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The virtue of defects

Speed read
  • NERSC supercomputers simulate defects in silicon solar cells.
  • The right kind of defect can improve panel efficiency.
  • Newly discovered principle also applicable to other photoelectrical technologies.

Scientists at the US Department of Energy's (DOE) National Renewable Energy Laboratory (NREL) are using supercomputers to study what may seem paradoxical: certain defects in silicon solar cells may actually improve their performance.

The findings, recently published in Applied Physics Letters, run counter to conventional wisdom, according to Paul 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 theoretical research suggests that defects with properly engineered energy levels can improve carrier collection out of the cell, or improve surface passivation of the absorber layer.

Researchers at NREL ran simulations to add impurities to layers adjacent to the silicon wafer in a solar cell. Namely, 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 proved to be beneficial.

The simulations were accomplished using NREL's supercomputer and resources at the National Energy Research Scientific Computing Center (NERSC), a DOE Office of Science User Facility located at Lawrence Berkeley National Laboratory. NERSC’s Hopper system was used to calculate various defect levels; the researchers ran a total of 100 calculations on Hopper, with each calculation taking approximately eight hours on 192 cores.

<strong>Amazing Grace. </strong>Hopper was NERSC's first petaFLOP system, a Cray XE6, with a peak performance of 1.28 PetaFLOPS, 153,216 compute cores, 212 TB of memory, and 2 PB of disk. Hopper placed fifth on the November 2010 Top500 Supercomputer list. Courtesy NERSC.

Finding the right defect was key to the process. To selectively collect one type of photocarrier and block the other 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 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 defects were then sorted according to their energy level and charge state.

The principles used in this study are applicable to other materials and devices, such as photoanodes and two-dimensional semiconductors. A recent study by the same authors showed 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 US 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. 

Adapted from a NREL news release. Read the orginal NERSC article here

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