Utah lens could boost cell efficiency

Researchers at the University of Utah have devised a transparent lens that could boost PV cell efficiency over 50% by "sorting and concentrating" incoming sunlight.

The ultra-thin "polychromat", which measures 50mm wide by 10mm long and is cut with three micrometer-wide light-separating grooves, was made through a photolithography process but could be mass-produced using an inexpensive mould and stamped out as with a DVD.

“Currently, high-efficiency solar cells are very expensive because they have to be carefully manufactured in a complex environment and are only cost-effective for space or defense applications like the Mars Rover,” states Rajesh Menon, a scientist at the Utah Science Technology and Research (UStar) centre at the university.

"We have designed a very cheap optical element that can be incorporated into the cover glass of a solar panel that will separate sunlight into various colours.”

PV cell performance hinges to the efficiency of converting sunlight into electricity, with most designs taking as a starting point the absorption of "one bundle of light", the photon, to generate electrical charge carriers in a layer of material within the cell to make electricity.

The UStar lens "selects" different wavelengths, ranging from ultraviolet to visible to infrared, to overcome the 33.5% theoretical limit on PV cell efficiency.

In the breakthrough study, electrical engineers squeezed 16% extra power efficiency out of a thin-film cell made up of two active layers – gallium indium phosphide to absorb visible light, and gallium arsenide to absorb infrared light – by overlaying the newly designed polychromat.

“These colors can be absorbed by appropriate solar cells to increase the efficiency of the overall process without increasing the cost,” says Menon.

The researchers also fleshed out computer simulations of a polychromat placed on a solar cell with eight different absorber layers to show a theoretical efficiency greater than 50%.

“With our approach, you can almost arbitrarily select the bands of light – within the laws of physics – to give more flexibility in the design of the panel,” adds Menon.

“With a normal lens it’s very difficult to actually separate the colors, and with a prism it’s difficult to get the separated light exactly where you want it.”

UStar is angling to now develop a commercial version of a polychromat-enhanced cell with a solar module manufacturer, with an eye mass production starting "in another five-10 years".

Tests of the new techology are cued up to start at the US Department of Energy's National Renewable Energy Laboratory.