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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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242 records · Page 14
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Hidden Local Symmetry Breaking in Silver Diamondoid Compounds is Root Cause of Ultralow Thermal Conductivity

Typically, conventional structure transitions occur from a low symmetry state to a higher symmetry state upon warming. In this work, an unexpected local symmetry breaking in the tetragonal diamondoid compound AgGaTe 2 is reported, which, upon warming, evolves continuously from an undistorted ground state to a locally distorted state while retaining average crystallographic symmetry. This is a rare phenomenon previously referred to as emphanisis. This distorted state, caused by the weak sd 3 orbital hybridization of tetrahedral Ag atoms, causes their displacement off the tetrahedron center and promotes a global distortion of the crystal structure resulting in strong acoustic–optical phonon scattering and an ultralow lattice thermal conductivity of 0.26 W m –1 K –1 at 850 K in AgGaTe 2 . Here, the findings explain the underlying reason for the unexpectedly low thermal conductivities of silver-based compounds compared to copper-based analogs and provide a guideline to suppressing heat transport in diamondoid and other materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

X-Ray and Electron Spectroscopy of the CdS/(Ag,Cu)(In,Ga)Se 2 Interface With RbF Treatment

The chemical and electronic structure of the CdS/(Ag,Cu)(In,Ga)Se 2 (CdS/ACIGSe) interface for thin-film solar cells, involving an absorber with a bulk [Ag]/([Ag]+[Cu]) (AAC) ratio of 0.06, a state-of-the-art RbF post-deposition treatment (PDT), and a chemical-bath deposited CdS buffer layer, is studied. To gain a detailed and depth-resolved picture of the CdS/ACIGSe interface, synchrotron- and laboratory-based hard X-ray, soft X-ray, and UV photoelectron spectroscopy, inverse photoemission spectroscopy, and X-ray emission spectroscopy are combined. Compared to the bulk of the absorber, a Cu- and Ga-poor ACIGSe surface is found, with a slightly increased AAC ratio. Strong evidence of a Rb–In–Se species (possibly with some Ag) at the absorber surface is compiled, with a corresponding band gap of 2.79 ± 0.12 eV. This finding is in clear contrast to comparable Ag-free Cu(In,Ga)Se 2 absorbers with RbF-PDT. The Rb–In–Se surface species is not removed by the (wet-chemical) CdS deposition process, while some Se diffuses into the CdS layer and segregates at its surface. The CdS buffer layer shows a band gap of 2.48 ± 0.12 eV, and a cliff (≈ -0.4 eV) is determined in the conduction band alignment at the interface between the Rb–In–Se species and the CdS buffer.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Potential-induced degradation of Cu(In,Ga)Se 2 can occur by shunting the front i-ZnO and by damaging the p-n junction

In this work we test field-relevant potential-induced degradation (PID) behavior by encapsulating laboratory Cu(In,Ga)Se 2 (CIGS) solar cells and applying +1000 V uniformly on the face of the front glass. In this configuration, we find that K-rich borosilicate glass reduces the extent of PID relative to Na-rich soda-lime glass. We also find that the standard testing protocol of stressing cells at short-circuit leads to faster PID than stressing cells at open-circuit. We characterize two types of CIGS PID: The first, front shunting PID, is driven by front-glass stress and occurs when alkali metal cations accumulate in the i-ZnO buffer, where they increase shunt conductance to reduce fill factor. The second, p-n junction PID, results from back-glass stress as alkali metal cations pile up near the CIGS surface/CdS buffer, where they reduce charge carrier concentration, open-circuit voltage, and fill factor to degrade efficiency ~160 times faster than front shunting PID.

