Tikalon Blog by Dev Gualtieri

Perfect Solar Absorber

October 17, 2014

Photovoltaics are a convenient way to harvest solar energy, but creating a photovoltaic solar cell is tricky business. Not all incident light will be converted to electricity. A photon incident on a solar panel might pass right through the material; or, it might be reflected away. Only at a range of wavelengths, as determined by the solar cell materials, will the photon create an electron-hole pair, so that these charge carriers have a reasonable probability of reaching terminating electrodes to create an electric current.

The photovoltaic effect was discovered in 1839 by Edmond Becquerel (1820 - 1891). Edmond was the father of Henri Becquerel (1852-1908), the discoverer of radioactivity and recipient of the 1903 Nobel Prize in Physics (see figure). The discovery of junction semiconductors marked a renewed interest in solar cells, and Bell Labs demonstrated a junction solar cell in 1951.

Edmond (left) and Henri Becquerel (right). Physics ran in their family; or, to borrow a line from the play, Arsenic and Old Lace, "It practically gallops." They were the middle two of four generations of physicists. Left image, lithograph of Edmond Becquerel by Pierre Petit (1832-1885), and right image, portrait of Henri Becquerel (1852-1908), circa 1905, by Paul Nadar (1856–1939), via Wikimedia Commons.

This early solar cell had an energy conversion efficiency of light to electrical power of just two percent. A few years later, in 1954, Hoffman Electronics Corporation (El Monte, California) produced solar cells with 4.5% efficiency, eventually attaining 14% in a commercial product by 1960. Hoffman Electronics solar cells powered the Vanguard 1 satellite, launched in 1958.

Vanguard 1 was the first solar-powered satellite, but the power was minuscule. The solar cells powered a 5 milliwatt transmitter. Telstar, launched in 1962, had much of its large outer shell covered with solar cells that generated about fourteen watts of electrical power. Compare this with today's solar-powered direct-broadcast satellites that power more than a kilowatt of transmitter power.

One interesting footnote of Telstar history is that Bell Labs engineer, John R. Pierce (1910 - 2002), who was executive director of the Bell Labs division in which Telstar was built, wrote science fiction. He used the pseudonym, "J.J. Coupling."[1] J-J coupling is a particular electron spin-orbit coupling of momentum that should be familiar to most physicists.

The efficiency of commercially available photovoltaics is now about 25%, but there are laboratory demonstrations of efficiency at nearly 50%. As I wrote in a recent article (Solar Steam, August 15, 2014), photovoltaics aren't the only route to efficient solar energy collection. One team of engineers has produced a black absorber that's a porous composite of graphite flakes and carbon foam that floats on water to convert solar energy to steam.[2-3]

A solar-black absorber will, by definition, absorb energy from the entire solar spectrum. As a consequence, the absorber will heat, giving you options as to how this heat can be used. A team of engineers from MIT's Solid State Solar Thermal Energy Conversion Center of its Micro and Nano Systems Laboratory and their colleagues has devised a nearly perfect solar absorber. The absorber, a metallic dielectric photonic crystal layer, is described in a recent paper by MIT postdoc Jeffrey Chou, MIT professors Marin Soljacic, Nicholas Fang, Evelyn Wang, and Sang-Gook Kim, along with five others presently at other institutions.[4-6]

Photonic crystals, the optical analogs of semiconductor crystals, allow control of light in the same way that semiconductors control electrons. The purpose of the photonic crystal in the solar absorber is to selectively allow absorption of solar energy while suppressing long wavelength (heat) emission. This allows construction of a solar energy harvester that operates by conversion of solar energy to thermal energy.[4-5] The photonic crystal layer has the additional advantages that it absorbs sunlight from a wide range of angles, it operates at high temperature, and it can be manufactured inexpensively.[6]

As in all photonic crystals, the structural dimension must be close to the wavelength of light, so the layer is an array of nanocavities in which the operating range is selected by the cavity size.[6] The photonic crystal layer is part of a solar-thermophotovoltaic (STPV) device in which solar energy is converted into heat. The heated material will emit infrared energy that's subsequently converted to an electrical current.[6] The device was initially built as hollow cavities. Says Jeffrey Chou, the paper's lead author, “They were empty, there was air inside... No one had tried putting a dielectric material inside, so we tried that and saw some interesting properties.”[6]

An artistic rendering of an idealized metallic dielectric photonic crystal layer for solar absorption.

(MIT image by Jeffrey Chou.)[6]

The solar absorber is designed to work at high temperatures in systems with sunlight-concentrating mirrors. The devices have survived 1000°C (1832 °F) for 24 hours without severe degradation.[6] Their solar collecting capability exists over a wide range of angles, so solar trackers wouldn't be required in some applications.[6] The device can be manufactured with the usual complement of silicon wafer processing tools. Such silicon wafers range in size up to twelve inches (300 millimeters) in diameter.

One stumbling point is the use of ruthenium, a relatively expensive metal, although substitutions might be possible. That's why the MIT group is investigating other metals. This research was funded by MIT's Solid State Solar Thermal Energy Conversion Center and the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.[4-6]

Scanning electron micrograph of MIT's metallic dielectric photonic crystal solar absorber. This image shows tungsten sputtered onto the alumina shells. The scale bar is 200 nm.

(Fig. S3 of ref. 5, Creative Commons license.)

Perfect Solar Absorber

References: