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

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 beta-phase in PFO upon PS addition. From the analysis of the absorption spectra, the fraction of the beta-phase induced by blending typically amounts to 13%. The strong PLED efficiency enhancement therefore arises from the combined effect of beta-phase formation (similar to 4 times) and trap dilution (similar to 2 times).