The University of Texas at Dallas

Nano & Beyond Lab

How grain boundaries affect the efficiency of poly-CdTe solar-cells

The Sunshot project scope is to develop next generation solar-cell devices with an installed cost for utility-scale solar systems of less than ~6ยข per kilowatt hour (kWh) or equivalently, $1 per watt for PV systems. In poly-crystalline CdTe PV devices, these goals can only be reached by developing a fundamentally new understanding of the role that grain boundaries play in limiting the efficiency to 17% and Voc to 850 mV. The aim of this collaborative research is to develop an atomic-scale understanding of the effects of grain boundaries in thin film CdTe on the minority carrier lifetime, Voc and Jsc, and thus the overall efficiency of the PV device. A collaborative research team (CRT) that combines their unique expertise in well-defined grain boundary fabrication at UT Dallas), II-VI thin film synthesis using molecular beam epitaxy (MBE) at UIC, atomic-resolution characterization in an aberration-corrected scanning transmission electron microscope (STEM) at UIC, as well as first-principles density functional theory (DFT) calculations at Argon National Lab).

(a) The cross-sectional HAADF-STEM image reveals the twin boundary at the bonded interface. (b) The atomic configuration of the bonded twin boundary reveals the two-atomic layer CdTe WZ structure between the CdTe ZB matrixes, and conduction band alignment between ZB CdTe and WZ CdTe. The red and blue spheres respectively represent Cd and Te atoms. HAADF intensity profiles (c) and (d) along the lines 1 and 2 in (a) reveal the atomic arrangement of the bonded interface.