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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Robust passivation of CdSeTe based solar cells using reactively sputtered magnesium zinc oxide

Magnesium zinc oxide (MZO, Mg x Zn 1-x O) is a leading emitter for CdTe-based solar cells due to its transparency and the ability to tune its conduction band offset with the absorber. Devices employing alloyed cadmium selenide telluride (CST, CdSe y Te 1-y ) absorbers achieved high efficiency (>19%) using MZO deposited by reactive sputtering over a broad composition range (3.68–3.92 eV, x: 0.20–0.35). Minimal differences in implied and measured open circuit voltage indicate that the contacts are well passivated and highly selective across the spectrum of MZO employed. Device performance insensitivity to MZO composition, which is not observed in CdTe devices, is attributed to the formation of an oxygenated interface layer. Se volatility creates a group VI deficiency at the interface that drives O migration from the MZO into the absorber. This introduces conductivity in the emitter not present in its as-deposited state, contributing to the exceptional performance observed. It is shown that the quality of device passivation depends on the oxidation state of the as-deposited MZO such that intelligent control and management of the reactive sputtering process is required.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Stable magnesium zinc oxide by reactive Co-Sputtering for CdTe-based solar cells

Magnesium zinc oxide (MZO) is a promising front contact material for CdTe solar cells. Due to its higher band gap than traditional CdS, MZO can reduce parasitic absorption to significantly increase short-circuit current density while also providing a benefit of conduction band offset tuning through Mg:Zn ratio optimization. MZO has been successfully implemented into CdTe devices, however its stability has been of concern. The MZO stability issue has been attributed to the presence of oxygen in the CdTe device processing ambient, leading to double-diode behavior (S-kink) in the current density-voltage curves. Here we report on MZO thin films deposited by reactive co-sputtering. The reactively co-sputtered MZO thin films have encouraging stability, show no significant variation in work function of the surface over a period of 6 months, as measured by Kelvin probe. Energy conversion efficiencies of around 16% have been achieved both with and without presence of oxygen in device processing ambients across multiple research facilities. Finally, these efficiencies should be possible to increase further by tuning of the thin film deposition and device processing parameters, especially through optimization of the back contact.

14 SOLAR ENERGY↗
DOE OSTI2021

Electro-Optical Stability in Gallium Magnesium Zinc Oxide Layers for CdTe Solar Cells

In CdTe solar cells, the emitter electron density, bandgap, and band alignment can be tuned by adjusting alloy composition and stoichiometry in gallium magnesium zinc oxide (GMZO) layers, thereby allowing for critical front interface engineering. For example, computational modeling indicates efficiency can change from 7% to 25% by adjusting these buffer properties even with fixed CdSeTe and CdTe material properties. However, the GMZO as-deposited electro-optical properties can shift during subsequent CdTe deposition, CdCl 2 annealing, and solar cell operation. Here, we examine these changes by sputtering GMZO films on glass, and measuring the electron density before and after post-deposition treatments over a range of temperatures and different ambients. We then compare GMZO to SnO 2 and MZO. The addition of Ga to MgZnO is found to have profound effects on both the electron density and electro-optical stability that should be advantageous for CdTe solar cells.

41 EE - Solar Energy Technologies Office (EE-4S)↗
DOE OSTI2020

A Density Functional Theory (DFT) Study on the Effect of Chlorine in the Magnesium Zinc Oxide (MZO) Front Interface for Cadmium Telluride (CdTe) Thin Film Solar Cells

Cadmium telluride photovoltaics has benefitted greatly from the cadmium chloride treatment which causes a remarkable improvement in cell efficiencies. This work investigates the effect of elemental chlorine at the interface between the magnesium zinc oxide front contact and the cadmium telluride absorber in such a device. Band alignment including band bending and interface states and its role in potential charge carrier transport is the key feature studied. It is seen that a small amount of chlorine is leads to a more desirable band alignment. Higher concentrations of chlorine result in unfavorable band characteristics.

