We demonstrate the epitaxial growth of tetragonal platinum monoxide (PtO) on MgO, TiO2, and β-Ga2O3 single-crystalline substrates by ozone molecular-beam epitaxy. We provide synthesis routes and derive a growth diagram under which PtO films can be synthesized by physical vapor deposition. A combination of electrical transport and photoemission spectroscopy measurements, in conjunction with density functional theory calculations, reveal PtO to be a degenerately doped p-type semiconductor with a bandgap of Eg ≈ 1.6 eV. Spectroscopic ellipsometry measurements are used to extract the complex dielectric function spectra, indicating a transition from free-carrier absorption to higher photon energy transitions at E ≈ 1.6 eV. Using tetragonal PtO as an anode contact, we fabricate prototype Schottky diodes on n-type Sn-doped β-Ga2O3 substrates and extract Schottky barrier heights of ϕB > 2.2 eV.

We utilize a combined computational-experimental approach to examine the influence of indium nanoparticle (NP) array distributions on deep-ultraviolet (UV) plasmon resonances. For photon energies < 5.7 eV, analysis of ellipsometric spectra reveals an increase in silicon reflectance induced by indium NP arrays on silicon. For various energies in the range 5.7–7.0 eV, a decrease in reflectance is induced by the NP arrays. Similar trends in reflectance are predicted from finite-difference time-domain (FDTD) simulations using NP size distributions extracted from atomic-force micrographs as input. In addition, in the energy range of 7.4–9.2 eV, the FDTD simulations reveal reflectance minima, characteristic of localized surface plasmon resonances. Electron energy-loss spectroscopy collected from individual indium NPs reveals the presence of LSPR at ≈ 8 eV, further supporting the promise of indium NP arrays on silicon for deep-UV plasmonics.

Thermal metamaterials have emerged as a powerful platform in the engineering of radiative heat transfer across a broad range of applications, including thermal imaging, passive cooling, and thermo-photovoltaics. Here, a novel application is presented for metamaterials: thermal dual barrier coatings (TDBCs). It is shown that the incorporation of carefully crafted metamaterials into widely used thermal barrier coatings (TBCs) offers a transformative potential to improve their thermal insulation performance. The metamaterial-based TDBC mitigates both conductive and radiative heat transfer, bridging a critical gap in traditional TBC technology, which focuses solely on thermal conduction. In designing the metamaterial, the concept of oxidation resistance through passivation and Pilling-Bedworth Ratio (PBR) engineering is brought to the forefront of material selection to enhance overall stability in harsh environments. The use of metal silicides in metamaterials is also proposed as an overlooked class of oxidation-resistant metallic building blocks. The study achieves high-temperature thermal metamaterials-stable at 1200 C even in ambient conditions. Merging the fields of thermal metamaterials and TBC technology may present a new frontier with transformative applications impacting energy sciences as well as the energy industry.

Carrier concentration (N) of indium tin oxide (ITO), poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), and low bandgap perovskite (FASnI3)0.6(MAPbI3)0.4 in the photovoltaic device configuration are determined using terahertz (THz) spectral range optical Hall effect measurements and analysis. Unlike direct electrical Hall effect, this technique is non-invasive as it does not need any direct electrical contacts and is able to probe free carrier transport properties of individual films inside the device stack. A modified Drude model is implemented to address the dominating free carrier absorption in the THz spectral range. N of the ITO, PEDOT: PSS, and (FASnI3)0.6(MAPbI3)0.4 are determined to be (2.8 +/- 0.6) x 1020 cm-3, (2.6 +/- 0.8) x 1022 cm-3, and (1.5 +/- 0.1) x 1018 cm-3, respectively. PV device performance is simulated with SCAPS employing the findings from optical Hall measurements. Open circuit voltage (VOC), short circuit current (JSC), fill factor (FF), power con-version efficiency (PCE) are found to be 0.865 V, 29.2 mAcm-2, 66.7%, and 16.8% respectively which are within 3% of corresponding experimentally measured values. These optically measured N serve as essential parameters for device modeling to understand its physics in device performance and to optimize PCE; consistency between simulated and experimental device performance parameters using these values supports optical Hall effect as a reliable technique to extract electronic transport properties.

