After Decades of Global Effort, Scientists Solve Mystery of Solar Cell Defect
Solar panels are among the most available system of generating energy through renewable sources due to their relative cost and consumer availability. However, the majority of solar cells only achieve 20 percent efficiency-for every kW of equivalent sunlight, about 200W of electrical power can be generated. An international team of researchers have found the key fundamental issue. A material defect which limits and degrades solar cell efficiency. Knowledge of this issue is not new; it has been known about and studied for over 40 years. There are more than 270 research papers attributed to the issue with no solution.
New research shows the first observation of a material defect that limits silicon solar cell efficiency.
Prof. Tony Peaker, who coordinated the research now published in the Journal of Applied Physics said: “Because of the environmental and financial impact, solar cells ‘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.”
“During the first hours of operation, after installation, a solar cells efficiency drops from 20 percent to about 18 percent. An absolute drop of 2 percent 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 energy cost of this defect across the globally installed solar capacity is in the 10’s of gigawatts.
That’s equivalent to more energy than the UK’s 15 nuclear power plants combine! Less sustainable energy options such as fossil fuels have to be implemented to bridge the gap because of this.
Researchers were able to identify the mechanism responsible for Light Induced Degradation (LID) thanks to the theoretical and experimental approach of many scientific disciplines. The team uncovered the existence of the defect, which initially lies dormant within the silicon used to manufacture the cells. A process known as ‘deep-level transient spectroscopy’ in which the electronic charge within the bulk of the silicon solar cell is transformed under sunlight. This is part of its energy generating process. The team found that this transformation involves a highly effective “trap” that prevents the flow of 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, used to determine the quality of silicon material, measures the lifetime of charge carriers. Higher quality material with fewer “traps” have a longer lifetime. Researchers in Manchester lead by professor Mathew Halsall found that their observations have strong correlation with this charge carrier lifetime. It was reduced significantly after transformation of the defect under illumination. They also noted that the effect was reversible. The lifetime increased again when the material was heated in the dark, a process commonly used to remove the “traps.”
The paper, “Identification of the mechanism responsible for the boron oxygen light induced degradation in silicon photovoltaic cells,” is published in the Journal of Applied Physics.