14 SOLAR ENERGY↗
DOE OSTI2025

Ferroelectricity in Atomic Layer Deposited Wurtzite Zinc Magnesium Oxide Zn 1– x Mg x O

Conformal deposition of wurtzite ferroelectrics, which is needed for their use in scaled nonvolatile memories, is challenging using current physical vapor deposition techniques. To overcome the conformality barrier, this work demonstrates ferroelectricity in wurtzite Zn 1− x Mg x O thin films prepared by plasma-enhanced atomic layer deposition, which is a non-line-of-sight deposition method. Films ranging in composition from x = 0.00 to x = 0.58 are predominantly wurtzite phase and exhibit a (0001)-texture. Increasing the magnesium content decreases the c/a ratio, increases the optical bandgap energy, increases the piezoelectric response, and enables polarization reversal. Clear polarization switching is demonstrated in 50 nm thick Zn 1− x Mg x O films by piezoresponse force microscopy in compositions containing x = 0.46 and x = 0.58.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Switching kinetics in ferroelectric zinc magnesium oxide thin films

The switching kinetics of RF-magnetron reactively sputtered ~200 nm thick Zn 1-x Mg x O (ZMO) ferroelectric thin films with x = 0.41 and x = 0.27 prepared on Pt/Ti/SiO 2 /Si substrates were studied at applied fields near the coercive fields, ranging from 3.4 to 5.1 MV cm -1 , and at temperatures ranging from room temperature to 100 °C. Polarization reversal in ZMO followss the Kolmogorov-Avrami-Ishibashi kinetics model for nucleation and growth at applied fields near the coercive field, and obeys an individual column switching (ICS) model at higher applied fields required to switch most of the spontaneous polarization. Switching in the high-applied field regime can be described using either the Gaussian or inverse gamma distribution functions depending on the mole fraction of magnesium in the film. The switching current transients of the high-Mg content ZMO film (x = 0.41) are always bi-modal, whereas the low Mg composition film (x = 0.27) is described by the Gaussian distribution initially following wake-up, and becomes bimodal with continued cycling. Rayleigh-like behavior of the dielectric constant revealed a simultaneous increase of the irreversible and decrease of the reversible contributions to the dielectric constant, which was ascribed to an increase in the density of mobile domain walls with cycling and resulted in faster switching.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2023

Wake up and retention in zinc magnesium oxide ferroelectric films

Zn 0.64 Mg 0.36 O (ZMO) is a newly discovered ferroelectric oxide with the wurtzite structure. Epitaxial Zn 0.64 Mg 0.36 O films from 0.036 to 0.5 μm in thickness are grown on Pt/sapphire with the crystallographic c-axis out of plane. At room temperature, the remanent polarization is ~80 μC/cm 2 and the coercive field is ~3 MV/cm. The coercive field is strongly temperature dependent up to 240 °C with a pseudo-activation energy of 23 ± 0.3 meV, suggesting that polarization reversal occurs through an extrinsic process such as domain wall motion. ZMO films can be woken up in 20 electric field cycles on driving near the coercive field; they wake up in a single loop at fields in excess of 4 MV/cm. A thermally activated fluid imprint process, with a pseudo-activation energy of 67 ± 8 meV, enlarges the coercive field by several hundred kV cm –1 after switching the polarization. Additionally, ZMO films exhibit excellent retention characteristics; no reduction in the polarization is observed up to 1000 h from room temperature to 200 °C bakes. Here, this current early generation of ZMO films can survive several thousand switching cycles before dielectric breakdown occurs.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Thickness scaling and ferroelectric switching in wurtzite structure zinc magnesium oxide thin films

This study reports the thickness scaling of sputtered ferroelectric Zn 0.61 Mg 0.39 O (ZMO) thin films down to 43 nm. Encapsulated IrO 2 /ZMO/Ir capacitors exhibited switchable polarizations exceeding 50 μC cm −2 and coercive fields that increased from 3.9 to 4.4 MV cm −1 as the thickness decreased. Switching kinetics are best described by the simultaneous non-linear nucleation and the growth model. Bimodal switching is prevalent at low thicknesses, with the fastest switching times measured to be approximately 400 ns. Device encapsulation made ZMO switching kinetics more abrupt, potentially due to changes in the concentration of atmosphere-induced defects such as hydroxides. These results demonstrate stable ferroelectricity and sub-microsecond switching in sub-50 nm wurtzite ZMO, highlighting its potential as a low-voltage ferroelectric for integrated nonvolatile memory applications.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Ferroelectrics everywhere: Ferroelectricity in magnesium substituted zinc oxide thin films

