Investigations of Hybrid Perovskite Thin Films and Interfacing TiO2 Electrode Surfaces: Chemical Origins of Nanoscale Structural and Electrical Properties for Photovoltaic Applications

Abstract

The over-arching theme of this dissertation is to explore critical chemical influencers of film growth and electrical properties, especially on nanometer length scales, of hybrid organic-inorganic perovskite (PVSK) photo-active layers and interfacing TiO2 electrode surfaces for use in photovoltaic (PV) devices. Current PVSK PVs, in which the PVSK-TiO2 heterojunction is used for photo-generated electron-hole charge separation, demonstrate significant spatial heterogeneity in photo-generated charge flow to and across the PVSK-TiO2 interface that likely limits device energy conversion efficiency. This dissertation addresses two hypothesized sources of this electrical heterogeneity: the nanoscale electrical properties of the TiO2 electrode surface itself, which may influence interfacial charge transfer efficiency, and the structural heterogeneity of crystals within the solution-cast PVSK polycrystalline layer, which may influence charge flow to the TiO2 interface. In the first results chapter (Chapter 3), we explore the first hypothesized area of concern – the nanoscale electrical properties of the TiO2 surface. By combining conductive atomic force microscopy (cAFM), to measure local current-voltage properties across the TiO2 surface, with X-ray photo-electron spectroscopy (XPS), to track near-surface composition and energetics, we examine how two common TiO2 surface treatments for PV contact formation (oxygen plasma treatment and UV light irradiation) modify the nanoscale distribution of electron injection barriers for this electrode surface. Our results show that these treatments differently affect the average electrical properties across the TiO2 surface by the selective removal or introduction of near-surface carbon or molecular oxygen adsorbates. For the most electrically-activated TiO2 samples, which we propose represents the native electrical state of the oxide, a significant degree of spatio-electrical heterogeneity is measured at nanometer length-scales. In addition to these results, a new approach to more reproducible cAFM measurements is introduced focusing on methods for maintaining consistent cAFM probe electrical activity when measuring such adsorbate-rich sample surfaces. In the second results chapter (Chapter 4), we explore the second hypothesized area of concern – structurally heterogeneous PVSK film growth on TiO2 surfaces. By employing a combination of X-ray Diffraction (XRD), Angle-Resolved Grazing Incidence Wide Angle X-ray Scattering (AR-GIWAXS), and cross-sectional electron imaging and elemental mapping, we analyze the structural evolution of freshly-cast PVSK films on TiO2 when allowed to “incubate” within residual solvent vapors entrapped from initial solution casting of the PVSK film. Significant changes in the average crystal orientation, phase, and grain size of incubated films, which appear to be initiated near the TiO2/PVSK interface, indicate the availability of overlooked thermodynamic structural states for these PVSK film types that varies as function of precursor solution composition and incubation conditions. In the final results chapter (Chapter 5), we explore changes in nanoscale photo-electrical properties in the combined TiO2/PVSK film system, the original motivator of all projects, as a function of incubation-induced changes to the PVSK film structure from the discoveries of Chapter 4. This project begins with describing the design of a unique illuminated sample holder in order to measure the nanoscale photo-electrical properties of the PVSK samples via a scanning AFM tip. Initial challenges with performing contact-based electrical measurements leads to the exploration of a non-contact “Surface Photo-Voltage” (SPV) measurement mode utilizing our prism-based illuminated sample holder. With this instrument design and methodology, we reveal that incubated PVSK films show similar nanoscale distributions of photo-voltage values, but films are highly susceptible to light-induced modification of measured surface voltage values which likely to originates from defect-mediated movement of ions within the irradiated PVSK film. The combined results of this dissertation improve our fundamental chemical understandings of both TiO2 and the PVSK materials especially as it applies to improving PVSK-based thin-film PV devices with TiO2 electron-collecting electrical contacts. This dissertation also offers some improved instrument methodologies and designs for performing reproducible surface analytical measurements with challenging chemical surfaces such as that of TiO2 and PVSK thin films.