Photoalignment and spatial patterning of liquid crystalline conjugated polymers for polarised light emission
Keywords: Conjugated Polymers
Optical Anisotropy
Soluble Semiconductors
Biomimetic Light Harvesting
Polymer Chain Orientation
Excited States
Meta-Materials
Molecular Electronic Materials &amp; Devices
Light-emitting Liquid Crystalline Conjugated Polymers
Polarised Light Emission
Förster Resonance Energy Transfer
Polymer Light Emitting Diodes
Photoalignment
Refractive Index Modification
Integrated Optical Circuits
Optoelectronic Devices
Organic Semiconducting Lasers
Two-photon Laser Direct Writing
Thin Film Transistors

Polymeric semiconductors have a number of benefits in the ease of processing and large-area production, tunability of the physical properties, low-cost, flexibility and biocompatibility. Although high-quality polymer alignment is highly favorable in terms of allowing access to fundamental optical/electrical anisotropy, the realisation of large-area extended uniform alignment and high-resolution spatial patterning of polymer chain-orientation remains a challenge. The purpose of this thesis is to investigate and optimise a non-contact photoalignment technique by exploiting the inherent long-range orientational ordering present in a liquid crystal (LC) mesophase to create highly aligned monodomains over cm2 areas with submicron patterning in thermotropic liquid crystalline conjugated polymers (LCCPs). The solution (water)-processable photoalignment material, azobenzene sulphonic dye (SD1), is presented within this thesis to be optically rewritable and remain stable at >300 °C temperatures, ensuring that it is suitable for orienting LCCP overlying films.
Two different photoalignment processes are explored in this thesis: the first involves using macroscopic linearly polarised ultraviolet (UV) light illumination to trigger alignment whereas the second method uses two-photon femtosecond direct laser writing (DLW), for the first time, to align/pattern the SD1 thin layers. The alignment conditions are optimised to enable ~100% order parameters for SD1-aligned nematic glass films of light-emitting LCCP fluorene-benzothiadiazole alternating copolymer F8BT. Uniform alignment (>10 dichroic ratios) is obtained for F8BT monodomain films with thicknesses in the range of 100 to 300 nm. Indirect photoalignment is also showcased using a 90 wt% F8BT:10 wt% non-LC Red-F binary blend film, whilst the photo-masked UV-alignment leads to a spatial pattern resolution of 2 - 4 µm.
Fine control of the domain structure and SD1-aligned long-range orientational ordering has been employed to tailor the order parameter in three types of F8BT films in order to study the structure-property relationships. The spin-coated amorphous F8BT reference film was found to possess no long-range structural order and exhibit the lowest photoluminescence (PL) lifetime and quantum efficiency (PLQE) of ~30%. It is also shown that LC alignment itself is responsible for a smeared X-ray diffraction ring and extra lower-energy absorbing species in the nematic polydomain F8BT glass film, in line with the formation of a minor (~5%) fraction of the locally and spontaneously oriented polymeric nanocrystals embedded in an amorphous matrix. It is suggested that this bi-phasic polydomain texture resembles a self-doped host-guest system in which nonradiative host-to-guest Förster resonance energy transfer and intrachain radiative recombination gives rise to a high PLQE of >70%, a value that approaches the PLQE limit measured for F8BT solutions. Our micro-PL spectral mapping results support that efficient radiative intrachain recombination most takes place in polymeric nanocrystals that act as low-energy acceptors and therefore dictates the PL behaviours in the self-organised F8BT nematic polydomain glass films. The SD1-aligned F8BT monodomain films have an order parameter of almost 1 and a >70% crystallinity, but show an intermediate PLQE of ~50% potentially due to the enhanced interchain conjugation length up to 20.5 nm and associated interchain PL emission.
Two-photon infrared laser writing using an ultrafast Ti:sapphire laser (785 nm, 250 fs, 80 MHz) has been utilised to inscribe bespoke photoalignment patterns with a resolution of 0.5 ~ 1 µm in F8BT monodomain glass films. Laser (re)writing enables a higher pattern resolution and integrated PL contrast of >15 than the polarised UV alignment. Laser writing and UV alignment induce different F8BT chain-conformations and interchain packing, as judged by the distinct polarisation-dependent vibronic transitions in the polarised micro-PL spectra. We elucidate that the efficient (>70% PLQE) and linearly polarised intrachain emission dominates the parallel-polarised PL// spectrum, whereas the red-shifted perpendicular-polarised PL⊥ spectrum indicates an interchain emission character with a significantly reduced PLQE of ~10%.
The anisotropic structural and spectral information presented in this thesis complements previous studies on non-oriented light-emitting conjugated polymers. Bringing structural order and creating spatial alignment-pattern offers great potential for the fabrication or performance improvement of various devices such as polarised/micro-organic light-emitting diodes, flexible thin-film transistors, microcavity lasers, meta/quantum-materials, and integrated optical circuits.