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

In this study, the use of intrinsic and highly insulating ZnO buffer layers to achieve high conversion efficiencies in CdSeTe/CdTe solar cells is reported. The buffer layers are deposited on commercial SnO2:F coated soda-lime glass substrates and then fabricated into arsenic-doped CdSeTe/CdTe devices using an absorber and back contact deposited by First Solar. The ZnO thickness is varied from 30 to 200 nm. The devices incorporating a 50 nm ZnO buffer layer achieved an efficiency of 21.23% without an anti-reflection coating. An improved efficiency of 21.44% is obtained on a substrate with a multilayer anti-reflection coating deposited prior to device fabrication. The highly efficient ZnO based devices are stable and do not develop anomalous J-V behavior following environmental tests. High resolution microstructural analysis reveals the formation of a high-quality ZnO/CdSeTe interface. Unusually, chlorine is not detected as a discrete layer at the interface, these observations point to a high-quality interface. The extrapolation of Voc to 0 K indicates that interface recombination dominates, suggesting that further improvement is possible. Using device modeling, an attempt is made to understand how this type of device performs so well. The incorporation of a ZnO buffer layer in the CdSeTe device structure has led to conversion efficiencies exceeding 21%. The ZnO carrier concentration is orders of magnitude lower than that of the conventional SnO2 buffer. ZnO creates a high-quality interface with CdSeTe, resulting in the unusual omission of Cl at the front interface.image