Effects of substrate orientation and solution movement in chemical bath deposition on Zn(O,S) buffer layer and Cu(In,Ga)Se2 thin film solar cells

With a colloid aggregation process, chemical bath deposited (CBD) Zn(O,S) thin films always consist of many particles or clusters on the surface and their removal is a long-standing problem. In this work, by varying the substrate orientations and solution movements, their effects on the quality of the Zn(O,S) layers, the formed junctions, and the performance of the Zn(O,S)-Cu(In,Ga)Se-2 (CIGS) solar cells have been systematically investigated. Unlike CBD-CdS growth for which stirring or rocking the solution is a common and useful practice for improved quality, surprisingly, for CBD-Zn(O,S) growth, solution movement plays a primary role and substrate orientation plays a secondary role to influence the film quality and the device performance. Equally surprising, the above effects strongly depend on the recipes and chemicals used for the CBD process. It has been identified that, independent of recipes, depositing Zn(O,S) films with the substrate inclined in a static solution can minimize the formation of particles/clusters, generate homogeneous and high quality buffer layers on CIGS with the best device performance. Disturbing the CBD solution would in general introduce large particles/clusters to adsorb on the Zn(O,S) films, leading to degraded junction quality, strong light soaking effect, and poor device performance.