III-V MIS Solar Cells
While the vast majority of solar cells use a p-n junction to induce the photovoltaic effect, alternative designs using a metal-semiconductor (MS) junction may be relevant for certain applications, including wide band gap solar cells or low-cost III-V-based thin film devices. Prior to modern epitaxial growth systems, nearly all PV devices were fabricated by diffusing dopants into a semiconductor at high temperatures. Doing so, however, can typically lead to an increase in point defects in the crystal, which act as recombination centers for minority carriers (i.e., not good). As an alternative to this high temperature process, several researchers over the years have investigated the MS, or Schottky, solar cell, whereby a thin semi-transparent metal layer is deposited on a doped semiconductor. The junction, then, is right at the surface of the semiconductor. Not only does this approach eliminate the high temperature diffusion, but it also reduces fabrication cost and processing time. However, given that a Schottky diode in the dark is driven by thermionic emission of majority carriers over a barrier, the reverse saturation current in these cells is typically high compared to p-n junctions. This correlates to a lower open circuit voltage. It has been shown, though, that a metal-insulator-semiconductor (MIS) solar cell can provide an alternative transport mechanism through tunneling that can significantly lower the reverse saturation current. This has been well demonstrated in silicon-based devices, but I am interested in the application for III-V-based materials, including wide band gap solar cells which commonly have a difficulty in doping either n- or p-type due to a phenomenon known as vacancy self-compensation. In addition, polycrystalline-based III-V materials for low-cost single junction solar cells could also benefit from the simplified fabrication involved in making an MIS-based device.
The Bottom Line
Silicon solar cells are cheap. Solar cells made from other materials are not so cheap, but can be more efficient than silicon solar cells. I want to make cheap solar cells that are as (or more) efficient than silicon solar cells. How? Minimize material usage and minimize processing/fabrication steps.