We demonstrate ferroelectricity in Mg-substituted ZnO thin films with the wurtzite structure. Zn 1–x Mg x O films are grown by dual-cathode reactive magnetron sputtering on (111)-Pt // (0001)-Al 2 O 3 substrates at temperatures ranging from 26 to 200 °C for compositions spanning from x = 0 to x = 0.37. X-ray diffraction indicates a decrease in the c-lattice parameter and an increase in the a-lattice parameter with increasing Mg content, resulting in a nearly constant c/a axial ratio of 1.595 over this composition range. Transmission electron microscopy studies show abrupt interfaces between Zn 1–x Mg x O films and the Pt electrode. When prepared at pO 2 = 0.025, film surfaces are populated by abnormally oriented grains as measured by atomic force microscopy for Mg concentrations >29%. Raising pO 2 to 0.25 eliminates the misoriented grains. Optical measurements show increasing bandgap values with increasing Mg content. When prepared on a 200 °C substrate, films display ferroelectric switching with remanent polarizations exceeding 100 μC cm –2 and coercive fields below 3 MVcm –1 when the Mg content is between ~30% and ~7%. Substrate temperature can be lowered to ambient conditions, and when doing so, capacitor stacks show only minor sacrifices to crystal orientation and nearly identical remanent polarization values; however, coercive fields drop below 2 MV/cm. Using ambient temperature deposition, we demonstrate ferroelectric capacitor stacks integrated directly with polymer substrate surfaces.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Combinatorial Study of MZO Emitters for CdTe-Based Solar Cells

Precise band alignment at the front interface of CdTe-based solar cells is necessary to reduce photoexcited charge carrier recombination and to enable higher energy conversion efficiencies. Magnesium zinc oxide is not only transparent but also offers the ability to tune the conduction band offset with CdTe at the front of the device stack. In this work, we describe reactive sputtering of Mg 1-x Zn x O using metal targets and its application in CdTe photovoltaics. Combinatorial libraries of Mg 1-x Zn x O were created to tune the conduction band alignment for the optimal performance of CdTe solar cells. Band gap variation of more than 0.4 eV is achieved across a 3-inch substrate. Kelvin probe measurements demonstrate variance of work function across the substrate. Device performance is optimized at band gaps of 3.5 eV for the CdTe device processing conditions employed in this study.

14 SOLAR ENERGY↗
DOE OSTI2022

MgZnO High‐Voltage Transparent Thin‐Film Transistors Built on Glass

Recently, there has been increasing interest in building‐integrated photovoltaic (BIPV), which enables harvesting solar energy effectively. Large‐area glass components such as windows are widely used in modern constructions. In addition to the PV on opaque components such as walls and roofs, the transparent photovoltaic (TPV) directly built on glass is complementary to fully utilize the PV energy in BIPV. A high‐voltage transparent thin‐film transistor (HVTTFT) built on glass is an ideal option for distributed microinverters for TPV modules. A wide‐bandgap oxide‐based HVTTFT on glass for this purpose is reported. The HVTTFT on glass uses ZnO‐based materials with different functions for two roles: a semiconductor Mg 0.01 Zn 0.99 O (MZO) as TFT channel and Al‐doped ZnO (AZO) as transparent conductive oxide (TCO) electrodes. The centrosymmetric circular structure of the MZO HVTTFT with a high‐k‐stacking gate dielectric enables a blocking voltage as high as ≈1 kV and an on/off ratio of 10 6 . The device exhibits an average optical transmittance of 81% over the visible spectrum.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2024

