Nanoscale Photovoltaic Performance of Thin Film Solar Cells by Atomic Force Microscopy

Research efforts have been going on for decades to improve the efficiency of photovoltaic devices in order to effectively compete with other energy sources. Considering the length scale of the underlying phenomena, scanning probe microscopy (SPM) based techniques are ideally suited for studying solar cells due to their ability to probe functioning materials and devices under operating conditions, and to directly correlate local film structure with local properties. Accordingly, Atomic Force Microscopy (AFM) techniques have been developed and applied in this thesis to investigate the photoelectrical properties of CdTe/CdS polycrystalline thin film solar cells, as well as an emerging technology that utilizes organometallic halide perovskites. As a first approach, a new technique, photoconductive AFM spectroscopy (pcAFMs) has been developed and performed on isolated, strain-relieved, photovoltaic (PV) micro-cells of polycrystalline CdTe in light and dark conditions. Performance metrics of these solar cells are mapped, revealing the behavior of individual grains, grain boundaries, and planar defects, achieving the requisite sub-10 nm spatial resolution. Same methodology has also been applied to spatially map the performance metrics of hybrid perovskite solar cells (PSCs), revealing substantial variations in the PV performance parameters that correlate with the thin-film microstructural features. Similar PSCs are also investigated using piezoforce microscopy (PFM), to show the presence of ferroelectric domains within high quality films, for the first time, as well as evidence for their reversible switching. Two additional AFM techniques are also developed within the scope of this work, for planarizing samples and ultimately achieving nanoscale milling and tomography. With periodic, or simultaneous functional imaging such as photoconductive AFM measurements during such AFM-NanoMilling (AFM-NM), 3-dimensional tomographic datasets are acquired, which revealed the 3-d network of photocarrier pathways in CdTe. In summary, the AFM methods developed and applied in this thesis provide a means to understand the fundamental transport mechanisms in photovoltaic systems with nanoscale resolution, with applicability to the knowledge-driven design of future devices that will have optimized materials and hence properties

Nanocharacterization of Porous Materials with Atomic Force Microscopy

Scanning Probe Microscopy techniques have proven very useful in the investigation of porous nanostructured surfaces. Especially, Atomic Force Microscopy (AFM) has been widely used due to its compatibility with non-conducting surfaces. In particular, AFM often complements other techniques like scanning and transmission electron microscopy by providing quantitative surface information coupled with nanoscale spatial resolution. Its ability to operate in fluid is also important, as this allows researchers to mimic the physiological environment of biological materials and systems. In this work, two main types of porous materials are studied with AFM, including Phosphoric Acid Fuel Cell (PAFC) electrode catalyst layers, and human molar dentin. Although these systems apply to very different areas of materials science, there are many commonalities in terms of feature sizes, surface morphology, and appropriate imaging methods

Long Distance Electron Transfer Across \u3e100 nm Thick Au Nanoparticle/Polyion Films to a Surface Redox Protein

Glutathione-decorated 5 nm gold nanoparticles (AuNPs) and oppositely charged poly(allylamine hydrochloride) (PAH) were assembled into {PAH/AuNP}n films fabricated layer-by-layer (LbL) on pyrolytic graphite (PG) electrodes. These AuNP/polyion films utilized the AuNPs as electron hopping relays to achieve direct electron transfer between underlying electrodes and redox proteins on the outer film surface across unprecedented distances \u3e100 nm for the first time. As film thickness increased, voltammetric peak currents for surface myoglobin (Mb) on these films decreased but the electron transfer rate was relatively constant, consistent with a AuNP-mediated electron hopping mechanism

Direct Observation of Ferroelectric Domains in Solution-Processed CH₃NH₃PbI₃ Perovskite Thin Films

A new generation of solid-state photovoltaics is being made possible by the use of organometal-trihalide perovskite materials. While some of these materials are expected to be ferroelectric, almost nothing is known about their ferroelectric properties experimentally. Using piezoforce microscopy (PFM), here we show unambiguously, for the first time, the presence of ferroelectric domains in high-quality β-CH₃NH₃PbI₃ perovskite thin films that have been synthesized using a new solution-processing method. The size of the ferroelectric domains is found to be about the size of the grains (∼100 nm). We also present evidence for the reversible switching of the ferroelectric domains by poling with DC biases. This suggests the importance of further PFM investigations into the local ferroelectric behavior of hybrid perovskites, in particular <i>in situ</i> photoeffects. Such investigations could contribute toward the basic understanding of photovoltaic mechanisms in perovskite-based solar cells, which is essential for the further enhancement of the performance of these promising photovoltaics

Mapping the Photoresponse of CH₃NH₃PbI₃ Hybrid Perovskite Thin Films at the Nanoscale

Perovskite solar cells (PSCs) based on thin films of organolead trihalide perovskites (OTPs) hold unprecedented promise for low-cost, high-efficiency photovoltaics (PVs) of the future. While PV performance parameters of PSCs, such as short circuit current, open circuit voltage, and maximum power, are always measured at the macroscopic scale, it is necessary to probe such photoresponses at the nanoscale to gain key insights into the fundamental PV mechanisms and their localized dependence on the OTP thin-film microstructure. Here we use photoconductive atomic force microscopy spectroscopy to map for the first time variations of PV performance at the nanoscale for planar PSCs based on hole-transport-layer free methylammonium lead triiodide (CH₃NH₃PbI₃ or MAPbI₃) thin films. These results reveal substantial variations in the photoresponse that correlate with thin-film microstructural features such as intragrain planar defects, grains, grain boundaries, and notably also grain-aggregates. The insights gained into such microstructure-localized PV mechanisms are essential for guiding microstructural tailoring of OTP films for improved PV performance in future PSCs

“Grafting-Through”: Growing Polymer Brushes by Supplying Monomers through the Surface

We introduce a “grafting-through” brush polymerization mechanism where monomers are supplied through the surface on which the initiators are attached rather than from solution as in the “grafting-from” technique. This is accomplished by attaching the initiator to the surface of a dialysis membrane and supplying monomers through the membrane to the growing brush. This avoids the growth of very long chains while promoting the growth of shorter chains by reversing the monomer concentration gradient found in the commonly used grafting-from technique, where monomer concentration is lowest at the substrate and highest in the surrounding solution. Reversing this monomer concentration gradient results in shorter chains experiencing a higher local monomer concentration than longer chains, thus speeding up their growth relative to the longer ones. It is shown by AFM that brush layers made by this method are thicker and have lower roughness than brushes made by a grafting-from approach. Coarse-grained molecular dynamics simulations of brush polymerizations with monomers supplied through a permeable substrate provide insight into the mechanism of the grafting-through brush growth process. Simulations show that it is possible to obtain a brush layer with a chain dispersity index approaching unity for sufficiently long chains. FTIR, contact angle measurements, SEM, and kinetic studies are used to characterize and elucidate the growth mechanism of brushes synthesized by the new grafting-through strategy