David Kirkpatrick

March 5, 2010

Silicon nanowires may improve solar costs

Silicon photovoltaics offer incredible solar cell efficiency and now it looks like nanotechnology may offer a way to add low production cost to that mix. This type of headway and improvement is what will make solar a market-viable power option.

The release:

Trapping Sunlight with Silicon Nanowires

MARCH 03, 2010

Lynn Yarris

This photovoltaic cell is comprised of 36 individual arrays of silicon nanowires featuring radial p-n junctions. The color dispersion demonstrates the excellent periodicity present over the entire substrate. (Photo courtesy of Peidong Yang)

This photovoltaic cell is comprised of 36 individual arrays of silicon nanowires featuring radial p-n junctions. The color dispersion demonstrates the excellent periodicity over the entire substrate. (Photo from Peidong Yang)

Solar cells made from silicon are projected to be a prominent factor in future renewable green energy equations, but so far the promise has far exceeded the reality. While there are now silicon photovoltaics that can convert sunlight into electricity at impressive 20 percent efficiencies, the cost of this solar power is prohibitive for large-scale use. Researchers with the Lawrence Berkeley National Laboratory (Berkeley Lab), however, are developing a new approach that could substantially reduce these costs. The key to their success is a better way of trapping sunlight.

“Through the fabrication of thin films from ordered arrays of vertical silicon nanowires we’ve been able to increase the light-trapping in our solar cells by a factor of 73,” says chemist Peidong Yang, who led this research. “Since the fabrication technique behind this extraordinary light-trapping enhancement is a relatively simple and scalable aqueous chemistry process, we believe our approach represents an economically viable path toward high-efficiency, low-cost thin-film solar cells.”

Yang holds joint appointments with Berkeley Lab’s Materials Sciences Division, and the University of California  Berkeley’s Chemistry Department. He is a leading authority on semiconductor nanowires – one-dimensional strips of materials whose width measures only one-thousandth that of a human hair but whose length may stretch several microns.

“Typical solar cells are made from very expensive ultrapure single crystal silicon wafers that require about 100 micrometers of thickness to absorb most of the solar light, whereas our radial geometry enables us to effectively trap light with nanowire arrays fabricated from silicon films that are only about eight micrometers thick,” he says. “Furthermore, our approach should in principle allow us to use metallurgical grade or “dirty” silicon rather than the ultrapure silicon crystals now required, which should cut costs even further.”

Yang has described this research in a paper published in the journal NANO Letters, which he co-authored with Erik Garnett, a chemist who was then a member of Yang’s research group. The paper is titled “Light Trapping in Silicon Nanowire Solar Cells.”

A radial p-n junction consists of a layer of n-type silicon forming a shell around a p-type silicon nanowire core. This geometry turns each individual nanowire into a photovoltaic cell.

A radial p-n junction consists of a layer of n-type silicon forming a shell around a p-type silicon nanowire core. This geometry turns each individual nanowire into a photovoltaic cell.

Generating Electricity from Sunlight

At the heart of all solar cells are two separate layers of material, one with an abundance of electrons that functions as a negative pole, and one with an abundance of electron holes (positively-charged energy spaces) that functions as a positive pole. When photons from the sun are absorbed, their energy is used to create electron-hole pairs, which are then separated at the interface between the two layers and collected as electricity.

Because of its superior photo-electronic properties, silicon remains the photovoltaic semiconductor of choice but rising demand has inflated the price of the raw material. Furthermore, because of the high-level of crystal purification required, even the fabrication of the simplest silicon-based solar cell is a complex, energy-intensive and costly process.

Yang and his group are able to reduce both the quantity and the quality requirements for silicon by using vertical arrays of nanostructured radial p-n junctions rather than conventional planar p-n junctions. In a radial p-n junction, a layer of n-type silicon forms a shell around a p-type silicon nanowire core. As a result, photo-excited electrons and holes travel much shorter distances to electrodes, eliminating a charge-carrier bottleneck that often arises in a typical silicon solar cell. The radial geometry array also, as photocurrent and optical transmission measurements by Yang and Garrett revealed, greatly improves light trapping.

“Since each individual nanowire in the array has a p-n junction, each acts as an individual solar cell,” Yang says. “By adjusting the length of the nanowires in our arrays, we can increase their light-trapping path length.”

While the conversion efficiency of these solar nanowires was only about five to six percent, Yang says this efficiency was achieved with little effort put into surface passivation, antireflection, and other efficiency-increasing modifications.

