Forum for Science, Industry and Business

Sponsored by:     3M 
Search our Site:

 

Down to the wire

01.09.2011
Berkeley Lab researchers develop inexpensive technique for making high quality nanowire solar cells

Solar or photovoltaic cells represent one of the best possible technologies for providing an absolutely clean and virtually inexhaustible source of energy to power our civilization.

However, for this dream to be realized, solar cells need to be made from inexpensive elements using low-cost, less energy-intensive processing chemistry, and they need to efficiently and cost-competitively convert sunlight into electricity. A team of researchers with the U.S. Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) has now demonstrated two out of three of these requirements with a promising start on the third.

Peidong Yang, a chemist with Berkeley Lab's Materials Sciences Division, led the development of a solution-based technique for fabricating core/shell nanowire solar cells using the semiconductors cadmium sulfide for the core and copper sulfide for the shell. These inexpensive and easy-to-make nanowire solar cells boasted open-circuit voltage and fill factor values superior to conventional planar solar cells. Together, the open-circuit voltage and fill factor determine the maximum energy that a solar cell can produce. In addition, the new nanowires also demonstrated an energy conversion efficiency of 5.4-percent, which is comparable to planar solar cells.

"This is the first time a solution based cation-exchange chemistry technique has been used for the production of high quality single-crystalline cadmium sulfide/copper sulfide core/shell nanowires," Yang says. "Our achievement, together with the increased light absorption we have previously demonstrated in nanowire arrays through light trapping, indicates that core/shell nanowires are truly promising for future solar cell technology."

Yang, who holds a joint appointment with the University of California (UC) Berkeley, is the corresponding author of a paper reporting this research that appears in the journal Nature Nanotechnology. The paper is titled "Solution-processed core–shell nanowires for efficient photovoltaic cells." Co-authoring this paper with Yang were Jinyao Tang, Ziyang Huo, Sarah Brittman and Hanwei Gao.

Typical solar cells today are made from ultra-pure single crystal silicon wafers that require about 100 micrometers in thickness of this very expensive material to absorb enough solar light. Furthermore, the high-level of crystal purification required makes the fabrication of even the simplest silicon-based planar solar cell a complex, energy-intensive and costly process.

A highly promising alternative would be semiconductor nanowires – one-dimensional strips of materials whose width measures only one-thousandth that of a human hair but whose length may stretch up to the millimeter scale. Solar cells made from nanowires offer a number of advantages over conventional planar solar cells, including better charge separation and collection capabilities, plus they can be made from Earth abundant materials rather than highly processed silicon. To date, however, the lower efficiencies of nanowire-based solar cells have outweighed their benefits.

"Nanowire solar cells in the past have demonstrated fill factors and open-circuit voltages far inferior to those of their planar counterparts," Yang says. "Possible reasons for this poor performance include surface recombination and poor control over the quality of the p–n junctions when high-temperature doping processes are used."

At the heart of all solar cells are two separate layers of material, one with an abundance of electrons that function as a negative pole, and one with an abundance of electron holes (positively-charged energy spaces) that function 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 p-n junction – the interface between the two layers - and collected as electricity.

About a year ago, working with silicon, Yang and members of his research group developed a relatively inexpensive way to replace the planar p-n junctions of conventional solar cells with a radial p-n junction, in which a layer of n-type silicon formed a shell around a p-type silicon nanowire core. This geometry effectively turned each individual nanowire into a photovoltaic cell and greatly improved the light-trapping capabilities of silicon-based photovoltaic thin films.

Now they have applied this strategy to the fabrication of core/shell nanowires using cadmium sulfide and copper sulfide, but this time using solution chemistry. These core/shell nanowires were prepared using a solution-based cation (negative ion) exchange reaction that was originally developed by chemist Paul Alivisatos and his research group to make quantum dots and nanorods. Alivisatos is now the director of Berkeley Lab, and UC Berkeley's Larry and Diane Bock Professor of Nanotechnology.

"The initial cadmium sulfide nanowires were synthesized by physical vapor transport using a vapor–liquid–solid (VLS) mechanism rather than wet chemistry, which gave us better quality material and greater physical length, but certainly they can also be made using solution process" Yang says. "The as-grown single-crystalline cadmium sulfide nanowires have diameters of between 100 and 400 nanometers and lengths up to 50 millimeters."

