White light electrophosphorescence from polyfluorene-based light-emitting diodes: Utilization of fluorenone defects
Poly(9,9-dioctylfluorene-co-fluorenone) with 1% fluorenone, (PFO−F(1%)), was synthesized as a model compound to investigate the optical and electrical effects of fluorenone defects in poly(9,9-dioctylfluoren-2,7-diyl), PFO. Photoluminescence (PL) and electroluminescence (EL) measurements demonstrate that PFO−F(1%) emits stable green light. PL and EL studies indicate that Förster energy transfer to and charge carrier trapping on fluorenone defects (with subsequent fluorenone emission) are responsible for the color degradation typically observed with the polyfluorenes. By utilization of “fluorenone defects” in PFO−fluorenone copolymers (PFO−F), white electrophosphorescent light-emitting diodes were fabricated. Polymer blends were spin-cast from solution containing PFO, PFO−F (1%), and tris[2,5-bis(9,9-dihexylfluoren-2-yl)pyridine-κ2NC3]iridium(III), Ir(HFP)3. The white emission turns on at approximately 5 V, with a luminance (L) of 6100 cd/m2 at 17 V. The luminous efficiency is 3 cd/A at current density of 8.5 mA/cm2 (L = 255 cd/m2). The white light exhibits stable color coordinates and stable color temperature and has a high color rendering index.