Efficiency enhancement of polyfluorene: Polystyrene blend light-emitting diodes by simultaneous trap dilution and β-phase formation

Abstract

By diluting poly(dioctylfluorene) (PFO) with low molecular weight polystyrene (PS), electron trapping and trap-assisted recombination can be strongly suppressed. For polymer light-emitting diodes (PLEDs) consisting of a PFO:PS (1:3) blend, a doubling of the efficiency is expected owing to trap dilution. Experimentally, we observe a much larger efficiency increase of nearly an order of magnitude. The electroluminescence and photoluminescence spectra of the PFO:PS blend show a sharpening of the vibrational peaks with regard to pristine PFO with the emission maximum at 436 nm. This spectral feature is characteristic of the formation of the β-phase in PFO upon PS addition. From the analysis of the absorption spectra, the fraction of the β-phase induced by blending typically amounts to 13%. The strong PLED efficiency enhancement therefore arises from the combined effect of β-phase formation (∼4 times) and trap dilution (∼2 times).