Detecting, correcting deficiencies: diagnostic imaging to boost solar cells
December 11, 2015 | By Barbara Vergetis Lundin
A new diagnostic imaging technique developed by the University of Maryland (UMD) promises to boost efficiencies of solar cells by making it possible to find and correct previously undetected ways that solar cells fall far short of theoretical efficiencies.
In theory, current solar cell technologies should convert solar energy to electrical energy with at least 30 percent efficiency, but the actual efficiencies of current cells is only around 20 percent. Consequently, solar panels produce one-third less power than they ideally could.
"With the new imaging technique our team has developed, academic and industry researchers will be able to diagnose where solar cells lose efficiency and close the gap between theory and the actual efficiencies experienced by consumers who install solar panels on their homes and businesses," said University of Maryland Assistant Professor Marina Leite, UMD Institute for Research in Electronics and Applied Physics (IREAP) and department of materials science & engineering in the A. James Clark School of Engineering.
Solar cell efficiencies depend on the maximum achievable open-circuit voltage generated by the device under illumination. Open-circuit voltage determines how well any photovoltaic device operates, and researchers must be able to measure and image it in order to diagnose which processes are adding to or detracting from cell efficiency.
The new, ambient temperature imaging technique is a variation of illuminated Kelvin Probe Force Microscopy -- a non-contact, non-destructive imaging technique used to determine the composition and electronic state of a surface.
Traditionally, this technique uses a laser diode to scan the surface of a solid and measure the potential difference between the tip of the probe and the surface of that material. However, Leite and her team take this conventional imaging method further to demonstrate a "direct correlation between Kelvin Probe Force Microscopy measurements (light- minus dark-KPFM) and the open-circuit voltage of photovoltaic devices through the measurement of the quasi-Fermi level splitting." -- allowing the UMD-led team to observe precisely [at nanoscale resolution] where the open-circuit voltage is changing.
Previous imaging techniques for determining solar cell efficiencies had to be performed under vacuum at very cold temperatures (-333 Fahrenheit or 70 Kelvin), according to the researchers, but their new technique fills an important gap in the literature surrounding solar cell efficiencies -- providing a "straightforward, universal, and accurate method to measure the open-circuit voltage... with high spatial resolution."
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