New Solar Cell Can Handle The Force Of 1,000 Suns

December 17th, 2014 by Tina Casey

Why are some of these new concentrating solar cell systems reminding us of the Eye of Sauron? Just wondering, because the National Renewable Energy Laboratory is out with a new solar cell specially designed for use in concentrating solar systems. The lab has already demonstrated the new solar cell at 700 suns concentration, and it is capable of getting up to — and over — 1,000 suns.

Concentrating solar tech has had its share of doubters in terms of cost effectiveness, but we’re thinking that the new solar cell could help lay some of those concerns to rest. At the concentration of 234 suns, the new cell checked out at a conversion efficiency of 45.7 percent, which puts it in the rarefied company of the most efficient solar cells of any type.

At 700 suns, the new solar cell also clocked in at an impressive 45.2 percent.

The New Solar Cell, Multijunction Style

NREL’s new solar cell is a multijunction type, which is fancyspeak for a solar cell made of layers, typically two or three.

Each layer is a different material. The topmost material is the most efficient as measured by its bandgap, and the lower layers have steadily decreasing bandgaps.

The layered effect lets multijunction cells skip over the theoretical limit for single junction cells, which is about 33.5 percent. According to the Energy Department, when exposed to concentrated sunlight a good multijunction cell can reach more than 43 percent.

After the 43 percent mark it’s a matter of incremental improvements, the challenge being to integrate different materials into a single, complex unit while maintaining their most efficient characteristics.

So let’s see how NREL got up to 45.7.

The New Solar Cell, Lattice-Mismatched Style

NREL calls the new cell 4JIMM for “four-junction inverted metamorphic,” meaning that it has four layers and some other stuff going for it.

The top layer is gallium indium phosphide, which compounds two materials (gallium and indium) favored by solar researchers for their high bandgaps. The next layer is gallium arsenide, and the bottom two layers are gallium indium arsenide.

The really tricky part is the bottom two layers, which are lattice-mismatched to the substrate. Lattice mismatch refers to the nanoscale structure of materials. The mismatch introduces defects that interfere with efficiency, so getting mismatched materials to behave is a particular focus of research at NREL.

The NREL scientist who designed the new solar cell, Ryan France, explains it thusly:

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.

Among many other improvements, the new solar cell also sports an advanced anti-reflective coating.

So…What About Concentrating Solar Systems?

Concentrating (or concentrator/concentrated) solar systems work by using mirrors or other reflective devices to focus more energy on a solar cell. That saves money on solar cell materials, but the relative complexity of a concentrating system also involves extra costs.

Despite concerns over the economics, the Energy Department tapped a suite of innovative utility scale concentrating solar systems for its loan guarantee program, including Ivanpah and Agua Caliente, which the agency has showcased as success stories.

So…like it or not the US is a leading force in the global concentrating solar technology field, and NREL’s new solar cell is going to help keep that engine humming right along, doubters or not.

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