Engineering Na-transport to achieve high efficiency in ultrathin Cu(In,Ga)Se2 solar cells with controlled preferred orientation

Abstract

Although the efficiency of Cu(In, Ga)Se2 (CIGS) solar cells have exceeded 22% for the absorber with thickness more than 2µm, thinning the absorber thickness has attracted attention to save the material costs of indium and gallium as well as to increase manufacturing throughput. However, a trade-off exists between efficiency and thickness of CIGS absorbers. In this work, we report a new insight to promote the efficiency of ultrathin (hundred-nanometer scale) CIGS solar cells by engineering Na-transport in a direct sputtering process with a quaternary CIGS target without post-selenization. We first demonstrate a feasibility to manipulate the preferred orientation of CIGS absorbers via controlled texture growth of the Mo underlayers. Then, we reveal that the amount of Na content within absorbers is strongly related to the preferred orientation and Cu-content of CIGS. The Cu-deficient CIGS absorbers grown on (211)-Mo underlayer show the strongest (220/204) CIGS texture and highest Na content, leading to enhanced efficiency. Through adequate Na-transport engineering and commonly used Ga gradient, we have significantly enhanced the efficiency of ultrathin (550nm) CIGS solar cells to 10.04%. Our results unveil the key factors of Na-transport in nano-scale and provide a strategy to achieve high-efficiency ultrathin CIGS solar cells.