The Origin of Non-Radiative Losses in Metal Halide Perovskites

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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.