Ultrafast Laser Scribing Cuts Cost, Hikes Solar Cell Efficiency

An advancement that will improve solar cell efficiency and reduce manufacturing costs involves using an ultrafast pulsed laser scribing technique to create more precise microchannels.

Microchannels are critical to cost and efficiency because they are needed to interconnect a series of solar panels into an array capable of generating usable amounts of power, said Yung Shin, a professor of mechanical engineering and director of Purdue University's Center for Laser-Based Manufacturing. Conventional scribing methods, which create the channels mechanically with a stylus, are slow and expensive and produce imperfect channels, impeding solar cells' performance.

"Production costs of solar cells have been greatly reduced by making them out of thin films instead of wafers, but it is difficult to create high-quality microchannels in these thin films," Shin said. "The mechanical scribing methods in commercial use do not create high-quality, well-defined channels. Although laser scribing has been studied extensively, until now we haven't been able to precisely control lasers to accurately create the microchannels to the exacting specifications required."

"The efficiency of solar cells depends largely on how accurate your scribing of microchannels is," Shin said. "If they are made as accurately as possible, efficiency goes up."

The work, funded by a three-year, $425,000 grant from the National Science Foundation, is led by Shin and Gary Cheng, an associate professor of industrial engineering. A research paper demonstrating the feasibility of the technique was published in Proceedings of the 2011 NSF Engineering Research and Innovation Conference in January. The paper was written by Shin, Cheng and graduate students Wenqian Hu, Martin Yi Zhang and Seunghyun Lee.

Research results have shown that the fast-pulsing laser accurately formed microchannels with precise depths and sharp boundaries. The laser pulses last only a matter of picoseconds. Because the pulses are so fleeting, the laser does not cause heat damage to the thin film, instead removing material in precise patterns in a process called cold ablation.

"It creates very clean microchannels on the surface of each layer," Shin said. "You can do this at very high speed, meters per second, which is not possible with a mechanical scribe. This is very tricky because the laser must be precisely controlled so that it penetrates only one layer of the thin film at a time, and the layers are extremely thin. You can do that with this kind of laser because you have a very precise control of the depth, to about 10 to 20 nanometers."

Traditional solar cells are usually flat and rigid, but emerging thin-film solar cells are flexible, allowing them to be used as rooftop shingles and tiles or building facades, or as the glazing for skylights. Thin-film solar cells account for about 20 percent of the photovoltaic market globally in terms of watts generated and are expected to account for 31 percent by 2013.

The researchers plan to establish the scientific basis for the laser ablation technique by the end of the three-year period. The work is funded through NSF’s Civil Mechanical and Manufacturing Innovation division.

For more information, visit: www.purdue.edu

Laser-Driven Electrons Observed in Real Time

The discovery will advance the development of new x-ray sources, the resolution of which will be much higher than current devices allow, according to physicists at the Laboratory of Attosecond Physics (LAP) at Max Planck Institute for Quantum Optics (MPQ) and Ludwig Maximilians University of Munich (LMU), in cooperation with colleagues from Friedrich Schiller University Jena.

In the researchers' experiments, when short laser pulses irradiate helium atoms, their structure is heavily disturbed. If the light is strong enough, electrons are pulled out of the atoms, and the helium atoms become ions. In this mixture, the electrons are much lighter than the helium ions and, as a result, are pushed aside.

Although the laser pulse sweeps across the system, the ions remain stationary and the released electrons oscillate around one location. Together, the particles form wave structures (electron plasma waves). In laser physics, this process and these waves are used under special conditions to rapidly accelerate a small number of the electrons to close to the speed of light and to control them.

In the plasma wave, gigantic electric fields are formed, which are 1000 times stronger than those generated in the world’s largest particle accelerators. A small number of the electrons take advantage of these fields, flying as a swarm behind the laser pulse in its slipstream and accelerating to close to the speed of light. In this process, every accelerated electron has almost the same energy.

Physicists have long been aware of this phenomenon, and it has been demonstrated in earlier experiments, but until now, it has been possible to individually observe only the electron swarm or the whole plasma wave with reduced resolution.

The laser physicists, including Ferenc Krausz and his employees Laszlo Veisz and Alexander Buck of LAP, succeeded in recording both phenomena with a high-resolution image of the plasma wave. The process was documented in snapshots with the same light pulse responsible for accelerating the electrons. The physicists previously split the laser pulse so that a small portion of it illuminated the system of free electrons and ions perpendicularly to the electron beam. The periodic structure of the plasma wave refracts and partially deflects the light.

"We observe the deflection and thereby image the plasma wave as a modulation of brightness onto a camera," said Veisz, the research group leader of the LAP team.

In doing so, the researchers can achieve a unique spatial and temporal resolution in the femtosecond range. The electron swarm produces strong magnetic fields that they also can record to determine its position and duration. Eventually, a film describing the acceleration of the electrons results from the combination of both measurement methods.

