Development of ZnO Buffer Layers for As-Doped CdSeTe/CdTe Solar Cells with Efficiency Exceeding 20%

The front buffer layer plays an important role in CdSeTe/CdTe solar cells and helps achieve high conversion efficiencies. Incorporating ZnO buffer layers in the CdSeTe/CdTe device structure has led to highly efficient and stable solar cells. In this study, the optimization of ZnO buffer layers for CdSeTe/CdTe solar cells is reported. The ZnO films are radio frequency sputter-deposited on SnO2:F coated soda-lime glass substrates. The substrate temperature for the ZnO deposition is varied from 22 to 500 degrees C. An efficiency of 20.74% is achieved using ZnO deposited at 100 degrees C. The ZnO thickness is varied between 40 nm and 75 nm. Following the ZnO depositions, devices were fabricated using First Solar's CdSeTe/CdTe absorber, CdCl2 treatment, and back contact. The optimal ZnO deposition temperature and thickness is 100 degrees C and 65 nm, respectively. The STEM-EDX analysis shows that within the detection limits, chlorine is not detected at the front interface of the devices using ZnO deposited at 22 degrees C and 100 degrees C. However, depositing ZnO at 500 degrees C results in chlorine segregation appearing at the ZnO/CdSeTe boundary. This suggests that chlorine is not needed to passivate the ZnO/CdSeTe interface during the lower temperature depositions. The nanocrystalline ZnO deposited at lower temperatures results in a high-quality interface.