Controlled assembly of conjugated polymers
Conjugated polymers and polyelectrolytes (CP/CPEs) exhibit excellent optoelectronic properties, which can be exploited to deliver advanced optical sensing platforms and organic solar cells. They offer the advantage of being compatible with “wet” processing technologies such as printing, providing a route to low cost thin film device fabrication. However, the nanoscale morphology of the resultant films will be highly dependent on the aggregation state of the polymer in the processing solution, which can be difficult to reproducibly control.
Our interest is in the use of solvent- and surfactant-mediation to control the self-assembly and aggregate structure of CP/CPEs in solution and subsequently deposited thin films. We use a combination of small-angle scattering (light/x-rays/neutrons), microscopy and spectroscopy to correlate the structural organization with the optoelectronic properties of the deposition solution. We then use this information to fabricate thin film devices with improved performance.
Self-Assembled Conjugated Polyelectrolyte-Surfactant Complexes as Efficient Cathode Interlayer Materials for Bulk Heterojunction Organic Solar Cells, M. Chevrier, J. E. Houston, J. Kesters, N. Van den Brande, A. E. Terry, S. Richeter, A. Mehdi, O. Coulembier, P. Dubois, R. Lazzaroni, B. Van Mele, W. Maes*, R. C. Evans* and S. Clément*, J. Mater. Chem. A, 2015, 3, 23905-23916.
All-Conjugated Cationic Copolythiophene “Rod-Rod” Block Copolyelectrolytes: Synthesis, Optical Properties and Solvent-Dependent Assembly. A. Thomas, J. E. Houston, N. Van den Brande, J. De Winter, M. Chevrier, R. K. Heenan, A. Terry, S. Richeter, A. Mehdi, B. Van Mele, P. Dubois, R. Lazzaroni, P. Gerbaux, R. C. Evans* and S. Clément*, Polym. Chem., 2014, 5, 3352-3362.
Luminescent solar concentrators and spectral converters to improve solar cell performance
Fundamental spectral losses constitute the largest source of inefficiency in every developed photovoltaic technology. These losses arise due to ineffective harvesting of high (UV) and low (near-IR) energy photons at the boundaries of the solar spectrum. This can be ameliorated through the application of a photoactive layer, that is extrinsic to the photovoltaic device itself, and is able to effectively harvest light in the UV or NIR regions and convert it to photons whose energies are more favourably matched to the spectral window of the solar cell. There are two photophysical approaches that can be exploited to achieve this: (i) luminescent solar concentration/down-shifting (LSC/LDS) and (ii) upconversion (UC).
Our group is interested in the development of novel multifunctional organic-inorganic hybrid materials for spectral conversion applications that specifically overcome the limitations of the materials traditionally employed. This includes: (1) new coordination and templating approaches to control lumophore placement and orientation within the host material, thereby limiting losses due to reabsorption; (2) the design of novel high refractive index materials to inhibit waveguide losses in LSCs; (3) new barrier/encapsulant materials to extend the device lifetime.
Targeted Design Leads to Tunable Photoluminescence from Perylene Dicarboxdiimide–Poly(oxyalkylene)/Siloxane Hybrids for Luminescent Solar Concentrators, I. Meazzini, N. Willis-Fox, C. Blayo, J. Arlt, S. Clément, R. C. Evans*, J. Mater. Chem. C., 2016, 4, 4049-4059.
Design and Response of High-Efficiency Planar Doped Luminescent Solar Concentrators using Organic-Inorganic Di-Ureasil Waveguides, A. Kaniyoor, B. McKenna, S. Comby, R. C. Evans*, Adv. Opt. Mater., 2016, 4, 444-456.
Multifunctional organic-inorganic hybrid materials
Organic-inorganic hybrids represent an elegant approach to manipulate the orientation and localisation of functional building blocks within a protective host material. Weak interactions (e.g. hydrogen bonding, ionic, pi-pi stacking) at the organic-inorganic interface (class I hybrids) can be harnessed to ensure homogenous mixing of the two components on the macroscale, whilst simultaneously promoting nanoscale phase separation and aggregation at the active interface. Alternatively, functional molecules can be covalently-grafted to the host material (class II) in order to control their placement and orientation. In addition, the host material itself may bring exploitable properties, which may yield hybrid materials with significantly superior properties due to a synergistic effect between individual components.
For example, we have recently shown that the immobilization of blue-emitting poly(fluorenes) within ureasil hybrids results in a dramatic enhancement of the photoluminescence quantum yield. Moreover, substitution with red- or orange-emitting conjugated polymers yields a solid-state white-light emitter with potential application in low energy indoor lighting.
Tunable White-Light Emission from Conjugated Polymer-Di-Ureasil Materials, N. Willis-Fox, M. Kraft, J. Arlt, U. Scherf, and R. C. Evans*, Adv. Funct. Mater., 2016, 26, 532–542.
Synergistic Photoluminescence Enhancement in Conjugated Polymer-Di-ureasil Organic-Inorganic Composites. N. Willis-Fox, A.-T. Marques, J. Arlt, U. Scherf, H. D. Burrows, L. D. Carlos and R. C. Evans*, Chem. Sci., 2015, 6, 7227-7237
Optical sensing, imaging and theranostics
The presence of lateral ionic chains on CPEs means that they show a tendency to undergo ionic self-assembly with oppositely charged species. This often leads to a change in the optical fingerprint of the CPE that can be exploited in optical sensing. We have previously used this approach to develop a sensor platform for the detection of anionic surfactants – commonly encountered pollutants in wastewater. More recently, we have developed a sensor that is able to sequentially detect key phase transitions occurring in model cell membranes. We are currently combining our expertise in CP self-assembly and organic-inorganic hybrid materials to develop core-shell organic-inorganic hybrid nanocapsules that are able to carry and transport optically-responsive molecules for application in sensing and imaging, with the end-goal of combining both functionalities in theranostic platforms.
Charge-Mediated Localization of Conjugated Polythiophenes in Zwitterionic Model Cell Membranes, J. E. Houston, M. Kraft, I. Mooney, A. E. Terry, U. Scherf and R. C. Evans*, Langmuir, 2016, 32, 8141–8153.
Sequential Detection of Multiple Phase Transitions in Model Biological Membranes Using a Red-Emitting Conjugated Polyelectrolyte, J. E. Houston, M. Kraft, U. Scherf and R. C. Evans*, Phys. Chem. Chem. Phys., 2016, 18, 12423-12427.
Cationic Polythiophene-Surfactant Self-Assembly Complexes: Phase Transitions, Optical Response, and Sensing. R. C. Evans*, M. Knaapila, N. Willis-Fox, M. Kraft, A. Terry, H. D. Burrows, U. Scherf, Langmuir, 2012, 28, 12348-12356.
Representative examples of some of the current projects being undertaken in the group may be found below.