Inorganic Nanocomposite Solar Cells

This project is a fundamental study into the development of low-cost, thin film solar cells. It explores the fabrication of semiconductor nanocomposites for photovoltaics using nanostructured inorganic materials and atomic layer deposition (ALD). In thin film technologies, there exists a common problem with conversion efficiency due to poor materials quality: the photogenerated electrons and holes cannot travel very far before recombination (short free-carrier diffusion lengths) and are hence lost for power conversion. If the solar cell can be made using nanoscale heterojunctions, then every photogenerated carrier will have less distance to travel, and the problem of recombination can be greatly reduced. The proposed designs allow collection of all photogenerated carriers even in poor quality materials, thus making low cost deposition routes acceptable. We have performed modeling studies on single and multijunction device geometries which show that the negative effects of short minority-carrier lifetimes, namely high dark current, will be exacerbated by the large increase in junction area that occurs with nanostructuring. As a result, it appears that a good candidate for nanostructuring would be a material where low mobility—not short lifetimes—is the reason for low diffusion length. Another solution is to relax the size scale for the nanostructuring, from 100 nm to 100’s of nanometers or even a few microns. This will allow a compromise to be reached between collecting the maximum number of carriers and minimizing the interface area. Current plans call for absorber material based on Cu2ZnSnS4 to be incorporated into nanostructured devices both by sputtering and chemical bath deposition (CBD), either through deposition on a nanostructured substrate or through nanostructuring of planar films. The opposite polarity material will be deposited by both ALD and by CBD. Nanostructured templates of both hole and pillar arrays have been successfully formed on silicon by using nanosphere lithography and reactive ion etching. Progress in development of low cost methods of deposition of the semiconductor thin film has been made, based on techniques that utilize chemical bath deposition, ion exchange, and sulfidization, as well as sputter deposition combined with sulfidization. Several tools have been developed to make or study the nanocomposite solar cells, including an ALD reactor, sputter deposition system, sulfidization apparatus, and a scanning probe microscope.