It’s not the absorbing of photons that matters most, it’s the emitting of them. That counterintuitive bit of wisdom about boosting photovoltaic cell efficiency is at the heart of work at Lawrence Berkeley National Laboratory (Berkeley Lab) that has led to record-breaking solar cell performances.
According to the lab, the most efficient solar cells on the market today are made from monocrystalline silicon wafers that operate at about 23 percent efficiency – that is, they convert about 23 percent of the energy hitting the cell into electricty. But based on work by the Berkeley researchers, the private company Alta Devices – founded by lead researcher Eli Yablonovitch – was able “to fabricate solar cells from gallium arsenide that have achieved a record conversion efficiency of 28.4 percent,” the lab said.
Gallium arsenide is considered a great material for making solar cells because, according to Yablonovitch, it “absorbs photons 10,000 times more strongly than silicon for a given thickness but is not 10,000 times more expensive.” But what the Berkeley researchers focused on was the next part of the process: Sunlight absorbed in a solar cell produces electrons that need to be pulled from the cell as electricity – and if they aren’t pulled quickly enough, the power dissipates, sometimes as heat, further degrading power output.
Calculations by co-researcher Owen Miller showed that if this released energy exits the cell as external fluorescence, it would boost the cell’s output voltage – bringing efficiency closer to the accepted efficiency limit (known as the Shockley-Queisser, or SQ , Limit) of 33.5-percent for a single p-n junction solar cell.
This theoretical knowledge was put to work by Alta Devices, which used a single-crystal thin-film technology developed earlier by Yablonovitch, called “epitaxial liftoff,” that squeezes more energy out of the cell. According the Berkeley Lab, the cells “not only smashed previous solar conversion efficiency records, but can be produced at well below the cost of any other solar cell technology.” And it won’t be long before these cells are on the market: Alta Devices expects to have gallium arsenide solar panels on the market within a year, the lab said.