This investigation aims to examine emission properties contributed from different spatial distribution of beta phase in poly(9,9-di-n-octyl-2,7-fluorene) (PFO)-based emitting layer. This beta phase is well-known to be solvent-induced and results in pure blue emission. To identify effects of spatial distribution of beta domains on emission properties, we have used mainly a practical method of dipping PFO thin films into solvent/nonsolvent (such as toluene/methanol) mixtures in contrast to the traditional way of using a low vapor pressure solvent, such as chlorobenzene. To further elucidate these effects, we have also introduced poly(3-n-hexyl-2,5-thiophene) (P3HT) as guest into PFO.
We dipped PFO thin films cast from different solvents into solvent/nonsolvent mixtures of different compositions. We found that films cast from THF after dipping into toluene/methanol mixtures gave stronger beta emission than those dipped in tetrahydrofuran/methanol (THF/MeOH) mixtures, indicating the key effect from toluene. When films cast from THF were immersed into a given toluene/MeOH mixture over various periods of time, we observed that (1) congregated 刍 globules (sub-micron in size as revealed by atomic force microscopy) were induced on the film surface where PFO is in contact with toluene molecules and (2) these aggregated 刍 domains tended to suppress further development of beta phase in the depth direction. Results of impedance measurements indeed indicated the presence of a thin (effectively a few nm) beta-rich zone at the surface. This is in clear contrast to extensively developed but dispersed rod-like beta domains induced using chlorobenzene as the solvent.
This difference in morphology results in only minor differences in emission behavior of the beta-containing PFO films, as beta-emission dominates. However, in the presence of a minor amount (0.5 wt%) of poly(3-n-hexylthiophene) (P3HT), significant differences in emission behavior can be observed. In the case of congregated beta-globules near the electrode interface (as obtained from dipping), electroluminescence (EL) spectrum shifts from pure blue (0.19, 0.13) in terms of CIE coordinates to whitish blue (0.23, 0.25) with increasing bias. P3HT molecules limited the formation of beta domains in beta-rich zone resulted in this trend. In contrast, the EL spectrum of films containing dispersed rod-like beta-domains (as obtained using chlorobenzene as the solvent) shifts from nearly white to blue with increasing bias. Emission behavior of EL devices fabricated from as-spun P3HT-doped PFO film, films dipped in toluene/MeOH 1/25 or 1/1 mixtures were further compared. For the as-spun film, we observed white emission (0.34, 0.34) under a 15 V bias. For the film dipped in toluene/MeOH 1/25 mixture, white light emission (0.34, 0.31) is reached at 13 V. In the case of the film dipped in toluene/methanol 1/1 mixture, there is only a gradual shift from blue (0.19, 0.13) to whitish blue (0.23, 0.25) emission with increasing bias due to the higher beta content. Thickness of beta-rich zone determines these different emission properties.
With all these observations, we conclude the following. Firstly, a thin surface layer of congregated beta-globules is induced at the PFO film surface by dipping into the toluene/methanol mixture. This thin layer of congregated beta-globules, with thickness and compactness of beta-globules determined by the toluene/methanol ratio, serves as a diffusion barrier for toluene and inhibits further development of beta-globules in the depth direction. The carrier-trapping characteristics of this interfacial beta-rich zone in the subsequently fabricated EL devices results in confined recombination of carriers within this region and hence dominating beta-emission. In the P3HT-doped case, the thin beta-rich zone provides adequate competition with P3HT for carrier trapping/recombination such that white light emission may become prohibited.

1. Bernius M. T.; Inbasekaran M.; O'Brien J.; Wu W. S. Advanced Materials 2000, 23, 1737-1750.
2. Furuta P.; Brooks J.; Thompson M. E.; Frechet J. M. J. J. Am. Chem. Soc. 2003, 43, 13165-13172.
3. Hwang D. H.; Park M. J.; Kim S. K.; Lee N. H.; Lee C.; Kim Y. B.; Shim H. K.
J. Mater. Res. 2004, 2081.
4. 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.
