Perovskites materials with ABX3 (A: monovalent cation; B: divalent cation; X: anion) structure manifest interesting properties and applications in materials science and photovoltaics (PV) as thin films and solar cells. Optimization of their application requires a profound understanding of optoelectronic and structural properties of these materials and correlation with device performance. This dissertation presents the wide range of spectroscopic ellipsometry studies performed on organic-inorganic metal halide-based perovskites films and solar cells, LaMnO3 complex oxide perovskite epitaxial films, and NiOx films as hole transport materials for perovskite-based PV devices.
Degradation of organic-inorganic metal halide-based perovskites films with exposure to relative humidity and temperature is studied by continuously tracking changes in structural and optical properties with time. Narrow bandgap perovskite devices incorporating different hole transport materials are aged in ambient air to track degradation and quantify losses towards front and back contact interfaces of the devices from external quantum efficiency simulations. Optical properties of LaMnO3 complex oxide perovskite are reported and interpreted. Optical properties and morphology of NiOx films prepared using nanoparticle, spray coating, and sputtering methods are determined to optimize preparation methods of NiOx for specific application.
Organic-inorganic metal halide ABX3 perovskites (A cation: methylammonium-MA, formamidinium-FA, cesium-Cs, rubidium-Rb; B cation: lead-Pb, tin-Sn; X anion: iodine-I, bromine-Br, chlorine-Cl) gained great interest recently in optoelectronic and photovoltaic applications due to their unique properties including high absorption coefficient, long charge carrier diffusion length, low-cost solution techniques for their fabrication, and high efficiency of the solar cell devices based on these materials. Aside from their great advantages, stability is a major challenge for commercialization of these materials. Understanding how they resist changes when exposed to external environment conditions is crucial to optimize their performance and solve some of the challenges that limit their applications. In situ real-time spectroscopic ellipsometry (RTSE) is performed on MAPbI3, MA0.7FA0.3PbI3, and (FAPbI3)0.95(MAPbI3)0.05 perovskite films when exposed to different levels of relative humidity at given temperatures over time. MAPbI3 and MA0.7FA0.3PbI3 films deposited on commercial fluorine-doped tin oxide coated glass are more stable than corresponding films deposited directly on soda lime glass. (FAPbI3)0.95(MAPbBr3)0.05 films on soda lime glass showed improved stability over the other compositions regardless of the substrate, and this is attributed to the preparation method as well as the final composition.
To overcome discrepancies in the reported optical properties of LaMnO3 complex oxide perovskite, spectroscopic ellipsometry and computational methods including Density Functional Theory (DFT), G0W0, and the Bethe-Salpeter Equation (BSE) approximations are used to determine optical properties of LaMnO3 epitaxial films on SrTiO3 and LaAlO3 single crystal substrates. Critical point energies (CPs) located from 0.91 to 1.49 eV are interpreted as energy separation between eg and t2g orbitals caused by the crystal field splitting. CPs from 3.31 to 3.53 eV and 3.75 to 5.23 are interpreted as Mn d exchange splitting and strong charge transfer transitions, respectively. Direct and indirect bandgaps extrapolated using Tauc-plots for epitaxial LaMnO3 on SrTiO3 are identified at 0.590 and 0.300 eV, respectively. For optically anisotropic epitaxial LaMnO3 on LaAlO3, direct bandgaps are found at 0.588 and 0.303 eV, and indirect bandgaps are found at 0.268 and 0.100 eV, for electric fields oscillating parallel and perpendicular to the sample surface, respectively.
To develop perovskite solar cells with optimized collection of photogenerated carriers, it is essential to understand how carrier collection losses, particularly near the front and back contacts, vary with different HTLs. In this dissertation, degradation of a narrow bandgap perovskite solar cells made with poly(3,4-ethylenedioxythiophene): polystyrenesulfonate (PEDOT:PSS), poly[3-(6-carboxyhexyl)thiophene-2,5-diyl] (P3CT), and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) hole transport layers (HTLs) aged in air ambient is investigated using spectroscopic ellipsometry measurements and data analysis followed by comparisons of measured and simulated external quantum efficiency spectra. Solar cell devices with P3CT and PEDOT: PSS HTLs show improved collection of photogenerated carrier near the front contact interface, better device performance and reduced electronic losses over a device with PTAA HTL.
The selection of proper hole transporting materials for perovskite solar cells (PSCs) is critical for device performance and long-term stability. Inorganic NiOx HTL is highlighted for improved PSC performance and stability. One of the key components in optimizing NiOx applications as HTLs in perovskite solar cells is to explore how preparation methods influence their optical properties and morphology, and the resulting interfacial formation between the HTL and perovskite absorber layers in PSCs as well as correlation with device performance. Complex optical properties for NiOx films prepared from nanoparticle, spray coating, and sputtering methods on soda lime glass have been measured by spectroscopic ellipsometry and relative thin film densities are determined. The nanoparticle and sputtered films are less dense, than that prepared using spray coating as deduced from using Bruggeman effective medium approximations. PSCs devices incorporating sputtered NiOx as the HTL have a slightly denser perovskite nucleation layer than when deposited on sprayed HTL, as determined from a reduced interaction of the sputtered NiOx in the latter case. Better device performance is observed with the spray coated NiOx HTL before heat. There are no substantial differences in the external quantum efficiency spectra, short circuit current, and power conversion efficiency between the perovskite solar cells incorporating spray coated and sputtered NiOx HTLs after heat treatment.