“With further improvements, most importantly in surface passivation, we think it is possible to push the efficiency to above 10 percent,” Yang says.

Combining a 10 percent or better conversion efficiency with the greatly reduced quantities of starting silicon material  and the ability to use metallurgical grade silicon, should make the use of silicon nanowires an attractive candidate for large-scale development.

As an added plus Yang says, “Our technique can be used in existing solar panel manufacturing processes.”

This research was funded by the National Science Foundation’s Center of Integrated Nanomechanical Systems.

Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research for DOE’s Office of Science and is managed by the University of California. Visit our website at http://www.lbl.gov.


Peidong Yang (Photo by Roy Kaltschmidt, Berkeley Lab Public Affairs)

Peidong Yang (Photo by Roy Kaltschmidt, Berkeley Lab Public Affairs)

Additional Information

For more about the research of Peidong Yang and his group, visit the Website at http://www.cchem.berkeley.edu/pdygrp/main.html

For more about the Center of Integrated Nanomechanical Systems (COINS) visit the Website at http://mint.physics.berkeley.edu/coins/

November 12, 2009

Silicon nanowires

Carbon gets most of the nanotech ink, but here’s some news on silicon nanowires.

The release:

Understanding mechanical properties of silicon nanowires paves way for nanodevices

IMAGE: These are silicon nanowires used in the in-situ scanning electron microscopy mechanical testing by Dr. Yong Zhu and his team.

Click here for more information.

 

Silicon nanowires are attracting significant attention from the electronics industry due to the drive for ever-smaller electronic devices, from cell phones to computers. The operation of these future devices, and a wide array of additional applications, will depend on the mechanical properties of these nanowires. New research from North Carolina State University shows that silicon nanowires are far more resilient than their larger counterparts, a finding that could pave the way for smaller, sturdier nanoelectronics, nanosensors, light-emitting diodes and other applications.

It is no surprise that the mechanical properties of silicon nanowires are different from “bulk” – or regular size – silicon materials, because as the diameter of the wires decrease, there is an increasing surface-to-volume ratio. Unfortunately, experimental results reported in the literature on the properties of silicon nanowires have reported conflicting results. So the NC State researchers set out to quantify the elastic and fracture properties of the material.

“The mainstream semiconductor industry is built on silicon,” says Dr. Yong Zhu, assistant professor of mechanical engineering at NC State and lead researcher on this project. “These wires are the building blocks for future nanoelectronics.” For this study, researchers set out to determine how much abuse these silicon nanowires can take. How do they deform – meaning how much can you stretch or warp the material before it breaks? And how much force can they withstand before they fracture or crack? The researchers focused on nanowires made using the vapor-liquid-solid synthesis process, which is a common way of producing silicon nanowires.

IMAGE: Dr. Yong Zhu and his research team stand front of a scanning electron microscope. From left to right, they are Feng Xu, Qingquan Qin and Yong Zhu.

Click here for more information.

 

Zhu and his team measured the nanowire properties using in-situ tensile testing inside scanning electron microscopy. A nanomanipulator was used as the actuator and a micro cantilever used as the load sensor. “Our experimental method is direct but simple,” says Qingquan Qin, a Ph.D. student at NC State and co-author of the paper. “This method offers real-time observation of nanowire deformation and fracture, while simultaneously providing quantitative stress and strain data. The method is very efficient, so a large number of specimens can be tested within a reasonable period of time.”

As it turns out, silicon nanowires deform in a very different way from bulk silicon. “Bulk silicon is very brittle and has limited deformability, meaning that it cannot be stretched or warped very much without breaking.” says Feng Xu, a Ph.D. student at NC state and co-author of the paper, “But the silicon nanowires are more resilient, and can sustain much larger deformation. Other properties of silicon nanowires include increasing fracture strength and decreasing elastic modulus as the nanowire gets smaller and smaller.”

The fact that silicon nanowires have more deformability and strength is a big deal. “These properties are essential to the design and reliability of novel silicon nanodevices,” Zhu says. “The insights gained from this study not only advance fundamental understanding about size effects on mechanical properties of nanostructures, but also give designers more options in designing nanodevices ranging from nanosensors to nanoelectronics to nanostructured solar cells.”

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The study, “Mechanical Properties of Vapor-Liquid-Solid Synthesized Silicon Nanowires,” was co-authored by Zhu, Xu, Qin, University of Michigan (UM) researcher Wei Lu and UM Ph.D. student Wayne Fung. The study is published in the Nov. 11 issue o fNano Letters, and was funded by grants from the National Science Foundation and NC State.

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