Diagnostic imaging technique aims to boost solar cell efficiencies
Theory indicates that existing solar cell technologies should able to convert solar energy to electrical energy with at least 30 percent efficiency, but the actual efficiencies of current cells is around 20 percent which means that solar panels produce one third less power than the theoretical maximum of these devices.
“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 (UMD) Assistant Professor Marina Leite, in the UMD Institute for Research in Electronics and Applied Physics (IREAP) and the 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 subtracting from cell efficiency.
The new, ambient temperature imaging technique presented by Leite and her team [Tennyson, et al] is a variation of illuminated Kelvin Probe Force Microscopy, which is 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. Tennyson, et al. takes 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’. The indirect measurement allows the UMD-led team to observe precisely [at nanoscale resolution] where the open-circuit voltage is changing.
The researchers claim that previous imaging techniques for determining solar cell efficiencies had to be performed under vacuum at cold temperatures (-333 Fahrenheit or 70 Kelvin). The technique fills an important gap in the literature surrounding solar cell efficiencies. The researchers said the technique provides "straightforward, universal, and accurate method to measure the open-circuit voltage… with high spatial resolution”.
The findings of Leite and her team are published in Advanced Energy Materials in a paper entitled ‘Nano-imaging of open-circuit voltage in photovoltaic devices’.
Munday, department of electrical and computer engineering and IREAP, UMD.
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