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NREL Findings Could Lead to More Reliable Solar Cells

October 24, 2017

Research at the Department of Energy’s National Renewable Energy Laboratory (NREL) shows how to increase reliability of copper indium gallium selenide (CIGS) thin-film solar cells. CIGS are today’s most-efficient thin-film photovoltaic technology with efficiencies exceeding 22 percent, but one of the challenges is to ensure long-term stability and reliability of solar panels. Solar cell efficiency and reliability depend on the semiconductor defects, and research at NREL identifies and provides a simple way to analyze key semiconductor defects in CIGS solar cells.

The research, Optically induced metastability in Cu(In,Ga)Se2 and published in Scientific Reports, holds the potential to enable wider use of the thin film photovoltaics (PV) technology, according to Darius Kuciauskas, lead author of the paper. Key to the improvement is the use of photoluminescence excitation spectroscopy to identify defects in the solar cell that lower its efficiency and cause degradation.

“It’s not a new optical technique, but it’s a new optical technique for photovoltaics,” said Kuciauskas, a senior scientist in NREL’s Spectroscopy and Photoscience group. His co-authors, all from NREL, are Soeren Jensen, Ana Kanevce, Lorelle Mansfield, Stephen Glynn, and Stephan Lany.

Powered by sunlight, electrons create an electrical current as they move through a semiconductor. Defects within the semiconductor create obstacles for the electrons. “A very common approach and a good approach is to look at defects by luminescence,” Kuciauskas said. “The problem, though, is that luminescence can only be used to measure defects that emit light. And usually if the defect is deeper, it doesn’t emit light.”

NREL scientists used time-resolved spectroscopy tuned to “sub-bandgap” energy levels—below the energy absorbed by the active layer of the solar cell. This approach, which directly excites specific defect states in CIGS absorber layers, allowed them to identify changes in the solar cell, such as a drop in voltage, and trace the properties of the defects.

Lany and Alex Zunger, a former NREL researcher and now a professor at the University of Colorado, theorized in a 2006 paper that divacancy defect states—when two atoms are missing—cause the metastability in CIGS solar cells. While their theory was widely accepted, until now there hasn’t been a measurement to test all predictions of the theory. With new characterization techniques developed at NREL, where light is directly absorbed by the defect states, the presence of divacancy defects and their effects on the solar cells can be monitored in relatively simple, contactless, and non-destructive measurements. This test for defects that cause metastability and degradation—which affects the efficiency of CIGS—can “take minutes instead of months,” Kuciauskas said. The theoretical analysis of the defects in CIGS was also updated and improved in this work.

Funding for the research came from the Department of Energy’s Solar Energy Technologies Office.

NREL is the U.S. Department of Energy's primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the Energy Department by The Alliance for Sustainable Energy, LLC.