Progress Report on the GCEP Project “Ultra High Efficiency Thermo-Photovoltaic Solar Cells Using Metallic Photonic Crystals

Our team aims to combine large-scale numerical simulations, with nanofabrication and characterization to develop the efficient intermediate absorbers and emitters that enables high efficiency solar thermo-photovolatic systems. In this first year, substantial progress has been made in developing designs that enable high efficiency TPV cells, as well as in experimental setup for fabricating and measuring of such devices. Introduction Thermal Photovoltaic (TPV) solar cells, where solar radiation is absorbed by an intermediate, which then emits thermal radiation towards a solar cell, is capable of achieving an extremely high efficiency using single-junction solar cells. The theoretical efficiency of 85% far exceeds the Shockley-Queisser limit. In order to approach such efficiency, however, there are very important constraints on the properties of the intermediate absorber and emitter. Until now, there is no known way to meet the requirements on the intermediate that are needed in order to reach efficiency beyond 30%. The aim of our project is to exploit emerging opportunities in the area of nanophotonic structure such as photonic crystals for TPV applications. It has been recently shown that both the absorption and thermal emission properties of nanophotonic structure can be tailored with appropriate design. Here, we aim to show that such structures can indeed be designed to enable ultra-high efficiency TPV solar cells. Specifically, we will demonstrate that crystal structures can be produced with low-cost self-assembly fabrication techniques using abundant materials such as Tungsten, that the crystals can provide broad-band absorption over the entire solar spectrum, and that the intermediate can be designed to enhance overall cell efficiency. Background Over the past few years, significant progress has been made on the fabrication of metallic photonic crystals with sub-micrometer dimensions. The most recent developments were just reported in Symposium J of the April 2009 MRS meeting (www.mrs.org). Braun and co-workers demonstrated that 3D self-assembled and holographically fabricated photonic crystal templates could be converted to refractory metals such as tungsten. The Norris group at the University of Minnesota presented their results on the 3D direct writing of silica photonic crystal woodpile structures followed by conversion of these structures to tungsten via chemical vapor deposition. Shawn-Yu Lin and his colleagues at RPI described the fabrication of iridium based photonic crystal thermal emitters. Iridium is interesting because of its theoretical potential to operate at shorter wavelengths than tungsten (below 1.8 micrometers) due to its fundamental optical properties. These and other rapid advances in the fabrication of metallic and metalodielectric photonic crystals should pay dividends for thermophotovoltaic energy conversion in the near future. Results Since the start of the project, we are making significant progresses regarding the design, measurement and fabrication of devices. Below we discuss in details progress along each directions.