RPF 2015/16 Solar energy conversion: Measurement of efficiency as a function of wavelength

Metal oxides are arguably the only viable class of material for solar to chemical energy conversion. This is primarily because metal oxides are abundant, inexpensive and are resistant to corrosion, supporting use on the global scale. However, a major challenge is to increase their conversion efficiency, which is currently too low.
Experiments with metal oxide systems in the RED group have reached the stage where it is necessary to routinely measure solar energy conversion efficiency as a function of wavelength. Current equipment includes light sources, power meter, potentiostats and automated gaseous product analysis by gas chromatography. This has allowed us to test materials and devices under broad band white light and determine an overall efficiency. However, to increase energy conversion, it is necessary to determine the wavelength dependence which provides critical information for targeting improvements. Critically, there is no deep understanding of which metal oxide materials may deliver the required efficiency and chemical treatments can be somewhat arbitrary. Therefore robust predictive theoretical models are required to direct synthetic chemists and materials scientists. This proposal is focussed on acquiring experimental data (acquired using the requested equipment) which will be used to test and validate theoretical models developed by the KPM group.
The RED group have recently prepared devices comprising a conducting macroporous host of fluorine doped tin oxide (FTO) which is used to support photoactive materials such as Fe2O3 and BiVO4. Recent unpublished work has shown that these simple devices are capable to producing hydrogen and oxygen under white light. However, we are unable to enumerate efficiency as a function of wavelength. This information is critical to formulating theoretical hypotheses by KPM which can guide future lines of research.
A major challenge is to extend the absorption of metal oxides across the visible spectrum. Absorption by materials and devices can be measured easily using existing equipment. However, increased absorption in metal oxides commonly does not translate to an increase in overall conversion because the origin of absorption is often localised, preventing extraction (or use of) photoexcited electrons or holes at the surface of a material or device. This proposal requests funds to purchase a computer controlled monochromator which will support the routine measurement of efficiency as a function of wavelength. The equipment if flexible, allowing illumination of wavelength ranges (5-20 nm) over the ultra violet and visible spectrum (300 – 1000 nm) which is required to obtain sufficient light intensity to measure a conversion efficiency. Furthermore, the monochromator is designed to integrate into existing apparatus and will be functional immediately.
The requested equipment will allow the differentiation between productive and unproductive photon absorption, which when coupled with other structural and spectroscopic data will provide a much clearer understanding of material performance. First principal theoretical modelling by KPM will be combined with experimental materials characterization offering a route to understand the factors responsible for reducing efficiency and to identify viable materials modification strategies to improve performance. This is the heart of this proposal, and the hypothesis we wish to validate. The combined theoretical and experimental approach is revolutionary and has potential to transform the design and optimisation of solar materials for a range of applications.

Project objectives
1.To measure the wavelength dependent efficiency of existing materials and devices. These will include photon-to-electron and photon-to-chemical efficiencies. This data can be used to improve the quality of publications and funding applications.
2.To provide data to help develop theoretical models to understand the photochemistry and photophysics of materials and devices.
3.To provide data to test and validate predictive theoretical models to identify new efficient materials and devices.
Awarded £14,992