Abstract
The goal of this work is to examine morphological characteristics and emission behavior of the beta phase in poly(9,9-di-n-hexyl-2,7-fluorene) (PFH) by means of microscopic and scattering techniques as well as spectroscopic methods. A dipping method is adopted to induce beta phase formation. We dipped PFH thin films cast from different solvents into mixed solutions (solvent/nonsolvent mixtures) and found that the PFH cast from tetrahydrofuran (THF) dipped into THF/methanol gave conspicuous beta phase contribution than other experimental conditions. It is well know that the beta phase in poly(9,9-di-n-octyl-2,7-fluorene) (PFO) exhibits an absorption peak at 430 nm and emits at 440 nm. Our optical absorption and photo-excited emission results indicate that the beta phase of PFH (different from PFO only in side-chain length by two carbons) behaves quite differently from its counterpart in PFO: the absorption and emission maxima are located at comparatively shorter wavelengths of 420 and 430 nm, respectively. We resolved the photoluminescence spectroscopy (PL) for beta-contained PFH thin film offered by alpha and beta phase, and found that characteristic peak would appear at 431, 458, and 490 nm which is blue-shift c.a. 6 nm compared with PFO. The morphology of pristine PFH thin film was very flat but forming globe-like aggregations (about 20 nm in diameter) after dipping treatment, so we hypothesized the beta phase would exist near the surface and formed a protective layer to prevent the solvent continuing diffusing into the inner pristine film. In single-layer architecture ITO/PEDOT/EML/Ca/Al, representative device performance, the pristine PFH cast from THF shifts from blue-emitting (0.17, 0.11) in CIE coordinates to blue-white light with increasing operating voltage. As blending poly(3-n-hexylthiophene) (P3HT) into PFH solution as guest, which has red-emitting (0.52, 0.47), the colors of the spectrum change to white-emitting but toward that in beta-contained case with increasing voltage.