Se Films Research Articles

The influence of Ga doping on preparation and thermoelectric properties of flexible Ag2Se films

Herein, we prepared Ga doped Ag2Se flexible films on porous nylon membrane. First, Se nanowires (NWs) were wet chemically synthesized, then different amounts of Ga doped Ag2Se (Ag2-xGaxSe, x = 0.01, 0.02, 0.04) NWs were prepared using the Se NWs as templates in solution at 80 °C, and finally the Ga doped Ag2Se films on porous nylon membrane were fabricated by vacuum assisted filtration and then hot pressing at 230 °C and 1 MPa for 30 min. The Ag1.98Ga0.02Se film exhibits a higher power factor of ~1162 μW m−1 K−2 at 300 K and a superior flexibility: after bending for 1000 cycles around a 4-mm-radius rod, about 97% of the initial electrical conductivity is maintained.

Employing Equivalent Circuit Models to Study the Performance of Selenium‐Based Solar Cells with Polymers as Hole Transport Layers

Selenium(Se)-based solar cells (SSCs), known as one of the oldest solar cells, have regained intense attention due to the advantages of Se including direct bandgap, good stability, and single absorber. Among all kinds of device structures, conventional n-i-p SSCs with top organic hole transport layers (HTLs) show great potential since organic HTLs could be well-designed to smoothly extract holes from the Se single absorber and protect the Se surface. However, till now, the performance of Se solar cells with organic HTLs is not as good as expected. To address this issue, herein, the SSCs are first presented with organic polymers as the HTLs with the improved efficiency up to 4.3%, which is the highest one in organic HTLs-based SSCs. Additionally, comparing with perovskite solar cells, it is found that the recombination process is the key factor that influences the performance of SSCs. It is believed that the further optimization of the Se active layer and the design of new and suitable organic HTLs for SSCs should be the main focus to suppress the undesired recombination processes of Se films. Such realization would boost the efficiency of the as-fabricated SSCs.

Hybrid Symmetry Epitaxy of the Superconducting Fe(Te,Se) Film on a Topological Insulator.

It is challenging to grow an epitaxial 4-fold compound superconductor (SC) on a 6-fold topological insulator (TI) platform due to the stringent lattice-matching requirement. Here, we demonstrate that Fe(Te,Se) can grow epitaxially on a TI (Bi2Te3) layer due to accidental, uniaxial lattice match, which is dubbed as "hybrid symmetry epitaxy". This new growth mode is critical to stabilizing robust superconductivity with TC as high as 13 K. Furthermore, the superconductivity in this FeTe1-xSex/Bi2Te3 system survives in the Te-rich phase with Se content as low as x = 0.03 but vanishes at Se content above x = 0.56, exhibiting a phase diagram that is quite different from that of the conventional Fe(Te,Se) systems. This unique heterostructure platform that can be formed in both TI-on-SC and SC-on-TI sequences opens a route to unprecedented topological heterostructures.

Enabling Solar Hydrogen Production over Selenium: Surface State Passivation and Cocatalyst Decoration

As the first material ever used in a solid-state solar cell, selenium (Se) has outstanding merits of light absorption, carrier mobility, intrinsic environmental stability, and straightforward film preparation, making it an attractive light-absorbing semiconductor for photovoltaic applications. However, the exploration of selenium for photoelectrochemical (PEC) cells remains vastly unreported. Here, we successfully enable selenium as an absorber in the photocathode for solar hydrogen production. A rapid thermal annealing process is adopted to prepare a tetragonal Se film with a preferred [100] orientation. Using a thin layer of TiO2 buffer and platinum decoration, a photocurrent density up to −7.2 mA/cm2 is achieved over the optimal Se/TiO2/Pt photocathode at 0 VRHE under simulated one sun illumination. The roles of surface state passivation by TiO2 and cocatalyst decoration by Pt in charge separation and transfer were thoroughly investigated by capacitance–voltage profiling, photoassisted Kelvin probe force microscopy, photoelectrochemical impedance spectroscopy, and also transient photovoltage decay. Our work provides Se as a promising candidate for solar hydrogen production and also highlights the effectiveness of surface state passivation and cocatalyst decoration in the fabrication of efficient PEC devices.