Strain Fluctuations Unlock Ferroelectricity in Wurtzites

Ferroelectrics are of practical interest for non-volatile data storage due to their reorientable, crystallographically defined polarization. Yet efforts to integrate conventional ferroelectrics into ultrathin memories have been frustrated by film-thickness limitations, which impede polarization reversal under low applied voltage. Wurtzite materials, including magnesium-substituted zinc oxide (Zn,Mg)O, have been shown to exhibit scalable ferroelectricity as thin films. In this work, the origins of ferroelectricity in (Zn,Mg)O are explained, showing that large strain fluctuations emerge locally in (Zn,Mg)O and can reduce local barriers to ferroelectric switching by more than 40%. Concurrent experimental and computational evidence of these effects are provided by demonstrating polarization switching in ZnO/(Zn,Mg)O/ZnO heterostructures featuring built-in interfacial strain gradients. These results open up an avenue to develop scalable ferroelectrics by controlling strain fluctuations atomistically.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Codesigning Alloy Compositions of CdSe y Te 1− y Absorbers and Mg x Zn 1− x O Contacts to Increase Solar Cell Efficiency

Thin‐film solar cells such as CdTe are a major commercial photovoltaic technology, with more than 25 GW installed worldwide and levelized costs of electricity competitive with fossil fuels. Further progress may result from integrating CdSe y Te 1− y absorbers with Mg x Zn 1− x O contacts, but the device efficiency is difficult to maximize due to coupled dependence on chemical composition of both alloys. Herein, a high‐throughput approach is demonstrated to codesign chemical compositions in alloyed Mg x Zn 1− x O/CdSe y Te 1− y thin‐film solar cells, using combinatorial libraries of PV devices with orthogonal composition gradients in CdSe y Te 1− y absorbers and Mg x Zn 1− x O contacts. It is found that the solar cell performance is a strong and coupled function of both elemental compositions, with efficiency up to 17.7% ( V OC = 836 mV, fill factor = 69%, J SC = 30.6 mA cm −2 ) at atomic compositions of Mg/(Mg + Zn) ≈18% and average Se/(Se + Te) ≈4%. These performance trends among >100 devices are explained by >100 ns lifetime of photoexcited charge carriers at the Mg x Zn 1− x O/CdSe y Te 1− y interface where strong Se accumulation is also observed. This study reports the optimal compositions of the commercially relevant Mg x Zn 1− x O/CdSe y Te 1− y solar cells and demonstrates a general approach to codesigning performance of alloyed thin‐film solar cells and other optoelectronic devices.

14 SOLAR ENERGY↗
DOE OSTI2021

Negative Capacitance MgZnO-Channel Thin-Film Transistor With Ferroelectric NiMgZnO in the Gate Stack

We demonstrate the negative capacitance thin-film transistor (NC-TFT) on glass substrate using the multifunctional MgZnO (MZO). The Mg 0.03 Zn 0.97 O semiconductor layer acts as the TFT channel for stable operation while the Ni-doped MZO, i.e. Ni 0.02 Mg 0.15 Zn 0.83 O (NMZO) serves as the ferroelectric layer in the NMZO/SiO 2 stacking gate dielectric structure. The subthreshold swing (SS) value is significantly reduced over the reference TFT without ferroelectric layer. The minimum SS value of the NC-TFT reaches 52 mV/dec while the on/off ratio of drain current ID reaches 10 9 .

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2022

Depth-resolved thermal conductivity and damage in swift heavy ion irradiated metal oxides

