Abstract
Here we report experimental results of tuning towards white light emission from blends of poly(9,9-di-n-octyl-2,7-fluorene) (PFO) or its copolymer Cz75PF (in which ca. 67 mol% of the monomers are grafted with a carbazole group at the end of its alkyl side-chain) with regio-regular poly(3-n-hexyl-2,5-thiophene) (P3HT). The blue-emitting PFO or Cz75PF serves as the host, whose emission spectrum significantly overlap with absorption spectra of the red-emitting P3HT guest. As these conjugated polymers are inherently immiscible, there should be limited presence of P3HT chains thermodynamically dissolved or kinetically trapped in the PFO or Cz75PF matrix. One would expect clear effects from phase heterogeneity, which can be manipulated thermodynamically by adjusting blend composition at low guest levels and kinetically by choosing solvents of different vapor pressures and hence different evaporation rates during film formation. In the first approach, we used different levels (0.15 to 5.0 wt%) of P3HT doping for the Cz75PF. Other processing parameters were kept fixed; specifically, chloroform was consistently adopted as the main solvent. In the second approach, we used a fixed level (0.5 wt%) of P3HT doping for the PFO matrix; the solvent used was then varied from chloroform, tetrahydrofuran (THF), to chlorobenzene in the order of decreasing vapor pressure. By means of ultraviolet-visible (UV-vis), phololuminescence (PL), excitation (PLE), electroluminescence (EL), and transient electroluminescence (TREL) spectroscopic analysis of the solutions and the cast films, we reach the following conclusions that satisfactorily describe variations in EL device performance. (1) Energy transfer (presumably the Förster-type) from PF chains to P3HT chains molecularly dispersed within the PF matrix is inherent in these blends. (2) Trapping and recombination of carriers on P3HT chains molecularly dispersed in the PF matrix exerts dominant effects in electroluminescence. (3) Macroscopically separated P3HT domains serve as in efficient drains of carriers. (4) Slow solvent evaporation results in formation of beta phase in the PFO matrix, which also allows for trapping and energy transfer from the PFO matrix composed dominantly of nematic glass.