Everyone loves a good sale as it lets you get more for your money. 'Buy one, get one free,' sales are perhaps the most intriguing of sales to consumers, as the same cost gets you double what you want. Researchers at the University of California, Davis have recently found a way to get just such a 'sale' within solar cells by having one photon create two electron-hole pairs, instead of the usual one.
When a photon strikes a silicon solar cell, depending on its frequency, it may cause an electron to be ejected from its atom and enter the conducting band, leaving a positively charged hole behind. By collecting these electrons one can create an electrical current, but a conventional silicon solar cell can achieve no better than 33% efficiency. What the researchers have discovered is a means to use a special form of silicon, called silicon BC8, to eject two electrons instead of one. This double ejection is due to an effect called 'quantum confinement,' which will typically only happen with ultra violet light. The researchers simulated how BC8 behaves though and found that nanoparticles of it should have the double ejection effect when visible hits it.
Potentially a solar cell that utilizes BC8 would have a maximum efficiency of 42%, but by concentrating the light that could shoot up to 70%. Of course a means of producing silicon BC8 will be needed first, but recent research from Harvard and MIT suggest this efficiently done using laser light.
Source: University of California, Davis