Session A60: Recent Advances in Materials and Devices for Energy Generation and Storage

Stanford R. Ovshinsky Sustainable Energy Fellowship Talk: Photonic structures for solar concentrators and photovoltaic modules

The solar spectrum is a broad and diffuse light source, but solar panels operate most efficiently at wavelengths near the semiconductor bandgap and over a limited range of incident angles. This talk will discuss the use of spectrally selective, photonic surfaces to harvest sunlight for photovoltaics. The first part will discuss the implementation of selective mirrors into luminescent solar concentrators (LSCs), which shift the inefficiently converted, high energy sunlight into a spectrally narrow, focused light source at wavelengths that are more efficiently utilized by the solar cell. The second part will discuss the integration of photonic mirrors into photovoltaic modules to reflect the infrared portion of the spectrum that contributes to solar cell heating.

This talk will discuss luminescent concentrators made from both Si nanocrystals and from CdSe/CdS core/shell nanocrystals. We will first discuss the fabrication and characterization of nanocrystal – polymer composites with low light scattering, and then the design of spectrally-selective mirrors on the top surface that transmit blue light and trap the luminescent light inside the concentrator. We will show how the design of the spectrally-selective mirror changes for different luminophore properties, as well as the design of back mirrors based on gradient metasurfaces to steer light toward the edges.

The second part of the talk will discuss spectrally-selective structures used for light management in photovoltaic modules. These structures are designed to enhance transmission of sunlight above the bandgap of Si to improve photocurrent, and enhance reflection of light below the bandgap to reduce module operating temperature. We will show how the structures change at different interfaces within the module, the importance of designing mirrors that operate under diffuse illumination, and the effect of geographical location on mirror performance.   

Ultralow Thermal Conductivity Materials for Thermoelectric Device Applications

High thermoelectric performance, or ZT, necessarily depends on achieving the contradictory combination of low thermal conductivity κ and high electrical conductivity (along with a large Seebeck coefficient). However, for ultralow κ materials (defined here as κ < 1 W/m-K at 300 K) the electrical conductivity requirement is proportionally reduced. There may therefore exist a new opportunity for such materials in the thermoelectric arena. Examples of such materials are Tl₃VSe₄ [1], AgBiSe₂ [2], and SnSe [3].

However, there are several scientific issues that must resolved prior to usage of any of these materials in an actual application. Ultralow κ materials tend to occur near to a ferroelectric transition or other structural instability, due primarily to the large anharmonicity near such an instability. This usually renders materials such as PbTe [4,5,6] unsuitable for device applications due to unfavorable mechanical properties. Is proximity to a structural instability a necessary condition for ultralow κ? Similarly, another ultralow κ route to high ZT is the use of mobile cations, such as Cu in Cu₂Se [7], which presents another hurdle for device applications. Can one create a robust thermoelectric device using a material with a structural instability, or mobile structural disorder, in the operating temperature range? The author will discuss these issues.

References

1. S. Mukhopadhyay, D. S. Parker, B. C. Sales, A. A. Puretzky, M. A. McGuire, and L. Lindsay. Science 360, 1455 (2018).
2. D.S. Parker, A. F. May, and D. J. Singh. Phys. Rev. Appl. 3, 064003 (2015).
3. L.-D. Zhao et al, Nature 508, 373 (2014).
4. D.S. Parker, X. Chen, and D. J. Singh. Phys. Rev. Lett. 110, 146601 (2013).
5. J.P. Heremans et al, Science 321, 554 (2008).
6. O. Delaire et al, Nature Materials 10, 614 (2011).
7. H. Liu et al, Nature Materials 11, 422 (2012).   

Recent Advances towards the Development of an Aqueous Direct Solar Battery

Energy supply by fossil fuels only has a limited lifetime and is responsible for an increasing environmental pollution, influencing also climate change. A rapid transition to more sustainable solutions is thus also in the focus of modern research. Renewable fuels are meant to circumvent supply fluctuations arising from energy sources like PV and wind, at the cost of infrastructural adaptations. Direct renewable energy storage is thus highly desirable, but expansive and complicated.
Herein, we report an earth-abundant polymeric material, which for the first time enables the synergistic coupling of two key functions of energy conversion within one single material: visible light harvesting and electrochemical energy storage. The “solar battery” anode material, a 2D cyanamide-functionalized polyheptazine imide (NCN-PHI), is the first of its kind being capable of storing large amounts of photogenerated electrons, operating even in aqueous conditions. Charge compensation and stabilization is realized by pseudocapacitive effects enabled by various aqueous alkali metal ions. Energetically, this occurs at energies well above the reversible hydrogen electrode, allowing for stable aqueous batteries with increased cell voltages.
Limitations arise from low conductivity and hole shuttling to a suitable cathode material. We herein present approaches that address these issues and present our work on hybrid material solutions for full aqueous solar batteries.
As such, we aim to provide a sustainable and cheap, earth abundant solution to overcome the intermittency of solar irradiation and other renewable electrical energy sources at a time.
Further information can be found in: “Toward an Aqueous Solar Battery: Direct Electrochemical Storage of Solar Energy in Carbon Nitrides”, F. Podjaski et al., Adv. Mater. 2018, 1705477.   

Power generation from direct conversion of infrared radiation from a thermal source.

In this talk, we will describe a new photo-thermoelectric conversion device that consists of a large-area nanoantenna-coupled metal-oxide-silicon (MOS) tunnel diode that directly converts infrared radiation into electrical power. This infrared photonic device needs only view a moderate temperature thermal source to generate a direct photocurrent. A resonant photonic frequency selective surface and a polar oxide material with longitudinal optical phonon resonance are arranged to spectrally overlap and enhance transverse field confinement in an extreme sub-wavelength MOS tunnel barrier. The enhanced field confinement is shown to drive photon-assisted tunneling resulting in large infrared photocurrent in the room temperature device. The measurement of photon-assisted tunneling photocurrent across a matched load impedance is shown to arise due to direct rectification of infrared radiation and a new radiative micropower supply will be described.   

Advanced Materials for Energy Storage

Batteries are central to fundamental changes in transportation through electric vehicles, in the electricity grid through renewable energy integration, replacement of natural gas peaker plants and customized electricity service, and in electric flight through drones, air taxis, package delivery and short-haul passenger service. Nest generation batteries to meet these diverse applications require new high-performance materials for anodes, cathodes, electrolytes and interfaces. Using new approaches for simulation and characterization at the atomic and molecular level, these materials can now be designed from the bottom up, atom-by-atom and molecule-by-molecule, where each atom or molecule plays a prescribed role in producing targeted overall materials behavior. Examples will be given for advanced materials for several beyond-lithium-ion batteries.