Polyoxometalate-Based Sensitizers for p-Type Dye-Sensitized Solar Cells

Photonic materials interact with light in interesting and useful ways. They are vital to many current and emerging technologies, such as biological imaging, optical data processing, telecommunications and solar energy. This project will investigate the properties of a promising, but little explored class of photonic materials and test them as a means to improve the performance of an emerging type of solar cell - the p-type dye sensitized photocathode (p-DSSC). In this way, our long-term goal is to develop low cost, high-efficiency solar energy devices which will help reduce carbon emissions and dependence on imported fossil fuels.

Dye-sensitized solar cells (DSSCs), based on a dye-sensitized n-type titanium dioxide photoanode, promise a low-cost alternative to conventional semiconductor photovoltaic (PV) materials like silicon. They function well in northern-European, low-light conditions but their peak power conversion lags far behind that of the best semiconductor designs, which combine several different semiconductors optimized to absorb different portions of the solar spectrum. DSSC performance may be improved through an analogous approach - tandem DSSCs which pair the usual dye-sensitized photoanode (n-DSSC) with a dye-sensitized photocathode (p-DSSC). With complementary absorption profiles, the n- and p-DSSCs absorb more sunlight together in the tandem DSSC than either can alone. Currently, though, the efficiency of the p-DSSC (record 1.3%) is far from matching that of n-DSSCs (10 to 15%). This means that tandem DSSCs perform worse than n-DSSCs by themselves, and p-DSSCs must improve dramatically for the tandem DSSC to become a viable device.

Both n- and p-DSSCs depend on efficient charge separation at the interface between a dye and a metal oxide support to generate electricity. n-DSSCs achieve useful efficiencies because light causes the dyes to rapidly inject electrons into an n-type (electron transporting) metal oxide. Transport of electrons through the metal oxide, and filling of "holes" formed in the dyes by electrons from a redox electrolyte, is much faster than recombination (return of electrons) to the dye from the metal oxide. p-DSSCs work in the opposite sense, injecting holes into a p-type (hole transporting) metal oxide, with the redox electrolyte taking electrons from the dyes. The problem for p-DSSCs is that transport of holes through oxides is slow, and recombination from the dye and electrolyte is fast. This leads to low efficiency.

In this project, we will synthesize a novel class of dye for the p-DSSC, based on connection of electron accepting multi-metallic clusters (polyoxometalates, POMs) to organic groups. By holding electrons away from the metal oxide surface, the POM electron acceptor groups will slow recombination and improve performance. The proposed POM-based sensitizers have an electronic structure that will favour charge separation, and are expected to have important advantages - in stability and ability to rapidly transfer electrons to the redox electrolyte - over the current purely organic materials.

The multidisciplinary nature of this project gives rise to potential for impact in three main areas. These are:

1. Economic and Social Impact: This project will develop new photonic materials, with the goal of improving the performance of an emerging type of solar cell. The economic impact of a low cost, high efficiency means to generate solar electricity cannot be overstated. It would reduce our dependence on costly fossil fuel imports, reduce the need for expensive and inefficient electricity transmission networks by enabling localized power generation, and provide the first step towards converting sunlight to chemical fuels. The new materials proposed are also potentially relevant to a whole range of technologies requiring the manipulation of light - for example optical switches, optical computing/telecommunications, optical power limiters, and imaging agents - all of which can provide economic impact.

In addition, the project contributes to supporting the UK economy by training highly skilled workers - as it involves synthesis, advanced photophysical measurement and device assembly/testing, the PDRA will gain skills which are widely marketable in academia and beyond. Such training can have a direct economic impact by forming individuals with high level problem solving and other transferrable skills, as well as specific scientific knowledge.

2. Environmental Impact: All photovoltaics have the potential to produce large quantities of low-to-zero carbon electricity, reducing our CO2 emissions and helping minimize climate change. Dye-sensitized solar cells (DSSCs) in general offer lower processing costs (both environmental and monetary) and embedded energy than traditional silicon-based PV, and often give better performance in northern-European light conditions. The efficient p-type DSSCs targeted here, through pairing with conventional n-type DSSCs in "tandem" devices, could lead to a step change in the applicability of DSSCs because once optimized, tandem DSSCs are expected to offer better performance than any other affordable PV technology in UK conditions. These factors give this project potential for a very positive environmental impact.

3. Social and Cultural Impact - through fundamental science: As the project involves synthesis of new materials and evaluating their properties, it clearly impacts on fundamental science. The photonic properties of the class of material I am interested in are almost completely unexplored, and the p-DSSC is at an embryonic stage. Therefore the discoveries we make in this project could influence science significantly, for some time. Topics like energy are an excellent vehicle for interesting students in science, and for this reason the original n-DSSCs have become an undergraduate experiment and textbook example of photochemistry. My hope and belief is that p-DSSCs will follow their example.