Now, metallic sandwich to triple the efficiency of solar cells

Princeton researchers led by electrical engineer Stephen Chou used nanotechnology to overcome two primary challenges that cause solar cells to lose energy: light reflecting from the cell, and the inability to fully capture light that enters the cell.

With their new metallic sandwich, the researchers were able to address both problems. The sandwich - called a subwavelength plasmonic cavity - has an extraordinary ability to dampen reflection and trap light.

The new technique allowed Chou's team to create a solar cell that only reflects about 4 per cent of light and absorbs as much as 96 per cent. It demonstrates 52 per cent higher efficiency in converting light to electrical energy than a conventional solar cell.

That is for direct sunlight. The structure achieves even more efficiency for light that strikes the solar cell at large angles, which occurs on cloudy days or when the cell is not directly facing the Sun.

By capturing these angled rays, the new structure boosts efficiency by an additional 81 per cent, leading to the 175 per cent total increase, researchers said in a statement.

Chou said the system is ready for commercial use although, as with any new product, there will be a transition period in moving from the lab to mass production.

The top layer, known as the window layer, of the new solar cell uses an incredibly fine metal mesh: the metal is 30 nanometres thick, and each hole is 175 nanometres in diameter and 25 nanometres apart.

This mesh replaces the conventional window layer typically made of a material called indium-tin-oxide (ITO).

The mesh window layer is placed very close to the bottom layer of the sandwich, the same metal film used in conventional solar cells.

In between the two metal sheets is a thin strip of semiconducting material used in solar panels. It can be any type - silicon, plastic or gallium arsenide - although Chou's team used an 85-nanometre-thick plastic.

The solar cell's features - the spacing of the mesh, the thickness of the sandwich, the diameter of the holes - are all smaller than the wavelength of the light being collected.

This is critical because light behaves in very unusual ways in subwavelength structures. Chou's team discovered that using these subwavelength structures allowed them to create a trap in which light enters, with almost no reflection, and does not leave.

The study was published in the journal Optics Express.