Nanoscale Tomographic Charge Transport in Polycrystalline Chalcogenide Absorbers: CDTE versus CIGS

Nanoscale charge transport is uniquely mapped in 3-dimensions for polycrystalline thin-film solar cell absorbers of cadmium telluride (CdTe) and copper indium gallium selenide (CIGS). Using a novel tomographic variation of Atomic Force Microscopy (CTAFM), we site selectively planarize photovoltaic materials while maintaining optoelectronic properties, thereby directly probing interfacial and bulk charge transport at and beneath the surface. This tomography reveals three notable differences and hence recommendations for CdTe versus CIGS. First, CIGS exhibits homogeneous charge transport in the bulk, as compared to CdTe which shows order-of-magnitude grain and grainboundary dependent performance. Grain boundary engineering approaches, such as passivation and doping, are thus proposed to be more effective for optimizing CdTe as opposed to CIGS. On the other hand, ordered defect compounds and deep acceptor states are detected between CIGS absorber and buffer (CdS) layers, emphasizing the importance of improving this interface for better CIGS efficiency. Lastly, CTAFM is found to enable site-selective comparisons of such local photovoltaic properties with grain orientations via EBSD, to directly explore and ultimately improve nanoand microscale connections between microstructure and solar cell performance.