High-Efficiency Multijunction Solar Cells
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photovoltaic systems. So, many companies are now developing concentrator systems to enable the use of these cells on earth. This article describes the physics behind these cells, emphasizing the value of combining multiple high-quality materials. We discuss the material characteristics that imply high quality and strategies for making alloys with a range of bandgaps and assembling these into high-efficiency solar cells.
Multijunction Solar Cells
Theoretical Considerations
Frank Dimroth and Sarah Kurtz Abstract The efficiency of a solar cell can be increased by stacking multiple solar cells with a range of bandgap energies, resulting in a multijunction solar cell with a maximum theoretical efficiency limit of 86.8%. III–V compound semiconductors are good candidates for fabricating such multijunction solar cells for two reasons: they can be grown with excellent material quality; and their bandgaps span a wide spectral range, mostly with direct bandgaps, implying a high absorption coefficient. These factors are the reason for the success of this technology, which has achieved 39% efficiency, the highest solar-toelectric conversion efficiency of any photovoltaic device to date. This article explores the materials science of today’s high-efficiency multijunction cells and describes challenges associated with new materials developments and how they may lead to next-generation, multijunction solar cell concepts.
Introduction As described in the introductory article by Slaoui and Collins, the photovoltaic (PV) industry is growing rapidly. A key strategy for increasing the industry growth is to reduce the amount of highpurity semiconductor material needed to make a solar cell by either thinning the active layers (shrinking the cell vertically) or using optics to focus the light on small solar cells (shrinking the cell laterally) (see Figure 1). If the semiconductor cost can be reduced to a small fraction of the system cost, then increasing the efficiency of the solar cell will add value to the rest of the system without changing the cost appreciably. In this case, higher-efficiency solar cells provide a pathway to lower cost even if the solar cell itself is more expensive. This concept of high-concentration photovoltaics is being pursued by companies around the world.1 To date, the highest solar-to-electric conversion efficiency achieved for any PV device is 39% under ⬃240 suns concentration.2 One sun is defined as 1 kW/m2. The basis of this technology is a dual-junction GaInP/GaAs solar cell that was originally invented and developed at the National Renewable Energy Laboratory. Today, triple-junction Ga0.5In0.5P/Ga0.99In0.01As/ Ge cells with an additional Ge junction are in production for space applications at
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Spectrolab, EMCORE, and AZUR Space Solar Power. Its high power-to-mass ratio outweighs its higher cost and drives the success of this solar cell technology in space. Such cells power the Mars Exploration Rovers, Spirit and Opportunity, and are the product of choice for most of today’s advanced satellites. S
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