Growth Mechanisms of Co-Evaporated Kesterite: A Comparison of Cu-Rich and Zn-Rich Composition Paths

Earth abundant kesterite solar cells have achieved 7-10% cell efficiency mostly by processes that separate the film deposition and the annealing into two sequential steps. In contrast, co-evaporation onto a high-temperature substrate, demonstrating previous success in chalcopyrite (Cu(In,Ga)Se ₂) solar cells, allows real-time composition control. Chalcopyrite research widely supports the model that Cu-rich growth conditions assist grain growth, and subsequently, the endpoint composition can be adjusted back to Cu-poor via monitoring the surface emissivity of the film. On the basis of the same intentions, the recent development of co-evaporated kesterite (Cu ₂ZnSnSe₄) adapts the concept and achieves 9.2% efficiency. To understand the effect of growth strategies, this study examines the phase evolution, grain morphology, and device performance in Cu-rich growth and other strategies (Zn-rich and close-to-stoichiometric). By characterizing films obtained from interrupted depositions and also interpreting the variation in surface emission during growths, this study found a subtle hindrance in the reaction of Cu_xSe_y and ZnSe possibly caused by the volatile nature of SnSe_x. The hindrance explains why, distinctive from chalcopyrite, little difference in grain size is observed between kesterite films made by Cu-rich versus Zn-rich growth at these deposition rates. At last, a Zn-rich growth 9.1% device, certified by the National Renewable Energy Laboratory, is presented, which equals the performance of the previously-reported Cu-rich growth device. At the present stage, we believe the Cu-rich and Zn-rich growth share equal promise for the optimization of kesterite solar cells.