Characterizing the Back-Contact Interface of Bi-Facial Poly-Crystalline CdTe Devices Using Transmission Electron Microscopy*

Poly-crystalline Cd(Se)Te based thin film solar cells have shown to be competitive in terms of efficiency and cost of electricity production. Yet, the presence of hetero-interfaces in Cd(Se)Te structure and low minority carrier lifetime have limited the thin film devices from reaching their maximum theoretical efficiency of approximately 30 percent. The back-contact of CdSeTe devices has been identified as one significant limitation to increased device performance since no metal has been identified that has a sufficiently high work function to create an Ohmic contact with the CdTe absorber at the back-surface of the film stack. Here, we will explore novel back-contact film layers in an effort to overcome this energy band mismatch. Atomic-resolution imaging in a scanning transmission electron microscope (STEM) combined with electron energy-loss spectroscopy (EELS) and energy-dispersive X-ray spectroscopy (XEDS) are used to characterize these devices and to inform the production process. The goal is to identify the ideal atomic and electronic structures, as well as any interfacial diffusion of elements.