Session D27: Invited Session: Materials for Energy Applications

The DOE SunShot Initiative: Science and Technology to enable Solar Electricity at Grid Parity

The SunShot Initiative's mission is to develop solar energy technologies through a collaborative national push to make solar Photovoltaic (PV) and Concentrated Solar Power (CSP) energy technologies cost-competitive with fossil fuel based energy by reducing the cost of solar energy systems by $\sim $ 75 percent before 2020. Reducing the total installed cost for utility-scale solar electricity to roughly 6 cents per kilowatt hour (1{\$}/Watt) without subsidies will result in rapid, large-scale adoption of solar electricity across the United States and the world. Achieving this goal will require significant reductions and technological innovations in all PV system components, namely modules, power electronics, and balance of systems (BOS), which includes all other components and costs required for a fully installed system including permitting and inspection costs. This investment will re-establish American technological and market leadership, improve the nation's energy security, strengthen U.S. economic competitiveness and catalyze domestic economic growth in the global clean energy race. SunShot is a cooperative program across DOE, involving the Office of Science, the Office of Energy Efficiency and Renewable Energy and ARPA-E.   

Integrating the multifunction necessary for electrochemical energy storage into energy- and size-scalable ultraporous nanoarchitectures

Designing high performance energy-storage devices that combine nanometric feature size with well-wired transport paths requires an architectural perspective. We chose carbon aerogel-like nanofoam papers as attractive plug-and-play electrode substrates because of such desirable properties as high specific surface area, electronic conductivity, and through-connected pore structure. Achieving this blend of desirable properties requires an optimal balance of critical architectural features: (1) open, 3D interconnected macropores sized at 100 to 300 nm (a difficult-to-obtain size range in porous carbons) and (2) pore walls of a size that reduce dead weight and volume (preferably ca. 20-nm wall thickness for 100- to 300-nm voids), yet retain mechanical strength and flexibility without compromising electronic conductivity (preferably ca. 20 S/cm). Carbon nanofoam papers provide a low cost and scalable nanocomposite that exists within this ``Goldilocks zone'' of desirable properties and which has catalyzed breakthroughs in our work with asymmetric electrochemical capacitors, air cathodes for metal/air batteries, lithium-ion batteries, 3D batteries, and semifuel cells. New charge-storage or catalytic functionality is imparted to internal carbon walls simply by transporting reactants within the 3D macroporous. Self-limiting modification strategies allow us to incorporate conformal, nanoscopic ``paints'' of metal (Mn, Ti, Ru, Fe) or polymer (redox-active or electron insulating) or to specifically adsorb metal nanoparticles (Pt, Au, Pd, Ag) throughout the macroscopic thickness (0.07 to 0.3 mm) of carbon nanofoam papers as dictated by the requirements of a specific end application. For instance, modification with 10-nm MnO$x$ increases the mass-, geometric-, and volume-normalized capacitance (2- to 10-fold) relative to the native carbon nanofoam without significantly altering its high-rate character and provides a structure that can be used in an asymmetric electrochemical capacitor or used to an air cathode in a Zn/air cell to electrocatalyze oxygen reduction and provide pulse power. Our redesigned carbon nanofoam offers a versatile design platform for much-needed advances in a broad range of multifunctional energy storage and conversion.   

Bio-inspired Approaches to Solar Energy Conversion

Natural photosynthesis is carried out by organized assemblies of photofunctional tetrapyrrole chromophores and catalysts within proteins that provide specifically tailored environments to optimize solar energy conversion. Artificial photosynthetic systems for practical solar fuels production must collect light energy, separate charge, and transport charge to catalytic sites where multi-electron redox processes will occur. The primary goal of our research in this field is to understand the fundamental principles needed to develop integrated artificial photosynthetic systems. These principles include how to promote and control: 1) energy capture, charge separation, and long-range directional energy and charge transport, 2) coupling of separated charges to multi-electron catalysts for fuel formation, and 3) supramolecular self-assembly for scalable, low-cost processing from the nanoscale to the macroscale. The central scientific challenge is to develop small, functional building blocks, having a minimum number of covalent linkages, which also have the appropriate molecular recognition properties to facilitate self-assembly of complete, \textit{functional} artificial photosynthetic systems. This lecture will describe our use of ultrafast optical spectroscopy and time-resolved EPR spectroscopy to understand charge transport in self-assembled structures for artificial photosynthesis.   

Quantum-well and quantum-dot structures for high-efficiency photovoltaics

Quantum-well and quantum-dot semiconductor heterostructures offer a variety of opportunities for achieving photovoltaic power conversion efficiencies in excess of the Shockley-Queisser limit for single-homojunction solar cells. However, realization of such efficiencies is likely to require a combination of very high quality epitaxial growth or nanostructure synthesis to minimize carrier trapping and recombination, detailed understanding and analysis of optical absorption and nonequilibrium carrier transport processes, and light trapping to enable efficient optical absorption in very thin device layers. We discuss work in which GaAs/InGaAs/InAs semiconductor quantum-well and quantum-dot solar cells are realized in designs that enable efficient collection of photogenerated carriers from quantum-wells and dots, and combined with subwavelength-scale metal and dielectric structures that enable incident photons to be scattered into guided optical modes within a thin-film device, thereby enabling increased absorption efficiency in very thin device layers. Several aspects of this work will be addressed. Measurement of electric-field-dependent photocurrent response enables design of structures in which photogenerated carriers are collected efficiently from quantum-well or quantum-dot structures in the intrinsic region of a p-i-n junction solar cell. Processing to remove epitaxially grown device layers from their original growth substrate enables metal and dielectric nanostructures to be designed and integrated with the semiconductor epitaxial layer structures to scatter incident photons into strongly guided optical modes within the semiconductor. Finally, detailed analysis of quantum-well and quantum-dot optical absorption as well as optical mode structure within the device enables optimization of the absorption and mode profiles to achieve maximum power conversion efficiency. Both computational and experimental results derived from these approaches will be described.   

Combined conversion of heat and light with Photon Enhanced Thermionic Emission for solar energy harvesting

Recently a new mechanism for solar energy harvesting based on photon-enhanced thermionic emission (PETE) was proposed. This two-step process uses photons to excite carriers into the conduction band, followed by thermionic emission from the conduction band, and electron collection at a low-workfunction anode. This process effectively combines both heat and light, and its efficiency was calculated to exceed ideal single junction photovoltaics since it harvests some of the heat that is normally lost within PV devices. Experimental measurements demonstrated this mechanism in GaN materials, yet the quantum efficiency was very low. Here we discuss the loss mechanisms in the PETE process, and several approaches to overcome them. In particular we focus on surface recombination effects and absorption losses, and demonstrate a heterostructured device that increases the quantum efficiency by two orders of magnitude. Prospects for combined-cycle devices incorporating a PETE converter as a topping cycle on conventional thermal cycles are also analyzed.