Zinc telluride deposition using close space sublimation to create back contacts for cadmium telluride solar cells
This thesis presents the optimization process of the close space sublimation (CSS) technique to deposit zinc telluride (ZnTe) on thin-film cadmium telluride (CdTe) solar cells. Previous research has demonstrated that creating a heavily doped ZnTe intermediate layer between the absorber-CdTe and the back contacts produces low-resistive CdTe contacts that overcome the Schottky barrier created at the interface of a metal and a semiconductor . ZnTe interlayer forms a tunnel barrier at the contact interface so the positive charges created at the p-CdTe/n-cadmium sulfide (CdS) junction can be easily collected at the back contact maximizing the current collection process. However, ZnTe contacting interlayer has to be deposited using expensive equipment and complex process, limiting its implementation in mass-production CdTe solar cells. In this thesis, it has been demonstrated that ZnTe can be deposited using the CSS technique, which is the most common method to deposit CdTe in both research and industry. In order to demonstrate the presence of ZnTe on top of CdTe photovoltaics, scanning electron microscopy (SEM), energy dispersive x-ray Spectroscopy (EDS), and x-ray diffraction (XRD) characterization techniques were used to study the morphology and composition of the deposited film. A requirement for this interlayer is that it has to be heavily doped. Therefore, the research presented in this thesis also includes the optimization of two copper (Cu) doping techniques to introduce impurities to the ZnTe layer in subsequent process and create the tunnel barrier at the contact interface. Previous research on ZnTe deposition used a ZnTe source already doped with Cu, limiting the optimization of the deposition parameters since Cu diffusion has to be controlled by these parameters [13, 14]. Cu is the best material to dope the ZnTe intermediate layer because of its conductive properties and because it replaces Zn atoms in the compound without altering the structure . The first Cu doping technique involves an immersion process in a cupric nitrate solution, proposed by a previous doping study that used ZnTe deposited on glass samples . However, the proposed technique had to be optimized to Cu dope ZnTe films deposited on complete and functional CdTe solar cells. The alternative technique uses copper telluride additives on conductive graphite paste to produce low-resistive back contacts and introduce Cu atoms into the bulk of the ZnTe layer to create the tunnel barrier by annealing the solar cell. This technique has been used in previous research to lower the back contact resistance ; however, it has not been used as a doping technique for ZnTe interlayer. Therefore, this technique had to be optimized to find the best copper telluride concentrations and the best annealing conditions to dope the ZnTe interlayer with copper. The resulting technique provided low-resistive and efficient back contacts that enhance the overall performance of the CdTe solar cells.
Romo, Luis C, "Zinc telluride deposition using close space sublimation to create back contacts for cadmium telluride solar cells" (2009). ETD Collection for University of Texas, El Paso. AAI1461184.