Phase evolution and local structure in [Cs, MA][Pb, Sr][Cl, Br]3 perovskites

• Main Author: Dintakurti, Sri Harsha Satya Sai
• Other Authors: Shen Zexiang
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Materials
• Online Access: https://hdl.handle.net/10356/157071
• Institution: Nanyang Technological University

Organic-inorganic hybrid lead halide perovskite materials have a wide flexibility for tuning crystal chemistry and adjusting properties. The prototype of this family is methylammonium lead triiodide (CH3NH3PbI3), but several chemical analogues where the organic component, Pb2+ or I-, are replaced by compatible chemical entities have been described. Recently, this class of compounds has garnered attention due to their potential as photoactive materials for deployment in low-cost solar cells, light emitting diodes and semiconductor lasers. However, prolonging the device lifetime requires overcoming challenges pertaining to phase stability, lead toxicity and moisture sensitivity. This thesis is concerned with investigating the phase evolution and local structure in lead halide perovskite materials. CH3NH3PbCl3 (MAPbCl3) and CH3NH3PbBr3 (MAPbBr3) adopt perovskite aristotype symmetry (cubic Pm-3m) at room temperature while their Cs analogues exist in the CaTiO3–type holotype (orthorhombic Pnma) structure. Methylammonium lead halide perovskites (MAPbX3) have been widely investigated for optoelectronic applications with the addition of Cs improving structural and thermal stability. This study maps the phase evolution during Cs substitution in MAPbCl3 and MAPbBr3. The complete A-site miscibility of cesium and methylammonium (MA) in the lead chloride and bromide perovskites is demonstrated with mechanochemically prepared samples having a nominal stoichiometric composition Csx(CH3NH3)1-xPb(Cl/Br)3 (x = 0, 0.13, 0.25, 0.37, 0.50, 0.63, 0.75, 0.87, 1). High resolution 133Cs/207Pb/13C/1H magic angle spinning nuclear magnetic resonance spectrometry (MAS-NMR) verified each composition was single-phase and highly ordered. Single resonances observed in the high resolution 133Cs and 207Pb MAS NMR confirm a continuous solid solution across the MAPbX3 – CsPbX3 join. Calibrating the compositional space to NMR chemical shifts, the 133Cs NMR can directly establish the Cs/MA ratio and probe sample homogeneity; this is especially valuable for nanoparticle forms where the X-ray reflections are less separated. In this manner, the advantages of solid-state NMR to validate perovskite homogeneity and chemistry simultaneously and non-destructively are demonstrated for this class of materials. The structural evolution from cubic to orthorhombic symmetry accompanying changes in composition and temperature reflects PbX6 octahedral tilting while maintaining perovskite framework topology. These polymorphic transitions were monitored by Rietveld refinement of powder X-ray diffraction (XRD) data to create phase diagrams in composition space. Timeresolved photoluminescence (TRPL) measurements of nanocrystalline Csx(CH3NH3)1-xPbBr3 revealed the optimized 13 mol% Cs nanoparticle composition exhibits the longest charge carrier lifetime and enhancement in radiative pathways with the highest photoluminescence quantum yield (PLQY) of ∼88%. The composition with superior cubic phase stability (Cs0.13MA0.87PbBr3) has been identified as having the greatest potential for LED applications. To date, efforts to replace lead with other ions in lead halide perovskites while maintaining high functionality in photovoltaic applications met with mixed success. This study explored the role of stereochemically active Pb2+ lone pair electrons in stabilizing atomic disorder across the CsPbCl3-CsSrCl3 and CsPbBr3-CsSrBr3 chemical joins. Continuous solid solutions of CsPbxSr1-xCl3 and CsPbxSr1-xBr3 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1) have been prepared using solid state synthesis techniques while the composition and homogeneity have been verified by solid-state NMR and powder X-ray diffraction studies. The 207Pb MAS NMR displayed 7 resonances with a binomial distribution arising from 0-6 nearest neighbour cations being replaced with Sr2+. The average chemical shift of each environment increases non-linearly with lead content. The presence of cross peaks in 207Pb-207Pb NOESY spectra further confirm that the resonances from various lead environments are arising from a solid-solution and not phase-separated compounds. 35Cl MAS NMR proved diagnostic of the disorder at the halide site, providing the relationship between lead content and halide ion disorder. This study points towards the structure directing role of stereoactive lone pair electrons (SLPe−) in determining halide ion disorder making them an essential feature to maintain high halide disorder. This thesis provides a comprehensive description of the local environment of lead in the plumbous perovskites Csx(CH3NH3)1-xPb(Cl/Br)3 and CsPbxSr1-x(Cl/Br)3 integrated with the nature of the average extended crystal structures. In so doing, a complete mapping of cubic Pm-3m phase stability as a function of temperature and composition was constructed. This was possible by deploying the less common mechanochemical alloying technique to prepare the perovskites without the loss of volatile components and maintain putative compositions. Solid state NMR has been established as a viable alternative for characterizing the composition in these materials. The structural changes correlate with improvements in optical properties highlighting the association between phase stability and optical performance. Future investigations should include A-site substitutions with multiple ions (formamidinium, methylammonium, Cs+, Rb+) to identify the most stable and high-functioning compositions. Detailed DFT modelling of the structure directing role of stereoactive lone pair electrons would support the experimental observations, however modelling the relativistic electrons in Pb remains challenging.