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October 2012

A Forest of Semiconductor Tree Trunks

Scientists prepare highly ordered semiconductor nanowires to harvest solar energy cheaply and efficiently

Emily Pentzer

Rows upon rows of gallium arsenide nanowires, which could improve the efficiency of solar cells, are quickly produced by this new technique.

Sunlight provides ample energy to meet our global needs, yet we currently harness only a small fraction of its potential. Moreover, optimization of solar cell device performance is of great interest to access a renewable energy source that gives off no carbon by-products. One avenue to improve device efficiency is to use semiconducting nanowires, which improve absorption of solar energy if the diameter of the structure is similar to the wavelength of light. The large-scale synthesis of these materials has been hampered by costly, time-consuming techniques − until now. As reported in Nano Letters, scientists at the Center for Energy Nanoscience (CEN) recently developed a new method that allows for the large-scale, cost-effective synthesis of vertically aligned semiconductor nanowires, akin to a densely packed forest of nano-tree trunks, which efficiently absorb incoming light.

To harvest sunlight efficiently, a material should absorb solar energy while minimizing energy reflection. The similar size of the nanowires and wavelength of the incident energy help that happen. "The light will go through multiple rounds of bouncing, being trapped in the nanowire arrays and have a better chance of getting absorbed," said author Maoqing Yao.This minimizes the amount of energy reflected and optimizes absorption, yielding more power output from the same amount of input energy. While such a system can give smaller, lighter devices to use in portable charging stations, they must also be cost-effective and scalable over a large area to compete commercially with coal and natural gas.

The CEN team prepared towers of gallium arsenide nanowires of controlled size and spacing rising from an underlying surface of the same material. To create this forest of nanowires, scientists first deposited tiny polymer spheres (100-890 nanometers) on a wafer, and then used oxygen plasma to uniformly shrink the spheres with space between them, giving what resembles a page of candy buttons. A layer of metallic aluminum or iron was put down to cover the entire surface, both the spheres and spaces between. Removal of the spherical nanoparticles by washing with an organic solvent revealed a regular pattern of gallium arsenide holes from which the nanowires grew by crystallization using a chemical vapor deposition method. This process gave a forest of evenly spaced tree trunks of semiconducting gallium arsenide nanowires, ideal for use in photovoltaic devices. 

While some defects were present in the fields, such as missing nanowires, the arrays of pillared semiconductors had excellent absorption and reflected less than 10 percent of incoming light.  Importantly, these values are similar to those observed for organized nanowires prepared from more expensive lithographic techniques. This novel method for the large-scale preparation of semiconducting gallium arsenide nanowires represents a great stride forward in the optimized harvesting of sunlight by photovoltaic devices and is currently being used to prepare patterned nanowire devices.

Acknowledgments

The Center for Energy Nanoscience, an Energy Frontier Research Center funded by the Department of Energy, Office of Science, Office of Basic Energy Sciences, supported this research. The University of Southern California’s Center for Higher Performance Computing and Communications provided computing resources.

More Information

Madaria AR, M Yao, CY Chi, N Huang, C Lin, R Li, ML Povinelli, PD Dapkus, and C Zhou. 2012. “Toward Optimized Light Utilization in Nanowire Arrays Using Scalable Nanosphere Lithography and Selected Area Growth.” Nano Letters 12:2839-2845. DOI: 10.1021/nl300341v

About the author(s):

  • Emily Pentzer is a postdoctoral associate at the Energy Frontier Research Center:  Polymer-Based Materials for Harvesting Solar Energy (PHaSE) at the University of Massachusetts, Amherst. Under the direction of Todd Emrick, Emily prepares hybrid organic materials for the optimization of harvesting solar energy.

Organizing Nanowires to Get Some Rays

A fast, inexpensive process creates patterns of tiny identical wires that capture sunlight for solar cells

Rows upon rows of gallium arsenide nanowires, which could improve the efficiency of solar cells, are quickly produced by this new technique.

Efficient solar cells require materials that absorb as much light as possible. One approach is to cover a surface in tiny wires, which dramatically increases the surface area, but this material must be accessed quickly and affordably. Scientists devised a new method that does just that by using a template with rows of tiny uniform spots on the desired surface from which metallic wires are grown. While some defects were present on the surface, such as missing nanowires, the arrays absorbed sunlight well, reflecting little back into the environment. This study reports, for the first time, a method to produce aligned nanowire materials at a large enough scale for use in solar cells and other light-driven technologies. The Center for Energy Nanoscience, led by the University of Southern California, conducted the work.

More Information

Madaria AR, M Yao, CY Chi, N Huang, C Lin, R Li, ML Povinelli, PD Dapkus, and C Zhou. 2012. “Toward Optimized Light Utilization in Nanowire Arrays Using Scalable Nanosphere Lithography and Selected Area Growth.” Nano Letters 12:2839-2845. DOI: 10.1021/nl300341v

Disclaimer: The opinions in this newsletter are those of the individual authors and do not represent the views or position of the Department of Energy.