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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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147 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

High-efficiency silicon heterojunction solar cells: Status and perspectives

Silicon heterojunction technology (HJT) uses silicon thin-film deposition techniques to fabricate photovoltaic devices from mono-crystalline silicon wafers (c-Si). This enables energy-conversion efficiencies above 21 %, also at industrial-production level. In this presentation we review the present status of this technology and point out recent trends. We first discuss how the properties of thin hydrogenated amorphous silicon (a-Si:H) films can be exploited to fabricate passivating contacts, which is the key to high- efficiency HJT solar cells. Such contacts enable very high operating voltages, approaching the theoretical limits, and yield small temperature coefficients. With this approach, an increasing number of groups are reporting devices with conversion efficiencies well over 20 % on n-type wafers, Panasonic leading the field with 24.7 %. Exciting results have also been obtained on p-type wafers. Despite these high voltages, important efficiency gains can still be made in fill factor and optical design. This requires improved understanding of carrier transport across device interfaces and reduced parasitic absorption in HJT solar cells. For the latter, several strategies can be followed: Short- wavelength losses can be reduced by replacing the front a-Si:H films with wider-bandgap window layers, such as silicon alloys or even metal oxides. Long-wavelength losses are mitigated by introducing new high-mobility TCO’s such as hydrogenated indium oxide, and also by designing new rear reflectors. Optical shadow losses caused by the front metalisation grid are significantly reduced by replacing printed silver electrodes with fine-line plated copper contacts, leading also to possible cost advantages. The ultimate approach to minimize optical losses is the implementation of back-contacted architectures, which are completely devoid of grid shadow losses and parasitic absorption in the front layers can be minimized irrespective of electrical transport requirements. The validity of this approach was convincingly demonstrated by Panasonic, Japan in 2014, reporting on an interdigitated back-contacted HJT cell with an efficiency of 25.6%, setting the new single-junction c-Si record. Finally, given the virtually perfect surface passivation and excellent red response of HJT solar cells, we anticipate these devices will also become the preferred bottom cell in ultra-high efficiency c-Si-based tandem devices, exploiting better the solar spectrum. Such tandem cells have the potential to overcome the fundamental single-junction limit of silicon solar cells (29.4%). Combining HJT cells with perovskite solar cells as top cell appears to be particularly appealing.

14 SOLAR ENERGY↗
DOE OSTI2016

Evaluating the oxidative, photothermal and electrical stability of colloidal nanocrystal solids

IV-VI quantum dot (QD) solids are a novel class of granular electronic materials with great technological potential (e.g., in photodetectors, field-effect transistors (FETs), and solar cells), but their oxidative and thermal instability present a barrier to practical applications [1]. Poor stability is a fundamental issue facing many nanoscale materials due to high surface area and surface energy. Basic studies are needed to elucidate the most important mechanisms of degradation and develop robust countermeasures if QD materials are to become technologically important. This project determined the degradation mechanisms of IV-VI QD solids (primarily PbSe and PbS) and introduced new chemical strategies to drastically improve their performance, stability, and operating lifetimes [2-5]. Our approach was based on (1) detailed testing of QD thin film materials (principally FETs and solar cells) as a function of oxidative and thermal stress, and (2) the use of organic and inorganic approaches to link the QDs into strongly electronically coupled, high-mobility films, prevent their oxidation, and eliminate internal degrees of freedom that lead to film instability and degradation in response to electrical and thermal stress. Stability against oxidation and thermal degradation was the major focus of this project. We have evaluated the stability of QD thin films and interfaces at temperatures less than 100°C (the regime most relevant to solar and transistor applications). Low-temperature oxidation and sintering of QD films have been investigated using optical absorption spectroscopy, Fourier transform infrared spectroscopy (FTIR), current-voltage scanning of transistors and solar cells, X-ray photoelectron spectroscopy, and scanning Kelvin probe microscopy (SKPM). SKPM was used to map the potential profiles of operating QD FETs and solar cells as a function of bias and illumination, which provides detailed information on how the work functions, potential drops and electric field within these devices determine device operation, and set the stage for future studies targeted at understanding and preventing device failure. We pursued two strategies to fabricate QD films with stable electrical characteristics: (1) the use of robust molecular surface ligands [2], and (2) “matrix engineering,” i.e., infilling the QD solid with metal oxide or metal sulfide matrices by low-temperature atomic layer deposition (ALD) to passivate surface states, prevent oxidation, lock the QDs into position, inhibit diffusion, and tune the height and width of the inter-QD potential barriers that govern charge transport [5,6]. Poor stability is a common feature of nanoscale electronic materials, yet stability is all too rarely the focus of basic research. Fundamental studies are therefore needed to elucidate the most important mechanisms of degradation and develop simple yet effective countermeasures. By revealing both how QD solids degrade in response to environmental stresses (oxidative, photothermal, and electric) and how to prevent this degradation, the project has greatly improved our ability to develop stable, high-performance QD materials for real-world applications.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Controlled Current Distribution in Anti-Hall bar Geometries

Quantum Hall measurements have been performed on high-mobility GaAs/AlGaAs (p-type) and (n-type) quantum wells using a Hall/Anti-Hall bar configuration having both inner and outer edges. The potential distribution and the current flow in the bulk can be controlled by the external magnetic field or the driving current. Extreme situations occur at the Quantum Hall states where the current, driven by leads connected to the outer edge, flows exclusively in one half of the sample. In these states, the chemical potential of the inner edge aligns itself with the edge at ground potential. Accumulation and depletion of carriers takes place at the edge whose carriers flow away from the current source.

Tarquini, Vinicio↗