Solution processing of conjugated polymers: the effects of polymer solubility on the morphology and electronic properties of semiconducting polymer films

It is becoming increasingly clear that the electronic properties of conjugated polymers are controlled by the way the films are cast: changing the solvent, spin speed or concentration changes the film morphology and thus the performance of devices based on these materials. In this paper, we show that the way a conjugated polymer is dissolved into solution also affects the interchain interactions and electronic behavior in the resulting film. Light scattering shows that even low molecular weight samples of poly(2,5-bis[N-methyl-N-hexylamino]phenylene vinylene) (BAMH-PPV) do not completely dissolve in the good solvent o-xylene, even after stirring for 2 days. The solutions behave more as a suspension of small pieces of polymer film, showing solid-state effects such as exciton-exciton annihilation; the corresponding cast films have a rough, agglomerated morphology. Complete dissolution of the polymer can be achieved either by heating the solutions while stirring for 2 days, or by stirring at room temperature for 2 weeks. In addition to aiding dissolution, heating is found to promote interactions between conjugated polymer chains, leading to films with a higher degree of exciton-exciton annihilation and devices with higher operating currents but lower electroluminescence quantum efficiencies than films cast from solutions that were fully dissolved but not heated. All the results suggest that understanding the details of how a conjugated polymer is dissolved into solution is critical to being able to reproducibly fabricate and optimize conjugated polymer-based devices. (C) 2001 Elsevier Science B.V. All rights reserved.