"The obtained improved knowledge about laser-driven electron acceleration helps us in the development of new x-ray sources of unprecedented quality, not only for basic research, but also for medicine," Krausz said.

The physicists describe their results in the scientific journal Nature Physics.

Laser heats up fusion quest

Physicists at the $3.5bn National Ignition Facility (NIF) say they have taken an important step in the bid to generate fusion energy using ultra-powerful lasers. By focusing NIF's 192 laser beams onto a tiny gold container, researchers have achieved the temperature and compression conditions that are needed for a self-sustaining fusion reaction – a milestone that they hope to pass next year.

Located at the Lawrence Livermore National Laboratory in California and officially opened last year, NIF will provide data for nuclear weapons testing as well as carry out fundamental research in astrophysics and plasma physics. The facility will also aim to fuse the hydrogen isotopes deuterium and tritium in order to demonstrate the feasibility of laser-based fusion for energy production.

These hydrogen isotopes will be contained within peppercorn-sized spheres of beryllium, which will be placed in the centre of an inch-long hollow gold cylinder – known as a hohlraum. By heating the inside of the hohlraum, NIF's laser beams will generate X-rays that cause the beryllium spheres to explode and, due to momentum conservation, the deuterium and tritium to rapidly compress. A shockwave from the explosion will then increase the temperature of the compressed matter to the point where the nuclei overcome their mutual repulsion and fuse.

One of the main aims of NIF is to achieve "ignition", which means that the fusion reactions generate enough heat to become self-sustaining. Researchers hope that by burning some 20–30% of the fuel inside each sphere the reactions will yield between 10 and 20 times as much energy as supplied by the lasers.

NIF first began testing the laser beams last year and now two groups at Lawrence Livermore have shown that they can obtain the desired conditions inside the hohlraum. They did this by using plastic spheres containing helium, rather than actual fuel pellets, since these were easier to analyse, and by combining their experimental measurements with computer simulations, the researchers found that the hohlraum converted nearly 90% of the laser energy into X-rays and that it heated up to some 3.6 million degrees Celsius. They also found that the sphere was compressed very uniformly, its diameter shrinking from around two millimetres to about a tenth of a millimetre.

"These results are better than we were hoping," says NIF boss Edward Moses. "People were concerned that we wouldn't be able to achieve the desired temperature and implosion shape, but those fears have proved unfounded." Moses says that the next step will be to replace the plastic spheres with beryllium ones containing unequal quantities of deuterium and tritium, in order to study how hydrodynamic stabilities might lead to asymmetrical implosions. The final step will then be to switch over to actual fuel pellets, which will contain equal quantities of the two hydrogen isotopes, and which, it is hoped, will ignite.

Moses says he hopes that ignition will take place in 2012. But he is keen not to raise expectations, having had to deal with many technical problems since construction started on NIF back in 1997. Indeed, he and his colleagues had predicted last January that ignition would be achieved by the end of 2010. "We might be able to reach ignition around spring or summertime next year," he says. "But there's a lot of physics that can run us off course in the meantime."

David Hammer, a plasma physicist at Cornell University in New York, says that the latest results are encouraging. However, he warns that the study was done without fully understanding the interactions taking place between the laser beams and plasma inside the hohlraum and that such interactions could wreck the very precise symmetry of the implosion needed for ignition.

The work is described in Phys. Rev. Lett. 106 085003 and 106 085004.

Ultra high speed film: Nano-scientists take snapshots of electronic states

German scientists in the team of Professor Michael Bauer, Dr. Kai Roßnagel and Professor Lutz Kipp from the Institute of Experimental and Applied Physics, together with colleagues from the University of Kaiserslautern and the University of Colorado in Boulder, U.S.A., are following the course of electronic switching processes which occur within fractions of a second (femtoseconds). The results of their research may trigger future developments of custom-made and ultra fast opto-electronic components in order to increase data transmission rates or to accelerate optical switches, to name just one example of potential areas of application.
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Two still frames recorded from the newly developed imaging method. The time interval betweenthe two frames is only 0.00000000000007 seconds. Recording: Rohwer et al., Copyright: CAU
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"These techniques that we have developed enables us to record films of extremely fast processes in a much more comprehensive manner than it was previously possible with similar techniques", Bauer explains. "We are able to, for example, directly track phase transitions in solids or catalytic reactions on surfaces."

To record the films, the Kiel scientists used ultra short flashes of light in the soft x-ray spectral region generated with a specific laser system. Bauer: "The amount of information gained from our pictures when played back in slow motion is vast. We will get completely new insights into most relevant electronic properties of solids which are important for a variety of current and future technologies, for example, in telecommunications."

More information: http://www.nature. … /nature09829