The cadmium sulfide nanowires were then dipped into a solution of copper chloride at a temperature of 50 degrees Celsius and kept there for 5 to 10 seconds. The cation exchange reaction converted the surface layer of the cadmium sulfide into a copper sulfide shell.

"The solution-based cation exchange reaction provides us with an easy, low-cost method to prepare high-quality hetero-epitaxial nanomaterials," Yang says. "Furthermore, it circumvents the difficulties of high-temperature doping and deposition for typical vapor phase production methods, which suggests much lower fabrication costs and better reproducibility. All we really need are beakers and flasks for this solution-based process. There's none of the high fabrication costs associated with gas-phase epitaxial chemical vapor deposition and molecular beam epitaxy, the techniques most used today to fabricate semiconductor nanowires."

Yang and his colleagues believe they can improve the energy conversion efficiency of their solar cell nanowires by increasing the amount of copper sulfide shell material. For their technology to be commercially viable, they need to reach an energy conversion efficiency of at least ten-percent.

This research was supported by the DOE Office of Science.

Lawrence Berkeley National Laboratory addresses the world's most urgent scientific challenges by advancing sustainable energy, protecting human health, creating new materials, and revealing the origin and fate of the universe. Founded in 1931, Berkeley Lab's scientific expertise has been recognized with 12 Nobel prizes. The University of California manages Berkeley Lab for the U.S. Department of Energy's Office of Science. For more, visit www.lbl.gov.

Lynn Yarris | EurekAlert!
Further information:
http://www.lbl.gov

More articles from Power and Electrical Engineering:

nachricht Open, flexible assembly platform for optical systems
24.01.2017 | Fraunhofer-Institut für Produktionstechnologie IPT

nachricht A big nano boost for solar cells
18.01.2017 | Kyoto University and Osaka Gas effort doubles current efficiencies

All articles from Power and Electrical Engineering >>>

The most recent press releases about innovation >>>

Die letzten 5 Focus-News des innovations-reports im Überblick:

Im Focus: Scientists spin artificial silk from whey protein

X-ray study throws light on key process for production

A Swedish-German team of researchers has cleared up a key process for the artificial production of silk. With the help of the intense X-rays from DESY's...

Im Focus: Quantum optical sensor for the first time tested in space – with a laser system from Berlin

For the first time ever, a cloud of ultra-cold atoms has been successfully created in space on board of a sounding rocket. The MAIUS mission demonstrates that quantum optical sensors can be operated even in harsh environments like space – a prerequi-site for finding answers to the most challenging questions of fundamental physics and an important innovation driver for everyday applications.

According to Albert Einstein's Equivalence Principle, all bodies are accelerated at the same rate by the Earth's gravity, regardless of their properties. This...

Im Focus: Traffic jam in empty space

New success for Konstanz physicists in studying the quantum vacuum

An important step towards a completely new experimental access to quantum physics has been made at University of Konstanz. The team of scientists headed by...

Im Focus: How gut bacteria can make us ill

HZI researchers decipher infection mechanisms of Yersinia and immune responses of the host

Yersiniae cause severe intestinal infections. Studies using Yersinia pseudotuberculosis as a model organism aim to elucidate the infection mechanisms of these...

Im Focus: Interfacial Superconductivity: Magnetic and superconducting order revealed simultaneously

Researchers from the University of Hamburg in Germany, in collaboration with colleagues from the University of Aarhus in Denmark, have synthesized a new superconducting material by growing a few layers of an antiferromagnetic transition-metal chalcogenide on a bismuth-based topological insulator, both being non-superconducting materials.

While superconductivity and magnetism are generally believed to be mutually exclusive, surprisingly, in this new material, superconducting correlations...

All Focus news of the innovation-report >>>

Anzeige

Anzeige

Event News

Sustainable Water use in Agriculture in Eastern Europe and Central Asia

19.01.2017 | Event News

12V, 48V, high-voltage – trends in E/E automotive architecture

10.01.2017 | Event News

2nd Conference on Non-Textual Information on 10 and 11 May 2017 in Hannover

09.01.2017 | Event News

 
Latest News

Breaking the optical bandwidth record of stable pulsed lasers

24.01.2017 | Physics and Astronomy

Choreographing the microRNA-target dance

24.01.2017 | Life Sciences

Spanish scientists create a 3-D bioprinter to print human skin

24.01.2017 | Health and Medicine

VideoLinks
B2B-VideoLinks
More VideoLinks >>>