Periodic Reporting for period 4 - CAPaCITy (Designing Conjugated Polymers for Photocatalysis and Ion Transport)
Reporting period: 2022-04-01 to 2024-01-31
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 are validated using experimental measurements of materials. CAPaCITy aimed to use the tools developed to help optimise to two particular applications in energy conversion, one, the conversion of solar energy to chemical energy using polymer photocatalysts, and two, the storage of energy in conjugated polymer electrodes by charging them using aqueous electrolytes. By identifying which material properties help optimise the efficiency of solar energy conversion and energy storage, the research helps to develop material design rules that lead to higher performing materials and devices.
The work 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 photocatalytic activity, by controlling the likelihood of the first step in the photocatalytic process where a polymer that has been excited by absorbing light transfers a charge to a molecule in the surrounding solvent (Figure 1). The work could explain why polymers that contain polar components outperform those that do not. We used this 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 conversion closer to the ideal limit. By adjusting the energies of the states involved in recombination, their brightness, and the strength of coupling to vibrational modes, the rate of energy loss through non-radiative recombination could be suppressed, improving the efficiency of molecular solar cells (Fig. 2).
Regarding electrochemical energy storage, we demonstrated a novel battery device using conjugated polymers with polar side chain that can transport and store ions (Figure 3). 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. Our research addressed the mechanical stability of the electrode materials and using multiscale modelling and experiments we showed that whereas polar groups are needed for the electrodes to charge in aqueous electrolytes, the mechanical stability is greatly helped by introducing a small fraction of non-polar side groups. An all-polymer version of the device was developed using a bio-derived polymer for the electrolyte which shows promising performance.
All these applications were underpinned by basic research into the relationship between polymer structure and the material’s electronic properties. In one important development we showed that ordered packing of polymer chains is not necessary for the material to show good electronic transport properties. In fact, for stiff and linear conjugated polymer chains, the highest electronic mobility is achieved in structures where chains cross at right angles, and this can be designed in via the polymer’s chemical structure. This introduced a new design approach for high performance conjugated polymers (Fig. 4).
The results have been disseminated through 60 publications with further work in review. Results have been widely presented at scientific conferences by PI and team members. The team collaborated on a record efficiency solar cell using knowledge developed in the project. Exploitation of the polymer battery device is being pursued.
Figure 1. 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.
Figure 2. Non radiative energy losses in solar cell
Figure 3. (a) schematic of the battery device based on conjugated polymer electrodes with a salt-water electrolyte. (b) reactions at anode and cathode under changing.
Figure 4. Connectivity in C16IDTBT
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 conversion closer to the ideal limit. By adjusting the energies of the states involved in recombination, their brightness, and the strength of coupling to vibrational modes, the rate of energy loss through non-radiative recombination could be suppressed, improving the efficiency of molecular solar cells (Fig. 2).
Regarding electrochemical energy storage, we demonstrated a novel battery device using conjugated polymers with polar side chain that can transport and store ions (Figure 3). 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. Our research addressed the mechanical stability of the electrode materials and using multiscale modelling and experiments we showed that whereas polar groups are needed for the electrodes to charge in aqueous electrolytes, the mechanical stability is greatly helped by introducing a small fraction of non-polar side groups. An all-polymer version of the device was developed using a bio-derived polymer for the electrolyte which shows promising performance.
All these applications were underpinned by basic research into the relationship between polymer structure and the material’s electronic properties. In one important development we showed that ordered packing of polymer chains is not necessary for the material to show good electronic transport properties. In fact, for stiff and linear conjugated polymer chains, the highest electronic mobility is achieved in structures where chains cross at right angles, and this can be designed in via the polymer’s chemical structure. This introduced a new design approach for high performance conjugated polymers (Fig. 4).
The results have been disseminated through 60 publications with further work in review. Results have been widely presented at scientific conferences by PI and team members. The team collaborated on a record efficiency solar cell using knowledge developed in the project. Exploitation of the polymer battery device is being pursued.
Figure 1. 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.
Figure 2. Non radiative energy losses in solar cell
Figure 3. (a) schematic of the battery device based on conjugated polymer electrodes with a salt-water electrolyte. (b) reactions at anode and cathode under changing.
Figure 4. Connectivity in C16IDTBT
The research resulted in detailed mechanistic analysis of polymer photocatalysis that set out the reasons why certain chemical structures work well as photocatalysts and others do not (e.g. S. A. J. Hillman et al., Journal of the American Chemical Society 144, 19382-19395 (2022)) leading to clear design rules that have been adopted more widely.
The project demonstrated the first electrochemical energy storage device using conjugated polymer electrodes and operating in a salt water electrolyte (D. Moia et al., Energy & Environmental Science 12, 1349-1357 (2019)). The material system uses no critical or hazardous materials, can be processed on flexible substrates, and shows promising performance as a pseudocapacitor.
An initially unplanned work addressed the limits to efficiency of solar energy conversion in molecular materials. A combined experimental and modelling study showed how loss-non-radiative recombination could be suppressed through changes in chemical design (M. Azzouzi et al., Physical Review X 8, 031055 (2018)), thus introducing new design guidelines for molecular solar cells.
In a collaborative work (L. Zhu et al., Nature Materials 21, 656 (2022)) the team achieved a record power conversion efficiency of over 19% for solar cells made from molecular semiconductors, achieved partly by suppressing non-radiative energy losses.
The study (Coker et al, Proc. Nat.Acad.Sci 121, e2403879121 (2024)) demonstrated a new paradigm in design rules for high electronic mobility in conjugated polymers. The study shows how high charge carrier mobility can result from high connectivity, rather than from crystalline order.
The project demonstrated the first electrochemical energy storage device using conjugated polymer electrodes and operating in a salt water electrolyte (D. Moia et al., Energy & Environmental Science 12, 1349-1357 (2019)). The material system uses no critical or hazardous materials, can be processed on flexible substrates, and shows promising performance as a pseudocapacitor.
An initially unplanned work addressed the limits to efficiency of solar energy conversion in molecular materials. A combined experimental and modelling study showed how loss-non-radiative recombination could be suppressed through changes in chemical design (M. Azzouzi et al., Physical Review X 8, 031055 (2018)), thus introducing new design guidelines for molecular solar cells.
In a collaborative work (L. Zhu et al., Nature Materials 21, 656 (2022)) the team achieved a record power conversion efficiency of over 19% for solar cells made from molecular semiconductors, achieved partly by suppressing non-radiative energy losses.
The study (Coker et al, Proc. Nat.Acad.Sci 121, e2403879121 (2024)) demonstrated a new paradigm in design rules for high electronic mobility in conjugated polymers. The study shows how high charge carrier mobility can result from high connectivity, rather than from crystalline order.