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Narrow bandgap mixed tin (Sn) + lead (Pb) perovskites are necessary for the bottom sub-cell absorber in high efficiency all-perovskite polycrystalline tandem solar cells. We report on the impact of mixed cation composition and atmospheric exposure of perovskite films on sub-gap absorption in films and performance of solar cells based on narrow bandgap mixed formamidinium (FA) + methylammonium (MA) and Sn + Pb halide perovskites, (FASnI3)x(MAPbI3)1−x. Structural and optical properties of 0.3 ≤ x ≤ 0.8 (FASnI3)x(MAPbI3)1−x perovskite thin film absorbers with bandgaps ranging from 1.25 eV (x = 0.6) to 1.34 eV (x = 0.3) are probed with and without atmospheric exposure. Urbach energy, which quantifies the amount of sub-gap absorption, is tracked for pristine perovskite films as a function of composition, with x = 0.6 and 0.3 demonstrating the lowest and highest Urbach energies of 23 meV and 36 meV, respectively. Films with x = 0.5 and 0.6 compositions show less degradation upon atmospheric exposure than higher or lower Sn-content films having greater sub-gap absorption. The corresponding solar cells based on the x = 0.6 absorber show the highest device performance. Despite having a low Urbach energy, higher Sn-content solar cells show reduced device performances as the amount of degradation via oxidation is the most substantial. [ABSTRACT FROM AUTHOR]

Hydrogenated silicon (Si:H) thin films have been prepared by radio frequency (RF) magnetron sputtering. The effect of hydrogen gas concentration during sputtering on the resultant film structural and optical properties has been investigated by real time spectroscopic ellipsometry (RTSE) and grazing incidence x-ray diffraction (GIXRD). The analysis of in-situ RTSE data collected during sputter deposition tracks the evolution of surface roughness and film bulk layer thickness with time. Growth evolution diagrams depicting amorphous, nanocrystalline and mixed-phase regions for low and high deposition rate Si:H are constructed and the effects of process parameter (hydrogen gas concentration, total pressure and RF power) variations on the deposition rate have been qualified. Virtual interface analysis of RTSE data provides nanocrystalline volume fraction depth profiles in the mixed-phase growth regime. GIXRD measurements show the presence of (111) and (220) oriented crystallites. Vibrational mode absorption features from Si-Hn bonding configurations at 590, 640, 2000 and 2090 cm-1 are obtained by ex-situ infrared spectroscopic ellipsometry. Hydrogen incorporation decreases as films transition from amorphous to nanocrystalline phases with increasing hydrogen gas concentration during sputtering [ABSTRACT FROM AUTHOR]

Ex-situ spectroscopic ellipsometry measurements are made on radio frequency magnetron sputtered oxygenated cadmium sulfide (CdS:O) thin films. Films are deposited onto glass substrates at room temperature and at 270 °C with varying oxygen to total gas flow ratios in the sputtering ambient. Ellipsometric spectra from 0.74 to 5.89 eV are collected before and after annealing at 607 °C to simulate the thermal processes during close-space sublimation of overlying cadmium telluride in that solar cell configuration. Complex dielectric function (ϵ = ϵ1 + iϵ2) spectra are extracted for films as a function of oxygen gas flow ratio, deposition temperature, and post-deposition annealing using a parametric model accounting for critical point transitions and an Urbach tail for sub-band gap absorption. The results suggest an inverse relationship between degree of crystallinity and oxygen gas flow ratio, whereas annealing is shown to increase crystallinity in all samples. Direct band gap energies are determined from the parametric modeling of ϵ and linear extrapolations of the square of the absorption coefficient. As-deposited samples feature a range of band gap energies whereas annealing is shown to result in gap energies ranging only from 2.40 to 2.45 eV, which is close to typical band gaps for pure cadmium sulfide. [ABSTRACT FROM AUTHOR]

A comprehensive database of the optical response in the form of complex dielectric function (ε) spectra of magnetron co-sputtered CdSe1−xSx alloy thin films is developed spanning the full 0 ≤ x ≤ 1 range of compositions. A parametric model is presented and used to determine ε describing each film while in both as-deposited and thermally annealed states. This model combines a critical point electronic transition lineshape and an Urbach tail in the above- and below-bandgap portions of the measured spectrum, respectively, while maintaining first derivative continuity. Additionally, this hybrid parametric description of ε automatically determines the Urbach energy (EU) describing the width of the sub-bandgap absorption tail, thereby providing a relative measure of the defect density of the modeled material. These as-deposited CdSe1−xSx films are generally found to be the most defective at intermediate compositions with some EU reaching energies > 200 meV. Annealing reduces EU in all films to a relatively uniform value < 100 meV for all compositions. Leveraging the full ε database in modeling CdS-CdSe bi-layer stacks is demonstrated to be effective in detecting the composition gradients resulting from inter-diffusion upon annealing. The accuracy of this technique is verified through excellent agreement with cross sectional energy dispersive X-ray spectroscopy measurements. [ABSTRACT FROM AUTHOR]

Thin films of transparent conducting fluorine doped tin oxide (SnO2:F) on soda lime glass and stainless steel substrates are prepared by spray pyrolysis at low temperature ≤250°C. Optical properties in the form of complex dielectric function (ε=ε1+iε2) spectra are obtained using spectroscopic ellipsometry from 0.035 to 5.89 eV for SnO2:F on glass and from 0.75 to 5.89 eV for SnO2:F on stainless steel. The absorption coefficient (a) spectra are extracted from spectra in e for the film on soda lime glass from 0.035 to 5.89 eV and on stainless steel from 0.75 to 5.89 eV. Direct band gaps for SnO2:F thin films on soda lime glass and stainless steel substrates are determined to be 3.45±0.01 and 3.60±0.01 eV, respectively. Infrared extended measurement shows an absorption feature at 0.063 eV (508 cm-1) which may be attributed to Sn–O and Sn–F bonding. A broad background phonon absorption feature due to Sn–OH and O–H bonding is also identified at 0.3 eV (2420 cm-1). Resistivity is deduced from ε. Improved agreement between optically and electrically determined resistivity is obtained when the infrared range spectrum is analyzed. The morphologies of both the films are studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). [ABSTRACT FROM AUTHOR]

