Transient Metastable Behavior Caused by Magnesium-Doped Zinc Oxide Emitters in CdSeTe/CdTe Solar Cells

Metastable behavior in highly efficient MZO/CdSeTe/CdTe solar cells has been reported previously. Different preconditioning procedures have been studied that are used to recover the performance of the devices. 11 wt% of MgO content in the MZO layer has shown to give optimized photovoltaic parameters in the device compared to other MZO compositions. J-V characteristics before preconditioning of the devices with higher MgO content show an "S " shaped behavior, which is removed during preconditioning. However, this recovery remained only for 3 days while the devices were stored under vacuum in the dark. Temperature-dependent J-V and capacitance measurements before and after preconditioning revealed the presence of recombination centers and defect levels at the MZO/absorber interface. Previous studies have shown degradation of MZO occurring if the layer is exposed to ambient atmosphere. Hall effect measurements on the MZO films showed no significant changes after any preconditioning or CdCl2 treatment. Secondary-ion mass spectrometry images show diffusion of oxygen from the MZO layer into the CdSeTe region after CdCl2 treatments. This likely enables the MZO to function as a buffer layer since it will increase the carrier concentration due to the formation of oxygen vacancies. As-deposited MZO thin films are insulating. However, the oxygen vacancies in the MZO layer also increase its reactivity and instability.