Buffer and Barrier Layers for CIGS Based Tandem Photovoltaics

Abstract

As the world’s energy consumption increases, a technology for energy production that does not release greenhouse gases has become desirable. Of these technologies, solar devices with an absorber composed of copper, indium, gallium, and selenium (CIGS) have become industrially viable. It is possible to increase the efficiency of a CIGS based device by implementing a so-called “tandem” configuration consisting of a wide bandgap device stacked on top of a narrow bandgap device, but actual demonstration of these devices remains elusive. This work seeks to address two concerns that arise during fabrication of this device. The first of these is the diffusion of cadmium from the buffer layer into the narrow bandgap absorber during top cell deposition. In this work, a diffusion barrier has been shown to be a partially effective mechanism for limiting the damages caused by this diffusion. The procedure for depositing the diffusion barrier and the electrical and chemical effects of the diffusion barrier are discussed. The second limitation to a tandem configuration discussed in this work is the optimization of the band structure between the top-cell wide bandgap absorber and the top-cell buffer layer. This interaction is measured directly, and the effect of varying the buffer layer on device efficiency is examined.