Selenium Films Research Articles

Angle-Dependent Photoinduced Changes of Near-Infrared Transmission in Amorphous Selenium Films

Angle-Dependent Photoinduced Changes of Near-Infrared Transmission in Amorphous Selenium Films

Processing of nanocrystalline thin films of selenium and formation of FTO/p-Se/n-CdS/In heterojunctions for photovoltaic response

Thin films of nanocrystalline selenium (Se) of different thicknesses were grown on annealing from 323 to 398 K. The crystallinity of the layers increased with annealing temperatures (TA). The temperature thickness correlations were established. The layers exhibited spherical-shaped particles at higher annealing temperatures. The grown layers possess hexagonal crystal system with space group P3121(152); it shows prominent planes along the a-axis. The bandgap (Eg) decreases from 2.18 to 1.94 eV, 2.12 to 1.82 eV, and 2.13 to 1.84 eV with changes in TA of 700, 270, and 150 nm thick layers, respectively. The high absorption coefficient (α) values ∼ 1 × 105 cm−1 observed in all films makes it suitable as an absorber layer in a solar cell structure and an optical sensor. The FTO/p-Se/n-CdS/In heterojunction diodes provide zero-bias barrier height (фbo) of 0.788(7) eV. The open circuit voltage (Voc) of FTO/p-Se (150 nm)/n-CdS(200 nm)/In a photovoltaic cell was observed to be 55 mV.

Synthesis and Characterization of Amorphous Selenium, Cadmium and Silver Selenide Thin Films on Polyamide-6

Increasing photon absorption by capturing light is an important way to increase the efficiency of photovoltaic devices. In this regard, the small optical band gap (Eg) and high absorption coefficient of Se-containing thin nanofilms make them ideal for next generation photovoltaic devices based on selenides. Amorphous selenium was introduced into polyamide-6 (PA 6) via a chemical synthesis in a bath and the influence of the products of its reaction with Cd2+ and Ag+ ions on the film phase composition, topographic and optical properties were evaluated. AFM data have revealed that the surface roughness of the a-Se/PA 6 composite noticeably increases compared to that of unreacted PA 6. However, at later stages of film deposition, the roughness decreases, and the thin film becomes smoother and uniform. The incorporation of solid inorganic nanoparticles into flexible polyamide network causes chain stretching, which has been confirmed by ATR-FTIR spectroscopy data. The data of X-ray diffraction analysis, depending on the stage of synthesis, showed the crystalline composition of the film with peaks of Se8, CdSe, Ag2Se and Ag, which may explain the observed optical properties. The optical properties of the composites indicate a shift in the band gap from 4.46 eV for PA 6 to 2.23–1.64 eV upon the stepwise deposition of amorphous Se, CdSe and Ag2Se. Eg is conveniently located in the visible region of solar energy, making the obtained nanofilms ideal for solar energy harvesting.

Enhanced carrier management and photogenerated charge dynamics for selenium (Se) film photovoltaics

Although power conversion efficiency (PCE) of selenium (Se) solar cells has been improved, the device performance is capped by unexpected open-circuit voltage(VOC), short-circuit current (JSC), and fill factor (FF). The Se solar cells with tin oxide (SnO2) as the n-type buffer layer (NBL) have been reported. Here, the polyacrylic acid with carboxylic functional groups was added into the Alfa-SnO2 colloid to impede SnO2 aggregation during spin-coating. The superior Alfa-SnO2 film compactness with improved film coverage can enhance carrier management. The decreased conduction band offset at NBL/Se can promote rapid carrier injection from Se to Alfa-SnO2. Furthermore, the electrostatic attraction between carboxylic groups and Se atoms will passivate interface defects and promote charge extraction. The increased external quantum efficiency with high photovoltaic parameters (VOC, JSC, and FF) will evidently improve the device performance. Ultimately, a high PCE of 5.82% (a VOC of 0.870 V, a JSC of 10.77 mA cm−2, and a FF of 0.621) for the best-performing Se solar cells is achieved. Also, the photogenerated carrier separation, transport, and extraction processes have also been measured to determine the charge dynamics. The conformal P-Alfa-SnO2 films can largely enhance the light-harvesting performance, and the colloidal Alfa-SnO2 will open up a simple and effective way for improving device performance of Se solar cells.

