE of (0.62, 0.35) for PFR1, 3.10 cd A(-1) and CIE of (0.63, 0.33) for PFR2, respectively, which are the best results for pure and saturated fluorescent red EL polymers reported so far.