Effects of Strain on Mass Transport in CsPbBr3; Insights from Kinetic Modelling

Abstract

Lead-halide perovskites (LHPs) have attracted much attention for their favourable optoelectronic properties, which in combination with their ease of fabrication justifies them being dubbed as the "poor man's high-performance semiconductors''. Indeed, they have been used successfully in various technologies, including photovoltaics, LEDs and photocatalysis. Thus far, the large-scale commercialisation of such devices has been hindered by the limited stability of the LHP layers therein.
Generally, the instability is observed as a decomposition of the ABX₃ perovskite to the BX₂ precursor salt. Although an atomic understanding of the exact decomposition mechanisms is still under development, it seems likely that such decomposition is facilitated by mass transport in the perovskite layer. Indeed, there has been a wealth of evidence indicating that LHPs exhibit significant ionic conduction, in addition to their favourable electronic conduction. In particular, the X-site anion has been shown to be rather mobile, though ionic migration of the A-site cation has also been observed.
In some cases, the stressors responsible for the observed instabilities can be mitigated using encapsulation, for example in the case of instabilities towards moisture and oxygen. In many other cases, for example in the case of instabilities towards heat and illumination, the stressors are naturally present under operation and cannot be avoided. Recently, a strong correlation between strain and stability has been observed. Several studies have shown that tensile strain can worsen the LHP degradation, while compressive strain has been linked to improved stability.
These observations beg the question to what extent ionic migration is influenced by strain, for which an atomistic understanding is equally elusive. This thesis has therefore been dedicated to unravel the effects of strain on the ionic migration in LHPs. We do so by looking at three representative types of strain in one, two and three dimensions, and compute the ionic mobility for both caesium (the A-site cation) and bromide (the X-site anion) in CsPbBr₃, which we consider as a representative material for the larger class of LHPs. We find that for both ions the directionality and magnitude of the mobility can change dramatically with strain, in particular for the A-site cation. In this thesis, we identify strain states that should be avoided experimentally and suggest more desirable strain states to reduce the ionic mobility, and potentially reduce the material instability under strain.

Date of Award	14 Sept 2022
Original language	English
Awarding Institution	University of Bath
Supervisor	Alison Walker (Supervisor), Matthew Wolf (Supervisor) & Petra Cameron (Supervisor)