PEPPERING solar cells with quantum dots could be the next step towards cheaper solar power, allowing basic devices to generate more electricity than ever before. The new technique borrows a trick from photosynthesis to convert light into electricity.
Like many solar cells, the new design consists of several layers of semiconductors, called a junction. The bottom layer is doped with impurities that provide excess free-moving electrons, while the top is capped with a layer that lacks free electrons and contains 鈥渉oles鈥� of positive charge. The movement of electrons and holes into their opposite layers helps to create an electric field across the junction.
Another semiconductor, which is sensitive to light and releases electrons when bombarded with photons, is sandwiched between these two layers. The electrons liberated by light then flow under the force of the electric field to produce a current.
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This configuration of semiconductors can absorb only a narrow range of light wavelengths, so manufacturers usually stack many different semiconductor junctions, each designed to absorb a different wavelength. But laying down many different sheets of semiconductor is costly.
A cheaper solution, according to Pavlos Lagoudakis at the University of Southampton in the UK, is to coat single junctions with quantum dots. 鈥淵ou can print them on virtually anything using ink-jet printing methods,鈥� he says.
Quantum dots are tiny crystals of a combination of cadmium, selenium and sulphur that trap electrons within their structure. These electrons can be excited by photons with a wide range of wavelengths. But previous attempts to use quantum dots to generate electricity have not worked because a viable way to channel the excited electrons鈥� energy from the crystal had not been found.
The new device overcomes this by placing the quantum dots right next to the underlying layers of semiconductor by positioning them in furrows that expose all three layers. The energy captured from the passing photon can then be transferred directly into the nearby semiconductor. Plants use a similar very efficient method to transfer energy between pigment molecules during photosynthesis.
In initial tests, the team recorded a whopping six-fold increase in current compared to a single junction without the quantum dots (Physical Review Letters, ). Further refinements could achieve an overall light-to-electricity efficiency of more than 31 per cent, which is comparable to the more expensive multi-junction devices, says Lagoudakis.
from Imperial College London is not convinced the technology will be easy to transfer to working solar cells. He says the team used high-power lasers to test the device and suspects that at lower intensities the results may not be so impressive.
