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.

1. S. H. Chen, A. C. Su, C. H. Su and S. A. Chen, J. Phys. Chem. B 2006, 110, 4007-4013.
2. D. Neher, Macromol. Rapid Commun. 2001, 22, 1365-1385.
3. U. Scherf and Emil J. W. List, Adv. Mater. 2002, 14, 477-487.
4. M. Grell, D.D.C. Bradley, X. Long, T. Chamberlain, M. Inbasekaran, E.P.Woo and M. Soliman, Acta Polym. 1998, 49, 439-444.
5. M. Grell, D. D. C. Bradley, G. Ungar, J. Hill, and K. S. Whitehead, Macromolecules 1999, 32, 5810-5817.
6. J. Morgado, L. Alcácer and A. Charas, Appl. Phys. Lett. 2007, 90; 201110.
7. H. H. Lu, C. Y. Liu, C. H. Chang, S. A. Chen, Adv. Mater. 2007, 19, 2574–2579.
8. Catherine C. Kitts and David A. Vanden Bout, Polymer 2007, 48, 2322-2330.
9. A. J. Cadby, P. A. Lane, H. Mellor, S. J. Martin, M. Grell, C. Giebeler, and D. D. C. Bradley, Phys. Rev. B 2000, 62, 15604-15609.
10. M. C. Hung, J. L. Liao, S. A. Chen, S. H. Chen and A. C. Su, J. Am. Chem.
Soc. 2005, 127, 14576-14577.
11. M. Ariu, M. Sims, M. D. Rahn, J. Hill, A. M. Fox, D. G. Lidzey, M. Oda, J. C. Gonzalez, D. D. C. Bradley, Phys. Rev. B 2003, 67, 195333.
12. Ho, G. K.; Meng, H. F.; Lin, S. C.; Horng, S. F.; Hsu, C. S.; Chen, L. C.; Chang S. M. Appl. Phys. Lett. 2004, 4576.
13. D. W. Bright, F. B. Dias and F. Galbrecht, unpublished result.
14. M. Knaapila, F. B. Dias, V. M. Garamus, L. Almásy, M. Torkkeli, K. Leppänen, F.
Galbrecht, E. Preis, H. D. Burrows, U. Scherf, and A. P. Monkman, unpublished result.
15. “ White light emission via manipulation of phase heterogeneity in light-emitting layer of immiscible blends” written by Chang Yung-Cheng.