Properties of CdSe_1−xS_x films by magnetron sputtering and their role in CdTe solar cells

CdS/CdSe composite window layer is a better candidate than CdS or CdSe single layer for CdTe solar cells. In order to find the underlying reason, the properties of co-sputtered CdSe_1−xS_x thin films with different compositions (\(0\le x\le 1\)) were examined. XRD patterns show that all CdSe_1−xS_x films (\(0\le x\le 1\)) are polycrystalline with hexagonal structures and the prominent peak position shows an increasing trend with the increase of x. This suggests that CdSe_1−xS_x window layer with a gradient composition, formed during the high-temperature growth and heat treatment of CdTe films, has the gradient lattice constant, meaning the less defect density. This is beneficial to the device performance. Hall measurements show that CdS_1−xSe_x films (x ≥ 0.772) are n-type semiconductors with carrier concentrations ranging from 10¹⁵ cm⁻³ to 10¹⁶ cm⁻³. It reveals that CdS/CdSe bi-layers can form a strong built-in electric field with CdTe layer as CdS does. The electronic band structure of CdSe_1−xS_x window layer with a gradient composition suggests that the spikes formed in the conduction band are lower than 0.3 eV, which would not impede the electron transport. The valence band alignment is beneficial for the hole transport. Therefore, an appropriate band alignment can be formed between CdSe_1−xS_x and CdTe layers. These results reveal that CdSe_1−xS_x film is a good candidate as the window layer for high-efficiency CdTe solar cells considering its structural, optical and electrical properties as well as its electronic structure.