Interrelation of the CdTe Grain Size, Postgrowth Processing, and Window Layer Selection on Solar Cell Performance

Recent improvements to the CdTe solar cell device structure have focused on replacing the CdS window layer with a more transparent material to reduce parasitic absorption and increase J_sc, as well as incorporating selenium into the absorber layer to achieve a graded band gap. However, altering the CdTe device structure is nontrivial due to the interdependent nature of device processing steps. The choice of the window layer influences the grain structure of the CdTe layer, which in turn can affect the chloride treatment, which itself may contribute to intermixing between the window and absorber layers. This work studies three different device architectures in parallel, allowing for an in-depth comparison of processing conditions for CdTe solar cells grown on CdS, SnO₂, and CdSe. Direct replacement of the CdS window layer with a wider band gap SnO₂ layer is hindered by poor growth of the absorber, producing highly strained CdTe films and a weak junction. This is alleviated by inserting a CdSe layer between the SnO₂ and CdTe, which improves the growth of CdTe and results in a graded CdSeTe_1- absorber layer. For each substrate, the CdTe deposition rate and postgrowth chloride treatment are systematically varied, highlighting the distinct processing requirements of each device structure.