We investigated thermal transport in swift heavy ion (SHI) irradiated insulating single crystalline oxide materials: yttrium aluminum garnet- Y 3 Al 5 O 12 (YAG), sapphire (Al 2 O 3 ), zinc oxide (ZnO) and magnesium oxide (MgO) irradiated by 167 MeV Xe ions at 10 12 – 10 14 ions/cm 2 fluences. Depth profiling of the thermal transport on nano- and micro- meter scales was assessed by time-domain thermoreflectance (TDTR) and modulated thermoreflectance (MTR) methods, respectively. This combination allowed us to isolate the conductivities of different sub-surface damage-regions characterized by their distinct microstructure evolution regimes. Thermal conductivity degradation in SHI irradiated YAG and Al 2 O 3 is attributed to formation of ion tracks and subsequent amorphization, while in ZnO and MgO it is mostly due to point defects. Additionally, notably lower conductivity when probed by very low penetrating thermal waves is consistent with surface hillock formation. An analytical model based on Klemens-Callaway method for thermal conductivity coupled with a simplified microstructure evolution capturing saturation in defect concentration was used to obtain depth dependent damage across the ion impacted region. The studies showed that YAG has the highest damage profile resulting in the less dependence of thermal conductivity with the depth, while MgO on the contrary has the strongest dependence. The presented work sheds new light on how SHI induced defects affect thermal transport degradation and recovery of oxide ceramics as promising candidates for next generation nuclear reactor applications.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Article with buffer layer

An article, for example a solar cell, includes a first substrate having a first surface and a second surface. An underlayer is located over the second surface. A first conductive layer is located over the underlayer. An overlayer is located over the first conductive layer. A semiconductor layer is located over the conductive oxide layer. A second conductive layer is located over the semiconductor layer. The first conductive layer can include a conductive oxide and at least one dopant selected from the group consisting of tungsten, molybdenum, niobium, and/or fluorine. The overlayer can include a buffer layer having tin oxide and at least one of zinc, indium, gallium, and magnesium.

14 SOLAR ENERGY↗
DOE OSTI2022

Prussian blue derived catalysts

Prussian blue analog derived catalysts having a composition of highly porous transition metal (“TM”) oxides with nano particle size. Such OER catalysts are synthesized from the PBA, containing cobalt, iron, nickel, copper, manganese, zinc, magnesium etc., as secondary building units (“SBUs”) coordinated by hexacyano-based ligands. Furthermore, the PBA-derived catalysts may also integrated into a highly graphitized carbon network to further improve the conductivity, mass transport and durability against oxidative corrosion.

Liu, Di-Jia↗
DOE OSTI2020

3D Printable Polymers for Radiological Applications

Additive manufacturing of polymers is a widely used and growing technology throughout the DOE complex. Composite filaments for Fused Deposition Modeling (FDM) printers are commonly used to enhance material properties such as mechanical strength, operating temperature and chemical resistance. Radio luminescent properties were added to 3D filaments by impregnating clear poly(ethylene terephthalate)-glycol (PETG) with X-ray scintillator powder. X-ray scintillators have unique, narrow emission spectra which can be imaged with low light DSLR cameras, photomultiplier tubes, or spectrometers. Filament Production: 1. Pellets are loaded into the hopper. 2. Temperature and extrusion speed are set on extruder. 3. Extruder turned on and filament is pulled from nozzle along air path to cool. 4. Spooler drive speeds set. 5. Filament is inserted through Filameasure then guided along drive wheels and into traverse. 6. Parameters adjusted to obtain 1.75 mm diameter 7. Filament is inserted into spooler wheel. Traverse speed set and slip nut adjusted to make tight and even roll. From Raw Materials to Printed Part: Clear PETG pellets were dipped in ZnS:Cu scintillator powder to coat pellets. Coated and uncoated pellets were mixed together before inserting into hopper. Filament showed trace amounts of powder and luminesced under UV light. Powder distribution is random and not reproducible thus providing a unique identification marker potentially useful for national security applications. Multiple X-ray scintillators can be arranged in a panel to discriminate incident X-ray photon energies. Variable panel arrangement: pixelated, layered, solid from homogenous filament. Ratio of emission intensities. Response varies with incident photon energy. Spectrally separate scintillators. Scintillators: Yttrium Oxide Europium Doped - Red, Gadolinium Oxysulfide Terbium Doped - Green, Barium Magnesium Aluminate Europium Doped - Blue, Zinc Sulfide Copper Doped - Green. Powder concentrations will need to be increased in the filament for use in the energy discriminating panel. Homogeneity in the filament will be a challenge that can be solved by turning the pellets into a powder and mixing it with scintillator powder prior to extrusion. The panels will be tested using x-ray booths and button sources such as Am-241, Cs-137, and Co-60.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