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

Abstract

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. Temperaturedependent 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 CdCl 2 treatment. Secondary-ion mass spectrometry images show diffusion of oxygen from the MZO layer into the CdSeTe region after CdCl 2 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.

Achieving 21.4% Efficient CdSeTe/CdTe Solar Cells Using Highly Resistive Intrinsic ZnO Buffer Layers

“…Although it has not been possible to measure carrier concentrations of as‐deposited MZO, it has been thought that doping may occur in the MZO layer, due to the formation of oxygen vacancies during the CdCl 2 activation treatment. [ 17 ] Device modeling suggests that higher values in the n ‐type buffer layer than the p ‐type absorber is a condition for high device efficiency. [ 29 ] We have used a high‐sensitivity parallel dipole line (PDL) Hall effect system [ 30 ] to measure the carrier concentration of very thin (50 nm) and highly resistive ZnO films.…”

“…[ 15 ] Although the ‘S’ shaped behavior can be removed after preconditioning with light soaking, the recovery typically only remains for about three days. [ 17 ] This instability is a severe drawback of using MZO in a device. It is vital to explore alternative materials that can replace existing buffer layers to further improve device efficiency while ensuring long‐term stability.…”