Evaluation of spectrum-splitting dichroic mirrors for PV mirror tandem solar cells

Single-junction solar cells are approaching their theoretical efficiency limit, and tandem or multi-junction architectures provide a route for further increasing efficiency. However, due to high material cost, these technologies have traditionally been restricted to high concentrations with the penalty of the loss of diffuse light in outdoor applications. We propose a new tandem concept called a PVMirror that makes use of the global spectrum. It utilizes PV cells as a three-in-one technology-they act as a concentrating mirror, spectrum splitter, and high-efficiency light-to-electricity converter. A key element of this technology is an effective spectrum-splitting dichroic mirror, and this paper evaluates three dichroic mirror designs. Prototype PVMirrors with these mirrors and silicon heterojunction solar cells were made, and their reflection and transmission spectra confirm the spectrum-splitting capability of the dichroic mirrors. Outdoor testing shows that reflected light is successfully concentrated to a focus at which the second sub-cell in the tandem is to be located.