Investigation of differences between high and low efficiency CIGS solar cell structures using surface analytical techniques

Abstract

CuInxGa(1−x)Se2 (CIGS) is one of the most promising thin film PV materials due to its high efficiency, variety of growth methods available, and compatibility with flexible substrates enabling roll-to-roll manufacturing. The goal for all PV is low cost per watt, the solar industry's key metric. CIGS offers similar manufacturing costs compared with other thin film PV but with the promise of higher efficiency. Significant effort has gone into reducing materials costs, manufacturing costs, and into improving efficiency. But what makes one cell efficient and the next cell less efficient when made using the same process? In this work we compare two CIGS structures, both grown using the same process. One was measured at 6% efficiency and the other was over 12% efficient. Why the difference? We used surface analytical techniques to examine the two cells. We compared layer structure, interfaces, composition, and contaminants looking for differences that might explain the efficiency difference. Can we determine with physical analysis why one solar cell is efficient, while another seemingly identical cell is less efficient? Some measurements showed no difference, some small differences, and some large differences. Identification of differences between high and low efficiency devices could help identify important process control variables.