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

Reporting period: 2018-10-01 to 2020-03-31

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, the major focus of the project is optimising a series of strategies to dye-sensitise the liquid-liquid interface. The project is mutli-disciplinary, with a range of experimental techniques bwing used to characterise a liquid-liquid interface, including electrochemical, spectroscopic and surface tension measurement methods.
The first Work-Package (WP) in this project involves the development of a host of novel experimental techniques to characterise materials self-assembled at electrified liquid-liquid interfaces. These techniques include UV/vis spectroscopy in total internal reflection (TIR) at an electrified liquid-liquid interface, confocal Raman at a liquid-liquid interface, photoelectrochemical characterisation of dye-sensitized liquid-liquid interfaces, scanning electrochemical microscopy (SECM) and surface tension measurements at electrified liquid-liquid interfaces (known as electrocapillary curves). Furthermore, anaerobic experiments are possible using electrodes fed into a glovebox. All the experimental setups described have now been realised and are being used to characterise the self-assembly and properties of porphyrin nanostructures at the liquid-liquid interface.
The second WP in this project is a series of experimental approaches to dye-sensitizing the liquid-liquid interface with the aim of maximise photoconversion at electrified soft interfaces. The UV/vis in TIR setup has proved particularly valuable to monitor the kinetics of porphyrin self-assembly. Furthermore, the photoelectrochemical setup designed to test traditional solar cells, such as dye-sensitized solar cells (DSSC), in a laboratory environment has proved superb at testing dye-sensitized liquid-liquid interfaces. Indeed, all photoelectrochemical methodologies, such as photocurrent transient or intensity modulated photocurrent spectroscopy (IMPS) measurements, applicable to a DSSCs have been demonstrated to be equally applicable at a dye-sensitized liquid-liquid interface.
Using a multi-technique approach, to date a precise set of experimental conditions has been determined to form ordered porphyrin nanostructures that are highly photoactive. Furthermore, the underlying mechanism giving rise to “dark” electrochemical signals in the presence of the porphyrin nanostructures has been identified. The use of superhydrophobic electron donors that do not partition to the aqueous phase has been particularly useful to eliminate artefacts due to ion transfer that may mask photo-induced electron transfer signals at the electrified liquid-liquid interface. Advanced models to determine the kinetics of each step in the photo-process (electron transfer, photoproduct separation, photoproduct recombination) have been developed and used to successfully model the IMPS data.
Future work remaining in the project will involve enhancing the conductivity of the porphyrin films by incorporation of conducting carbon nanomaterials and enhancing the photocurrents plasmonically by integrating metallic nanoparticles within the porphyrin films. Photoconversion efficiencies far beyond the current state-of-the-art using electrified soft interfaces have already been attained, but scope remains to further improve them and scale-up the self-assembled photoactive biphasic system for real-world deployment.
Significant progress has been made to understand the mechanism of porphyrin nanostructure formation at immiscible liquid-liquid interfaces. To achieve this, a unique UV/vis spectroscopy setup in total internal reflection (TIR) at a liquid-liquid interface has been developed. By monitored the shape and magnitude of the porphyrin Soret band upon adsorption of a porphyrin at the liquid-liquid interface, we can determine whether the porphyrins remain in a monomeric form at the interface or proceeds to form a nanostructure (so either a J- or H-aggregate). Thus, this UV/vis setup provides us with a powerful diagnostic tool to evaluate the changes in the type or rate of interfacial nanostructure formation upon introduction of the porphyrin species to the aqueous phase as a function of pH, porphyrin concentration, electrolyte composition, ionic strength, temperature, etc. Also, the influence of the nature of the substituent on the porphyrin on interfacial nanostructure formation is easily accessible. The more crystalline the porphyrin nanostructure, the better the photoconversion efficiency in theory. In other words, photo-generated excitons can migrate greater distances before being quenched in comparison to in disordered aggregates.
The interfacial porphyrin nanostructures developed thus far in the project are highly photoactive, as shown in the Figure. Major progress has been achieved to fully elucidate the mechanism of photocurrent generation in terms of the influence of the applied interfacial potential in particular. This has been achieved by firstly developing an understanding of the mechanism underpinning the electrochemical response of the interfacial porphyrin nanostructures in the dark. The latter stems from an adsorption/ion exchange mechanism involving both the interfacial porphyrin nanostructures, aqueous protons and organic cations. By understanding the origin of the dark electrochemical signals, they were suppressed using a careful choice of experimental conditions. This allows the full potential window to be accessed with which to study the influence of the applied electric field on the magnitude and kinetics of the photocurrents generated in the presence of an electron donor specie sin the organic phase.
Future work in the project will involve developing prorphyrin nanocomposites with conductive carbon nanomaterials, including those sources sustainably such as lignin. Also, attempts will be made to enhance the photoconversion efficiencies using plasmons generated at the interface in the presence of metallic nanoparticles. Ultimately, scale-up of the dye-sensitized liquid-liquid interfaces is envisioned to convert solar energy to chemical energy using a biphasic system that self-assembles spontaneously upon contact of the two immiscible phases.
Mechanism of photocurrent generation at a dye-sensitized liquid-liquid interface.