Quantum dot solar cells emerged in 2010 as the newest technology at the US Department of Energy's (DOE) National Renewable Energy Laboratory (NREL) with the initial lead sulfide quantum dot solar cells having an efficiency of 2.9 percent. The improvement from the initial efficiency to the previous record of 12 per cent last year came from better understanding of the connectivity between individual quantum dots, better overall device structures and reducing defects in quantum dots.
The new record comes from using a different quantum dot material, cesium lead triiodide (CsPbI3), and is within the recently emerging family of halide perovskite materials. In quantum dot form, CsPbI3 produces an exceptionally large voltage (about 1.2 volts) at open circuit.
"This voltage, coupled with the material's bandgap, makes them an ideal candidate for the top layer in a multijunction solar cell," said Joseph Luther, a senior scientist and project leader in the Chemical Materials and Nanoscience team at NREL. The top cell must be highly efficient but transparent at longer wavelengths to allow that portion of sunlight to reach lower layers. Tandem cells can deliver a higher efficiency than conventional silicon solar panels that dominate today's solar market.
The multijunction approach is often used for space applications where high efficiency is more critical than the cost to make a solar module. The quantum dot perovskite materials developed by Luther and the team at NREL and the University of Washington could be paired with cheap thin-film perovskite materials to achieve similar high efficiency but built at even lower costs than silicon technology--making them an ideal technology for both terrestrial and space applications.
"Often, the materials used in space and rooftop applications are totally different. It is exciting to see possible configurations that could be used for both situations," said Erin Sanehira, a doctoral student at the University of Washington who conducted research at NREL.