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Carey Reich1 Drew Swanson2 Tushar Shimpi1 Ali Abbas3 Zachary Holman2 Walajabad Sampath1

1, Colorado State University, Fort Collins, Colorado, United States
2, Arizona State University, Tempe, Arizona, United States
3, Loughborough University, Loughborough, , United Kingdom

Cd1-xMgxTe (CMT) is a wide bandgap semiconductor which is optimal for use with silicon (Si) bottom cells in a tandem configuration for photovoltaic (PV) applications. This is due to its tunable bandgap with Mg content and high transmission of sub-bandgap photons. Additionally, it takes advantage of CdTe PV manufacturing techniques with proven scalability. By pairing the existing production capabilities of Si PV with the scalable production that CdTe manufacturing offers, devices of greater efficiency than that of a single junction device can be produced while maintaining or reducing cost of power. However, fabrication of high efficiency CMT devices necessary for this application has not yet been achieved. This is often attributed to issues in post-deposition CdCl2 processing of the CMT films. It has been shown that CdCl2 promotes recrystallization and grain growth of CdTe, as well as acting as a source of chlorine for diffusion along the grain boundaries. These effects are responsible for eliminating detrimental defects, growing large grains, and allowing grain boundaries to assist in current collection. The combination of these effects is attributed with bringing 1% efficient CdTe devices up to 13% or greater. When CdCl2 processes are applied to CMT, the increase in device performance is not achieved. It has been reported that the process causes loss of Mg, degrading the optical and electrical properties which had made it ideal for use in tandem PV. It has been postulated that MgCl2 passivation processes will produce CMT devices of higher performance without removing Mg from the film. It is thought that the loss reaction will be limited by removing the additional Cd of CdCl2 needed to form CdTe during Mg loss. Devices that have undergone both processes are characterized using current density-voltage, quantum efficiency, cross-sectional transmission electron microscopy (TEM), and cross-sectional energy dispersive spectroscopy (EDS) measurements. This work shows that CdCl2 treatments on CMT slightly improved device efficiency, but optically degraded the CMT film. Additionally, it is shown that a large numbers of voids developed after processing and there was loss of Mg from the grain boundaries and close to the junction. It was found that MgCl2 passivation did not optically degrade the film, but it has not yielded the desired device performance.

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