University of Minnesota Researchers Clear Major Hurdle in Road to High-efficiency Solar Cells

MINNEAPOLIS/ST. PAUL, MN — A team of University of Minnesota-led researchers has cleared a major hurdle in the drive to build solar cells with potential efficiencies up to twice as high as current levels, which rarely exceed 30 percent.

By showing how energy that is now being lost from semiconductors in solar cells can be captured and transferred to electric circuits, the team has opened a new avenue for solar cell researchers seeking to build cheaper, more efficient solar energy devices. The work is published in this week’s Science.

A system built on the research could also slash the cost of manufacturing solar cells by removing the need to process them at very high temperatures.

The achievement crowns six years of work begun at the university Institute of Technology (College of Science and Engineering) chemical engineering and materials science professors Eray Aydil and David Norris and chemistry professor Xiaoyang Zhu (now at the university of Texas-Austin) and spearheaded by U of M graduate student William Tisdale.

In most solar cells now in use, rays from the sun strike the uppermost layer of the cells, which is made of a crystalline semiconductor substance—usually silicon. The problem is that many electrons in the silicon absorb excess amounts of solar energy and radiate that energy away as heat before it can be harnessed.

An early step in harnessing that energy is to transfer these “hot” electrons out of the semiconductor and into a wire, or electric circuit, before they can cool off. But efforts to extract hot electrons from traditional silicon semiconductors have not succeeded.

However, when semiconductors are constructed in small pieces only a few nanometers wide — “quantum dots” — their properties change.

“Theory says that quantum dots should slow the loss of energy as heat,” said Tisdale. “And a 2008 paper from the University of Chicago showed this to be true. The big question for us was whether we could also speed up the extraction and transfer of hot electrons enough to grab them before they cooled. ”

In the current work, Tisdale and his colleagues demonstrated that quantum dots—made not of silicon but of another semiconductor called lead selenide — could indeed be made to surrender their “hot” electrons before they cooled. The electrons were pulled away by titanium dioxide, another common inexpensive and abundant semiconductor material that behaves like a wire.

“This is a very promising result,” said Tisdale. “We’ve shown that you can pull hot electrons out very quickly – before they lose their energy. This is exciting fundamental science.”

The work shows that the potential for building solar cells with efficiencies approaching 66 percent exists, according to Aydil.

“This work is a necessary but not sufficient step for building very high-efficiency solar cells,” he said. “It provides a motivation for researchers to work on quantum dots and solar cells based on quantum dots.”

The next step is to construct solar cells with quantum dots and study them. But one big problem still remains: “Hot” electrons also lose their energy in titanium dioxide. New solar cell designs will be needed to eliminate this loss, the researchers said.

Still, “I’m comfortable saying that electricity from solar cells is going to be a large fraction of our energy supply in the future,” Aydil noted.

The research was funded primarily by the U.S. Department of Energy and partially by the National Science Foundation. Other authors of the paper were Brooke Timp from the University of Minnesota and Kenrick Williams from UT-Austin.

University of Minnesota
www1.umn.edu/twincities/index.php

MIT Sucessfully Prints Solar Cell On Everyday Paper

Scientists at the Massachusetts Institute of Technology have successfully coated paper with a solar cell, part of a suite of research projects aimed at energy breakthroughs.

Susan Hockfield, MIT’s president, and Paolo Scaroni, CEO of Italian oil company Eni, on Tuesday officially dedicated the Eni-MIT Solar Frontiers Research Center. Eni invested $5 million into the center, which is also receiving a $2 million National Science Foundation grant, said Vladimir Bulovic, the center’s director.

The printed solar cells, which Bulovic showed at a press conference Tuesday, are still in the research phase and are years from being commercialized.

However, the technique, in which paper is coated with organic semiconductor material using a process similar to an inkjet printer, is a promising way to lower the weight of solar panels. “If you could use a staple gun to install a solar panel, there could be a lot of value,” Bulovic said.

The materials MIT researchers used are carbon-based dyes and the cells are about 1.5 percent to 2 percent efficient at converting sunlight to electricity. But any material could be used if it can be deposited at room temperature, Bulovic said. “Absolutely, the trick was coming up with ways to use paper,” he said.

MIT professor Karen Gleason headed the research and has submitted a paper for scientific review but it has not yet been published. MIT and Eni said this is the first time a solar cell has been printed on paper.

During the press conference, Scaroni said that Eni is funding the center because the company understands that hydrocarbons will eventually run out and believes that solar can be a replacement. At the same time, he said, current technologies are not sufficient.

“We are not very active (in alternative energy) today because we don’t believe today’s technologies are the answer of our problems,” he said.

Quantum dots
The paper solar cells are one of many avenues being pursued around nanoscale materials at the Eni-MIT Solar Frontiers Center. Layers of these materials could essentially be sprayed using different manufacturing techniques to make a thin-film solar cell on a plastic, paper, or metal foils.

