A recent advance by Arizona State University researchers in developing nanowires could lead to more efficient photovoltaic cells for generating energy from sunlight and to better LEDs. Electrical engineers Cun-Zheng Ning and Alian Pan are working to improve quaternary alloy semiconductor nanowire materials. Nanowires are tens of nanometers in diameter and tens of microns in length. Quaternary alloys are made of semiconductors with four elements, often made by alloying two or more compound semiconductors. Semiconductors are the material basis for technologies such as solar cells, and for high-efficiency LEDs, and visible and infrared detectors. One parameter of semiconductors that determines the feasibility for these technologies is the bandgap, which determines whether a given wavelength of sunlight is absorbed or left unchanged by the semiconductor in a solar cell. Bandgap also determines the color light that an LED emits. To make solar cells more efficient, the range of bandgaps must increase to match the entire solar spectrum, said Ning, a professor in the School of Electrical, Computer and Energy Engineering, a part of the university’s Ira A. Fulton Schools of Engineering. More bandgaps, more colors In LED applications, having more available bandgaps means that more colors could be emitted, providing more flexibility in color engineering or color rendering of light; for example, different proportions of red, green and blue colors would mix with various white colors, enabling the whites to be adjusted to suit various situations or individual preferences, Ning said. The researchers, led by Ning and Pan, an assistant research professor, say the hurdle is that every manmade or naturally occurring semiconductor has only a specific bandgap. One standard way to broaden the range is to alloy two or more semiconductors. By adjusting the relative proportion of two semiconductors in an alloy, it is possible to develop new bandgaps between those of the two semiconductors. But accomplishing this requires lattice constant matching, a process that requires similar interatomic spaces between two semiconductors to be grown together. “This is why we cannot grow alloys of arbitrary compositions to achieve arbitrary bandgaps,” Ning said. "This lack of available bandgaps is one of the reasons current solar cell efficiency is low, and why we do not have LED lighting colors that can be adjusted for various situations." In recent attempts to grow semiconductor nanowires with “almost” arbitrary bandgaps, the researchers used a new approach to produce an extremely wide range of bandgaps. They alloyed two semiconductors, zinc sulfide and cadmium selenide, to produce the quaternary semiconductor alloy ZnCdSSe, which produced continuously varying compositions of elements on a single substrate (a material on which a circuit is formed or fabricated). 350- to 720-nm light emissions Ning said this is the first time a quaternary semiconductor has been produced in the form of a nanowire or nanoparticle. By controlling the spatial variation of various elements and the temperature of a substrate (called the dual-gradient method), the team produced light emissions that ranged from 350 to 720 nm on a single substrate only a few centimeters in size. The color spread across the substrate can be controlled to a large degree, and Ning believes that this dual-gradient method can be applied to produce other alloy semiconductors or to expand the alloys’ bandgap. To explore the alloys’ use in making photovoltaic cells more efficient, the researchers developed a lateral multicell design combined with a dispersive concentrator, a concept that has been explored for decades. But the typical application uses a separate solar cell for each wavelength band. With the new materials, the team is working on the design and fabrication of a monolithic lateral supercell that would contain multiple subcells in parallel, each optimized for a given wavelength band. The subcells would absorb the entire solar spectrum. Such solar cells would be able to achieve extremely high efficiency with low fabrication cost. Similarly, the new quaternary alloy nanowires with a large wavelength span can be explored for color-engineered light applications. The researchers have demonstrated that color control through alloy composition control can be extended to two spatial dimensions, a step closer to color design for direct white light generation or for color displays. The research was supported by Science Foundation Arizona and by the US Army Research Office. For more information, visit: nanophotonics.asu.edu