Rational design of solid-state semiconductor-sensitized solar cells: from materials modelling to device fabrication

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Due to the growing global demand for energy, the development of efficient ways of harnessing solar power has become a key scientific challenge. Among promising low-cost alternatives to silicon photovoltaics, nanostructured solar cells based on porous metal oxides films coated with an extremely thin film of light absorbing semiconductors have gained prominence due to their relatively high energy conversion efficiencies, as compared to many new low cost concepts. Despite the prominent role of materials interfaces in these advanced solar cell concepts, very little is known about their electronic and optical properties at the nanoscale, and most of the current research relies on a Edisonian trial-and-error approach. The key idea of this project is to develop a rational approach to the design and fabrication of nanostructured solar cells based on semiconducting inorganic sensitizers, using a combination of quantum-mechanical atomistic materials modelling, materials synthesis and characterization, device fabrication and characterization, and advanced spectroscopy. Indeed characterization techniques and computer modelling can nowadays address similar length-scales (sub-nm to a few nm's), hence it is now the perfect time to use experiment and modelling synergistically in order to accelerate discovery in nanoscale solar energy research. The vision underpinning this project is that within 10 years it will be possible to design, optimize, and fabricate nanostructured solar cells in a way similar to what happens in rational drug design and bioinformatics. In order to achieve this goal our strategic asset will be a very close cooperation between leading materials modellers, nanotechnologists, and device engineers. The rational design of new solar cells will require the computational study and the experimental control of many aspects, including the optical properties of the sensitizer, the interfacial energy-level alignment, the charge injection/recombination rates, and the carrier mobilities. In this project we take the first step along this direction by focussing primarily on the electronic energy-level alignment at the sensitizer/oxide interface. The interfacial energy level alignment is directly related to the open-circuit voltage of sensitized solar cells and is a key design parameter for improving cell efficiencies. Our proposed rational design will consist of the following steps: (i) identify promising sensitizers via computational modelling, (ii) synthesize and characterize the selected materials, (iii) fabricate and optimise the solar cells, and (iv) perform advanced spectroscopy to understand the fundamental operation and limiting factors to performance in complete solar cells. This synergistic use of first-principles modelling and experiment has not been attempted so far in nano-photovoltaics research and has the potential of revolutionizing the field. Owing to our complementary skills, our research team is unique in the UK and EU arenas and this project holds the promise for revolutionizing our understanding of sensitized solar cells at the nano scale, and introducing and developing paradigm-shifting technology. In this project we will focus specifically on solid-state semiconductor-sensitized solar cells. These devices are an evolution of the concept of dye-sensitized solar cells whereby the dye sensitizer is replaced by a semiconductor quantum dot or a nanoscale semiconducting film. This choice has three advantages: (I) the expensive transition-metal based dye sensitizer is replaced by a inexpensive light-absorber obtained by colloidal synthesis (ii) the optical properties of the sensitizer can be tuned by exploiting quantum size effects, and (iii) in comparison to conventional thin film photovoltaics, there is a much broader library of materials which may work effectively as semiconductor sensitizers.