Significant Passivation Effect of Cu(In, Ga)Se2 Solar Cells via Back Contact Surface Modification Using Oxygen Plasma

Molybdenum back contact plays a significant role in chalcopyrite Cu(In, Ga)Se-2 (CIGS) solar cells. A molybdenum oxide intermediate layer is applied to optimize the back contact from the aspect of absorber crystal growth regulation and back surface field reconstruction. A novel molybdenum oxide preparation method is introduced through oxygen plasma treatment of the molybdenum layer. CIGS film is obtained by sputtering from a quaternary target with post-selenization. The shortage of Cu and Se in the middle of the absorption layer causes crystallization stratification. A bidirectional regulation method for controlling intermediate fine grains is proposed. Superior crystallinity is produced to facilitate carriers' transport and reduce recombination at grain boundaries. Back passivation is achieved by forming a reverse p-n junction on the back surface. Despite the formation of this reverse junction, the efficiency of the device is still improved because the transport of holes is assisted by the molybdenum oxide's gap states. Thus, back surface recombination is effectively reduced by back passivation, leading to the increase in open-circuit voltage (V-oc) and fill factor (FF). The power conversion efficiency of treated solar cells increases by 34.1%. An innovative direction is proposed to optimize the back contact of CIGS solar cells in this study.

Automated summary

Molybdenum back contact is crucial for CIGS solar cells. A novel molybdenum oxide preparation method was introduced through oxygen plasma treatment of the molybdenum layer. Back passivation was achieved by forming a reverse p-n junction, and the transport of holes was assisted by the molybdenum oxide's gap states, leading to an efficiency improvement of 34.1% in treated solar cells.