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January 28, 2016

NREL: Solar Cell Defects Might Improve Solar Cells

Originally published on CleanTechnica – January 20th, 2016 by Glenn Meyers

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-defectNREL 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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