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NREL's New Microscopy Technology Reveals Path to Cheaper Solar Cells

May 17, 2011

The solar cell industry is looking to cut costs in production and manufacturing, and one avenue is to use lower-cost materials—such as low-cost, solar-grade silicon. However, silicon of this grade has more impurities than higher-cost silicon, and those impurities can cause weak junctions and weak regions in the cell (called "shunts") that can lower reliability of solar modules and lead to their destruction. For example, when a solar module is partially shaded, the solar cells in the shaded area go into reverse bias mode, in which current can potentially leak between the positive and negative sides of the cell. Under high reverse bias mode, a solar cell with shunts will allow an extremely high current to be driven through the region. This high current heats up and breaks down the area, causing the module to fail.

To avoid such a scenario, researchers at the National Renewable Energy Laboratory (NREL) in Golden, Colorado, knew they had to examine this breakdown process at the microscopic level. But traditional imaging tools, thermography and luminescence imaging, did not show the shunt areas clearly—all that could be seen was a blurred spot at the shunt location. Therefore, a research team, led by Manuel Romero decided to create a new optical tool. Romero's team is part of the Measurements & Characterization group in the National Center for Photovoltaics, which is based at NREL. The team includes Steve Johnston, Mowafak Al-Jassim, Kirstin Alberi, Charles Teplin, David Young, and Howard Branz.

Together, they developed a near-field scanning optical microscope (NSOM). This microscope employs special optics technology to allow observation of the heat in the shunt areas at extremely high resolution, that is, at a scale of 50 to 100 nanometers. The NSOM has an optical fiber tip, which researchers use to scan the solar cell at close range. The tip captures different colors of the light emitted from the shunted area, and those colors reveal, with unprecedented precision, where the breakdown originates and how it expands to cover the surrounding area.

The NSOM allowed the team to characterize junction breakdown in solar cells and to identify the microstructural defects that caused the degradation in open-circuit voltage and high dark currents in epitaxial silicon solar cells. The team also discovered the manufacturing processes and issues that caused the defects.

Romero says,"The use of high-resolution microscopies helps our understanding of the causes behind these shunts, and our findings are shared with our industrial partners to improve solar cells and modules."

And they aren't stopping there. Romero says he and his team are working hard to develop more instrumentation to meet the ever-increasing demands imposed by the exploratory research in photovoltaics—research that is driven by NREL’s and DOE’s missions.

"We continue to develop new techniques in the area of electron microscopy and scanning probe microscopy to measure all the physical properties of solar cells with high resolution," Romero says. It is this kind of scientific innovation that will enable the development of more economical, market-viable solar cells, and advance the deployment of solar technology across the nation and the globe.

To read related technical material, see the summaries of the Applied Physics Letter, Nanoscale measurements of local junction breakdown in epitaxial film silicon solar cells, and the paper presented at the IEEE 2010 Photovoltaic Specialists Conference, Novel Applications of Near-Field Scanning Optical Microscopy: Microluminescence from Local Junction Breakdown in Solar Cells.

—Karen Atkison