Underlying Material Defect Uncovered in Silicon Solar Cells Limits Performance

June 04, 2019 by Scott McMahan

A team of researchers at the University of Manchester have found the underlying material defect which limits and degrades silicon solar cell efficiency. The issue has been known and studied for more than 40 years, with over 270 research papers about the issue with no solution.

The team was the first to observe what they say is a previously unknown material defect that limits silicon solar cell efficiency.

Prof Tony Peaker, who co-ordinated the research now published in the Journal of Applied Physics explained, "During the first hours of operation, after installation, a solar panel's efficiency drops from 20% to about 18%. An absolute drop of 2% in efficiency may not seem like a big deal, but when you consider that these solar panels are now responsible for delivering a large and exponentially growing fraction of the world's total energy needs, it's a significant loss of electricity generating capacity."

The research team notes that the energy cost of this shortfall across the world's installed silicon solar cell capacity measures in the 10's of gigawatts. They pointed out that this is equivalent to more energy than is produced by the UK's combined total of 15 nuclear power plants.

The multi-disciplinary experimental and theoretical approach that the researchers used identified the mechanism responsible for Light Induced Degradation (LID).

Using a specialized electrical and optical technique, known as ‘deep-level transient spectroscopy' (DLTS), the team found a previously unknown material defect which initially lies dormant within the silicon used to make the cells.

"Because of the environmental and financial impact solar panel ‘efficiency degradation' has been the topic of much scientific and engineering interest in the last four decades. However, despite some of the best minds in the business working on it, the problem has steadfastly resisted resolution until now," Prof Tony Peaker said.

The electronic charge in the bulk of the silicon solar cell is converted under sunlight as part of its energy-generating process. The team found that this transformation involves a highly effective 'trap' that blocks the flow of photo-generated charge carriers (electrons).

Dr Iain Crowe said, "This flow of electrons is what determines the size of the electrical current that a solar cell can deliver to a circuit, anything that impedes it effectively reduces the solar cell efficiency and amount of electrical power that can be generated for a given level of sunlight. We've proved the defect exists, it's now an engineering fix that is needed."

The industry standard technique to determine the quality of the silicon material measures the 'lifetime' of charge carriers, which is longer in high-quality material. The higher the quality of the material, the longer the lifetime.

The researchers lead by Prof Matthew Halsall of the University of Manchester found that their observations were strongly (inversely) correlated with this charge carrier lifetime, which was reduced significantly after transformation of the defect under illumination. The more 'traps,' that were created after just hours of exposure to illumination, the shorter the lifetime tended to be.

They also noted that the effect was reversible. The lifetime increased again after the material was heated in the dark, a process commonly known to increase the efficiency (at least temporarily). It essentially annihilated the traps.

The researchers published their findings in the Journal of Applied Physics in a paper titled, "Identification of the mechanism responsible for the boron oxygen light induced degradation in silicon photovoltaic cells."

Engineers still have to figure out a way of solving the issue in silicon solar panels.

Reference Material

Vaqueiro-Contreras, M., Markevich, V. P., Coutinho, J., et al. Identification of the mechanism responsible for the boron oxygen light induced degradation in silicon photovoltaic cells. Journal of Applied Physics. 125, 185704 (2019) DOI: 10.1063/1.5091759