Study of linear and non-linear optoelectronic properties of thermally grown thin films of amorphous selenium doped with graphene, multiwalled carbon nanotubes, and silver nanoparticles

The present research covers the impact of doping of different nano-materials [Graphene, Multiwalled Carbon Nanotubes (MWCNTs), and Silver nanoparticles (Ag: NPs)] on the optoelectronic properties of selenium glass. For this, we have prepared the amorphous thin films (ATFs) of pure selenium and nano-additives doped Se and the systematic optical characterization of ATFs of as-prepared samples has been done. We have examined the optical characteristics of prepared ATFs by collecting the experimental spectral data of reflection/transmission in the range of 400 nm to 2500 nm. Knowing the spectral variation of the dispersion parameters, we have calculated the values of the optical band gap Egopt and Urbach tail Ee. The indirect optical transition is found in the present samples that are confirmed by the transition power factor m. The values of real component (i.e., ε') and imaginary components (i.e., ε″) of the dielectric constant and its value in the high-frequency range (i.e., ε∞), as well as single-oscillator dispersion parameters, are also determined. The value of infinite dielectric constant (ε∞) of ATF of parent selenium (ε∞ = 2.65) is increased significantly after the doping of graphene (ε∞ = 3.82), MWCNT (ε∞ = 4.37), and Ag: NPs (ε∞ = 6.19). Further, we have investigated the dependence of tan δ (dissipation factor), τ (relaxation time), optical/electrical conductivities, and volume/surface energy loss functions on photon energy hν. The nonlinear properties of the samples are discussed in terms of their susceptibility and nonlinear refractive index.

Desorption characteristics of selenium and tellurium thin films

The temperature-dependent desorption behavior of selenium and tellurium is investigated using a heated quartz crystal microbalance. Prior to heating the quartz crystal microbalance, selenium and tellurium films with varying thickness were deposited using thermal effusion cells in a molecular beam epitaxy system for subsequent determination of temperature-dependent mass loss of the deposited films. The desorption rate for tellurium was found to exhibit one sharp peak around 190 °C, indicating the loss of the entire film irrespective of film thickness within a temperature window of 20 °C, which was completely evaporated at 200 °C. Similar experiments for selenium revealed that the thermal desorption took place via a two-stage process with a smaller portion of the material desorbing within an even narrower temperature window of 5 °C at a much lower peak temperature of 65 °C, while most selenium desorbed within a temperature range of 10 °C around 90 °C. This two-stage behavior indicated the presence of at least two chemically distinct selenium species or binding states. The direct and quantitative determination of the chalcogen desorption process provides important insights into the kinetics of chalcogenide-based film growth and is in addition of applied benefit to the research community in the area of Se/Te capping and decapping of air sensitive materials as it provides temperature ranges and rates at which full desorption is achieved. Our work furthermore points toward the need for a more detailed understanding of the chemical composition state of atomic and molecular beams supplied from thermal evaporation sources during growth.

Surface Morphology and Optical Properties of Glassy Selenium Films after Laser Modification

Surface Morphology and Optical Properties of Glassy Selenium Films after Laser Modification

Strong bulk photovoltaic effect and second-harmonic generation in two-dimensional selenium and tellurium

