The cost of solar power is beginning to reach price parity with cheaper fossil fuel-based electricity in many parts of the world, yet the clean energy source still accounts for slightly more than 1% of the world’s electricity mix.
To boost global solar power generation, researchers must overcome some of the technological limitations that are preventing solar power from scaling up even further, which includes the inability to develop very high-efficiency solar cells – solar cells capable of converting a significant amount of sunlight into usable electrical energy – at very low costs.
A team of researchers from the Masdar Institute and the Massachusetts Institute of Technology (MIT) may have found a way around the seemingly inseparable high-efficiency and high-cost linkage through an innovative multi-junction solar cell that leverages a unique “step-cell” design approach and low cost silicon. The new step-cell combines two different layers of sunlight-absorbing material to harvest a broader range of the sun’s energy while using a novel, low-cost manufacturing process.
The team’s step-cell concept can reach theoretical efficiencies above 40% and estimated practical efficiencies of 35%
In order to increase the global share of solar power, solar photovoltaics (PV) need to move away from traditional silicon crystalline solar cells, which have been touted as the industry’s gold standard in terms of efficiency for over a decade. In fact, some estimates report that over 90% of global solar PV installations are single-junction, crystalline silicon solar cells.
This is because silicon-based solar cells are relatively cheap to manufacture, but the problem is that they are not very efficient at converting sunlight into electricity. On average, solar panels made from silicon-based solar cells convert between 15% and 20% of the sun’s energy into usable electricity.
Silicon’s low sunlight-to-electrical energy efficiency is partially due to its bandgap; the bandgap prevents the semiconductor from efficiently converting higher energy photons, such as those emitted by blue, green and yellow light waves, into electrical energy. Instead, only the lower energy photons, such as those emitted by the longer red light waves, are efficiently converted into electricity.
To harness more of the sun’s higher energy photons, scientists have explored different semiconductor materials, such as gallium arsenide and gallium phosphide. While these semiconductors have reached higher efficiencies than silicon, the highest efficiency solar cells have been made by layering different semiconductor materials on top of each other and fine-tuning them to absorb a different slice of the electromagnetic spectrum.
These layered cells, known as multi-junction solar cells, can reach theoretical efficiencies upwards of 50%, but their very high manufacturing costs are preventing them from entering the mainstream solar cell market, relegating their use to niche applications, like satellites and other specialized applications where high costs are less important than low weight and high efficiency.
The Masdar Institute-MIT step-cell, which can be manufactured at a fraction of the cost of traditional multi-junction solar cells, may be the critical solution needed to boost commercial applications of high-efficiency, multi-junction solar cells at the industrial level.
The innovative “step-cell” is made by layering gallium arsenide phosphide-based solar cells, a semiconductor material that absorbs and efficiently converts higher energy photons, on a low cost silicon solar cell, creating a tandem solar cell that could ultimately achieve a practical power efficiency of approximately 35%.
The step-cell creates a literal “step” between the top gallium arsenide phosphide layer and the bottom silicon layer. The silicon layer is exposed, appearing like a bottom step. This intentional step design allows the top gallium arsenide phosphide layer to absorb the high energy photons (from blue, green and yellow light) leaving the bottom silicon layer free to absorb not only lower energy photons (from red light) transmitted through top layers, but also the entire visible light spectrum.
The unique design ensures that the silicon cell below can receive more photons in the exposed “step” part, increasing the solar cell’s efficiency. This “step” can be used as a new design optimization parameter, with the added benefits of low-cost manufacturing process.
