The Origin of Non-Radiative Losses in Metal Halide Perovskites

Solar cells and light-emitting diodes (LEDs) made from novel, inexpensive materials have the potential to be low-cost, clean and scalable solutions to supply our growing electricity and lighting demands. While solar cells convert sunlight into electrical energy, LEDs are the reverse, with electrical energy transformed into emitted light. Metal halide perovskites are extremely promising materials for both applications. Perovskite solar cells have improved their power conversion efficiency from 3% to 22% in just three years, approaching that of the market-leading technology, silicon (25%). Early reports of perovskite LEDs are also encouraging though relatively unexplored. Perovskite ingredients are abundant and can be combined inexpensively into thin films with a crystalline structure similar to silicon. Rolls of thin, flexible perovskite film could one day be rapidly spooled from a special printer to make lightweight, bendable, and colourful solar and light-emitting sheets.

Nevertheless, the full potential of perovskites has not yet been realised. Strong light emission is essential for both solar cells and LEDs to reach their theoretical efficiency limits, but emission and therefore performance is still limited by parasitic emission loss pathways that are still poorly understood. The films are made up of densely packed crystals (grains) and we hypothesise that each grain has slightly different local chemistry and structural properties, some of which are defective. The ultimate aim of this work is to determine the fundamental origin of these loss pathways in perovskite films and full devices by elucidating which are the optimal chemical and structural properties, and using this information to achieve optimal films.

This aim will be achieved by measuring the grain-to-grain emission using a novel microscope system which will allow rapid imaging of the emission with high spatial resolution. Most microscopic emission measurements on perovskites to date have employed confocal microscopes in which the emission is mapped by taking sequential measurements of the spectra of adjacent regions and moving the sample point by point until the region of interest has been covered. On the other hand, imaging consists of focusing the image of a sample on a detector and measuring for each pixel the intensity of light at one particular wavelength, much like taking a photograph, but at a single wavelength. In some applications, the power of the laser used in imaging can be orders of magnitude higher than in mapping, since the power is spread over the whole region instead of a single point, thus allowing measurement under device-like conditions. Imaging also permits a higher resolution and reduces the acquisition time by orders of magnitude.

Emission images under both light (photoluminescence) and when applying an electrical bias (electroluminescence) will be acquired. The emission images of the same scan area will then be directly correlated with maps of the local grain-to-grain chemistry using electron microscopy techniques including energy-dispersive X-Ray (EDX) spectroscopy and local structural measurements using a nano-X-Ray Diffraction (n-XRD) beamline at the Diamond synchrotron.

The work is highly timely and the results will provide a platform for efforts to take perovskites to their efficiency limits. The work will reveal the specific preferred chemistry and structural properties which must be targeted for growth of higher performing perovskite films and also reveal insights into potential post-treatments capable of healing defects in the perovskite materials. This will be of strong interest to a range of academic researchers in the perovskite field as well as industrial entities such as UK-based Oxford PV, which is leading the current commercialisation efforts of this exciting technology. Finally, the project will allow the PI to establish his team as a world-leading group with a cutting-edge programme and toolset.

The work has the potential for significant economic impact through important fundamental science discoveries relating to the origins of loss mechanisms in perovskite films and devices. These results will lead directly into wider programmes to exploit this knowledge to rationally eliminate the losses through passivation treatments and/or improved crystal growth. Many of these efforts will be UK-based owing to significant relevant local experience. It is likely that several routes to enhance perovskite performance along these lines will be discovered in the proposed work alone. These findings and any associated IP will be of strong interest to companies focusing on commercialisation of the perovskite technology, and the PI and the University will undertake licensing discussions with relevant entities such as the UK-based companies Oxford PV and Heliochrome. Success in the project and any subsequent licensing agreement will push the perovskite technology ever closer to commercialisation, increasing the value of these indigenous UK companies by attracting further international investment. The technology has enormous scope for international scale and could therefore generate substantial value for the UK economy.

The work would provide the fundamental science that, through the above commercialisation routes, could lead to low-cost, clean renewable solar power and lighting technologies. This would have enormous benefits for UK society by generating strong PV and lighting industries with significant job creation. The next decade will be decisive for the future of the UK PV industry and this project is suitably timely to provide an important contribution to this sector and to energy-based policy. Low-cost power and light-emission technologies will also help the UK and other countries to meet their emissions and renewable energy target, limiting the damaging effects of climate change for future generations. The move from resource-based fossil fuels to sustainable renewables will have positive implications for energy security and the geo-politics. Furthermore, a low-cost energy source could provide power for the 1.3 billion people in the developing world who currently lack access to electricity. Solar power has the potential to do for developing communities what mobile phones have done for telecommunications: they allow several phases of infrastructure to be bypassed, permitting communities without grid electricity to realise dramatic improvements in quality of life. Likewise, demonstration of perovskite lighting at quality comparable to today's technologies will lead to opportunities for deployment of low-cost lighting solutions.