In -situ real-time spectroscopic ellipsometry (RTSE) measurements are performed on a growing amorphous vanadium oxide (a-VO x ) film to determine the complex dielectric function ( e = e 1 + i e 2 ) spectra, structure, and oxygen content ( x ) with depth during deposition. VO x with x -2 is annealed to produce polycrystalline VO 2 and characterized by near -infrared to ultraviolet (0.75 - 5.9 eV) spectroscopic ellipsometry during heating and cooling from room temperature (RT) to 343 K to RT to track hysteresis effects in the semiconducting -to -metal transition (SMT). Spectra in e are measured for as -deposited and annealed films prepared with varying deposition parameters to determine the as -deposited film x and verify the SMT. Temperature -dependent IR extended spectroscopic ellipsometry is performed from 0.06 to 0.74 eV to determine the IR e spectra of polycrystalline VO 2 across the SMT. As -deposited amorphous VO x films with x from 1.89 <= x <= 2.14 transition to polycrystalline VO 2 after annealing and exhibit the SMT when heated from RT to 343 K which are verified by substantially increased e 2 magnitude at >= 343 K. By identifying x of a-VO x from e spectra, a shortened path of qualifying a-VO x films which will crystallize to phase change VO 2 is developed.

An in-depth analysis of the optical properties of epitaxial (001) oriented LaMnO3films grown on SrTiO3and LaAlO3single crystal substrates is performed by spectroscopic ellipsometry to determine the complex dielectric function (ε = ε1 + iε2) spectra from 0.12 to 5.89 eV. Density Functional Theory and many body perturbation theory within the G0W0 and the Bethe-Salpeter Equation approximations, are also employed to generate theoretical spectra in ε. Critical point energies (CPs) from 0.91 to 1.49 eV are interpreted as energy separation between eg and t2g orbitals. CPs from 3.31 to 3.53 eV and 3.75 to 5.23 are interpreted as Mn d exchange splitting and strong charge transfer transitions, respectively. Direct and indirect bandgaps from Tauc-plots are identified in the range of 0.100–0.594 eV. Anisotropy presents in LaMnO3 on LaAlO3 is attributed to substantial distortion in the out-of-plane epitaxial strain compared to LaMnO3 on SrTiO3. These findings provide a comprehensive understanding of the origins and characteristics of features observed in optical properties of epitaxial LaMnO3, offering insights for development of optical and electronic applications based on these materials.

Determining exactly how the performance of a thin-film photovoltaic device is limited by a particular recombination mechanism can be difficult, particularly in the case of CdTe solar cells. As a result, efforts are being made to improve all parts of the device without good knowledge of which improvements are necessary. To understand where the device limitation is, the recombination current densities of at least one on of the interfaces must be known. Here, we present a method to determine which recombination mechanisms is limiting. First, back illuminated quantum efficiency measurements are used to determine the key parameters of the back interface – the back surface recombination velocity and the band bending near the back surface. Once these back interface parameters are determined, the recombination current densities can be calculated for a front illuminated device to determine the limiting mechanism. The validity of the approach is tested using previously reported data.

It has been suggested that Ba3In2O6might be a high-Tcsuperconductor. Experimental investigation of the properties of Ba3In2O6was long inhibited by its instability in air. Recently epitaxial Ba3In2O6with a protective capping layer was demonstrated, which finally allows its electronic characterization. The optical bandgap of Ba3In2O6is determined to be 2.99 eV in-the (001) plane and 2.83 eV along thec-axis direction by spectroscopic ellipsometry. First-principles calculations were carried out, yielding a result in good agreement with the experimental value. Various dopants were explored to induce (super-)conductivity in this otherwise insulating material. NeitherA- norB-site doping proved successful. The underlying reason is predominately the formation of oxygen interstitials as revealed by scanning transmission electron microscopy and first-principles calculations. Additional efforts to induce superconductivity were investigated, including surface alkali doping, optical pumping, and hydrogen reduction. To probe liquid-ion gating, Ba3In2O6was successfully grown epitaxially on an epitaxial SrRuO3bottom electrode. So far none of these efforts induced superconductivity in Ba3In2O6,leaving the answer to the initial question of whether Ba3In2O6is a high-Tcsuperconductor to be 'no' thus far.

