22 March 2010


Quaternary semiconductor nanowire promises higher-efficiency solar cells

Cun-Zheng Ning and Alian Pan of Arizona State University (ASU) have fabricated what are claimed to be the first quaternary semiconductor nanowire materials, which could lead to more efficient photovoltaic cells and possibly also to better LEDs for replacing less energy-efficient incandescent light bulbs.

Ideally, the highest solar cell efficiency is achieved by having a wide range of energy bandgaps that matches the entire solar spectrum, explains Ning, a professor in the School of Electrical, Computer and Energy Engineering (part of ASU’s Ira A. Fulton Schools of Engineering). “The lack of available bandgaps [due to lattice mismatch preventing the combination of different-bandgap materials] 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,” says Ning.

In recent attempts to grow semiconductor nanowires with ‘almost’ arbitrary bandgaps, the research team led by Ning and assistant research professor Pan have used a new approach to produce an extremely wide range of bandgaps. They alloyed the two binary semiconductors zinc sulfide (ZnS) and cadmium selenide (CdSe) to obtain the quaternary semiconductor alloy ZnCdSSe, which produced continuously varying compositions of elements on a single substrate. Ning says 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 the substrate (the dual-gradient method), the team has produced light emission ranging in wavelength from 350nm to 720nm on a single substrate just a few centimeters in size. The color spread across the substrate can be controlled to a large degree, and Ning says he believes that the dual-gradient method can be more generally applied to produce other alloy semiconductors or expand the bandgap 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 typical applications use a separate solar cell for each wavelength band.

With the new materials, Ning aims to fabricate a monolithic lateral super-cell containing multiple subcells in parallel, each optimized for a given wavelength band. The multiple subcells can collectively absorb the entire solar spectrum. Such solar cells should be able to achieve extremely high efficiency with low fabrication cost, it is believed. The team is currently working on both the design and fabrication of such solar cells.

Similarly, the large wavelength span of the new quaternary alloy nanowires can be explored for color-engineered light applications, add the researchers. The team has 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.

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