Impact of RbF and NaF Postdeposition Treatments on Charge Carrier Transport and Recombination in Ga‐Graded Cu(In,Ga)Se2 Solar Cells

AbstractTwo key strategies for enhancing the efficiency of Cu(In,Ga)Se2 solar cells are the bandgap gradient across the absorber and the incorporation of alkali atoms. The combined incorporation of Na and Rb into the absorber has brought large efficiency gains compared to Na‐containing or alkali‐free layers. Here, transient absorption spectroscopy is employed to study the effect of NaF or combined NaF+RbF postdeposition treatments (PDT) on minority carrier dynamics in different excitation volumes of typical composition‐graded Cu(In,Ga)Se2 solar cells. Electron lifetimes are found to be highly dependent on the film composition and morphology, varying from tens of nanoseconds in the energy notch to only ≈100 ps in the Ga‐rich region near the Mo‐back contact. NaF PDT improves recombination lifetimes by a factor of 2–2.5 in all regions of the absorber, whereas the effectiveness of the RbF PDT is found to decrease for higher Ga‐concentrations. Electron mobility measured in the absorber region with large grains is promoted by both alkali PDTs. The data suggest that NaF PDT passivates shallow defect states (Urbach tail) throughout the Cu(In,Ga)Se2 film (including the interior of large grains), whereas the additional RbF PDT is effective at grain boundary surfaces (predominantly in regions with medium to low Ga‐concentrations).

Indentation-induced cracking behavior of a Cu(In,Ga)Se2 films on Mo substrate

Abstract Cu(In,Ga)Se2 (CIGS) light absorption films were synthesized by co-sputtering method on Mo/soda-lime glass, and the mechanical behavior of the films was investigated by nanoindentation. The Young's modulus and hardness of the films were approximately 88.5 ± 2.2 and 5.89 ± 0.23 GPa, respectively. The comparison graph of indentation load–displacement curve with the curve based on the theoretical elastic films, while considering the curvature of the indenter tip, revealed that cracks initiated in the CIGS films at approximately 25 mN indentation load. The transmission electron microscopy (TEM) analysis revealed that most of the cracks exhibited intergranular fracture, and the fracture toughness of the films was ~0.22 MPa√m, which is greater than the brittleness of soda-lime glass. Both Palmqvist radial and lateral cracks are observed. Results also reveal that the indentation pressure caused grain subdivision (before cracking initiated), and indentation stress may be absorbed by relatively less dense microstructures of the films.

Impact of Thickness on the Optical Properties of Selenium Thin Films

Objectives: The primary objective of this investigation is to make a systematic study on the impact of thickness on optical properties, such as energy gap, absorption coefficient, optical density etc., for selenium thin films. Understanding of the band gap energy and its influence on film thickness is of utmost importance in acquiring the intended electrical characterization of semiconducting films. Materials and methods: Ultra-purity selenium (99.99 %) was deposited on glass substrates. During deposition, the glass substrate with its holder were rotated with constant speed to have a smooth coating. Results and discussions: The XRD findings indicate that selenium is amorphous in nature. The optical band gap energy is found to be decreasing form (2.3 to 2eV) with the rise of film thickness in interval (200 to 1000 nm). The band gap energy obeys inverse square law with respect to thickness. Conclusion: We have properly grown thin films of Se below the De Broglie wavelength limit by thermal evaporation in vacuum. The optical density varies directly with film thickness. The absorption coefficients were in the interval (0.5 to 4) × 107m-1. The AFM results confirmed that the Se nano-size increases with the increase in thickness. Both the grain boundaries and sub-grain regions are clearly visible in the SEM micrographs

Fabrication of highly flexible electromagnetic interference shielding polyimide carbon black composite using hot-pressing method

Traditionally, electromagnetic interference (EMI) shielding material has been occupied by metallic materials due to its high electric conductivity and EMI shielding effect. However, with rising demands from technology development for light-weight, highly durable, and easily moldable materials, metallic materials are gradually being substituted by polymer composite materials. Still, there are limitations on polymer composite as EMI shields: High ratio of fillers in composite achieves excellent EMI shielding effect but it severely compromises its mechanical behaviors. In many studies, they have failed to fabricate the high-ratio filler films with good mechanical durability, but here the initially fragile high-ratio film has been transformed to a flexible film by a very simple method: Hot-pressing. This approach not only solved the problems mentioned above but further provided a possible breakthrough point for composite study by solving threshold of the maximum loading of the fillers in polymer matrix. The stiff composite of polyimide sponge with exceeding carbon black loading became a highly flexible EMI SE film with doubled tensile strength and enhanced EMI SE. As a result, a new way of fabricating flexible EMI shielding polymer composite is demonstrated with great potential applications in aerospace and wireless communications.

