Scientists at the U.S. Department of Energy's (DOE) National Renewable Energy Laboratory (NREL) recaptured the record for highest efficiency in solar hydrogen production via a photoelectrochemical (PEC) water-splitting process. NREL researchers Myles Steiner (left), John Turner, Todd Deutsch and James Young stand in front of an atmospheric pressure MDCVD reactor used to grow crystalline semiconductor structures. Courtesy of Dennis Schroeder. The new solar-to-hydrogen efficiency record is 16.2 percent, topping a reported 14 percent efficiency in 2015 by an international team made up of researchers from Helmholtz-Zentrum Berlin, TU Ilmenau, Fraunhofer ISE and the California Institute of Technology. A paper in Nature Energy titled "Direct solar-to-hydrogen conversion via inverted metamorphic multijunction semiconductor architectures" outlines how NREL's new record was achieved. The authors are James Young, Myles Steiner, Ryan France, John Turner and Todd Deutsch, all from NREL, and Henning Döscher of Philipps-Universität Marburg in Germany. Döscher has an affiliation with NREL. The record-setting PEC cell represents a significant change from the concept device Turner developed at NREL in the 1990s. Both the old and new PEC processes employ stacks of light-absorbing tandem semiconductors that are immersed in an acid/water solution where the water-splitting reaction occurs to form hydrogen and oxygen gases. But unlike the original device made of gallium indium phosphide grown on top of gallium arsenide, the new PEC cell is grown upside-down, from top to bottom, resulting in a so-called inverted metamorphic multijunction device. This advancement allowed the NREL researchers to substitute indium gallium arsenide for the conventional GaAs layers, improving the device efficiency considerably. A second key distinguishing feature of the new advancement was depositing a very thin aluminum indium phosphide "window layer" on top of the device, followed by a second thin layer of gallium indium phosphide. These extra layers served both to eliminate defects at the surface that otherwise reduce efficiency and to partially protect the critical underlying layers from the corrosive electrolyte solution that degrades the semiconductor material and limits the lifespan of the PEC cell. Before the PEC technology can be commercially viable, the cost of hydrogen production needs to come down to meet DOE's target of NREL is the DOE’s primary national laboratory for renewable energy and energy efficiency research and development. NREL is operated for the Energy Department by the Alliance for Sustainable Energy LLC.