Fabrication of Microscale Back-Contact Arrays for Local Charge Transport Measurements

Remarkable progress has been achieved in CdTe thin-film solar cells by maximizing open-circuit voltage (
$V_{oc}$
) and short-circuit current (
$J_{sc}$
) via bandgap optimization and interfacial engineering. Fundamental knowledge of the local carrier dynamics at microstructures in advanced PV architectures is required for further improvement of the performance. Scanning probe techniques have been extensively used for investigating inhomogeneous microstructural properties of CdTe solar cells. While extremely useful, the tip-based platforms are often limited to surface roughness, slow scan speed, and narrow field of view (< 10's of
$\mu\mathrm{m}$
). In this work, we report the fabrication of microscale back-contact array to measure the optoelectronic properties of individual grains and grain boundaries. This platform allows the intergranular (across a grain boundary) and intragranular (within a grain) local transport measurements. The laser-beam lithography developed in this work enables the parallel production of many identical microscale contacts, providing sufficient datasets for statistical analysis. We discuss the critical lithography conditions (e.g., exposure power) to optimize the fabrication processes for rough surface CdTe solar cells. As proof of concept, we design and fabricate micro-contacts on individual grains of CST (CdSe
0.1
Te
0.9
) grown by colossal grain growth. We show intragranular charge transport of this sample is promoted compared to intergranular transport under 1-sun illumination. Our results confirm that innate grain boundaries of CGG-grown CST film act as an active barrier, often observed in conventional polycrystalline semiconductors. The fabrication versatility of microscale contact arrays offers a robust measurement platform for studying inhomogeneous local carrier transport in CdTe-based solar cells and other types of thin-film PVs (e.g., CIGS, CZTS, perovskites).