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Making Photovoltaic Solar Cells More Efficient

The maximum use of incoming solar radiation relies on using the full spectrum of the light and infrared radiation. There’s a lot of power in the infrared – not something the current photovoltaic cell is harvesting.

Photovoltaic solar cells convert energy from the sun into electricity by absorbing light. However, different materials absorb light at different wavelengths. The most efficient cells are made of multiple materials that together can capture a greater portion of the electromagnetic radiation in sunlight. The best solar cells today are still missing a material that can make use of a portion of the sun’s infrared light.  Close, but leaving a huge part of the energy unharvested.  What’s missing is the layer that converts infrared into electricity.

In what could be a step toward higher efficiency solar cells, an international team including University of Michigan professors has invalidated the most commonly used model to explain the behavior of a unique class of materials called highly mismatched alloys. These mismatched alloys, which are still in the experimental stages of development, are combinations of elements that won’t naturally mix together using conventional crystal growth techniques.

Professor Rachel Goldman, a professor in the departments of Materials Science and Engineering, and Physics at the University of Michigan compares them to some extent to homogenized milk, in which the high-fat cream and low-fat milk that would naturally separate are forced to mix together at high pressure.

Goldman says new mixing methods such as “molecular beam epitaxy” are allowing researchers to combine disparate elements. The results are more dramatic than smooth milk. “Highly mismatched alloys have very unusual properties,” Goldman said. “You can add just a sprinkle of one element and drastically change the electrical and optical properties of the alloy.”
Goldman’s team made samples of gallium arsenide nitride, a highly mismatched alloy that is spiked with nitrogen, which can tap into that underutilized infrared radiation.

Goldman’s team includes researchers from Tyndall National Institute in Ireland with other UM physicists and engineers.  The team used the new molecular beam epitaxy to coax the nitrogen to mix with their other elements. Molecular beam epitaxy involves vaporizing pure samples of the mismatched elements and combining them in a vacuum.

Next, the team measured the alloy’s ability to convert heat into electricity. They wanted to determine whether its 10 parts per million of nitrogen were distributed as individual atoms or as clusters. They found that in some cases, the nitrogen atoms had grouped together, contrary to what the prevailing “band anti-crossing” model predicted.

Goldman explains, “We’ve shown experimentally that the band anti-crossing model is too simple to explain the electronic properties of highly mismatched alloys.  It does not quantitatively explain several of their extraordinary optical and electronic properties. Atomic clusters have a significant impact on the electronic properties of alloy films.”

If researchers can learn to control the formation of these clusters, they could build materials that are more efficient at converting light and heat into electricity, Goldman said. “The availability of higher efficiency thermoelectrics would make it more practical to generate electricity from waste heat such as that produced in power plants and car engines.”

The team’s research is newly published online in Physical Review B.  The paper is entitled “Nitrogen Composition Dependence of Electron Effective Mass in Gallium Arsenide Nitride.”

The photovoltaic world has been at lower key since 2008 when the claims and new releases came fast and furious.  Back then the highest claims were at 40% of the spectrum.  Adding infrared to the spectrum is going to increase the claims dramatically.

Should research push molecular beam epitaxy, or one of the other processes far enough to build molecules for infrared harvesting with improved photovoltaic reliably and at lower cost the photovoltaic business could get a massive boost.  There’s a huge performance difference when collecting a great bulk of the radiation.  Today thermal solar can make viable energy production from home heating up to utility scale thermal power plants using the infrared part of the spectrum alone.  Photovoltaic needs this research to succeed to compete. The industry needs to drive to lower investment costs and more robust panels.

Photovoltaic is getting closer to wide ranging viability, and not a competitive moment to spare.

By. Brian Westenhaus

Source: On the Path to Full Spectrum Photovoltaic Solar Cells




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