Blue Light-Emitting Diodes with Good Spectral Stability Based on Blends of Poly(9,9-dioctylfluorene):  Interplay between Morphology, Photophysics, and Device Performance

Spectrally stable blue electroluminescence (EL) is obtained from single-layer polymer light-emitting diodes fabricated from binary blends of conjugated poly(9,9-dioctylfluorene) (PFO) with either thermally stable poly(vinyl diphenylquinoline) (PVQ) or polystyrene. The brightness and EL efficiency of the polymer blend LEDs were enhanced by a factor of 5−14 compared to the PFO homopolymer devices. The additional green emission observed in the EL spectra of pure PFO devices was substantially suppressed in the blend LEDs. The electrical characteristics of the diodes and electric-field-modulated PL spectroscopy results indicate increased spatial confinement induced exciton stability and electron−hole recombination efficiency in the blend devices. The variation of the device performance with blend composition is related to the phase-separated morphology of the blends. The origin of the spectral stability lies in the improved thermal stability of the PFO:PVQ blends due to the high glass transition temperature (Tg) of PVQ (185 °C). These results demonstrate that blending of PFO with high Tg charge transport or charge blocking polymers is a simple strategy to overcoming the problem of poor spectral stability of blue-emitting polyfluorenes.