Researchers develop new technology for cheaper, more efficient solar cells

Posted: Feb 20th, 2011

(Nanowerk News) The sun provides more than enough energy for all our needs, if only we could harness it cheaply and efficiently. Solar energy could provide a clean alternative to fossil fuels, but the high cost of solar cells has been a major barrier to their widespread use.

Stanford researchers have found that adding a single layer of organic molecules to a solar cell can increase its efficiency three-fold and could lead to cheaper, more efficient solar panels. Their results were published online in ACS Nano on Feb. 7.

Professor of chemical engineering Stacey Bent first became interested in a new kind of solar technology two years ago. These solar cells used tiny particles of semiconductors called "quantum dots." Quantum dot solar cells are cheaper to produce than traditional ones, as they can be made using simple chemical reactions. But despite their promise, they lagged well behind existing solar cells in efficiency.

"I wondered if we could use our knowledge of chemistry to improve their efficiency," Bent said. If she could do that, the reduced cost of these solar cells could lead to mass adoption of the technology.

Bent will discuss her research on Sunday, Feb. 20, at 8:30 a.m. Eastern, at the annual meeting of the American Association for the Advancement of Science in Washington, D.C.

In principle, quantum dot cells can reach much higher efficiency, Bent said, because of a fundamental limitation of traditional solar cells.

Solar cells work by using energy from the sun to excite electrons. The excited electrons jump from a lower energy level to a higher one, leaving behind a "hole" where the electron used to be. Solar cells use a semiconductor to pull an electron in one direction, and another material to pull the hole in the other direction. This flow of electron and hole in different directions leads to an electric current.

But it takes a certain minimum energy to fully separate the electron and the hole. The amount of energy required is specific to different materials and affects what color, or wavelength, of light the material best absorbs. Silicon is commonly used to make solar cells because the energy required to excite its electrons corresponds closely to the wavelength of visible light.

But solar cells made of a single material have a maximum efficiency of about 31 percent, a limitation of the fixed energy level they can absorb.

Quantum dot solar cells do not share this limitation and can in theory be far more efficient. The energy levels of electrons in quantum dot semiconductors depends on their size – the smaller the quantum dot, the larger the energy needed to excite electrons to the next level.

So quantum dots can be tuned to absorb a certain wavelength of light just by changing their size. And they can be used to build more complex solar cells that have more than one size of quantum dot, allowing them to absorb multiple wavelengths of light.

Because of these advantages, Bent and her students have been investigating ways to improve the efficiency of quantum dot solar cells, along with associate Professor Michael McGehee of the department of Materials Science and Engineering.

The researchers coated a titanium dioxide semiconductor in their quantum dot solar cell with a very thin single layer of organic molecules. These molecules were self-assembling, meaning that their interactions caused them to pack together in an ordered way. The quantum dots were present at the interface of this organic layer and the semiconductor. Bent's students tried several different organic molecules in an attempt to learn which ones would most increase the efficiency of the solar cells.

But she found that the exact molecule didn't matter – just having a single organic layer less than a nanometer thick was enough to triple the efficiency of the solar cells. "We were surprised, we thought it would be very sensitive to what we put down," said Bent.

But she said the result made sense in hindsight, and the researchers came up with a new model – it's the length of the molecule, and not its exact nature, that matters. Molecules that are too long don't allow the quantum dots to interact well with the semiconductor.

Bent's theory is that once the sun's energy creates an electron and a hole, the thin organic layer helps keep them apart, preventing them from recombining and being wasted. The group has yet to optimize the solar cells, and they have currently achieved an efficiency of, at most, 0.4 percent. But the group can tune several aspects of the cell, and once they do, the three-fold increase caused by the organic layer would be even more significant.

Bent said the cadmium sulfide quantum dots she is currently using are not ideal for solar cells, and the group will try different materials. She said she would also try other molecules for the organic layer, and could change the design of the solar cell to try to absorb more light and produce more electrical charge. Once Bent has found a way to increase the efficiency of quantum dot solar cells, she said she hopes their lower cost will lead to wider acceptance of solar energy.