5. Huang J.; Li G.; Wu E.; Xu Q.; Yang Y. Advanced Materials 2006, 1, 114.
6. Xu Y.; Peng J.; Mo Y.; Hou Q.; Cao Y. Appl. Phys. Lett. 2005, 16, 163502.
7. Gong X.; S. Wang; Moses D.; Bazan G. C.; Heeger A. J. Advanced Materials 2005, 17, 2053.
8. Xu Q.; Duong H. M.; Wudl F.; Yang Y. Appl. Phys. Lett. 2004, 16, 3357.
9. Gong X.; Ma W. L.; Ostrowski J. C.; Bazan G. C.; Moses D.; Heeger A. J. Advanced Materials 2004, 7, 615.
10. Liao L. S.; Fung M. K.; Lee C. S.; Lee S. T.; Inbasekaran M.; Woo E. P.; Wu W. W.
Appl. Phys. Lett.2000, 76(24), 3582.
11. Dias F. B.; Morgado J.; Macanita A. L.; da Costa F. P.; Burrows H. D.; Monkman A. P. Macromolecules 2006, 39, 5854-64.
12. Azuma H.; Asada K.; Kobayashi T.; Naito H. Thin Solid Films 2006, 509, 182-4.
13. Winokur M. J.; Slinker J.; Huber D. L. Phys Rev B 2003, 67, 184106/1-184106/11.
14. Khan A. L. T.; Sreearunothai P.; Herz L. M.; Banach M. J.; Kohler A. Phys Rev B 2004, 69, 085201/1-085201/8.
15. Cadby A. J.; Lane P. A.; Wohlgenannt M.; An C.; Vardeny Z. V.; Bradley D. D. C.
Synth. Met. 2000, 111-112, 515-8.
16. Grell M.; Bradley D. D. C.; Ungar G.; Hill J.; Whitehead K. S. Macromolecules 1999, 32, 5810-7.
17. Grell M.; Bradley D. D. C.; Long X.; Chamberlain T.; Inbasekaran M.; Woo E. P. et al. Acta Polym. 1998, 49, 439-44.
18. Cadby A. J.; Lane P. A.; Mellor H.; Martin S. J.; Grell M.; Giebeler C.; Bradley D. D. C. Phys Rev B 2000, 62, 23, 15604.
19. Kitts C. C.; Vanden Bout D. A. Polymer 2007, 2322-2330.
20. Morgadoa J.; Alcácer L.; Charas A. Appl. Phys. Lett.2007, 90, 201110.
21. H. H. Lu; C. Y. Liu; C. H. Chang; S. A. Chen. Advanced Materials 2007, 19, 2574-2579.
22. Khan A. L. T.; Sreearunothai P.; Herz L. M.; Banach M. J.; Köhler A. Phys Rev B 2004, 69, 085201.
23. Hansen. Journal of Paint Technology 1967, 39, 505.
24. Easteal A. J.; Woolf L. A. J. Chem. So. Faraday Trans I, 1984, 80, 1287-1295.
25. Gong X.; Moses D.; Heeger A. J.; Xiao S. Synth. Met. 2004, 141, 17-20.
26. S. H. Chen; A. C. Su; S. A. Chen. Journal of Physical Chemistry B 2005, 109(20),
10067-10072.
27. Habibur R. M.; C. Y. Chen; S. C. Liao; H. L. Chen; C. S. Tsao; J. H. Chen; J. L. Liao; Ivanov V. A.; S. A. Chen. Macromolecules 2007, 40(18), 6572-6578.
28. C. C. Hsiao; C. H. Chang; T. H. Jen; M. C. Hung; S. A. Chen. Appl. Phys. Lett. 2006, 88, 033512.
29. “White light emission via manipulation of phase heterogeneity in light-emitting layer of immiscible blends” by Yung-Cheng Chang, 2007