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Abstract:

Systematic tuning of chemical and physical structure allows fine control over the desired electronic and optical properties of many molecular materials, including conjugated polymer semiconductors. In the case of physical structure, molecular orientation via liquid crystalline alignment allows access to fundamental optical anisotropies and the associated refractive index modification offers great potential for the fabrication of photonic structures. In this paper, we report on the use of photoalignment to orient the liquid crystalline conjugated polymer poly(9,9-dioctylfluorene-co-benzothiadiazole), specifically involving two-photon infrared laser writing of patterns in an azobenzene sulphonic dye photoalignment layer. These patterns are transferred into the overlying film by thermally orienting the polymer chains in their nematic phase and are then frozen in place by quenching the film to room temperature as a nematic glass. Optimization of the laser power and scan speed allows features to be achieved with linewidths down to 1 um or less. Photoluminescence (PL) peak anisotropy values reach PL_para / PL_perp = 13 for laser writing, compared with PL_para / PL_perp = 9 for polarized UV-LED exposure of the same azobenzene sulphonic dye alignment layer. The two approaches also result in different film microstructures as evidenced by characteristic changes in PL spectra. The anisotropic PL spectra provide information on the emissive excited states that complements previous studies on non-oriented poly(9,9-dioctylfluorene-co-benzothiadiazole) and related copolymers, also suggesting two emissive state but with more complex spectral signatures than previously considered.