Five new thermally robust electroluminescent fluorene-based conjugated copolymers, including poly[2,7-(9,9-dioetylfluorene)-co-4,7-15,6-bis(3,7-dimethyloctyloxymethy l)-2,1,3-(benzothiadiazole))] (PFO-P2C(10)BT) were synthesized and used to fabricate the efficient polymer light-emitting diodes (PLEDs). The glass transition temperatures of the polymers were found to be higher than that of poly(9,9-dialkyl-fluorenes) and are in the range 113-165 degrees C. We fabricated PLEDs in indium-tin oxide/PEDOT/light-emitting polymer/cathode configurations using either double-layer LiF/Al or triple-layer Alq(3)/LiF/Al cathode structures. The new copolymers were found to have emission colors that vary from greenish blue (491 nm) to green (543 nm) depending on the copolymer composition. The maximum brightness and luminance efficiency of these PLEDs were found to be up to 5347 cd/m(2) and 1.51 cd/A at 10 V, respectively. (c) 2008 Wiley Periodicals, Inc.