A proof-of principle photoelectrochemical cell capable of capturing excess photon energy that is normally lost to generating heat could produce solar fuels. A lead sulfide quantum dot solar cell developed by researchers at NREL. Photo by Dennis Schroeder. Scientists at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL) used quantum dots (QDs) and Multiple Exciton Generation (MEG) to push the peak external quantum efficiency for hydrogen generation to 114 percent. This could significantly boost the production of hydrogen from sunlight by using the cell to split water at a higher efficiency and lower cost than current photoelectrochemical approaches. "The major difference here is that we captured that MEG enhancement in a chemical bond rather than just in the electrical current," said researcher Matthew Beard. "We demonstrated that the same process that produces extra current in a solar cell can also be applied to produce extra chemical reactions or stored energy in chemical bonds." The maximum theoretical efficiency of a solar cell is limited by how much photon energy can be converted into usable electrical energy, with photon energy in excess of the semiconductor absorption band edge lost to heat. The MEG process takes advantage of the additional photon energy to generate more electrons increasing chemical and electrical potential without generating heat. QDs, which are spherical semiconductor nanocrystals (2 to 10 nm in diameter), enhance the MEG process. Multiple electrons, or charge carriers generated through the MEG process within the QDs are captured and stored within the chemical bonds of an H2 molecule. NREL researchers devised a cell based upon a lead sulfide (PbS) QD photoanode. The photoanode involves a layer of PbS QDs deposited on top of a titanium dioxide/fluorine-doped tin oxide dielectric stack. The chemical reaction driven by the extra electrons demonstrated a new direction in exploring high-efficiency approaches for solar fuels. Details of the research are outlined in the journal Nature Energy (doi:10.1038/nenergy.2017.52).