Besides cost, the most fundamental issue in assessing photovoltaic solar cells is efficiency – how much of the sunlight that falls on the cell can it convert to electricity? For the second time in two years, Kin Man Yu and Wladek Walukiewicz of the Materials Sciences Division, working with colleagues from Berkeley Lab and other institutions, have announced a new solar cell material that may be able to achieve extraordinary efficiency. In every other way these discoveries are different, however.
Berkeley, California – April 9, 2004 [SolarAccess.com] The only thing the two materials have in common,” says Yu, “is that they both try to capture as much of the solar spectrum as possible.” In 2002, the researchers learned that indium gallium nitride (InGaN) would respond to different wavelengths of light if the proportions of indium and gallium in the alloy were adjusted. Thus it might be possible to create a photovoltaic cell sensitive to the full solar spectrum by stacking multiple negatively and positively doped layers to form several current-producing junctions. In their latest discovery – what Yu calls “a totally new material concept”, the researchers treat the alloy zinc manganese tellurium (ZnMnTe) in such a way that a single junction of the material may be able to respond to virtually the entire solar spectrum. “This isn’t a multijunction material,” says Walukiewicz, “it’s even more interesting: a multigap material” – a single semiconductor with multiple band gaps. A solar cell with the simplest possible physical structure could achieve 50 percent efficiency or better, far higher than any yet demonstrated in the laboratory. How solar cells work Sunlight comes in many colors, combining low-energy infrared photons with high-energy ultraviolet photons and all the visible-light photons between. Each photovoltaic material responds to a narrow range of these energies, corresponding to its characteristic band gap. The band gap is the amount of energy, expressed in electron volts (eV), required to kick an electron from a semiconductor’s valence band, which is chock full of electrons bound to atoms, into its empty conduction band, where electrons are free to move. (The bands are graphical representations, not physical spaces.) If the semiconductor is doped with impurity atoms to form an n-type, electrically negative material, it already has a few electrons in the conduction band; conversely p-type (positive) material has been doped to leave missing electrons, or holes, in the valence band. A junction between n- and p-type creates a voltage bias; when incoming photons are absorbed, electrons migrate toward the positive side of the junction and holes toward the negative side, forming an electric current. Photons with energy lower than the band gap escape unabsorbed; photons with higher energy are absorbed, but most of their energy is wasted as heat. Crystalline silicon, the leading solar cell material, has a band gap of only about 1.1 eV; most solar photons are much more energetic. Crystalline-silicon solar cells are about 25 percent efficient at best.