Source: Stanford University Source Article [Source URL] Effects of Self-Assembled Monolayers on Solid-State CdS Quantum Dot Sensitized Solar Cells Abstract Full Text HTML Hi-Res PDF[5091 KB] PDF w/ Links[1086 KB] Supporting Info Figures Pendar Ardalan†, Thomas P. Brennan†, Han-Bo-Ram Lee†, Jonathan R. Bakke†, I-Kang Ding‡, Michael D. McGehee‡, and Stacey F. Bent†* † Department of Chemical Engineering ‡ Department of Materials Science and Engineering Stanford University, Stanford, California 94305, United States ACS Nano, Article ASAP DOI: 10.1021/nn103371v Publication Date (Web): February 7, 2011 Copyright © 2011 American Chemical Society *Address correspondence to sbent@stanford.edu. Abstract Quantum dot sensitized solar cells (QDSSCs) are of interest for solar energy conversion because of their tunable band gap and promise of stable, low-cost performance. We have investigated the effects of self-assembled monolayers (SAMs) with phosphonic acid headgroups on the bonding and performance of cadmium sulfide (CdS) solid-state QDSSCs. CdS quantum dots 2 to 6 nm in diameter were grown on SAM-passivated planar or nanostructured TiO2 surfaces by successive ionic layer adsorption and reaction (SILAR), and photovoltaic devices were fabricated with spiro-OMeTAD as the solid-state hole conductor. X-ray photoelectron spectroscopy, Auger electron spectroscopy, ultraviolet−visible spectroscopy, scanning electron microscopy, transmission electron microscopy, water contact angle measurements, ellipsometry, and electrical measurements were employed to characterize the materials and the resulting device performance. The data indicate that the nature of the SAM tailgroup does not significantly affect the uptake of CdS quantum dots on TiO2 nor their optical properties, but the presence of the SAM does have a significant effect on the photovoltaic device performance. Interestingly, we observe up to 3 times higher power conversion efficiencies in devices with a SAM compared to those without the SAM. Keywords (): quantum dot sensitized solar cells; self-assembled monolayers; successive ionic layer adsorption and reaction; cadmium sulfide; titanium dioxide; nanostructure

Solar Tech Finding Claims 50% Efficiency

by Susan DeFreitas, August 6th, 2010

The “black swan theory” refers to real events beyond the realm of normal expectations–and that’s how an international team of researchers at SciTech Solar (lead by a former professor emeritus from Penn State) are referring to their latest breakthrough, a technology that allows for ultra high-efficiency solar cells to generate DC, or direct current electricity.

Why is this such a big deal in the solar world? Apparently because it makes possible the design and fabrication of a new class of solar energy converters which could allow for a dramatic increase in energy conversion efficiency and cost savings of solar cells. According to Penn State, this is a technology that can successfully compete with today’s semiconductor-based solar cells while exceeding efficiencies and decreasing costs. They’re hailing it as a scalable, sustainable, adaptable and environmentally-friendly technology that will allow manufacturers to quickly and economically shift to new materials if a shortage of any one type occurs.

image via SciTech Associates

The technology is based on a new “optical rectification” process that makes use of a simple, cost-effective, single element system that extracts energy from the solar spectrum from the infrared through the visible light spectrum. This broad-spectrum absorption significantly contributes to the gain in efficiency as compared to today’s solar cells, allowing these single element cells to act simultaneously as both a receiving antenna and as a rectifier to absorb and convert solar energy to an electric current. Such a device is historically termed a “rectenna” and was developed for microwave power transmission.