An in-depth analysis of the optical properties of epitaxial (001) oriented LaMnO3 3 films grown on SrTiO3 3 and LaAlO3 3 single crystal substrates is performed by spectroscopic ellipsometry to determine the complex dielectric function (epsilon = epsilon 1 1 + i epsilon 2) 2 ) spectra from 0.12 to 5.89 eV. Density Functional Theory and many body perturbation theory within the G0W0 0 W 0 and the Bethe-Salpeter Equation approximations, are also employed to generate theoretical spectra in epsilon. Critical point energies (CPs) from 0.91 to 1.49 eV are interpreted as energy separation between eg g and t 2g orbitals. CPs from 3.31 to 3.53 eV and 3.75 to 5.23 are interpreted as Mn d exchange splitting and strong charge transfer transitions, respectively. Direct and indirect bandgaps from Tauc-plots are identified in the range of 0.100-0.594 eV. Anisotropy presents in LaMnO3 3 on LaAlO3 3 is attributed to substantial distortion in the out-of-plane epitaxial strain compared to LaMnO3 3 on SrTiO3. 3 . These findings provide a comprehensive understanding of the origins and characteristics of features observed in optical properties of epitaxial LaMnO3, 3 , offering insights for development of optical and electronic applications based on these materials.

The complex dielectric function ( ε = ε 1 + iε 2 ) spectra of epitaxial In 0.53 Ga 0.47 As and In 0.52 Al 0.48 As in multilayer stacks were extracted by spectroscopic ellipsometry. Unlike previous studies over more limited spectral ranges, these analyses span from the THz to ultraviolet and provide a non-contacting method of determining optical and electronic transport properties from ε of different layers. The direct band gap of these In 0.53 Ga 0.47 As and In 0.52 Al 0.48 As films were determined to be 0.73 and 1.44 eV, respectively. Above band gap critical point transition energies were measured to be 1.22, 2.53, and 2.87 eV for In 0.53 Ga 0.47 As and 1.74 and 2.95 eV for In 0.52 Al 0.48 As, respectively. Biaxial tensile stress in epitaxial In 0.53 Ga 0.47 As and In 0.52 Al 0.48 As films were measured to be 0.15 and 0.11 GPa, respectively. The amplitude of phonon modes in In 0.53 Ga 0.47 As and In 0.52 Al 0.48 As at 252 and 351 cm −1 were determined to be 105 and 99, respectively. Resistivity ( ρ ), scattering time ( τ ), carrier mobility ( μ ), and carrier concentration ( N ) were determined using the Drude model, yielding ρ = 0.00412 Ωcm, τ = 238 fs, μ = 9975 cm 2 /Vs, and N = 1.52 × 10 17 cm −3 for In 0.53 Ga 0.47 As and ρ = 0.026 Ωcm, τ = 189 fs, μ = 4435 cm 2 /Vs, and N = 5.3 × 10 16 cm −3 for In 0.52 Al 0.48 As. The transport properties measured from ellipsometry are in close agreement with Hall effect measurement and reported results in literature. Optoelectronic parameters from these analyses can serve as input to model In 0.53 Ga 0.47 As and In 0.52 Al 0.48 As based optoelectronic devices to optimize their performance and to understand device physics.

Glancing angle deposition (GLAD) of CdTe can produce a cubic, hexagonal, or mixed phase crystal structure depending upon the oblique deposition angles (Φ) and substrate temperature. GLAD CdTe films are prepared at different Φ at room temperature (RT) and a high temperature (HT) of 250 °C and used as interlayers between the n-type hexagonal CdS window layer and the p-type cubic CdTe absorber layer to investigate the role of interfacial tailoring at the CdS/CdTe heterojunction in photovoltaic (PV) device performance. The Φ = 80° RT GLAD CdTe interlayer and CdS both have the hexagonal structure, which reduces lattice mismatch at the CdS/CdTe interface and improves electronic quality at the heterojunction for device performance optimization. The device performance of HT CdS/CdTe solar cells with Φ = 80° RT with 50 to 350 nm thick GLAD CdTe interlayers is evaluated in which a 250 nm interlayer device shows the best device performance with a 0.53 V increase in open-circuit voltage and fill-factor product and a 0.73% increase in absolute efficiency compared to the HT baseline PV device without an interlayer.

