Development of new atomic force microscopy methods to investigate interfaces of semiconductor materials
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Abstract
This PhD Thesis investigates nanoscale structures in semiconductor devices and their structure
property relationships, with a particular focus on perovskite solar cells (PSCs). Over the past two decades, PSCs have attracted significant attention due to their high power conversion efficiency (PCE) and low production costs, positioning them as promising alternatives to traditional silicon solar cells. However, to fully exploit their potential for commercial applications, a fundamental understanding of their nanoscale physical properties is essential. Features such as grain boundaries (GBs) and crystal lattice and interfacial defects has been observed to function as recombination centers for charge carriers. This property is critical to the functionality of the device. However, direct visualization at this scale has remained challenging.
The aim of this PhD Thesis is to develop tools and methods that enable a deeper understanding through visualization of promising semiconductor materials and their buried interfaces at the nanoscale. This knowledge will facilitate targeted engineering of surfaces and defects to optimize device performance without compromising efficiency.
To probe electronic properties at the nanoscale, I employed conventional Kelvin Probe Force Microscopy (KPFM) alongside two newly developed Atomic Force Microscopy (AFM) techniques based on Electrostatic Force Microscopy (EFM) principles. The first method, nanoscale surface photovoltage spectroscopy (nano-SPV), enables the study of charging and discharging processes in PSCs. The second method, multi-frequency heterodyne Electrostatic Force Microscopy (MFH-EFM), enhances the spatial resolution of the second capacitance derivative signal, allowing for more detailed investigation of interfaces in semiconductor devices.
As demonstrated by KPFM measurements, passivation molecules have been found to preferentially accumulate at grain boundaries, where dangling bonds are present. Utilizing tr-KPFM and the newly developed nano-SPV method has revealed that surface passivation not only leads to more homogeneous extraction and recombination of charge carriers but also increases
recombination times, indicating fewer defects that hinder carrier extraction.
The novel MFH-EFM method enabled superior localization of dielectric properties compared to standard approaches and the possibility to do dielectric spectroscopy.
This PhD thesis presents new functional methods with improved resolution for the AFM and
further improves the versatility of the EFMs for nanocharacterization of new energy materials.