Few-layer selenium and tellurium films have been recently prepared, and they provide a new platform to explore novel properties of two-dimensional (2D) elemental materials. In this work, we have performed a systematic first-principles study on the electronic, linear, and nonlinear optical (NLO) properties of atomically thin selenium and tellurium films within the density-functional theory with the generalized gradient approximation plus scissors correction using the band gaps from the relativistic hybrid Heyd-Scuseria-Ernzerhof functional calculations. Interestingly, we find that few-layer Se and Te possess large second-harmonic generation (SHG), linear electro-optic effect, and bulk photovoltaic effect. In particular, trilayer (TL) Te possesses large SHG coefficient, being more than 65 times larger than that of GaN, a widely used NLO material. Bilayer (BL) Te has huge static SHG coefficient, being more than 100 times larger than that of GaN. Furthermore, monolayer (ML) Se possesses large SHG coefficient. Moreover, we predict that TL Te exhibits strong bulk photovoltaic effect (BPVE), being greater than that of GeS, a polar system with the largest BPVE found so far. Although the shift current conductivities of bulk and 2D Se are comparable, the shift current conductivities of TL Te are five times larger than those of bulk Te. Finally, an analysis of the calculated electronic band structures indicates that the strong NLO responses of 2D Se and Te materials are primarily derived from their low-dimensional structures with high anisotropy, directional covalent bonding, lone-pair electrons, and relatively small band gaps. These findings provide a practical strategy to search for excellent NLO and BPVE materials.

High-energy micrometre-scale pixel direct conversion X-ray detector.

The objective of this work was to fabricate and characterize a new X-ray imaging detector with micrometre-scale pixel dimensions (7.8 µm) and high detection efficiency for hard X-ray energies above 20 keV. A key technology component consists of a monolithic hybrid detector built by direct deposition of an amorphous selenium film on a custom designed CMOS readout integrated circuit. Characterization was carried out at the synchrotron beamline 1-BM-B atthe Advanced Photon Source of Argonne National Laboratory. The direct conversion detector demonstrated micrometre-scale spatial resolution with a 63 keV modulation transfer function of 10% at Nyquist frequency. In addition, spatial resolving power down to 8 µm was determined by imaging a transmission bar target at 21 keV. X-ray signal linearity, responsivity and lag were also characterized in the same energy range. Finally, phase contrast edge enhancement was observed in a phase object placed in the beam path. This amorphous selenium/CMOS detector technology can address gaps in commercially available X-ray detectors which limit their usefulness for existing synchrotron applications at energies greater than 50 keV; for example, phase contrast tomography and high-resolution imaging of nanoscale lattice distortions in bulk crystalline materials using Bragg coherent diffraction imaging. The technology will also facilitate the creation of novel synchrotron imaging applications for X-ray energies at or above 20 keV.

Mass Transfer Study on Improved Chemistry for Electrodeposition of Copper Indium Gallium Selenide (CIGS) Compound for Photovoltaics Applications.

Copper indium gallium selenium (CIGS) films are attractive for photovoltaic applications due to their high optical absorption coefficient. The generation of CIGS films by electrodeposition is particularly appealing due to the relatively low capital cost and high throughput. Numerous publications address the electrodeposition of CIGS; however, very few recognize the critical significance of transport in affecting the composition and properties of the compound. This study introduces a new electrolyte composition, which is far more dilute than systems that had been previously described, which yields much improved CIGS films. The electrodeposition experiments were carried out on a rotating disk electrode, which provides quantitative control of the transport rates. Experiments with the conventional electrolyte, ten times more concentrated than the new electrolyte proposed here, yielded powdery and non-adherent deposit. By contrast, the new, low concentration electrolyte produced in the preferred potential interval of −0.64 ≤ E ≤ −0.76 V vs. NHE, a smooth and adherent uniform deposit with the desired composition across a broad range of rotation speeds. The effects of mass transport on the deposit are discussed. Sample polarization curves at different electrode rotation rates, obtained in deposition experiments from the high and the low concentration electrolytes, are critically compared. Characterization of the overall efficiency, quantum efficiency, open circuit voltage, short circuit current, dark current, band gap, and the fill factor are reported.