Borosilicate glass is of significant interest and utility to for a variety of applications, often for its robust ability to withstand significant thermal stresses resulting from a low coefficient of thermal expansion. Here, ellipsometric spectra of borosilicate glass from the millimeter wavelength range (terahertz) to the ultraviolet are collected from rotating compensator configuration ellipsometers. Specifically, spectra from three instruments each measuring a separate spectral range (0.4-4.1 meV; 0.035-0.75 eV; 0.75-5.89 eV) are modeled simultaneously. Continuous parameterization of the complex dielectric function (ε = ε1 + iε2) spectra from 0.4 meV to 5.89 eV describing the optical response of the glass is determined by modeling the measured spectra. Additionally, intensity-based transmittance measurement spanning 0.75-5.89 eV is used to determine the absorption coefficient at photon energies greater than 4 eV. Phonon absorption features are identified at low energies in the infrared and terahertz ranges. [ABSTRACT FROM AUTHOR]

The anisotropic optical properties of single crystal SrLaAlO4 are investigated using spectroscopic ellipsometry over the range from 0.74 to 5.90 eV. Divided spectral range analysis of ellipsometric and Mueller matrix spectra are used to obtain the sample structural parameters (surface roughness thickness and void fraction). The ordinary and extra-ordinary optical properties in the form of complex dielectric function, ϵ = ϵ1 + i ϵ2 (or complex index of refraction, N = n + i k), spectra are extracted from the same analysis. The observed birefringence is ∼ 0.025 in the transparent range. An upper limit for the indirect gap at 3.01 ± 0.01 eV and a critical point at 5.54 ± 0.01 eV are identified within the measured spectral range. [ABSTRACT FROM AUTHOR]

Optimization of thin film photovoltaics (PV) relies on characterizing the optoelectronic and structural properties of each layer and correlating these properties with device performance. Growth evolution diagrams have been used to guide production of materials with good optoelectronic properties in the full hydrogenated amorphous silicon (a-Si:H) PV device configuration. The nucleation and evolution of crystallites forming from the amorphous phase were studied using in situ near-infrared to ultraviolet spectroscopic ellipsometry during growth of films prepared as a function of hydrogen to reactive gas flow ratio R = [H2]/[SiH4]. In conjunction with higher photon energy measurements, the presence and relative absorption strength of silicon-hydrogen infrared modes were measured by infrared extended ellipsometry measurements to gain insight into chemical bonding. Structural and optical models have been developed for the back reflector (BR) structure consisting of sputtered undoped zinc oxide (ZnO) on top of silver (Ag) coated glass substrates. Characterization of the free-carrier absorption properties in Ag and the ZnO + Ag interface as well as phonon modes in ZnO were also studied by spectroscopic ellipsometry. Measurements ranging from 0.04 to 5 eV were used to extract layer thicknesses, composition, and optical response in the form of complex dielectric function spectra (ε = ε1 + iε2) for Ag, ZnO, the ZnO + Ag interface, and undoped a-Si:H layer in a substrate n-i-p a-Si:H based PV device structure. [ABSTRACT FROM AUTHOR]

In situ real-time spectroscopic ellipsometry (RTSE) measurements have been conducted on MAPbI3, MA0.7FA0.3PbI3, and (FAPbI3)0.95(MAPbBr3)0.05 perovskite thin films when exposed to different levels of relative humidity at given temperatures over time. Analysis of RTSE measurements track changes in the complex dielectric function spectra and structure, which indicate variations in stability influenced by the underlying material, preparation method, and perovskite composition. MAPbI3 and MA0.7FA0.3PbI3 films deposited on commercial fluorine-doped tin oxide coated glass are more stable than corresponding films deposited on soda lime glass directly. (FAPbI3)0.95(MAPbBr3)0.05 films on soda lime glass showed improved stability over the other compositions regardless of the substrate, and this is attributed to the preparation method as well as the final composition. [ABSTRACT FROM AUTHOR]

The unsatisfactory performance of low‐bandgap mixed tin (Sn)–lead (Pb) halide perovskite subcells has been one of the major obstacles hindering the progress of the power conversion efficiencies (PCEs) of all‐perovskite tandem solar cells. By analyzing dark‐current density and distribution, it is identified that charge recombination at grain boundaries is a key factor limiting the performance of low‐bandgap mixed Sn–Pb halide perovskite subcells. It is further found that bromine (Br) incorporation can effectively passivate grain boundaries and lower the dark current density by two–three orders of magnitude. By optimizing the Br concentration, low‐bandgap (1.272 eV) mixed Sn–Pb halide perovskite solar cells are fabricated with open‐circuit voltage deficits as low as 0.384 V and fill factors as high as 75%. The best‐performing device demonstrates a PCE of >19%. The results suggest an important direction for improving the performance of low‐bandgap mixed Sn–Pb halide perovskite solar cells. Charge recombination at grain boundaries is a key factor limiting the performance of low‐bandgap mixed tin–lead halide perovskite solar cells. It is found that bromine incorporation can passivate grain boundaries and lower the dark current density by two to three orders of magnitude. The champion cell shows an open‐circuit voltage deficit of 0.384 V and power conversion efficiency exceeding 19%. [ABSTRACT FROM AUTHOR]