Understanding the Role of CdTe in Polycrystalline CdSexTe1-x/CdTe-Graded Bilayer Photovoltaic Devices

Grading of bandgap by alloying CdTe with selenium to form a CdSexTe1-x/CdTe-graded bilayer device has led to a device efficiency over 19%. A CdSexTe1-x absorber would increase the short-circuit current due to its lower bandgap but at the expense of open-circuit voltage. It has been demonstrated that adding a CdTe layer at the back of such a CdSexTe1-x film reduces the voltage deficit caused by the lower bandgap of absorber from selenium alloying while maintaining the higher short-circuit current. This leads to a photovoltaic device that draws advantage from both materials with an efficiency greater than either of them. Herein, a detailed account using device data, ultraviolet photoelectron spectroscopy, electron microscopy, and first-principles density functional theory modeling is provided, which shows that CdTe acts as an electron reflector for CdSexTe1-x.