Simulation and Experimental Validation of the Uniformity of Thermally Evaporated Amorphous Selenium Films for Large-Area Imaging and Radiation Detection Applications

Amorphous selenium is one of the best photoconductors with a wide range of applications, from its early use in the photocopy industry to its present application in X-ray imaging. Low melting point and high vapor pressure of amorphous selenium enable uniform and economical deposition of this photoconductive material up to few millimeters thick. However, there is a tradeoff between deposition rate and film uniformity. In this article, a thermal evaporation system for amorphous selenium deposition was developed. The system geometry was simulated to determine the optimized source–substrate throw distance for uniform film thickness of amorphous selenium (a-Se) on a substrate with up to 4-in diameter. Boat and crucible source configurations of equal volume (100 cc) were tested. The film was fabricated, characterized, and the uniformity and thickness were measured and compared with simulation results.

Electroplating Kinetic of Nanostructured Selenium Films from Citrate Bath

In this work; Cyclic-Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) were used to study the electrodeposition kinetic of selenium films in potentiostatic mode from aqueous solution containing selenium dioxide and sodium citrate at pH = 4.2. Semiconducting proprieties of obtained deposits were investigated by Mott-Schottky measurements whereas the optical ones were performed by UV–Visible spectrophotometry. The morphological characterization was carried out using the scanning electron microscopy (SEM). The obtained results showed that the electrodeposition process of selenium films in citrate bath occurred under diffusional regime as rate-limiting step. Deposition rate of selenium layers on platinum substrate is much superior than in the case of ITO substrate and up to a value of 0.65 μg/cm2 s. The HSeO3−/Se system becomes more rapid with the increase of the bath temperature. Obtained deposits are photoactive films that belong to p-type semiconductors with number of charge carriers in order of 1021/cm3 and energy band gap about 1.7 eV. The grains of electroplated films have spherical forms, nanometric sizes and strong adhesion on the substrate surface.

ELECTROCHEMICAL SYNTHESIS OF IRON MONOSELENIDE THIN FILMS

In the presented research work, the kinetics and mechanism of the deposition process of thin iron, selenium and Fe-Se films have been studied by recording a cyclic and linear polarization curves by potentiodynamic method using Pt and Ni electrodes. Individual and co-deposition potential areas of the components of the electrolyte on the Pt electrode were determined. In order to determine the optimal electrolysis mode and electrolyte composition, the effect of various factors (concentration of initial components, temperature, etc.) on the co-electrodeposition process of Fe-Se was studied. In addition, Fe-Se samples deposited on the surface of Ni electrodes were thermally treated at 4500C and studied by SEM and X-ray phase analysis methods. Elemental analysis of the films shows that they contain 42.2% Fe and 57.8% Se.

Aperiodic Schrodinger Crystals

In 1945, E. Schrödinger predicted the possibility of the existence of “aperiodic crystals,” that is, more and more increasing crystalline aggregates, but without a simple lattice repetition mechanism. In the present work, such structures are experimentally prepared using selenium as an example. By thermal gradient treatment of the amorphous selenium film, we have obtained nano-thin spatial dissipative structures (SDS) of hexagonal selenium, the lattice of which undergoes non-uniform rotational curvature around, in the general case, three mutually perpendicular directions. The formation of these nano-thin SDS of hexagonal selenium occurs as a result of cooperative rotations of selenium macromolecules around, in general, three mutually perpendicular directions. Since nano-thin SDS after hardening in air have all the features of crystals each of the above nano-thin SDS of hexagonal selenium obtained at different annealing temperatures corresponds to the concept of “aperiodic crystals” Thus, “aperiodic crystals” were obtained experimentally, the possibility of the existence of which was predicted by E. Schrödinger.

