Engineering Hybrid Halide Perovskites for Enhanced Stability: A Structural and Spectroscopic Study

<p dir="ltr">Semiconductors are the backbone of modern technology, driving innovations in medical devices to clean energy. Semiconductor devices based on silicon technology have been a major driving force behind the electronic revolution in the past few decades. Recently, a class of materials that has emerged as a promising candidate for many optoelectronic applications is organic lead halide perovskites. The remarkable surge in perovskite-related research is attributed to their high absorption coefficient, long charge carrier mobility, facile bandgap tuning, easy solution processability, high photoluminescence quantum yield, etc. However, organic lead halide perovskites possess stability issues under various environmental stressors such as temperature, moisture, and light. They suffer from temperature-dependent structural phase transitions, which can affect the optoelectronic properties of the compound. While under continuous light irradiation, perovskite shows halide segregation and metallic lead formation, which degrade the performance of the device. On the other hand, moisture-induced decomposition of perovskite materials can produce contaminated chemicals apart from the reduction of the device performance. Overcoming issues like structural phase transitions and degradation is crucial for future advancements. This thesis deals with the structural and spectroscopic investigations of composition- and additive-engineered organic lead halide perovskite materials. </p><p dir="ltr">The results and discussions for the investigating systems, along with the introduction, methodology, and experimental techniques, are organized into seven chapters of the thesis. </p><p dir="ltr">Chapter 1 is a brief introduction and motivation for the research work carried out in the thesis. </p><p dir="ltr">Chapter 2 includes discussions on sample synthesis techniques and various experimental setups and protocols utilized for the investigations presented in the thesis. </p><p dir="ltr">In Chapter 3, structural, optical, chemical, and electronic properties of A-site substituted MA1-xFAxPbBr3 (x= 0, 0.5, and 1) single crystals are presented. This chapter is divided into two sections. In the first section, detailed temperature-dependent structural characterization confirms the formation of cubic Pm3 ̅m symmetry at room temperature in all three compounds. At lower temperatures, all three compounds show phase transition to tetragonal to orthorhombic phases. Temperature-dependent Raman spectroscopy shows stronger interaction between MA+ cations and Br¯ anions in the low temperatures of the orthorhombic phase in MAPbBr3, while no such interactions were noticed in the other two compounds. Temperature-dependent photoluminescence study shows dual emission behavior of the compounds, while broad-band emission in the low temperature of MAPbBr3. Detailed study suggests that the dual emission behavior is due to the formation of direct-indirect state formation, while the broad-band emission is the result of the formation of the intrinsic self-trapped excitons. In the second section, a time-dependent XPS study was performed to understand the light stability of the compound under continuous exposure to high-energy X-rays. It was noticed that FA+ incorporation inhibits the metallic lead formation in perovskites. </p><p dir="ltr">Chapter 4 discusses the results from X-site substituted MAPb(Cl1-xBrx)3 single crystals. This chapter is divided into two sections. In the first section, detailed temperature-dependent structural characterization confirms the formation of cubic Pm3 ̅m symmetry at room temperature in all three compounds. MAPbCl3 attains tetragonal and orthorhombic symmetry under changes in temperature. In the orthorhombic phase, below 160 K, a single orthorhombic Pnma symmetry cannot be fitted with small values of reliability parameters, indicating the presence of two orthorhombic phases. MAPbCl1.5Br1.5 maintains cubic Pm3 ̅m symmetry till 120 K, while MAPbBr3 attains its tetragonal I4/mcm symmetry and orthorhombic Pnma symmetry respectively. Temperature-dependent Raman spectroscopy shows much stronger interaction between MA+ cations and Cl¯ anions in the low temperatures of the orthorhombic phase in MAPbCl3 compared to MAPbBr3, while no such interactions were noticed in the mixed compound. Temperature-dependent photoluminescence study shows light-induced halide segregation in the mixed compound, while broad-band emission in the low temperature of MAPbBr3 and higher temperatures of MAPbCl3 was observed. Detailed study suggests that the broad-band emission is the result of intrinsic self-trapped excitons formation. The formation of double unit cells in the low temperatures of MAPbCl3 stabilizes the lattice fluctuation, while smaller sizes of Cl¯ ions make compact unit cells at the higher temperature with stronger MA+–Cl¯ interaction, which gives rise to the broad-band emission. In the second section, an XPS study was performed to understand the light stability of the compound under continuous exposure to high-energy X-rays. All three compounds show light-induced Pb0 formation under continuous X-ray irradiation. </p><p dir="ltr">In Chapter 5, five single crystals of dual-site compositionally engineered perovskite MA1-xFAxPb(I0.95Br0.05)3 (0 ≤ x ≤ 1) compounds are presented. Structural study confirms the presence of cubic and tetragonal phases in MAPb(I0.95Br0.05)3, while hexagonal and cubic phases in MA0.25FA0.75Pb(I0.95Br0.05)3 and FAPb(I0.95Br0.05)3. Complete cubic structure is found in MA0.75FA0.25Pb(I0.95Br0.05)3 and MA0.5FA0.5Pb(I0.95Br0.05)3 compounds. UV-visible spectroscopy suggests that FA-rich compounds with hexagonal phases are partially photoinactive. All the compounds show excellent light stability without halide segregation. Time-dependent photoluminescence suggests enhancement in intensity over time, which is due to the Pb-interstitial defects occupied by the superoxide molecules. FA-rich compounds show excellent stability under continuous X-ray exposure. </p><p dir="ltr">In Chapter 6, two ionic liquids 2-chloro-1-methypyridinium iodide (CNMP) and 1,4-dimethylpyridinium iodide (DMPI) were used as additives with different concentrations in the precursor solution of MA0.4FA0.6PbI2.8Br0.2 perovskite and thin films were fabricated. Time-dependent XPS study was performed to understand the stability of the perovskite layer. It was found that DMPI-added thin films with concentrations of 1 mg/ml and 1.5 mg/ml show excellent X-ray high-energy beam stability under 30 minutes of continuous exposure. A time-dependent photoluminescence study shows that DMPI-added films with concentrations of 1 mg/ml and 1.5 mg/ml maintain the intensity of photoluminescence, suggesting defect passivation in the thin films. Scanning electron microscopy also suggests that the DMPI additive enhances the film morphology. The thin films were kept under continuous light exposure at room temperature and high temperature, and in both cases, the thin film without the DMPI additive changed its color from black to yellow, while DMPI added thin film maintained its black color. XRD study suggests the formation of PbI2 in the bare film without DMPI additive. Further, photodetectors were fabricated with or without DMPI-added perovskite precursor solution. A summary and future scopes are discussed in Chapter 7.</p>