Azobenzene Sulphonic Dye Photoalignment as a Means to Fabricate Liquid Crystalline Conjugated Polymer Chain‐Orientation‐Based Optical Structures

Abstract

layers. [9][10][11][12][13][14][15][16][17][18] This has allowed the demo nstration of polarized polymer light emitting diodes (LEDs), [7,12,19] highly polarized microcavity emission sources, [20] enhanced thin film optical gain media and microcavity lasers, [21][22][23] and opti mized charge carrier transport, including within field effect transistor (FET) device structures. [24][25][26] Rubbed alignment layers have, how ever, several potential drawbacks in rela tion to the resulting surface roughness, the presence of debris and electrostatic charges, and a limited ability to spatially pattern the subsequent liquid crystal orientation. Transfer printing can allow directional/local placement of oriented films but this approach is not conducive to straightforward fabrication. [27] Noncon tact photoalignment provides a different approach and a variety of polymeric and crosslinkable alignment layer materials have been used to align LCCPs and related oligomers for LED and FET applications. [28][29][30][31][32] For example, polarized electrolu minescence was demonstrated by Sainova et al., [29] albeit with relatively modest efficiency (≤0.1 cd A −1 ) and at relatively high drive voltage (≥10 V), additionally requiring doping of the alignment layer with a significant fraction of a hole transport The use of a noncontact photoalignment method to fabricate in-plane optical structures, defined by the local uniaxial ordering of liquid crystalline conjugated polymer chains, is reported. Molecular orientation is demonstrated for both green-light-emitting fluorene-benzothiadiazole alternating copolymer F8BT and F8BT/red light emitting complex copolymer Red-F binary blendfilms deposited on a well-known azobenzene sulphonic dye photoalignment material SD1. Absorption anisotropy ratios of up to 9.7 are readily achieved for 150 nm thickness F8BT films. Spatial pattern definition, afforded by masking the UV polarized light exposure of the photoalignment layer, allows the fabrication of optical structures with a resolution down to the micron scale. The alignment process is further extended to enable the serial, independent orientation of films deposited on top of each other and to permit the molecular orientation to follow curvilinear patterns. In the former case, this allows F8BT bilayer structures to be fabricated that show even higher absorption anisotropy ratios, up to ≈12, close to the theoretical limit for the previously deduced ≈22° optical transition dipole moment angle relative to the chain axis.

Two‐Photon Laser‐Written Photoalignment Layers for Patterning Liquid Crystalline Conjugated Polymer Orientation

“…This is consistent with results presented by Zhang et al. [ 27 ] The corresponding emission spectra recorded with excitation polarized perpendicular ( ⊥ ) to the polymer chains are shown in Figure S8a, Supporting Information; no spectral differences are evident but, as expected, weaker emission results because of the weaker alignment between the excitation beam polarization and the absorption dipole moment orientation for F8BT; angle β ≈ 22° relative to the chain axis. [ 20 ]…”

“…Also shown in Figure 3e is the nonaligned PL spectrum from an F8BT film following heating and quenching for a location atop a nonaligned SD1 layer; the spectrum peaks at 546 nm and generally matches PL spectra previously reported for nonaligned F8BT and other dialkylfluorene‐benzothiadiazole containing conjugated molecules. [ 25,27,43,44 ]…”

“…Photoalignment layers offer a promising alternative to rubbed high‐temperature polyimides, with previous studies demonstrating the alignment of LCCPs for a range of device applications. [ 8,16,21,26–28 ] One such photoalignment layer, sulphonic azo‐benzene dye (SD1), has received considerable past attention for use in LC devices [ 29–31 ] as well as for indirect alignment of functional nanostructures. [ 32–36 ] Recently, SD1 has also been shown to be capable of orienting LCCPs such as poly(9,9‐dioctylfluorene‐co‐benzothiadiazole) (F8BT) and other fluorene‐based polymers, yielding optical anisotropies comparable to those observed using rubbed high‐temperature polyimide alignment layers.…”

“…[ 32–36 ] Recently, SD1 has also been shown to be capable of orienting LCCPs such as poly(9,9‐dioctylfluorene‐co‐benzothiadiazole) (F8BT) and other fluorene‐based polymers, yielding optical anisotropies comparable to those observed using rubbed high‐temperature polyimide alignment layers. [ 1,18,19,25,27,28 ]…”

“…Microscopic features can be formed by masking the exposure of the layer to polarized light as demonstrated by Zhang et al. [ 27 ] who reported linear and curvilinear LCCP orientation features with linewidths down to ≈3 µm. A maskless approach using laser writing in turn offers improved pattern resolution and greater flexibility to reconfigure the pattern design by programming the laser or sample stage movements; such direct laser writing has been shown to be a powerful tool for producing high‐resolution spatial‐patterning in a number of functional optical materials.…”