Abstract
Solar cells made from Cu 2 ZnSn(S,Se) 4 (CZTS)-derived materials have been widely studied for their favourable material properties utilized in photovoltaic energy conversion. Drawbacks of the materials are associated with low open circuit voltage (V oc) resulting from non-radiative recombination at grain boundaries and interfaces. Considerable work has focused on the incorporation of sodium (Na), which is found to passivate trap states and reduce electronic losses. Here we present evidence that Na + as well as several ionic species (Se 2À and Zn 2+), do not remain stationary after device fabrication, but in fact migrate under electrical biasing. Furthermore, this ionic migration can be manipulated at room temperature by exposing the device to an external electric forming field. We outline a novel procedure that can effectively control and adjust ionic movement and associated local distribution in fully fabricated devices. Our results show that this simple treatment leads to favourable improved device performance and provides insight into light-induced reduction in performance which may be partially reversible.