| Solar cells could one day see a boost in their theoretical maximum efficiency from 31 per cent to 66 per cent, thanks to a novel way of harnessing electrons whose energy is normally lost as heat. The most efficient silicon solar cells turn 25 per cent of the incoming light into electricity, but even with further improvements these cells will reach a theoretical limit at 31 per cent, because incoming light creates large numbers of extremely energetic electrons. These "hot" electrons lose their energy in less than a picosecond – too rapidly to be harnessed. In 2001, Arthur Nozik of the National Renewable Energy Laboratory (NREL) in Golden, Colorado, suggested that quantum dots – nanoscale chunks of semiconducting material – could help slow the rate at which hot electrons lose energy as heat. That's because the energy levels within quantum dots are widely spaced, making it difficult for electrons to jump between them. The energy levels are more closely packed in larger chunks of semiconductor such as the silicon wafers often used in solar cells, so jumping levels and losing energy as heat is easier. Quantum dots Now, Xiaoyang Zhu at the University of Texas in Austin and his colleagues have shown that those longer-lived hot electrons can pass from quantum dots to a semiconducting wafer before the electrons give up their energy as heat – a step towards a solar cell that can harness hot electrons for their energy. To do this, the researchers coated a wafer of semiconducting titanium dioxide with quantum dots of lead selenide and shone light on it. A change in the optical properties of the wafer showed that electrons had entered it from the quantum dots. But the materials were engineered such that the electrons could enter the substrate only when hot. "So any transfer that we observe is conclusively hot electron transfer," says team member Eray Aydil of the University of Minnesota, Minneapolis. Cheap and efficient The researchers caution that this is just a first step in a complicated process to convert the hot electrons into electric current. However, the theoretical maximum efficiency of a solar cell that can harness hot electrons jumps to about 66 per cent. "Of course, we'll never get that high. There are a lot of engineering issues where we will lose energy," says Zhu. "But the sheer fact that it's more than twice the theoretical limit of conventional solar cells does make it attractive." Matthew Beard of NREL, a member of Nozik's team, says the work represents "one more step towards the goal of radically increasing solar energy conversion efficiencies", – while echoing the team's caution that much is still to be done. If it works out, the process of manufacturing these new solar cells could be significantly cheaper than existing methods for boosting solar cell efficiency. The most efficient solar cells today are made by stacking layers of different semiconductors, each processing different wavelengths of light, leading to efficiencies of about 40 per cent. Manufacturing such stacks is extremely expensive, so they are used mainly in space and military applications. Coating a semiconductor with quantum dots, however, is much cheaper. "The fabrication part is extremely easy," says Zhu "That's the beauty of quantum dots."
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