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Solar Energy Conversion without Solid State Architectures: Pushing the Boundaries of Photoconversion Efficiencies at Self-healing Photosensitiser Functionalised Soft Interfaces

Periodic Reporting for period 4 - SOFT-PHOTOCONVERSION (Solar Energy Conversion without Solid State Architectures: Pushing the Boundaries of Photoconversion Efficiencies at Self-healing Photosensitiser Functionalised Soft Interfaces)

Reporting period: 2021-10-01 to 2023-03-31

Solar energy has an important role to play in meeting growing global demand for energy, yet conventional methods of making solar cells have some significant limitations. The conventional method of making solar cells is by using inorganic materials to make solid state architectures, through which light is harvested and converted into chemical energy. However, this approach has some shortcomings which limit the effectiveness of solar cells (high processing costs, occasionally the use of toxic materials, and defects/impurities that reduce photoconversion efficiencies). The research in SOFT-PHOTOCONVERSION is built on Dr Scanlon's expertise in controlling an electric field at a water-oil interface. The main objective is to explore a new paradigm in solar energy conversion by achieving efficient charge separation at these soft liquid-liquid interfaces, without solid electrodes. The ultimate goal is to push the boundaries of the maximum photoconversion efficiencies possible at soft interfaces (currently unsatisfactorily < 1 %). To achieve this goal, unprecedented levels of electrochemical control over photosensitiser assembly at soft interfaces must be attained, generating photoactive films with unique photophysical properties. By shining light on the dye-sensitized interface, electron transfer is promoted from a “donor” molecule in the oil to an “acceptor” molecule in the water. This leads to photoproducts (oxidized and reduced species) that are separated at the interface based on their affinity to water, with one side of the interface very hydrophilic, while the other is very hydrophobic. The concentration of dye at the interface is an important factor determining the solar conversion efficiency. Thus, a major focus of the project is optimising a series of strategies to dye-sensitise the liquid-liquid interface. The project is multi-disciplinary, with a range of experimental techniques being used to characterise a liquid-liquid interface, including electrochemical, spectroscopic, and surface tension measurement methods.
Overview of key results:
(1) The self-assembly of carboxyphenyl-substituted porphyrins into highly ordered nanostructures selectively at the interface between two immiscible liquids (Journal of Physical Chemistry C, 2020, 124, 6929). The unique physicochemical properties of the liquid-liquid interface are exploited to provide a one-step method in which the self-assembly of porphyrin molecules into crystalline nanostructures is triggered by manipulating the pH of the porphyrin aqueous solution. The photoactivity of the these free-floating porphyrin films is demonstrated in situ by illumination of the electrified liquid-liquid interface.
(2) The electrochemical analysis of the kinetics and thermodynamics of ion intercalation into solid matrices through purely ionic processes, not linked to a redox reaction. The dynamics of ion intercalation into a floating film of inorganic zinc porphyrin interfacial nanostructures are driven by electrifying an immiscible liquid-liquid interface (Journal of Physical Chemistry C, 2020, 124, 18346). Modelling using a Frumkin isotherm suggested a positive cooperativity mechanism for ion intercalation linked with structural rearrangements of the porphyrins within the nanostructures as a function of potential applied at the liquid-liquid interface.
(3) The reversible structural rearrangement of a soft porphyrin membrane under an electrical potential stimulus in the absence of solid-state architectures (Chemical Science, 2021, 12, 10227). The free-floating porphyrin membrane lies at the interface between immiscible aqueous and organic electrolyte solutions and is formed through interfacial self-assembly of zinc(II) meso-tetrakis(4-carboxyphenyl)porphyrins. In situ UV/vis absorbance and polarisation-modulated fluorescence spectroscopies in total internal reflection mode show that ionic intercalation and exchange involving the organi electrolyte cation induces a structural interconversion of the individual porphyrin molecules in the membrane from an H- to a J-type molecular configuration. These structural rearrangements are reversible over 30 potential cycles. Soft molecular assemblies that respond reversibly to external stimuli are attractive materials as on/off switches, in optoelectronic, memory and sensor technologies.
(4) The first conclusive proof that pathway complexity occurs at interfaces, in this case an interface formed between two immiscible liquids, and not just in bulk solutions (Journal of the American Chemical Society, 2021, 143, 9060). The consequences of this insight are profound since, by understanding the kinetics of the pathway complexity process, we can adjust the experimental conditions to favour one self-assembly pathway over another. This means that nanostructures with widely diverging physiochemical properties, such as their morphology, may be formed from the same initial monomers at the liquid|liquid interface. The model species studied is zinc(II) 5,10,15,20-(tetra-4-carboxyphenyl)porphyrin (ZnTPPc), a dye molecule of particular interest for solar energy conversion and storage applications. ZnTPPc has a strong absorbance in the visible range and is therefore highly amenable to UV/vis spectroscopic analysis. By combining a custom in situ UV/vis spectroscopy in total internal reflection methodology (TIR-UV/vis) methodology at the liquid-liquid interface and use of advanced chemometric tools to analyse the spectra, the complex mechanism by which self-assembly proceeds using kinetic model calculations (isodesmic and cooperative, respectively) was explained. Two parallel and competing pathways leading to the different ZnTPPc nanostructures were revealed.
(5) Electropolymerisation of thin films of free-standing conducting polymers at an electrified water-oil interface (Journal of the American Chemical Society, 2022, 144, 4853). Poly(3,4-ethylenedioxythiophene), known as PEDOT, the most commerically exploited polymer, was used as a model system. In a major breakthrough, the precise experimental conditions to electropolymerise a thin film of PEDOT were optimised using an aqueous oxidant (cerium) to oxidise the EDOT monomer exclusively at the electrified water-oil interface. This unique “direct-to-2D” method yielded PEDOT thin films with beyond-the-state-of-the-art biocompatability and outstanding conductivity.
Two novel methods to create photoactive electrified liquid-liquid interfaces have been developed; firstly the self-assembly of porphyrins into highly ordered nanostructures selectively at the interface (Journal of Physical Chemistry C, 2020, 124, 6929) and secondly the electrosynthesis of photoactive conducting polymers directly at the electrified water-oil interface (manuscripts in preparation).
The underlying mechanism of the nature of the photocurrents is elucidated by developing a model using a purely numerical, as opposed to analytical, approach (manuscripts in preparation).
The photoactive conductive polymer film can achieve photocurrent magnitudes of >80 microA/cm2 (manuscript in preparation). This is the highest ever photocurrent recorded at an electrified liquid-liquid interface.
Ultimately, scale-up of the photoactive conductive polymer-sensitized liquid-liquid interfaces is envisioned to convert solar energy to chemical energy using a biphasic system that can be electrosynthesised in a single step (with no solid electrodes required) between the two immiscible phases.
Mechanism of photocurrent generation at a dye-sensitized liquid-liquid interface.