High-efficiency solar cells

Using arrays of long, thin silicon wires embedded in a polymer substrate, a team of scientists from the California Institute of Technology (Caltech) has created a new type of flexible solar cell that efficiently converts photons into electrons.

‘These solar cells have, for the first time, surpassed the conventional light-trapping limit for absorbing materials,’ said Harry Atwater, Howard Hughes professor of applied physics and materials science at Caltech.

The light-trapping limit of a material refers to how much sunlight it is able to absorb.

The silicon-wire arrays absorb up to 96 per cent of incident sunlight at a single wavelength and 85 per cent of total collectible sunlight.

The silicon-wire arrays are able to convert between 90 and 100 per cent of the photons they absorb into electrons.

The key to the success of the solar cells is their silicon wires, each of which, said Atwater, are independent highly efficient and high-quality solar cells.

When brought together in an array, however, they’re even more effective, because they interact to increase the cell’s ability to absorb light.

Light comes into each wire and a portion is absorbed and another portion scatters.

The collective scattering interactions between the wires make the array very absorbing.

This effect occurs despite the sparseness of the wires in the array – they cover only between two to 10 per cent of the cell’s surface area.

The new solar cells also use only a fraction of the expensive semiconductor materials required by conventional solar cells.

Just two per cent of the cell is silicon, while 98 per cent is polymer.

Since the silicon material is an expensive component of a conventional solar cell, a cell that requires just two per cent of the amount of semiconductor material will be much cheaper to produce.

The composite nature of these solar cells, Atwater added, means that they are also flexible. ‘Because flexible thin films can be manufactured in a roll-to-roll process, it is an inherently lower-cost process than one that involves brittle wafers like those used to make conventional solar cells,’ he said.

The next steps, Atwater said, are to increase the operating voltage and the overall size of the solar cell. ‘The structures we’ve made are square centimetres in size,’ he explained. ‘We’re now scaling up to make cells that will be hundreds of square centimetres – the size of a normal cell.’

Atwater says that the team is already ’on its way’ to showing that large-area cells work just as well as these smaller versions.

Schematic diagram of the light-trapping elements used to optimise absorption within a polymer-embedded silicon wire array. [Credit: Caltech/Michael Kelzenberg]
Schematic diagram of the light-trapping elements used to optimise absorption within a polymer-embedded silicon wire array [Credit: Caltech/Michael Kelzenberg]