The Department of Materials Science and Engineering welcomed Bryan Huey from the University of Connecticut for a seminar titled “Nanoscale 2- Dimensional and Tomographic Photovoltaic Property Mapping.” Below is the absract:
The local materials properties for many classes of solar cells are often limiters for ultimate performance and reliability. Using Atomic Force Microscopy (AFM) with a conducting probe serving as a finely positionable top electrode, as well as in-situ illumination, the local photovoltaic performance of several solar cell systems are demonstrated including Silicon PERC, MAPbI3, CIGS, and CdTe thin film cells. With particular emphasis on grain boundaries and interfaces, local variations are reported in the effective short circuit current, open circuit potential, and fill factor. Uniquely, PV property measurements have even been extended into 3-D by developing CT-AFM, yielding nanoscale insight throughout the thickness of working polycrystalline solar cells in particular. The resulting photovoltaic performance maps reveal tremendous heterogeneity, both at the surface and as a function of depth. This includes surprising order-of-magnitude enhancements for certain grains, hypothesized to result from current percolation pathways more than compositional or orientational variations. The work also directly supports separate, indirect studies which suggest preferential electron conduction along grain boundaries. Finally, CTAFM uniquely resolves photoactive planar defects, identified via cross-sectional TEM as twins and stacking faults, which are proposed to enhance instead of degrading CdTe solar cell efficiency by providing orthogonal channels for preferential hole transport. Such investigations can literally provide a new perspective on nanoscale photovoltaic properties into the depth of operating solar cells. The resulting insight suggests new pathways to improve photovoltaic designs, efficiencies, and lifetimes.