Controlling silicon bottom cell lifetime variance in II-VI/Si tandems
Kevin D Tyler1, Madhan K Arulanandam1, Ramesh Pandey2, Jennifer Drayton2, James Sites2, Richard R King1
1Arizona State University, Tempe, AZ, United States
/2Colorado State University, Fort Collins, CO, United States

An intricate look is taken at the steps and methods used to account for variance in minority-carrier lifetime in the silicon bottom cell of II-VI/Si tandem solar cells.  A discussion on the modeling is provided, analyzing the amount of variance acceptable for the overall tandem cell.  The difference between wafer-to-wafer lifetime variance and lateral lifetime variance across one wafer indicates that greater experimental accuracy can be gained by using separate pieces of one wafer for the different experimental conditions.  A detailed analysis of the crucial cleaning procedure is given, as well as the use of photoluminescence imaging as a contamination control check.  Finally, despite a small sample size and the substantial variance of starting lifetime in silicon wafers, this procedure of control samples and variance control allows statistically significant conclusions to be drawn on the main factors affecting lifetime, and Si samples with CdTe deposition and CdCl2 treatment to maintain over 1.5 ms lifetime, enabling the Si bottom cell in II-VI/Si tandem cells to reach state-of-the-art performance.  CdCl2 treatment has the largest effect on the degradation of silicon lifetime, while higher temperatures have a moderate effect, and thicker IZO layers can have a mild effect, depending on the conditions of deposition.