14 SOLAR ENERGY↗
DOE OSTI2024

Colloidal AInSe 2 (A = K, Rb, Cs) Nanocrystals with Tunable Crystal and Band Structures

Wide band gap AInSe 2 (A = K, Rb, Cs) is an important interlayer material for improving the efficiency of Cu(In,Ga)(S,Se) 2 (CIGS) solar cells. Compared to high-vacuum deposition and solid-state synthesis, a less energyintensive method is of interest for its fabrication. Herein, we present the rapid, low-temperature colloidal synthesis of AInSe 2 nanocrystals that opens a pathway for convenient solution processing. The crystal structures and electronic band structures of the nanocrystals were studied, and their particle morphology was found to be dependent on the choice of alkali metal and selenium precursors. Homogeneous solid solution (K,Rb,Cs)InSe 2 nanocrystals were synthesized using a mixture of alkali metal precursors. Their compositions, lattice parameters, and band gaps were easily tuned based on the K:Rb:Cs precursor ratio, providing potential for interface engineering of CIGS nanocrystal-based solar cells.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

In situ Al 2 O 3 incorporation enhances the efficiency of CuIn(S,Se) 2 solar cells prepared from molecular-ink solutions

Here, we report an efficiency enhancement of solution-processed CuIn(S,Se) 2 (CISSe) thin film solar cells via in situ incorporation of Al 2 O 3 . These films were produced using inks containing CuCl, InCl 3 , AlNO 3 (Al/Al + In: 0.1) and thiourea dissolved in methanol. After spin coating of these solutions in air, samples were subjected to a selenization process. Auger electron spectroscopy depth-profiling analysis showed that Al is evenly distributed throughout the bulk of the film. Transmission electron microscopy revealed that AlNO 3 precursor reacted with oxygen to form nanosized amorphous Al 2 O 3 grains located within the bulk and grain boundaries of CISSe, as well as at both the top and bottom interfaces. Power conversion efficiency (PCE) as high as 11.6% (JS C : 35.8 mA cm –2 , V OC : 518 mV, FF: 62.2%, no anti-reflection coating) was achieved with Al–CISSe solar cell devices integrated with CdS (chemical bath deposition, thickness: 80 nm) and ZnO/ITO bilayers (sputtered, thickness: 300 nm). The average PCE (10.1%, $\langle$J SC $\rangle$: 34.5 mA cm –2 , $\langle$V OC $\rangle$: 491 mV, $\langle$FF$\rangle$: 59.8%) was nearly 4% (absolute) higher than that measured on CISSe baseline cells fabricated from solutions without Al ($\langle$PCE$\rangle$ = 6.4%, $\langle$J SC $\rangle$: 32.8 mA cm –2 , $\langle$V OC $\rangle$: 410 mV, $\langle$FF$\rangle$: 47.3%). This in situ Al 2 O 3 incorporation is speculated to play a role in the enhancement of the VOC and FF of the devices through passivation of defects in CISSe reducing interface and bulk recombination, as evidenced by a reduced defect density and an increased activation energy of the dominant recombination mechanism from capacitance and temperature-dependent VOC measurements, respectively.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Multi-junction solar cells paving the way for super high-efficiency

In order to realize a clean energy society by using renewable energies, high-performance solar cells are a very attractive proposition. The development of high-performance solar cells offers a promising pathway toward achieving high power per unit cost for many applications. As state-of-the-art of single-junction solar cells are approaching the Shockley–Queisser limit of 32%–33%, an important strategy to raise the efficiency of solar cells further is stacking solar cell materials with different bandgaps to absorb different colors of the solar spectrum. The III–V semiconductor materials provide a relatively convenient system for fabricating multi-junction solar cells providing semiconductor materials that effectively span the solar spectrum as demonstrated by world record efficiencies (39.2% under one-sun and 47.1% under concentration) for six-junction solar cells. This success has inspired attempts to achieve the same with other materials like perovskites for which lower manufacturing costs may be achieved. Recently, Si multi-junction solar cells such as III–V/Si, II–VI/Si, chalcopyrite/Si, and perovskite/Si have become popular and are getting closer to economic competitiveness. Here, we discuss the perspectives of multi-junction solar cells from the viewpoint of efficiency and low-cost potential based on scientific and technological arguments and possible market applications. In addition, this article provides a brief overview of recent developments with respect to III–V multi-junction solar cells, III–V/Si, II–VI/Si, perovskite/Si tandem solar cells, and some new ideas including so-called 3rd generation concepts.

14 SOLAR ENERGY↗