Combinatorial study of MZO emitters for CdTe-based solar cells
Yegor Samoilenko1, Gavin Yeung1, Andriy Zakutayev2, Matthew O. Reese2, Colin A. Wolden1
1Colorado School of Mines, Golden, CO, United States
/2National Renewable Energy Laboratory, Golden, CO, United States

Precise band alignment at the front interface of CdTe-based solar cells is necessary to reduce recombination and enable higher efficiencies. Magnesium zinc oxide is not only transparent but also offers the ability to tune the conduction band offset with CdTe at the front of the device stack. To date most efforts have employed MZO targets which limits one to discrete compositions. The goals of this work are two-fold. First, we demonstrate successful synthesis of MZO films through reactive co-sputtering of metal targets. Uniform films are deposited with substrate rotation while combinatorial libraries are created with the substrate fixed to fine-tune the optimal MZO composition for CdTe-based solar cells. This technique allows us to probe multiple compositions, and therefore, band gaps of MZO, on a single substrate, reducing the effect of processing variability. Combinatorial libraries were created to tune the conduction band alignment for the optimal performance of CdTe solar cells. Band gap variation of more than 0.4 eV is achieved across a 3-inch substrate. As-deposited films show a decrease in electron affinity with increasing band gap as obtained by Kelvin probe, which is consistent with theory. Device performance is optimized at band gaps of 3.5-3.6 eV for the processing conditions employed.