Clean Power

Published on January 20th, 2016 | by Glenn Meyers


NREL: Solar Cell Defects Might Improve Solar Cells

January 20th, 2016 by  

The time-honored adage that we sometimes learn best by the mistakes we’ve made is now being applied by scientists at the National Renewable Energy Laboratory (NREL) in their study of defects in solar cell defects, stating the results may lead to improved performance.

The study reports about certain defects in silicon solar cells which may eventually improve their overall performance. The findings run counter to conventional wisdom, according to Pauls Stradins, the principal scientist and a project leader of the silicon photovoltaics group at NREL.

NREL cell defect 20160111-solar-defect

Schematic of a ‘good’ defect (red cross), which helps collection of electrons from photo-absorber (n-Si), and blocks the holes, hence suppresses carriers recombination.The findings run counter to conventional wisdom, according to Pauls Stradins, the principal scientist and a project leader of the silicon photovoltaics group at NREL.

Deep-level defects frequently hamper the efficiency of solar cells, but NREL’s theoretical research suggests such defects with properly engineered energy levels can sometime improve carrier collection out of the cell, or “improve surface passivation” of the absorber layer.

NREL researchers conducted simulations to add impurities to layers adjacent to the silicon wafer in a solar cell. Specifically, they introduced defects within a thin tunneling silicon dioxide (SiO2) layer that forms part of “passivated contact” for carrier collection, and within the aluminum oxide (Al2O3) surface passivation layer next to the silicon (Si) cell wafer. In both cases, specific defects were identified to be beneficial.

According to NREL press information, the research by Stradins, Yuanyue Liu, Su-Huai Wei, Hui-Xiong Deng, and Junwei Luo, “Suppress carrier recombination by introducing defects: The case of Si solar cell,” appears in Applied Physics Letters.

Finding the correct defects to examine

The researchers state finding the right defect was key to their research process.

“To promote carrier collection through the tunneling SiO2 layer, the defects need to have energy levels outside the Si bandgap but close to one of the band edges in order to selectively collect one type of photocarrier and block the other. In contrast, for surface passivation of Si by Al2O3, without carrier collection, a beneficial defect is deep below the valence band of silicon and holds a permanent negative charge. The simulations removed certain atoms from the oxide layers adjacent to the Si wafer, and replaced them with an atom from a different element, thereby creating a “defect.” For example, when an oxygen atom was replaced by a fluorine atom it resulted in a defect that could possibly promote electron collection while blocking holes.”

The referenced defects were then sorted according to their energy level and charge state. It is believed more research is needed in order to determine which defects will ultimately produce the best results.

A recent study by the same authors has shown that the addition of oxygen could improve the performance of those semiconductors. For solar cells and photoanodes, engineered defects could possibly allow thicker, more robust carrier-selective tunneling transport layers or corrosion protection layers that might be easier to fabricate.

The research was funded by the U.S. Department of Energy SunShot Initiative as part of a joint project of Georgia Institute of Technology, Fraunhofer ISE, and NREL, with a goal to develop a record efficiency silicon solar cell. The SunShot Initiative is a collaborative national effort that aggressively drives innovation to make solar energy fully cost-competitive with traditional energy sources before the end of the decade.

Graphic via NREL

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About the Author

is a writer, producer, and director. Meyers was editor and site director of Green Building Elements, a contributing writer for CleanTechnica, and is founder of Green Streets MediaTrain, a communications connection and eLearning hub. As an independent producer, he's been involved in the development, production and distribution of television and distance learning programs for both the education industry and corporate sector. He also is an avid gardener and loves sustainable innovation.

  • Riely Rumfort

    Wonder what percentage gains we’re talking, and therein if it’s worth the possible added complexity of manufacturing.

  • JamesWimberley

    Not really comprehensible to non-hysicists and a long way from application. My takeaway is that there are still things to learn at a quite fundamental level about solar cells. I’d be surprised if the same were true of wind. The limits of technology are those of the fundamental science on which it is based. So we can reasonably hope for a good few decades of progress in solar efficiency and cost.

    • Roger Lambert

      It’s similar to climate science itself – we are learning (a lot) as we go along.

    • Glenn Meyers

      Very astute observation.

    • A different Adrian

      James, I disagree about the wind; the two major areas that would seriously advance the economics of wind are improvements in aerodynamics and increases in blade length.

      Regarding aerodynamics we’re still very far from understanding boundary layers, turbulence, and how to control them; there are huge research programmes (more directed at aircraft than wind turbines) looking at active control of boundary layers e.g. with microscopic piezo electric actuators, and at the use of compliant surfaces and active shape changes. There’s also a lot of on-going work just understanding the structure of turbulence e.g. Lagrangian structures, and how turbulence changes at different length scales. If we can improve aerodynamics and introduce more / better active control we’ll get some more efficiency at present speeds, but more importantly we’ll significantly expand the lowest and highest wind speeds that turbines can operate at, which will increase the capacity factor noticeably.

      Regarding increasing blade length, this has been *the* primary driver of wind’s improved economics over the past years. Blade lengths are currently approaching the limits of materials in terms of both engineering properties e.g. strength to weight ratio, and manufacturing properties e.g. requirements for vacuum, heat treatment, post-production finishing, etc, so fundamental advances in materials technology are a major requirement to make turbines bigger and more economic. I don’t know exactly what those technologies will be (if I did I’d be a billionaire hedge-fund owner!) but there are really big advances going on in the world of ceramics e.g. ceramic matrix composites, carbon fibre e.g. graphene and carbon nano-fibres, and nanotechnology generally e.g. it’s already possible to create metals (by controlled deposition of different layers) that have properties beyond any current alloy and which can’t be created by simply melting a mixture and applying any heat treatment you care to name (Modumetal is one company I know of who does this, and for sure many others exist too). Additionally, there’s a lot of activity in researching nano-fillers to improve the properties of the plastics in blades – something that accounts for a lot of their mass and cost. And then there’s the huge potential of 3-D printing to improve blades’ engineering properties and reduce production costs – but that 3-D printing requires advances in controlling the deposition of different materials (even down to the atomic scale) and understanding the effect on resultant bulk properties.

      Sure, it’s going to take time and resources to make progress, but what we *don’t* know about aerodynamics and materials (and also the application of that knowledge) is occupying tens of thousands of researchers and technologists around the world, and I see every bit as much potential to uncover fundamental unknowns in wind as there is in solar.

      • MeToo

        What “A different Adrian” said!!!

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