Modelling charge carrier dynamics in metal halide perovskites for next-generation solar cells

Metal halide perovskites have attracted significant research attention in recent years, owing to their potential for use in the active layer in high-efficiency and simple-to-fabricate solar cells. However, there remain significant gaps in the understanding of the optoelectronic properties of these materials and the nature of charge transport within them. Examples of such gaps include the debate over the physical origin of the unusual temperature-dependence of experimental mobility measurements and the slow cooling of hot carriers compared with conventional semiconductors. Various claims have been made regarding the essential physics giving rise to such observations.

Previous work from the group has scrutinised some of these assertions. In particular, the importance of the formation of large polarons and Fröhlich coupling to multiple polar optical phonon modes to an understanding of mobility measurements in these materials has been studied. The significance of carrier-carrier scattering within the context of transient dynamics has also been explored, and some investigation has been performed into the consequences of screening of the Fröhlich scattering of carriers. The group has developed a world-leading ensemble Monte Carlo code, BoltMC, with which such investigations are performed. This allows for modelling in between the atomistic and continuum length scales, an approach that is essential for a proper understanding of device physics.

The initial phase of this research project in year 1 will involve developing a full understanding of BoltMC as it is currently written and continuing work from the last year investigating the Fröhlich coupling of charge carriers to multiple polar optical phonon modes in methylammonium lead triiodide, from which a publication is expected to result. BoltMC will be extended to account for polaronic scattering. Following this subproject, previous work by the group relating to transient carrier dynamics will be advanced, with a collaboration with the experimental group of Felix Deschler at the University of Munich in development. It is planned that the screened Fröhlich scattering of carriers will also be studied further, since early results suggest this may be important in resolving ongoing questions surrounding the temperature-dependence of the mobility in this class of materials. In the longer term, this project will look to significantly extend BoltMC to allow for the investigation of heterogeneity and the coupling of electronic and ionic motion. The latter of these will require the marriage of the Monte Carlo approach of BoltMC and drift-diffusion modelling. There is also the further possibility of extending the code to enable full device-scale modelling.

Overall, this project has the potential to provide important developments in the understanding of the operation of halide perovskite solar cells. This is an important step on the road to ultimately establishing the commercial viability of this technology.