Wide bandgap Cd1-xZnxTe and Cd1-xMgxTe have drawn attention as top cells in tandem devices. These materials offer flexibility of tuning the band gap over a wide range by controlling the Zn (or Mg) concentration in CdTe with little alteration to its properties. Historically, CdS has been extensively used as a heterojunction partner for CdTe based devices. However, use of CdS as a window material has shown many drawbacks in these devices. Due to the small band gap, photons with higher energy wavelengths (> 2.4 eV) absorbed by CdS do not contribute to the photocurrent of the completed devices. Another drawback is that, CdS forms an energetic “cliff” in the conduction band at the interface between CdTe and CdS resulting in increased recombination, which reduces the open circuit voltage, fill factor, and overall device efficiency. Since the wider bandgaps of Cd1-xZnxTe and Cd1-xMgxTe are mostly formed due to an increase in energy of the conduction band relative to CdTe, this cliff with CdS will be even larger in these devices, indicating that a CdS hetero-partner for these devices can severely degrade the performance.
Recent work shows that appropriate band alignment at the interface between the window layer and the absorber results in significant improvements in device performance. While using wide band gap oxides to create this energetic “spike” at the window layer-absorber interface yields these advances, the interface between the transparent conducting oxide and window layer also needs to be optimized to allow barrier free electron flow to the front contact. In this study, we will use SCAPS 1D software to model the wider band gap Cd1-xZnxTe and Cd1-xMgxTe devices to determine the appropriate alignment between the absorber and the window layer. We will also investigate how the material properties of TCO and window layer will affect the front contact alignment to determine the optimized device structure for high efficiency Cd1-xZnxTe and Cd1-xMgxTe.