Two-stage annealing-induced secondary grain growth for enhanced efficiency in CdTe solar cells on flexible polyimide foils

Flexible CdTe solar cells integrated onto polyimide foils are a promising renewable power generation technology. Optimizing crystal structure and reducing grain boundary recombination by CdCl₂ annealing are the key approaches to increase carrier concentration and improve device performance. However, the inherent thermal constraints of polyimide substrates limit the feasibility of achieving full annealing. In response, we introduce a novel two-stage annealing process: a primary recrystallization at 350 °C followed by a secondary recrystallization at 380 °C. Employing a rigorous investigation of surface and interface energy minimization, strain energy minimization, and the growth kinetics of Cl drag phenomena, this method induces significant secondary grain growth, surpassing the film thickness itself. The culmination of these efforts yields an extraordinary power conversion efficiency (PCE) of 12.2 %, marking a historic achievement for sputtered CdTe solar cells on polyimide substrates. This study not only unveils a pioneering approach to grain boundary engineering but also offers insights into optimizing the CdCl₂ annealing process for CdTe absorbers, ultimately facilitating the realization of high-efficiency, flexible CdTe solar cells.