Silicon, the predominant material for solar cells, is durable and is made from abundant materials. Many companies sell or are developing thin-film solar cells, which are less efficient but are cheaper to manufacture.

During a tour, Bulovic showed one of the center’s labs, where researchers use a laser to blast light at nanomaterials for picoseconds. A picosecond is one trillionth of a second. The laser provides data on how the light excites electrons in the material, which will provide clues as to whether it will make a good solar cell material, he explained.

MIT is focusing much of its effort on quantum dots, or tiny crystals that are only a few nanometers in size. A human hair is about 50,000 to 100,000 nanometers thick.

By using different materials and sizes, researchers can fine-tune the colors of light that quantum dots can absorb, a way of isolating good candidates for quantum dot solar cells.

Researchers at the center are also looking at different molecules or biological elements which can act as solar cell material. These cheap thin-film materials can be used on their own or added to silicon-based solar panels to enhance the efficiency, Bulovic said.

If 0.3 percent of the U.S. were covered with photovoltaics with 10 percent efficiency, solar power could produce three times the country’s needs, including a transition to electric vehicles, Bulovic said. For example, the easement strip on highways could be coated with material that could capture energy from the sun.

web.mit.edu

First Solar-Powered Airplane Takes Flight

The Solar Impulse lifted off from a military airport at a speed no faster than 28 mph (45 kph) after briefly accelerating down the runway. It slowly gained altitude above the green-and-beige fields and eventually faded into the horizon as villagers watched from the nearest hills.

During Wednesday’s 90-minute flight, the plane completed a series of turns by gently tilting its black-and-white wings, which are as wide as those of a 747 jumbo jet. It climbed nearly a mile above the Swiss countryside. The weather was sunny, and there was little wind — obvious advantages for a plane so light and dependent on the sun.

Engineers on the $93.5 million (euro70 million) project have been conducting short tests since December, taking the plane no higher than 2 feet and flying no more than 1,000 feet in distance. A night flight is planned before July, and then a second plane will be built based on the results of those tests.

That plane will be the one to attempt the round-the-world flight planned for 2012.

Aviation experts said they see a future for renewable fuels in commercial aviation, but they predicted that biofuels from plants, algae or other sources were more likely to succeed than solar power.

Test pilot Markus Scherdel said Wednesday’s flight proved that the plane could take off and land safely and handles like a passenger jet.

While the next plane will have an outer shell, the current prototype has an open cockpit — sort of the aeronautic version of a convertible.

Using almost 12,000 solar cells, rechargeable lithium batteries and four electric motors, Piccard and co-pilot Andre Borschberg plan to take the plane around the world. They will make regular stops to switch places and stretch after long periods in the cramped cockpit — and to show off their aircraft.

The circumnavigation will take time. With the engines providing only 40 horsepower, the plane will perform like a moped in the sky, at an average flight speed of 44 mph (70 kph). The trip will be divided into five stages — keeping the plane in the air for up to five days at a time.

Solar flight isn’t new, but Piccard’s project is the most ambitious.

In 1980, a fragile ultra-lightweight experimental solar plane called the Gossamer Penguin flew short demonstration flights with one pilot on board. A bigger project called the Solar Challenger flew a single pilot from France to England in 1981 in a trip lasting more than five hours.

Solar plane technology recalls the early days of manned flight, and the slow ascent of the Solar Impulse was somewhat reminiscent of the Wright brothers’ pioneering experiments in 1903.

Wilbur and Orville Wright also progressed from short hops to longer flights after 1905, reaching average speeds above 30 mph (48 kph) and only slightly slower than the Solar Impulse.

The loud clicks of the Swiss plane’s four propellers added another hint of nostalgia. And designers acknowledged the same worries that preoccupied the first fliers.

On Wednesday, the Solar Impulse reached an altitude of 5,500 feet. After a gentle landing, Scherdel emerged from the cockpit with his arms raised, and the team broke open bottles of champagne.

When the plane attempts to circle the globe, the team will have to monitor conditions closely to ensure the aircraft follows the best weather. Ground crews will stay close to provide service at each stop, he said.

news.yahoo.com

Solar Panels Inspire LED Breakthrough

Light bulbs that last 100 years and fill rooms with brilliant ambiance may become a reality sooner rather than later, thanks to a National Renewable Energy Laboratory discovery.

NREL scientists found a way to generate a tricky combination of green and red that may just prove to be the biggest boost for illumination since Edison’s light bulb.

Green isn’t just a symbol of environmentalism, it is a real color, and a desperately needed one for researchers looking for a way to light homes, streets and buildings at a fraction of today’s costs.

LEDs — light-emitting diodes — are the promise of the future because unlike tungsten bulbs or compact fluorescent bulbs, they deliver most of their energy as light, rather than heat. An extra plus is that they don’t contain dangerous mercury.