Photovoltaic characteristics and computational simulation of samarium-ion doped Cu(In, Ga)Se2 thin films prepared via a non-vacuum coating process

The effects of samarium ions on the photovoltaic performance of Cu(In, Ga)Se2 (CIGS) films were investigated through experiments and computational simulation. Compared with pristine CIGS films, the conversion efficiency of the prepared solar cells increased by 26.6% (from 8.62% to 10.91%) when 0.5 mol% samarium ions were doped into CIGS films. The incorporation of 0.5 mol% samarium ions facilitated the formation of the Cu2−xSe phase during the selenization reaction. The Cu2−xSe phase as the flux agent melted at high temperatures and thus promoted the growth of CIGS grains and reduced the number of surface pores on CIGS films. The smooth morphology of 0.5 mol% samarium-ion doped CIGS films improved the coverage of the buffer layers and suppressed the formation of additional shunt paths, thereby decreasing defect density in the CIGS/cadmium sulfide (CdS) interface and the absorber layer. The simulated results revealed the incorporation of 0.5 mol% samarium ions reduced the defect density from 1 × 1011 cm−3 to 1 × 109 cm−3 in the CIGS/CdS interface and from 3 × 1017 cm−3 to 4 × 1016 cm−3 in the absorber layers. The shunt conductance and saturation current of the prepared solar cells also decreased, from 4.38 mS/cm2 to 2.60 mS/cm2 and from 7.25 × 10−3 mA/cm2 to 1.31 × 10−3 mA/cm2, respectively. Excess doping of samarium ions caused a residual Cu2−xSe phase in CIGS films and resulted in the formation of additional shunt paths to deteriorate the performance of CIGS solar cells. The incorporation of appropriate concentrations of samarium ions into CIGS films was an effective approach to improve the photovoltaic properties of CIGS solar cells.

Valence Band Structure of Chalcogenide Obtained by X‐Ray Photoelectron Spectroscopy and Etching Technique

Chalcogenide glass, such as GeSe, has been widely used in the selector device for high‐density vertically stackable memory application due to their nonlinear electrical property. The electronic structure of the valence band, from which the nature of the bonding, the short‐range structure, and the Fermi level can be obtained, is of great importance to understand their unique electrical behavior. However, the surface oxidation issue makes it difficult to obtain the accurate valence band structure by X‐ray photoelectron spectroscopy (XPS). Herein, XPS depth profiles using the combination of the monatomic and cluster ion etching are performed to determine the valence band structures of amorphous Ge–Se films capped with a thin carbon layer. The completely different etching behavior dependent on composition in the depth profiles may be closely associated with the intrinsic bonding configurations of the amorphous films. After obtaining the fresh surface, the intrinsic valence band structures of amorphous Ge–Se samples demonstrate the different bonding behavior and short‐range structure. Most importantly, the Fermi level of amorphous GexSe100−x compounds is also determined.

Eco-friendly and cost-efficient inks for screen-printed fabrication of copper indium gallium diselenide photoabsorber thin films

Given the societal concerns about the use of toxic chemicals and costly fabrication of functional materials and devices for photovoltaic applications, it is important to develop alternative sustainable methodologies. Previous studies have shown that cost-effective printing fabrication of Cu(In,Ga)Se2 thin film photovoltaics represents an interesting alternative to energy-demanding vacuum-based deposition methods, commonly used to produce Cu(In,Ga)Se2 photovoltaics. To enrich the field of printed Cu(In,Ga)Se2 photoabsorber thin films and to develop associated eco-friendly solutions, two novel inks, consisting of non-toxic reagents and readily available oxide materials, are reported. Screen printing of the inks over fluorine-doped tin oxide conductive substrates followed by swift selenization of the resultant patterns provides a straightforward route to phase-pure, uniform, and compact Cu(In,Ga)Se2 films with thickness and band gap energies ranging from 2.5 µm to 3.5 µm and from 0.97 eV to 1.08 eV, respectively. The present approach represents an important step forward in the sustainable fabrication of Cu(In,Ga)Se2 photovoltaics, where the physical properties of the photoabsorber can be easily adjusted by tuning the conditions of the screen printing process and the metal ratios in the inks.

