Two predominant ways to get white light from LEDs now exist. In one, a blue or UV-emitting LED is combined with a phosphor to produce an approximation of the white-light spectrum. In the other, the light from separate blue-, green-, and red-emitting LED chips are mingled to create white. Researchers at North Carolina State University (Raleigh, NC) and the U.S. Army Research Office (Research Triangle Park, NC) have developed a third approach, at least in theory.

Indium gallium nitride/gallium nitride (InGaN/GaN) LEDs can emit blue or green strongly, and red only weakly, with very poor efficiency. This occurs because a large internal field keeps the wave functions of holes and electrons apart; the effect results from a lattice mismatch between the well and the GaN quantum barrier. The researchers propose adding aluminum (Al) to the structure, replacing the GaN barrier with AlInGaAn and tailoring lattice mismatch. Simulations show that, with proper design, a single LED with three structures can produce blue, green, and red light of similar intensity and good efficiency. The researchers would like to fabricate such a device; one possible source of funding is the U.S. Department of Energy program on solid-state white-light sources, when it materializes. Contact Ki Wook Kim at kwk@ncsu.edu.

If made practical, amorphous and polycrystalline sheet solar cells could end up perched atop many a commercial or residential roof. But the devices, which can range up to a square meter in size, suffer from reduced efficiency caused by micron-scale nonuniformities in grain size and chemical composition. Scientists at the University of Toledo (Toledo, OH) have developed a technique in which a thin-film semiconductor structure, when immersed in an electrolyte and exposed to light, modifies itself to compensate for the nonuniformities.

In one example, a cadmium telluride/cadmium sulphide (CdTe/CdS) photovoltaic device contains a 0.15-µm-thick layer of CdS and a 3.5- to 4-µm-thick layer of CdTe on glass. The electrolyte consists of sodium chloride, aniline, and p-toluenesulfonic acid in deionized water. When the electrolyte is applied to one side of the structure and tungsten-halogen lamp light to the other, changes occur that the researchers posit are the elimination of weak microdiodes and the depositing of a protective layer. The process improved the efficiency of CdTe/CdS cells from 1%–3% to 11%–12%. Contact Yann Roussillon at yroussi@uoft02.utoledo.edu.