Mar
25
Better Light Emitting Diodes and Solar Cells
March 25, 2010 | 6 Comments
Researchers Cun-Zheng Ning and Alian Pan at Arizona State University are developing nanowires could lead to better light-emitting diodes (LEDs) that could replace less energy-efficient incandescent light bulbs and more efficient photovoltaic cells for generating energy from sunlight.
The research effort is to improve quaternary alloy semiconductor nanowire materials. 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, high-efficiency LEDs for lighting, and for visible and infrared detectors.
Quaternary Semiconductor Alloy ZnCdSSe Nano Wires. Click image for more info.
One critical parameters of semiconductors to determine the feasibility for these technologies is called the band gap. The band gap of a semiconductor determines, for example, if a given wavelength of sunlight that is absorbed or left unchanged by the semiconductor in a solar cell and determines what color of light an LED emits. To gain efficiency and light spectrum emissions, it’s necessary to increase the range of band gaps.
The problem is that every manmade or naturally occurring semiconductor has only a specific band gap.
One standard way to broaden the range of band gaps is to alloy two or more semiconductors. By adjusting the relative proportion of two semiconductors in an alloy, it’s possible to develop new band gaps between those of the two semiconductors. But accomplishing this requires a condition called lattice constant matching, which requires similar inter-atomic spaces between two semiconductors to be grown together.
Ning explains, “This is why we cannot grow alloys of arbitrary compositions to achieve arbitrary band gaps. This lack of available band gaps is one of reasons current solar cell efficiency is low, and why we do not have LED lighting colors that can be adjusted for various situations.”
During the latest attempts to grow semiconductor nanowires with “almost” arbitrary band gaps, the research team used a new approach to produce an extremely wide range of band gaps. They alloyed two semiconductors, zinc sulfide (ZnS) and cadmium selenide (CdSe) 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). Ning says this 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 nanometers 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 says he believes this dual-gradient method can be more generally applied to produce other alloy semiconductors or expand the band gap range of these alloys.
To explore the use of quaternary alloy materials for making photovoltaic cells more efficient, the team has developed a lateral multi-cell design combined with a dispersive concentrator. The concept of dispersive concentration, or spectral split concentration, has been explored for decades. But the typical current design application uses a separate solar cell for each wavelength band, an expensive proposition.
With the new materials, Ning hopes to build a monolithic lateral super-cell that contains multiple subcells in parallel, each optimized for a given wavelength band. The multiple subcells can absorb the entire solar spectrum. Such solar cells will be able to achieve extremely high efficiency with low fabrication cost. The team is working on both the design and fabrication of such solar cells.
Parallel to that the new quaternary alloy nanowires with large wavelength span can be explored for color-engineered light applications in light emitting diodes. 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 news is built on prior work. The most interesting is published at the Journal of the American Chemical Society “Quaternary Alloy Semiconductor Nanobelts with Bandgap Spanning the Entire Visible Spectrum”. In another paper published at the American Chemical Society’s Nano “Spatial Composition Grading of Quaternary ZnCdSSe Alloy Nanowires with Tunable Light Emission between 350 and 710 nm on a Single Substrate” the team discusses growing spatially composition-controlled alloys by combining spatial source reagent gradient with a temperature gradient.
The research is far from commercial, but the discussion’s bait is low cost. Should this get to scales of manufacturing in either LEDs or photovoltaic cells both solar production and energy use would have considerable improvements in efficiency.
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I’ve recently started a blog, the information you provide on this site has helped me tremendously. Thank you for all of your time & work.
LED is the way to go for lighting. This is a better innovation as it is highly energy efficient and cost-effective. Nice research!