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