Polycrystalline CdTe-based photovoltaic (PV) devices currently account for ~5-10% of the PV market. This is despite the fact that the open circuit voltage (Voc) of these devices is only ~71 % of its detailed-balanced limit. Increasing the Voc will lead to further efficiency gains and potentially reduce the already low cost per Watt production. To improve the Voc will require reducing recombination, likely at the back interface for the current generation of Cu-doped devices. To do this, we employ sputtered oxide buffer layers, specifically, ${\mathrm{A}1_{2}\mathrm{O}_{3}}$ and ${\text{CuAlO}_{2}}$ . The high bandgap and valence band positions deeper than that of CdTe suggest that these materials have the potential to reduce the recombination at the back interface. When thick oxide layers are used, we observe and s-kink in the current density-voltage curve, but Voc values above 900 mV are observed for both materials. As the thickness of the oxide buffers decreases the Voc decreases with an increase in the fill factor. These results point to a pathway to achieve high Voc while maintaining a high fill factor.

A low bandgap mixed tin-lead (Sn-Pb) halide perovskite based thin film solar cell is a multilayer stack consisting of indium tin oxide (ITO) as the transparent front electric contact, poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) as the hole transport layer, low bandgap perovskite (FASnI3)0.6(MAPbI3)0.4 as the absorber layer, fullerene (C60) as the electron transport layer, bathocuproine (BCP) as the hole blocking layer, and Ag as the back contact. Determination of carrier concentration (N) of these individual layers in the solar cell stack structure using direct electrical measurements is not accessible. Here, N for some component layers in a low bandgap organic inorganic (FASnI3)0.6(MAPbI3)0.4 absorber based perovskite solar cell are determined using non-contacting magnetic field dependent terahertz (THz) range spectroscopic ellipsometry for optical Hall effect measurement to ascertain the free carrier optical absorption and corresponding N using the Drude model. From THz spectral range optical Hall effect measurements and analysis, N of the ITO, PEDOT:PSS, and low bandgap (FASnI3)0.6(MAPbI3)0.4 are determined to be (2.8 ± 0.6), 1020 cm −3 , (2.6 ± 0.4), 1022 cm −3 , and (1.5 ± 0.1), 1018 cm −3 , respectively. All values are within expectations for this device design. These results demonstrate the capability of THz range optical Hall effect measurements to determine transport properties of layers within complete thin film polycrystalline solar cells, and the properties determined can be used as input for photovoltaic device modeling to understand device physics and optimize performance.

There are limited choices for front-surface, electron-selective contacts (emitters) for CdTe solar cells, thus hindering scientific and technical development. Here we investigate the photovoltaic performance of devices fabricated with (InxGa1-x)(2)O-3 (IGO) emitters with varying In-to-Ga ratios prepared by cosputtering. In agreement with predictions, an IGO emitter with a 4.03 eV bandgap (x = 0.36) allowed fabrication of devices with efficiencies of 16%. Increasing the performance to higher values will be enabled by increasing the transmission through the IGO-coated substrate and decreasing the bulk and back interface recombination. These findings demonstrate IGO materials as effective emitters in high-efficiency CdTe-based solar cells.

External quantum efficiency (EQE) of copper indium gallium diselenide (CIGS) based solar cells with different antireflection coatings (ARC) has been evaluated under the solar irradiance of airmasses (AM) 0 and 1.5G. The simulations are performed in the wavelength range of 300 - 2500 nm to investigate the absorptance and reflectance features below and above the band gap of the absorber layer. Short circuit current density is increased the most for AM 0 and 1.5G using MgF2 ARCs. However, these ARCs also reduce reflectance below the band gap energy of CIGS which will lead to absorption in other component layers, device heating, and lower operating efficiency.

Optical properties and band gap energy of solution processed p-type transparent Cu x AlO y thin film deposited on sodalime glass are determined using spectroscopic ellipsometry. This is a promising material for a p-type transparent back contact and passivation layer for thin film CdTe based solar cells. The direct optical band gap obtained from Tauc plot is found to be $3.64 \pm 0.01\ \text{eV}$ . Further characterization is also done with this layer in the CdTe / CdS device stack. Cu x AlO y conformally coats and smoothens the CdTe surface on the CdTe / CdS device stack indicating improved surface quality and surface passivation.