Terahertz Emission and Ultrafast Carrier Dynamics of Ar-Ion Implanted Cu(In,Ga)Se2 Thin Films

We investigated THz emission from Ar-ion-implanted Cu(In,Ga)Se2 (CIGS) films. THz radiation from the CIGS films increases as the density of implanted Ar ions increases. This is because Ar ions contribute to an increase in the surface surge current density. The effect of Ar-ion implantation on the carrier dynamics of CIGS films was also investigated using optical pump THz probe spectroscopy. The fitted results imply that implanted Ar ions increase the charge transition of intra-and carrier–carrier scattering lifetimes and decrease the bandgap transition lifetime.

MATHEMATICAL MODELING AND OPTIMIZATION OF THE LECTRODEPOSITION PROCESS OF ANTIMONY-SELENIUM SYSTEM

Antimony selenide (Sb2Se3), is an excellent photovoltaic absorber due to its high absorption coefficient (> 105 cm–1) at the visible region and 1.17 eV band gap. In recent years, the power conversion efficiency of Sb2Se3 thin film solar cells has gradually enhanced. Therefore, given the great interest in this material, this work is devoted to the study of a mathematical model for the optimization of the preparation of thin Sb–Se films by the electrochemical method. The study was conducted by potentiodynamic, potentiostatic and galvanostatic methods carried out under different conditions at Pt, Cu and Ni elec-trodes. The kinetics and mechanism of the electroreduction of antimony and selenite ions in the tartaric acid were studied separately for the electrochemical deposition. On the basis of cyclic polarization, X-ray phase and SEM-EDX analyses, it is found that Sb–Se thin films are deposited on Pt and Ni electrodes, but not on Cu electrode. The mathematical calculations were performed in the OptimME software package using specially developed software for this process. By studying the effects of various factors (concentration of the initial components, temperature, current density, etc.), the optimal electrolysis mode and electrolyte composition for the co-deposition process were selected. Based on these results, Student and Fisher criteria were assigned for future purposes and regression coefficients were estimated. The obtained regression equation determines the electrolyte content and the electrolysis conditions, which allows precipitating the Sb–Se alloy containing the required amount of Sb. Calculations and experimental results show that the error of the regression equation for obtaining the Sb–Se alloy is =6.4%.

Enhancing Surface Properties for Electrodeposited Cu(In,Ga)Se2 Films by (NH4)2S Solution at Room Temperature

Fabricating chalcopyrite Cu(In,Ga)Se2 (CIGS) films from electrodeposition is one of the most effective ways to lower the production cost and energy consumption compared with that from vacuum method...

Surface Morphology and Phase Composition of (Zn,Sn)Se Thin Films, Obtained by Chemical-Molecular Beam Deposition

(Zn,Sn)Se films of different chemical composition ratio Zn/Sn were obtained by chemical molecular beam deposition (CMBD) method on borasilicate glass with variation the ratio of the partial pressure of ZnSe and SnSe in vapour phase during growth. Chemical and phase composition, as well as surface morphology of samples were investigated. The samples were found to be a thin films with a thickness of 2–3 μm with and evenly distribution of chemical elements. Grain structure, phase composition and surface roughness parameters dependencies on the elemental composition are determined.