Nanofine spatial dissipative structures with azimutal lattice curvation

The nanofine spatial dissipative structures (NSDS) were characterized by transmission electron microscopy and electron diffraction. The NSDS obtained by thermogradiently processing an amorphous selenium film by unilateral heating of its lower surface at T = 413 K preliminarily. The results indicate that the NSDS of hexagonal selenium obtained in an amorphous film possess an azimuthal curvature of the lattice and a nonlinear fan-shaped system of flexural contours on their electron microscopic image. The lattice of the above NSDS undergoes elastic – plastic rotational curvature around three mutually perpendicular directions. The lattice rotation angles of hexagonal selenium NSDS reach: around [001], – 25°, around the direction perpendicular [001] and lying in the plane of the amorphous film – 32°, around the direction perpendicular to the first two ones and not lying in the plane of the amorphous film - azimuthal curvature of the lattice, – 35°. Thus, as a result of electron-microscopic and microdiffraction studies, it was found that NSDS of hexagonal selenium with azimuthal curvature of the lattice, causing the curvature of its habitus, are in a nonequilibrium state.

Inverted Solar Cells with Thermally Evaporated Selenium as an Active Layer

Because of its less toxicity and direct band gap, selenium (Se) has been considered as a promising single-element absorber in photovoltaics. In this work, a vacuum-evaporated selenium film has been...

Laser-induced modification of optical properties of glassy selenium films synthesized by vacuum thermal evaporation

The influence of laser radiation at the wavelength of 632.8 nm on the optical properties of ~ 1 µm thick Se (a-Se) glassy films synthesized by vacuum thermal evaporation on quartz substrates has been studied. It is established that an insignificant decrease in their transmittance coefficients (T) and optical gap energy (Eg) is observed after the laser irradiation of the obtained films with a power density of 80 W⋅cm−2. For the first time, an increase in the reflection coefficient (R) and refractive index (n) of the films was detected in the course of long-term relaxation of their amorphous matrix to a more stable state caused by phase changes in a-Se films. It was established that a significant decrease in transmittance and reflection coefficients, refractive index, and Eg occurs after the laser irradiation of a-Se films with a higher power density (5300 W⋅cm−2).

Evidence for narrow mid-gap defect band in amorphous selenium from low-temperature anomalous electrical properties

Electrical properties of amorphous selenium films (~2.1 μm thick) were investigated at different temperatures between 225 K and room temperature. The samples were prepared using thermal evaporation method. The capacitance and conductance were measured and analyzed to study their changes with temperature and voltage. An anomalous peak is found in the conductance- and capacitance-temperature curves at ~245 K. The phenomenon can be linked to the distribution of defect states near the mid-gap and the change in ‘effective transport energy’ at which the transport takes place.

Laser-induced crystallization of monoclinic nanowires in glassy selenium films

The authors investigate influence of 632.8 nm laser irradiation on structural properties of thin glassy (amorphous) Se films synthesized on quartz substrates using vacuum thermal evaporation. They first propose the fact that low power laser irradiation of Se films (power density PS=80 W cm−2) initiates phase transformations, accompanied by growth of monocrystalline nanowires of metastable phase of monoclinic beta-Se8 in the amorphous matrix at room temperature. New structural properties of Se films are proven to be long term stable at room temperature. It is shown that laser irradiation at PS ≥ 800 W cm−2 induces synthesis of a mixture of nanocrystallites of trigonal Se and monoclinic alpha-Se8 in the amorphous films and their mass crystallization with growth of spherulites at room temperature.

Defect absorption in selenium films by photothermal deflection spectroscopy

Sub-gap absorption spectra of selenium films are investigated by photothermal deflection spectroscopy. The selenium films are prepared by vacuum evaporation of selenium pellets. Raman spectroscopy reveals that as-deposited films are amorphous, and the films annealed at 100 °C are trigonal crystal. Photothermal deflection spectroscopy is extended to infrared light of 0.31 eV with maintaining high sensitivity, and detects weak absorption at energies below the band gap. Five absorption peaks and tail absorption are observed in selenium films, and the absorption peak energies are 1.32, 1.08, 0.47, 0.41 and 0.34 eV, respectively. These absorption tail and peaks are derived from selenium, and the origin of these absorptions is explained based on the oxygen impurity and the defect structure of the selenium film.