Hybrid halide perovskites have attracted huge attention ever since it was shown that they could be used in highly efficient photovoltaic devices produced via low-cost deposition methods. Their exceptional attributes, including high carrier mobility, an adjustable spectral absorption range, long diffusion lengths, and the simplicity and affordability of fabrication make them one of the most exceptional and market-competitive optoelectronic materials for applications in photovoltaic, light emitting diodes, photodetectors, lasers and more.
Despite the phenomenal properties of hybrid perovskites, several crucial issues still need to be tackled before their industrial-scale, including toxicity, instability and an anomalous hysteresis in the current-voltage curves. Whereas an ever growing number of studies focus on improving optoelectronic properties, there is still an urgent need for a detailed understanding of these materials at a more basic level. For example, the nature (organic or inorganic) and ionic size of the perovskite A-site cation leads to strikingly different trends in the properties of these materials. This project is framed in this context, and its principle goal consists in understanding the intrinsic properties of halide perovskites and how they affect the devices performances.
Since the project targets intrinsic properties, an early goal will be to grow and characterize high-quality polycrystalline and single-crystal samples of hybrid or inorganic materials, and to understand the mechanisms that lead to good materials stability. As an example, very recently the addition of a small percentage of caesium and bromine in the FAPbI3 compound allowed a more stable perovskite structure to be obtained. A second early challenge will be to perfect the growth of single crystals (presently just in the early stages), since these samples are highly desirable to investigate intrinsic properties of perovskites, due to their low trap density and absence of grain boundaries. Moreover, some of the most incisive techniques exploiting neutron and synchrotron sources require single crystals. We will also explore the possibility of growing entirely new hybrid or inorganic materials for optoelectronic applications, by combining the insight provided by Density Functional Theory (in collaboration with Prof. F. Giustino's group) with synthesis and characterisation.
The overarching goal of the experiments we will perform on these samples is to understand structure-property relationships in both hybrid and fully inorganic systems over the entire phase diagrams (temperature, composition, pressure), and to guide their rational design and fine tuning of their optoelectronic properties. In particular, for hybrid materials, we will investigate the interaction between organic and inorganic species to better use the advantages of both components. With this intent, we will deploy a panoply of characterisation techniques on both polycrystalline samples and single crystals, including: neutron and x-ray diffraction (lab and synchrotron), ferroelectricity and dielectric spectroscopy measurements, coherent and incoherent inelastic neutron scattering, and X-ray spectroscopy (XAS, EXAFS), all in combination with the classic light spectroscopy techniques that are widely employed to study photo-carriers in these materials.
The direct impact will be to deliver efficient materials for the device architecture and to gain a deeper understanding of the relationship between the material properties and the devices. The academic impact will be a number of high profile scientific papers. This research falls within the EPSRCS research areas of Materials for Energy Applications, and Solar Technologies.