Impact of Dynamics and Disorder on Structure and Electronic Levels of Hybrid Organic-Inorganic Perovskites

Abstract

Hybrid Organic-Inorganic Perovskites (HOIPs) have been attracting significant attention in photovoltaic and various light-emission fields due to their excellent semiconductor characteristics and high structural versatility. Structure determines electronic properties, which in turn dictate the optoelectronic properties. While the electronic structure of static HOIPs has been extensively studied, there has been less focus on understanding the dynamics and atomic disorder in HOIPs through first-principles calculations. Gaining a deeper understanding of the impact of these effects on electronic and optoelectronic properties is crucial for improving the optoelectronic, spintronic, and other properties of HOIPs for future practical applications.The electronic properties of HOIPs can be effectively tuned by atomic disorder, whether induced extrinsically through alloying or inherently via thermal vibration. For instance, the tunable band gap of the mixed halide perovskite (PEA)2Pb(I1−xBrx)4 as a function of x was predicted by first calculating its phase diagram using semi-local density functional calculations with van der Waals dispersion correction. By utilizing the lowest energy structures, the trend of tunable band gaps is predicted with hybrid DFT incorporating spin-orbit coupling effects, aligning with the photoluminescence spectra observations. Beyond inorganic halide alloying, the introduction of mixed organic cations offers another route to modulate the electronic and optoelectronic behaviors in 2D HOIPs. This is evident when comparing [S-MePEA][C4A]PbBr4 and [S-MePEA][C4A]PbI4 to their purely chiral cation analogs, [S-MePEA]2PbBr4 and [S-MePEA]2PbI4. Such variations are supported by atomic structures determined experimentally via XRD and by theoretical electronic and spin texture calculation results. Next, the thesis investigates atomic disorder induced by inherent thermodynamic effects, as well as their impacts on the electronic and optoelectronic properties of HOIPs. For example, a detailed examination of local and dynamic disorders in MAPbBr3 is conducted, with parameterization through Pb-Br pair distribution function (PDF), PbBr62- octahedron distortion, and MA+ dynamics derived from AIMD simulation trajectories at various temperatures. These findings reveal that the structural disorder in MAPbBr3 primarily stems from thermally activated anharmonic dynamics, not static disorder, aligning with experimental observations. The dynamic spin splitting in (PEA)2PbI4 is studied using AIMD and DFT-PBE+SOC band structure calculations. The results show that the average spin-splitting energy in the dynamic structures is not zero but is localized and short-lived. This dynamic spin-splitting characteristic diminishes over extensive time and spatial scales. Electron-phonon interactions and phonon-polaronic coupling effects play pivotal roles in understanding the fundamental electronic and charge-carrier properties of HOIPs, providing active mechanisms to manipulate charge carrier transportation or polaron oscillation properties. For instance, it is demonstrated that the electron-phonon coupling (AE2T)2AgBiI8 can induce coherent charge transfer kinetics (AE2T)2AgBiI8. This occurs through the oscillation of energy levels between the organic and inorganic components, which then causes the hole population to shift controllably between these components. When simulating polaron populations with static exciton calculations combined with phonon calculations for CsPbBr3, it's indicated that the stretching phonon mode at 140 cm-1, which is responsible for the dephasing of polaron oscillation, remains stable even at high excitation densities. This stability suggests a weak coupling between the stretching modes and the low-energy rocking modes. Experimental evidence shows that the rocking modes are strongly coupled to the polaronic system, protecting its coherent oscillation from dephasing and leading to the phenomenon of superfluorescence. Furthermore, chiral phonon calculations in [S-MePEA]2PbI4 predict the presence of both transverse and longitudinal chiral phonons, in alignment with experimental observations. These chiral phonons are predominantly found in the acoustic phonon branches near the Γ point, signifying that they are long-range, low-frequency phonon modes, which is consistent with experimental findings. This thesis has unraveled the profound influence of both extrinsic and inherent thermodynamic atomic disorders on the electronic properties of HOIPs. Moreover, this thesis illuminated the potential of leveraging electron-phonon coupling to intentionally modulate atomic disorder and thereby to manipulate the electronic properties of HOIPs with precision and control. The findings underscore the great potential of HOIPs in optoelectronic applications.