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Designing Conjugated Polymers for Photocatalysis and Ion Transport

Periodic Reporting for period 3 - CAPaCITy (Designing Conjugated Polymers for Photocatalysis and Ion Transport)

Reporting period: 2020-10-01 to 2022-03-31

Solar energy conversion will play an essential role in the future supply of clean energy. Secure access to energy sources will require energy conversion technologies that are low impact, distributed and accessible both technically and financially. Molecular electronic materials embody these possibilities, offering facile synthesis, low energy production and the versatility to allow performance to be maximized for specific applications. Moreover, they bring appealing similarities with nature’s intrinsically low impact energy conversion materials. The goal of CAPaCITy is to develop a modelling framework to understand the behaviour of molecular electronic materials, like conjugated polymers, when applied to solar energy conversion and storage. It is also intended to help the design of new materials. The underlying idea is that the properties of the molecular solids can be understood by modelling the properties of the individual molecules that make up the material, and the interactions between them. The project goals are to build tools to model the structure of molecular solids, their interaction with light and transport of electrical charge, and to use those tools to simulate and design materials for solar-to-chemical energy conversion and for charge storage. The models should be validated using experimental measurements of materials.
The work to date has provided computationally efficient tools to model the structure and electronic properties of molecular electronic materials, to design new materials and to understand the behaviour of such materials when applied to solar energy conversion and charge storage. In the case of polymer photocatalysts, we combined different modelling tools to show that the interactions between the polymer and its liquid environment control the likelihood of the first step in the photocatalytic process (Figure 1). We used the understanding to design better performing materials. Previous approaches had not considered the polymer-solvent interactions. In a separate study we developed a new model of charge recombination at a molecular interface, something which is relevant to both photovoltaic and photochemical solar energy conversion. We discovered that the non-radiative recombination, the main loss pathway in molecular materials, is controlled partly by the brightness of the excited states at the interface, and that this can in turn be controlled by choosing the energy levels of the materials. This represents a design rule to bring molecular solar energy energy conversion closer to the ideal limit. Finally, we demonstrated a novel battery device using conjugated polymers with polar side chain that can transport and store ions (Figure 2). The device could charge and discharge very rapidly compared to state-of-the art lithium ion devices and operates in a safe, salt-water electrolyte.

Figure 1. Snapshots of molecular dynamics simulation showing that whilst a polar polymer (b) photocatalyst avoids water, a non-polar polymer (a) locates at the interface between water and the sacrificial electron donor, triarylamine. In the latter case, the presence of the water increases the driving energy for charge separation, which is the first stage of the photocatalytic process.

Figure 2. (a) schematic of the battery device based on conjugated polymer electrodes with a salt-water electrolyte. (b) reactions that occur at the anode and cathode under changing and discharging.
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