Property correlations in materials for energy applications utilising advanced X-ray and photophysical techniques
Date
2024
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Abstract
Hybrid-perovskites are an emerging material class, with optoelectronic properties highly suited for photovoltaics (PV), light emitting diode, and phosphor applications. Among PVs they have seen a meteoric rise in photon conversion efficiency (PCE) over the last decade, exceeding that of thin-film and multi-crystallite Si PV. Despite the rapid evolution of PCE within these materials, environmental and thermal stability remain the major impediment for their commercial viability. The use of A site cation substitution is an approach to enhance performance and stability in the versatile family of hybrid perovskites.
In this thesis, the effect of A site cation substitution on the phase transitions and stability is explored, for both cations which appropriate for the APbX3 structure, and a larger organo-ammonium cation (A′ ) associated with the formation of lower dimensionality hybrid perovskite derivative structures. Both approaches are reported in literature to provide exceptional stability and PV PCE improvements.
The hybrid perovskite (F AP bI3)0.85(MAP bBr3)0.15 was characterised using in-situ variable-temperature X-ray diffraction (VT-XRD) data. Sequential Rietveld refinements were employed to explore phase stability, and structural characteristics of this data (FA = formamidinium, MA = methylammonium). A more gradual α → β phase transition is identified, compared to the single cation systems in literature. The effect of Caesium (Cs) substitution on (F AP bI3)0.85(MAP bBr3)0.15 has been characterised across a series of VT-XRD measurements through parametric Rietveld refinement. The introduction of Cs is correlated to divergence between the observed onset temperatures for the cubic to tetragonal and a 0a 0a 0 to a 0a 0 c + (Glazer notation) transitions in the systems, which occur for the α/β phase transition. Beyond the performance improvements, the inclusion of larger A′ cations, have been associated with structural defect passivation, and electronic property tuning due to quantum confinement effects in perovskite derived structures.
Hybrid perovskite based systems containing the A′ cation, propylammonium are studied to gain insights on phase stability and phase formation, and optical properties in these systems. The phase composition, structural III properties, and optical properties of (PABr)2x(MABr)1−xPbBr2, 0 ≤ x ≤ 1 and P ABr/MAP bBr3 systems were studied using ex-situ XRD, optical absorbance spectroscopy and photoluminescence measurements (PA = propylammonium). These systems were fabricated by spin coating, with the former from a single well mixed precursor solution and latter a 2-stage process, where PABr was added in various concentrations to a fully formed MAPbBr3 film. Mixed phase compositions were formed of (P A)2MAn−1P bBr3n+1 2- dimensional (2D) and quasi-2D hybrid-perovskite derivative phases, where n determines the layer thickness and n = ∞ is a MAPbBr3 phase. The difference in the kinetic and thermodynamic aspects of film formation in these systems was leveraged to propose a set of controlling factors responsible for the phase formation behaviour of the varied formation conditions.
Description
A thesis submitted in fulfilment of the requirements for the degree of Doctor of Philosophy to the Faculty of Science, School of Chemistry, University of the Witwatersrand, Johannesburg, 2023
Keywords
Hybrid-perovskites, Correlations, Property correlations