In extensive computer simulations, scientists from United States, Belgium and Korea performed quantum-mechanical calculations that agree with the rectification results of the actual operation of the device, showing rectification of light throughout the visible region and a significant DC current output. The scientists obtained efficiencies comparable to and exceeding those of current solar cell devices (efficiencies as high as 50% were recorded). The scientists are currently developing prototype devices which include more robust antenna structures and plasmonic effects to enhance output and efficiency.

Source

United States Patent Application	20090308443
Kind Code	A1
Cutler; Paul H.	December 17, 2009

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Apparatus and system for a single element solar cell

Abstract

A device for receiving and converting incident radiation into DC current, the device including a transparent conductor, at least one point-contact diode, the at least one point-contact diode having a nanowire/mCNT providing a receiving antenna function and a rectification function, a thin insulating layer situated between the transparent conductor and the nanowire/mCNT, and a point contact junction at which the nanowire/mCNT contacts the thin insulating layer.

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Inventors:	Cutler; Paul H.; (State College, PA)
Correspondence Address:	KENYON & KENYON LLP ONE BROADWAY NEW YORK NY 10004 US
Serial No.:	157842
Series Code:	12
Filed:	June 13, 2008

Current U.S. Class:	136/256
Class at Publication:	136/256
International Class:	H01L 31/00 20060101 H01L031/00

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Claims

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1. A device for receiving and converting incident radiation into DC current, the device comprising:a transparent conductor; at least one point-contact diode, the at least one point-contact diode having a nanowire providing a receiving antenna function and a rectification function; a thin insulating layer, situated between the transparent conductor and the nanowire; and a point-contact junction, at which the nanowire contacts the thin insulting layer.

1-Solar Cell 2-Solar Radiation 3-Transparent Cover 4-Metal Electrode 5-Thin Insulating Layer 6-mCNT Rectifying Antenna
7-Contact Area 8-Sharp Edge 9-High Frequency Diode

See also WO2009152435

CNT Solar Cell Rectenna

United States Patent Application 20100133513
Kind Code A1
Schmidt; Howard K. June 3, 2010

NANOPARTICLE / NANOTUBE-BASED NANOELECTRONIC DEVICES AND CHEMICALLY-DIRECTED ASSEMBLY THEREOF

Abstract
According to some embodiments, the present invention provides a nanoelectronic device based on a nanostructure that may include a nanotube with first and second ends, a metallic nanoparticle attached to the first end, and an insulating nanoparticle attached to the second end. The nanoelectronic device may include additional nanostructures so a to form a plurality of nanostructures comprising the first nanostructure and the additional nanostructures. The plurality of nanostructures may arranged in a network comprising a plurality of edges and a plurality of vertices, wherein each edge comprises a nanotube and each vertex comprises at least one insulating nanoparticle and at least one metallic nanoparticle adjacent the insulating nanoparticle. The combination of at least one edge and at least one vertex comprises a diode. The device may be an optical rectenna.

Inventors: Schmidt; Howard K.; (Cypress, TX)
Correspondence Name and Address:
WINSTEAD PC
P.O. BOX 50784
DALLAS
TX
75201
US
Assignee Name and Adress: William Marsh Rice University
Houston
TX

Serial No.: 910521
Series Code: 11
Filed: February 2, 2007
PCT Filed: February 2, 2007
PCT NO: PCT/US07/61563
371 Date: October 2, 2007
U.S. Current Class: 257/25; 257/E21.04; 257/E31.033; 438/57
U.S. Class at Publication: 257/25; 438/57; 257/E31.033; 257/E21.04
Intern'l Class: H01L 31/0352 20060101 H01L031/0352; H01L 31/18 20060101 H01L031/18

Goverment Interests
STATEMENT OF GOVERNMENT SPONSORSHIP

[0002]The present invention was made in part with United States Government support under a grant awarded by the Office of Naval Research, Grant No. N00014-04-1-0765 and the Department of Energy, Grant No. DE-FC36-050015073. The U.S. Government has certain rights in this invention.

