Tandem Solar Cells:
Traditional solar cells with a single band gap utilize less than half the energy in the solar spectrum. Energy is lost because low-energy photons are not absorbed, and high-photons are absorbed inefficiently. These deficiencies – intrinsic to traditional semiconductors – limit the maximum potential solar cell efficiency.
We’re developing foundational technologies to enable stacked — so-called “tandem” — solar cells. These solar cells aim to convert a greater percentage of incident sunlight into electricity, in some cases with marginal additional cost. In parallel, we’re performing bottom-up cost modeling to determine under what conditions tandem solar cells make economic sense. This effort is led by research scientist Dr. Marius Peters. We’re just getting started, so stay tuned for more exciting research updates!
 H. Liu et al., “The realistic energy yield potential of GaAs-on-Si tandem solar cells: a theoretical case study,” Optics Express 23, A382 (2015); http://dx.doi.org/10.1364/OE.23.00A382
 C. Bailie et al., “Semi-transparent perovskite solar cells for tandems with silicon and CIGS,” Energy & Environmental Science 8, 956 (2015); http://dx.doi.org/10.1039/C4EE03322A
Nature makes extensive use of solar power. In the process of photosynthesis, plants use solar power to split water, combining the products with CO2 to produce a sugar-like fuel that can be stored for use when the sun is not shining. This basic concept is appealing to scientists and engineers, as even the most efficient solar panel cannot provide power when the sun goes down.
Creating an “artificial leaf” that splits water at efficiencies and costs that enable widespread deployment requires two different technologies: a power source (to provide the energy required to split water) and a catalyst (a compound that acts as a sort of chemical lubricant, allowing the water-splitting reaction to proceed more easily). Ideally, we would like to mimic nature with an artificial leaf in which we dunk a solar cell in water, shine light on it, and produce a hydrogen-based fuel. However, the solutions that allow the water-splitting reaction to proceed efficiently tend to be highly corrosive to solar cell materials.
The PV Lab is working to solve this problem for silicon-based solar cells. Silicon solar cells are appealing because they are the most-widely used and best-understood solar technology. Using new catalysts developed by the Nocera group in the Chemistry department at Harvard and careful engineering of the silicon-catalyst interfaces, we have successfully demonstrated that silicon can be integrated into an efficient water-splitting process , modeling the system  and achieving 10% solar-to-fuel conversion efficiency with all Earth-abundant elements .
 J.J.H. Pijpers et al.; “Light-induced water oxidation at silicon electrodes functionalized with a cobalt oxygen-evolving catalyst,” Proceedings of the National Academy of Sciences 108 (2011); http://dx.doi.org/10.1073/pnas.1106545108
 M.T. Winkler et al., “Modeling integrated photovoltaic–electrochemical devices using steady-state equivalent circuits,” Proceedings of the National Academy of Sciences 110, E1076 (2013); http://dx.doi.org/10.1073/pnas.1301532110
 C.R. Cox et al., “Ten-percent solar-to-fuel conversion with nonprecious materials,” Proceedings of the National Academy of Sciences 111, 14057 (2014); http://dx.doi.org/10.1073/pnas.1414290111