The era of LEDs is fast approaching. The U.S. Department of Energy expects to phase out tungsten bulbs in four years and compact-fluorescents in 10 years. That will leave LEDs with virtually 100 percent of the market.

To make an LED that appears white, researchers minimally need the colors red, green and blue. The white light from the sun is really all the colors of the rainbow. Without at least red, blue and green from the spectrum, no lighting device will be practical for home or office use.

Red proved easy to generate, and about 15 years ago, Japanese scientists found a way to generate blue, thus providing two of the key colors from the spectrum of white light.

But green has been elusive. In fact, the $10 LEDs that people can buy now are made to look white by aiming the blue light at a phosphor, which then emits green. It works OK, but the clunky process saps a big chunk of the efficiency from the light.

NREL Jumps into LED Research via Solar Cells

Along came NREL, a world leader in designing solar cells, but a neophyte in the lighting realm.

NREL scientist Angelo Mascarenhas, who holds patents in solar-cell technology, realized that an LED is just the reverse of a solar cell. One takes electricity and turns it into light; the other takes sunlight and turns it into electricity.

Indeed, Mascarenhas found it. NREL had won major scientific awards with its inverted metamorphic solar cells, in which the cells are built by combining layers of different lattice sizes to optimally capture solar energy. In fact, an NREL-produced IMM cell set a world record by converting 40 percent of absorbed sunlight into electricity.

Solving a Decade-Old Conundrum

For a decade, LED researchers had tried and failed to make a reliable efficient green light by putting indium into gallium nitride.

He and his fellow solar-cell researchers had dealt with the same problem trying to build a solar cell with gallium indium phosphide. When the lattices created by molecular gases don’t match up with the lattices of the layer below, it can’t grow well and the efficiency is very, very poor.

NREL’s solar cell experts found a way around that. They put in some extra layers that gradually bridge the gap between the mismatched lattices of the cell layers.

The researchers deposited layers that had lattice patterns of atoms close to, but not exactly matching, the layers below. The tiny gap in size was at the so-called “elastic limit” of the material — close enough that the lattices bonded to each other and impurities were deflected away.

Then, add a third layer, this one again at the precise “elastic limit” of the one below. After about seven microns of layering, the result is a solar cell with a firm bond and almost no impurities.

Why not try that same process, only in reverse, to make a reliable deep-green LED using indium gallium phosphide?

A Deep Green on the Very First Try

Astonishingly, once the concept was understood, Mascarenhas’s team produced a radiant deep green on their very first try — without any money backing the effort.

The aim now is to provide a fourth color to make that white light even whiter.

NREL plans to use a slightly deeper red and a lemony green, which would then be combined with a blue and a very deep green made using the gallium nitride based technology.

In three years, NREL should have a bi-colored device that when teamed with blue and deep green can produce a sterling LED with a color-rendering index well over 90.

And, by the way, the move toward all LEDs all the time will save some $120 billion in electricity between now and 2030, the Department of Energy forecasts. Not to mention tens of millions of tons of greenhouse gases.

www.nrel.gov

Asthetically Pleasing Solar Cells Save Energy

Solar engineers have long sought to develop an energy-generating glazing that is as capable of producing power as it is easy on the eyes. The feat may just have been accomplished by The Center for Architecture Science and Ecology (CASE), who have developed a concentrating solar system that is not only modern and attractive but extremely efficient and cost effective. The system is made up of rows of pyramid-shaped glass receptors that move with sulight throughout the day, magnifying the incoming light and capturing it in a small photovoltaic cell located in the center of each pyramid.

CASE’s receptors can be fitted to existing buildings or built into new designs. The concentrating solar cells are strung on wires with tracking mechanisms that turn the receptors in the direction of the sun throughout the day. Since they are made from glass they are transparent and allow light to pass through windows, which makes them perfect for adding solar power to building facades while maximizing available daylight.

The glass pyramid shape actually serves to magnify light and increase the natural lighting inside a building while decreasing the need for artificial light. The design is also meant to capture thermal energy trapped inside the glass pyramids that is not converted into electricity to be used for heating and cooling systems.

The receptors aren’t commercially available just yet, but a manufacturer has been lined up and the hope is to bring these cells to market as soon as the manufacturing process is finalized. The estimated cost return is less than two and a half years in a sunny place like Los Angeles and less than a decade in a foggy place like San Francisco.

www.case.rpi.edu

Obama Gives $2.3B In Tax Credits to Solar & Wind Energy

President Barack Obama last week announced the award of US $2.3 billion in Recovery Act Advanced Energy Manufacturing Tax Credits. One hundred eighty three projects in 43 states will receive that funds that are meant to help create tens of thousands of jobs through the domestic manufacturing of advanced clean energy technologies including solar, wind and efficiency and energy management technologies.

While projects selected for this tax credit generally must be placed in service by 2014, approximately 30 percent of them will be completed in 2010.