The DoE Just Broke the World Record in Solar Cell Efficiency

The Department of Energy (DoE) continues to make gains on research efforts to improve the efficiency of solar cells. The agency’s National Renewable Energy Laboratory (NREL) recently achieved an efficiency of 45.7% in a solar cell, one of the highest cell efficiencies achieved to date.

The work surpasses the current world record in efficiency of 44.7% achieved last year by a group of German and French scientists at the Fraunhofer Institute for Solar Energy Systems ISE, Soitec, CEA-Leti, and the Helmholtz Center Berlin. That research, as well as the NREL's, involved a four-junction solar cell, which means the cell has four p-n junctions, or interfaces, between two types of semiconductor material. The cells harvest sunlight by dividing the solar spectrum into portions and each junction is tuned to a different light wavelength, increasing the efficiency of the overall cell.

A diagram shows the latest in research on solar-cell efficiency and the institutions working on these projects.
Click to enlarge

(Source: Department of Energy National Renewable Energy Laboratory)

Solar cell efficiency is the ratio of the electrical output of a solar cell to the incident energy in the form of sunlight. Before achieving 44.7% efficiency, the European collaborators had developed a solar cell that achieved a 43.6% efficiency.

The new solar cell designed by NREL -- the DoE’s primary national laboratory for renewable energy and energy efficiency research and development -- demonstrated its efficiency level at 234 suns concentration. The cell itself was designed to be operated in a concentrator photovoltaic (CPV) system where it can receive more than 1,000 suns of concentrated sunlight.

The system achieved its efficiency improvement through the addition of a high-quality sunlight absorber layer, said NREL scientist Ryan France, who designed the cell.

"The distinction of this multijunction device is the very high quality of the lattice-mismatched subcells," he said in a press statement. "Lattice-mismatched materials require the introduction of defects, called dislocations, into the device, which can drastically hinder device performance. NREL has learned to control and confine these dislocations to inactive regions of the device, allowing even highly mismatched material to be used in a multijunction cell."

Specifically, the new design is comprised of a gallium indium phosphide (GaInP) junction, a gallium arsenide junction, and two gallium indium arsenide junctions that are lattice-mismatched to the substrate.

The cell's peak efficiency of 45.7 ±2.3% was measured under the AM1.5 direct spectrum at 234 suns concentration, researchers said. However, the device performed well even at higher concentrations, with only slightly less efficiency -- 45.2% -- at 700 suns concentration.

In addition to the absorber layer, other improvements to the design over previous cells include a broadband four-layer anti-reflection coating, a new metamorphic tunnel junction interconnect, and the best performance to date from the GaInP top cell, which has both higher voltage and the reduced series resistance essential for high efficiency at high solar concentrations.

The NREL’s cell measurements lab validated the efficiency of the new cell, but the cell is being evaluated and tested for confirmation by an external accredited laboratory.

The Alliance for Sustainable Energy LLC operates NREL for the DoE. NREL’s solar-cell research is supported by the Energy Department's SunShot Initiative, a national effort to make solar energy costs comparable to traditional energy sources by the end of the decade. Through SunShot, the DoE provides funding for private companies, universities, and national laboratories that are working to drive down the cost of solar electricity to $0.06 per kilowatt-hour.

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