Voltage-dependent capacitance and conductance in Se films

Voltage-dependent capacitance and conductance in Se films

ICMMS-2: Influence of Gamma Radiation on Nonlinear Optical, Semiconducting and Dielectrical Properties of In0.95Mn0.05Se Thin Films

In0.95Mn0.05Se films with of thickness 750 nm were evaporated by using thermal evaporation technique, this film was irradiated by γ radiation with doses (0,40 and 120 KGy). Both of dispersion energy (Ed) and oscillating energy (Eo) were determined. The values of lattice dielectric constant (eL) and free carrier concentration/effective mass) (N/m*) were calculated. On the other hand, the values of first order of moment (M-1), the third order of moment (M-3) and static refractive index (no), were determined. Both of dielectric loss (e) and dielectric tangent loss (e\) for these films increased with photon energy (hν). Also, the same behavior was noticed for the real part of optical conductivity (σ1) and imaginary part of optical conductivity (σ2). The Linear optical susceptibility (χ(1)) increases with (hν)for all compositions. The nonlinear optical parameters such as, nonlinear refractive index (n2), the third-order nonlinear optical susceptibility (χ(3)), non-linear absorption coefficient (βc) , were determined theoretically. Both of the electrical susceptibility (χe) and relative permittivity (er) increase with photon energy and had a highest value near the energy gap. The semiconducting results such as, density of the valence band, conduction band and Fermi level position (Ef) were calculated.

Challenges in the deposition of (Ag,Cu)(In,Ga)Se2 absorber layers for thin-film solar cells

The partial replacement of Cu by Ag in Cu(In,Ga)Se2 thin-film solar cells is strategically interesting to achieve smooth devices with high conversion efficiencies. Yet, the industrial exploitation requires further understanding of the deposition process and control of the absorber layer properties. In this study, three-stage co-evaporation of (Ag,Cu)(Ga,In)Se2 films with [Ag]/([Ag] + [Cu]) contents up to 0.2 was investigated. Deep crevices and voids, sometimes extending down to the rear contact, were found. They mainly occur for high Ag contents and excessive group-I richness during the second stage of the deposition. The formation of cavities is attributed to the segregation of Ag–Se phases and slow Ag diffusion into the chalcopyrite during the deposition. Another identified challenge is the flattening of the desired bandgap grading which is correlated with the Ag content. Optimized process conditions allow fabrication of smooth (Ag,Cu)(Ga,In)Se2 films in a manufacturing-like inline deposition with cell efficiencies up to 20.5%.

Insights from Transient Absorption Spectroscopy into Electron Dynamics Along the Ga‐Gradient in Cu(In,Ga)Se2 Solar Cells

AbstractCu(In,Ga)Se2 solar cells have markedly increased their efficiency over the last decades currently reaching a record power conversion efficiency of 23.3%. Key aspects to this efficiency progress are the engineered bandgap gradient profile across the absorber depth, along with controlled incorporation of alkali atoms via post‐deposition treatments. Whereas the impact of these treatments on the carrier lifetime has been extensively studied in ungraded Cu(In,Ga)Se2 films, the role of the Ga‐gradient on carrier mobility has been less explored. Here, transient absorption spectroscopy (TAS) is utilized to investigate the impact of the Ga‐gradient profile on charge carrier dynamics. Minority carriers excited in large Cu(In,Ga)Se2 grains with a [Ga]/([Ga]+[In]) ratio between 0.2–0.5 are found to drift‐diffuse across ≈1/3 of the absorber layer to the engineered bandgap minimum within 2 ns, which corresponds to a mobility range of 8.7–58.9 cm2 V−1 s−1. In addition, the recombination times strongly depend on the Ga‐content, ranging from 19.1 ns in the energy minimum to 85 ps in the high Ga‐content region near the Mo‐back contact. An analytical model, as well as drift‐diffusion numerical simulations, fully decouple carrier transport and recombination behaviour in this complex composition‐graded absorber structure, demonstrating the potential of TAS.

Stimulated Emission of Thin Cu(In, Ga)Se2 Films Irradiated by Protons

Spontaneous and stimulated emission (SE) of thin Cu(In,Ga)Se2 films, deposited on sodium-containing glass substrates and irradiated by protons with an energy of 2.5 keV and doses of 1014–1017 cm–2, were investigated upon excitation by nanosecond laser pulses with the power density from 5 to 100 kW/cm2. An increase in the intensity and a decrease in the SE appearance threshold were found for the films irradiated by protons with doses of 1014–1015 cm–2, in comparison with nonirradiated films. An increase in the SE threshold and a decrease in the intensity of SE and spontaneous emission were observed at the irradiation dose of 1016 cm–2. After a dose of 1017 cm–2 the intensity of emission decreased sharply and the SE threshold was not reached. Possible reasons of the observed effects are discussed.