Spatial Patterning of the β‐Phase in Poly(9,9‐dioctylfluorene): A Metamaterials‐Inspired Molecular Conformation Approach to the Fabrication of Polymer Semiconductor Optical Structures

Abstract

Materials in which sub‐wavelength physical structures, rather than variations in chemical composition, are used to modify the nature of their interaction with electromagnetic radiation form the promising new class of metamaterials. For molecular materials one has an intriguing alternative, namely structuring the conformation or physical geometry of the molecule. In order for this to be an effective methodology one needs the change in conformation i) to engender a significant change in electromagnetic properties and ii) to be spatially controllable to allow patterning of practical structures. In this paper the potential of such an approach is demonstrated through spatial patterning, via masked solvent vapor exposure, of the β‐phase conformation in poly(9,9‐dioctylfluorene) (PFO). Significantly the conformation change approach preserves a planar film format and is found not to negatively impact on optical gain properties, both very attractive features for optoelectronic and photonic lightwave circuit applications. As a specific demonstration the ability to spatially control the lasing wavelength for samples in which a β‐phase conformation is selectively patterned in a glassy PFO film spin coated atop a one‐dimensional distributed‐feedback grating etched into a spectrosil substrate is shown.

Tuning the Exciton Diffusion Coefficient of Polyfluorene Based Semiconducting Polymers

“…The PL spectra of all polyfluorene derivatives is dominated by a peak very close to 423 nm, with a further peak around 447 nm and a shoulder around 480 nm. This emission is characteristic of the glassy phase of polyfluorene . We measured the solid‐state photoluminescence quantum yield (PLQY) using an excitation wavelength of 375 nm and the results are given in Table .…”

Spectroscopic properties of poly(9,9‐dioctylfluorene) thin films possessing varied fractions of β‐phase chain segments: enhanced photoluminescence efficiency via conformation structuring

“…The recently demonstrated spatial patterning of the β‐phase conformation in thin films of PFO 15 and PFO derivatives on length scales ≤500 nm has been proposed as an attractive approach to the design and fabrication of novel nanophotonic elements . A versatile imaging method is therefore needed to determine the spatial distribution of β‐phase fraction within such samples to optimize and control the simultaneously induced local change in refractive index .…”

“…The profiles for both 1581 cm −1 /1606 cm −1 and 1257 cm −1 /1606 cm −1 ratios are identical and indicate that the β‐phase fraction smoothly increases then decreases across the stripe, following a “semicircular” variation with its maximum value at the stripe‐center. The change in refractive index due to the presence of β‐phase chain segments will follow a similar graded profile; an advantageous situation for light propagation since scattering losses are thereby reduced relative to a step index profile …”

Sub‐Micrometer Patterning of Amorphous‐ and β‐Phase in a Crosslinkable Poly(9,9‐dioctylfluorene): Dual‐Wavelength Lasing from a Mixed‐Morphology Device

“…Although there has been a recent report by Ryu et al on spatial patterning of a PFO fi lm into aand β -phase areas, respectively, by masked solvent exposure, this method does not allow for sub-wavelength patterning of the fi lm, e.g., the preparation of diffraction gratings or photonic crystal structures. [ 6 ] It also remains problematic to integrate β -phase PFO into solution-processed multilayer devices, since subsequent solvent and heat treatment will destroy the fragile β -phase morphology.…”