We recently demonstrated that $\left(\mathbf{In}_{\mathbf{x}} \mathbf{Ga}_{\boldsymbol{1-}\mathbf{x}}\right)_{\boldsymbol{2}} \mathbf{O}_{\boldsymbol{3}}$ (IGO) alloys have the potential to be high-performing emitters in CdTe based photovoltaic devices, readily producing devices with efficiencies in excess of 16%. Here we present characterization data for the $(\mathbf{In}_{\mathbf{x}}\mathbf{Ga}_{\boldsymbol{1-}\mathbf{x}})_{\boldsymbol{2}}\mathbf{O}_{\boldsymbol{3}}$ (IGO) films as x was varied from 0 to 1. As grown IGO films exhibited band gaps ranging from 3.3 eV to 4.77 eV and were amorphous and highly resistive. After heating through a temperature profile that would be experienced during CdTe deposition, Hall effect measurements found n-type conductivity and carrier concentrations ranging from $\boldsymbol{10^{19}}$ to $\boldsymbol{10^{20}}\ \mathbf{cm}^{\boldsymbol{-3}},$ . The best performing solar cell was fabricated with $\mathbf{x}\boldsymbol{=0.36}$ , which showed a bandgap of 4.02 eV, a carrier concentration of $\boldsymbol{2.5} \ \mathbf{x}\ \boldsymbol{10^{19}}\ \mathbf{cm}^{\boldsymbol{-3}}$ , and a mobility of $\boldsymbol{9.1}\ \mathbf{cm}^{\boldsymbol{2}}/\mathbf{V}.\mathbf{s}$ . PL measurements showed the brightest emission for this same composition.

We have studied the growth of epitaxial films of stannate pyrochlores with a general formula A2Sn2O7 (A = La and Y) and find that it is possible to incorporate ∼25% excess of the A-site constituent; in contrast, any tin excess is expelled. We unravel the defect chemistry, allowing for the incorporation of excess A-site species and the mechanism behind the tin expulsion. An A-site surplus is manifested by a shift in the film diffraction peaks, and the expulsion of tin is apparent from the surface morphology of the film. In an attempt to increase La2Sn2O7 conductivity through n-type doping, substantial quantities of tin have been substituted by antimony while maintaining good film quality. The sample remained insulating as explained by first-principles computations, showing that both the oxygen vacancy and antimony-on-tin substitutional defects are deep. Similar conclusions are drawn on Y2Sn2O7. An alternative n-type dopant, fluorine on oxygen, is shallow according to computations and more likely to lead to electrical conductivity. The bandgaps of stoichiometric La2Sn2O7 and Y2Sn2O7 films were determined by spectroscopic ellipsometry to be 4.2 eV and 4.48 eV, respectively.

Antimony selenide (Sb2Se3) is a highly promising solar cell absorber material with excellent optoelectronic properties including high absorption coefficient in the visible energy range. Here, we investigate the optical and electronic properties of thin film polycrystalline Sb2Se3 deposited on glass. The indirect bandgap of 1.117 ± 0.001 eV, direct optical gap of 1.175 ± 0.002 eV, and Urbach energy of 21.1 ± 0.6 meV are determined from the absorption coefficient spectra obtained from photothermal deflection spectroscopy. Complex dielectric function (ε = ε1 + iε2) spectra are determined using through-the-glass spectroscopic ellipsometry measurements in 0.75 – 4 eV spectral range due to the roughness of the Sb2Se3 film. These spectra in ε along with the solar cell component layer thicknesses are used as input parameters for external quantum efficiency (EQE) simulation to investigate electronic losses in substrate type Sb2Se3 based solar cells. The difference between simulated EQE for Sb2Se3 based PV assuming complete carrier collection in absorber layer Sb2Se3 and measured EQE are evaluated to obtain 97.0 ± 0.2% carrier collection probability in Sb2Se3 at the heterojunction interface and a 400 nm carrier collection length throughout the Sb2Se3 absorber layer. The difference between measured short circuit current density and that simulated assuming no electronic losses shows that 5.4 mA/cm 2 is lost due to incomplete carrier collection.