Putting a dent in solar cells may actually make them more efficient. It could even pave the way to solar cells that break a fundamental limit on how much energy the material can absorb.
Solar cells work via the photovoltaic effect, in which light imparts energy to electrons, allowing them to move around and create electrical current. Most modern solar cells place two different types of semiconductor materials next to each other, which directs the electrical current to flow from one material to the other.
These solar cells are limited by how much energy the electrons can absorb from sunlight. f each particle of light has too little energy to excite the electrons, they don鈥檛 absorb any of it. But too much, and the extra goes unused.
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at the University of Warwick in the UK and his colleagues have come up with a new way to generate energy from sunlight within just one material 鈥� and it might be able to make use of more of the sun鈥檚 energy.
In a single material, electrical current can only flow when the molecular structure is not perfectly symmetrical. In symmetrical materials, electrons can jostle around but there鈥檚 nothing directing or organizing their motion to make it useful.
Pressing matters
Alexe and his colleagues found a way to make any semiconductor into a solar cell: simply break its molecular symmetry. By pressing the rounded tip of an atomic force microscope into a sample of a symmetrical semiconductor, they squeezed some of the molecules closer together.
鈥淭hink about if I have a completely symmetrical cube of jelly,鈥� says Alexe. 鈥淚f I poke the jelly with my finger, I will make a dip in it and then there will be a strain all around that dip. This is not symmetric anymore.鈥�
The strain causes electrons to flow towards that dip and create a current. The effect may not generate enough energy on its own to be cost-effective, but Alexe says that if it is used with standard two-layer materials, solar cells could breach the constraint on possible energy intake that currently limits their performance.
The maximum efficiency of current silicon solar cells is about 25 per cent, says at the Colorado School of Mines in Golden. Creating an array of nubs that poke into the material of a solar cell could increase efficiency by a few per cent, Alexe says, though the researchers haven鈥檛 measured the exact amount.
鈥淚f you鈥檙e going to have to cover a large area with nanoindenters, it鈥檚 going to be terribly expensive to scale up,鈥� says Taylor. But if somebody could actually break the limit on energy a material can absorb from the sun in a real cell, 鈥渢hat would be a significant leap,鈥� he says.
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Read more: Solar cell you wear like a bandage can power a watch