Claims

1. A nanoelectronic device, comprising:a first nanostructure, comprising:a nanotube having first and second opposing ends;a metallic nanoparticle attached to the first end; and an insulating nanoparticle attached to the second end.

2. The nanoelectronic device according to claim 1, wherein the nanotube is conducting.

3. The nanoelectronic device according to claim 1, wherein the nanotube absorbs light.

4. The nanoelectronic device according to claim 3, wherein the nanotube comprises an antenna.

5. The nanoelectronic device according to claim 4, wherein the length between the first and second ends is about half a wavelength of the light.

[0046]The nanoelectronic device may be arranged so as operate as an optical rectenna. The optical rectenna may display frequency response some 10,000 times higher than achieved with prior optical rectennae and deliver useful power conversion well into the optical regime at 30,000 TH. The optical rectenna may find application in photovoltaics for conversion of light to electricity.

Source

Amazing - and as from ANI's grandfather, of interest here. Will ANI participate????

Hybrid Material Could Vastly Increase Energy Potential From the Sun

10.17.2008 9:03 AM

Harnessing Light's Full Spectrum: Scientists Claim Solar Power Breakthrough

By Dan Shapley

Chemists at Ohio State University say they have produced a next-generation material that not only absorbs the full spectrum of sunlight, but also make makes the electrons generated more easy to capture.

The hybrid material -- a combination of electrically conductive plastic and metals like molybdenum and titanium -- is the first of its kind to capture the full solar spectrum, according to Malcolm Chisholm, one of the authors of the paper describing the research, which appears in Proceedings of the National Academy of Sciences. Solar panels in use today capture only a small fraction of the energy contained in sunlight.

The material is years from being made into a commercial product, but is another example of how innovations in the field of solar energy could make vastly more of the sun's energy available for human use. Recent action by Congress to extend industry tax incentives should keep companies investing in new technology research and development. And according to the Department of Energy, "Under the ongoing global financial crisis, a lack of available credit is causing projects to be delayed or canceled, but the clean energy sector is continuing to attract substantial amounts of investment capital."

If coupled with new battery technology, solar energy technology has the potential to revolutionize the way we generate electricity. Millions of homes could be outfitted with their own power sources, and they could store enough electricity -- if efficient enough -- to eliminate the need for power plants in the residential sector.

That's been the promise of solar energy for a long time. Breakthroughs like this one announced by Ohio State brings the vision that much closer to reality.

Here's how the university described the breakthrough:

The material generates electricity just like other solar cell materials do: light energizes the atoms of the material, and some of the electrons in those atoms are knocked loose.

Ideally, the electrons flow out of the device as electrical current, but this is where most solar cells run into trouble. The electrons only stay loose for a tiny fraction of a second before they sink back into the atoms from which they came. The electrons must be captured during the short time they are free, and this task, called charge separation, is difficult.

In the new hybrid material, electrons remain free much longer than ever before.

To design the hybrid material, the chemists explored different molecular configurations on a computer at the Ohio Supercomputer Center. Then, with colleagues at National Taiwan University, they synthesized molecules of the new material in a liquid solution, measured the frequencies of light the molecules absorbed, and also measured the length of time that excited electrons remained free in the molecules.

They saw something very unusual. The molecules didn't just fluoresce as some solar cell materials do. They phosphoresced as well. Both luminous effects are caused by a material absorbing and emitting energy, but phosphorescence lasts much longer.

To their surprise, the chemists found that the new material was emitting electrons in two different energy states -- one called a singlet state, and the other a triplet state. Both energy states are useful for solar cell applications, and the triplet state lasts much longer than the singlet state.

Electrons in the singlet state stayed free for up to 12 picoseconds, or trillionths of a second -- not unusual compared to some solar cell materials. But electrons in the triplet state stayed free 7 million times longer -- up to 83 microseconds, or millionths of a second.

When they deposited the molecules in a thin film, similar to how they might be arranged in an actual solar cell, the triplet states lasted even longer: 200 microseconds.

Source