Se Films Research Articles

Enhanced thermoelectric properties in Cu₁.₈Se thin films: Achieving superior power factor through phase control and optimal deposition temperature

Thermoelectric thin films are vital for efficient energy conversion and thermal management. XRD analysis reveals that Cu₁.₈Se films exhibit a mixed α-phase when deposited at room temperature, 100 °C, and 300 °C, but transition to a pure β-phase at 200 °C. The formation of the pure β-Cu₁.₈Se phase at 200 °C significantly improves the crystallinity of the films. Increased annealing temperatures lead to greater surface roughness and grain size as observed by AFM and FESEM. Electrical conductivity decreases with higher measurement temperatures, reflecting degenerate semiconductor behavior due to Cu vacancies. The sample deposited at 200 °C, exhibiting the pure β-phase, achieves the highest power factor of 5456 μWm⁻1K⁻2 and improved Seebeck coefficient, underscoring the importance of phase purity and controlled surface roughness for optimal thermoelectric performance.

Standing 1D Chains Enable Efficient Wide-Bandgap Selenium Solar Cells.

The recent surge in tandem solar cells and indoor photovoltaics has renewed interest in selenium (Se), the world's first photovoltaic material, due to its intrinsic wide bandgap of ≈1.9eV, high stability, and non-toxicity in small quantities when applied in photovoltaics. However, with a 1D chained crystal structure, Se tends to grow crystalline films with a lying orientation-chains parallel to substrates arising from the low surface energy; this results in poor carrier transport across chains held together by weak van der Waals forces. Here a substrate-heating strategy that facilitates the interfacial bonding between Se and substrate is introduced, enabling the growth of Se films with a standing orientation-chains perpendicular to substrates. This achieves efficient carrier transport along covalently bonded chains. The resulting Se films thereby exhibit a fourfold increase in carrier mobility compared to lying-oriented Se films. Consequently, Se solar cells are achieved with the highest power conversion efficiency of 8.1% under AM1.5G 1-sun illumination. The unencapsulated devices exhibit negligible efficiency loss after 1000 h of storage under ambient conditions.

Lanthanide‐Like Contraction Enables the Fabrication of High‐Purity Selenium Films for Efficient Indoor Photovoltaics

AbstractThe lanthanide contraction involves a reduction in atomic radius among f‐block elements below the expected level. A similar contraction is observed in group‐16 elements. The atomic radius of Se (117 pm) is slightly larger than that of S (104 pm) arising from the presence of d electrons, compared to the significant increase in atomic radius from O (73 pm) to S. This lanthanide‐like contraction contributes to Se's robust oxidative resistance. Here we report a selective oxidation strategy utilizing Se's strong antioxidative property to remove coexisting narrow‐band gap Te impurities from Se feedstocks. This strategy selectively oxidizes volatile Te impurities into involatile TeO2 that remains in the evaporation source, while only volatile Se deposits onto the substrate during the thermal‐evaporation deposition process. This enables the fabrication of high‐purity Se films possessing a wide band gap of 1.88 eV, ideally suited to the optimal band gap for indoor photovoltaics (IPVs). The resulting Se photovoltaics exhibit an efficiency of 20.1 % under 1000‐lux indoor illumination, outperforming market‐dominant amorphous silicon and all types of lead‐free perovskite IPVs. Unencapsulated Se devices show no efficiency degradation after 20,000 hours of storage in ambient atmosphere.

Lanthanide-Like Contraction Enables the Fabrication of High-Purity Selenium Films for Efficient Indoor Photovoltaics.

The lanthanide contraction involves a reduction in atomic radius among f-block elements below the expected level. A similar contraction is observed in group-16 elements. The atomic radius of Se (117 pm) is slightly larger than that of S (104 pm) arising from the presence of d electrons, compared to the significant increase in atomic radius from O (73 pm) to S. This lanthanide-like contraction contributes to Se's robust oxidative resistance. Here we report a selective oxidation strategy utilizing Se's strong antioxidative property to remove coexisting narrow-band gap Te impurities from Se feedstocks. This strategy selectively oxidizes volatile Te impurities into involatile TeO2 that remains in the evaporation source, while only volatile Se deposits onto the substrate during the thermal-evaporation deposition process. This enables the fabrication of high-purity Se films possessing a wide band gap of 1.88 eV, ideally suited to the optimal band gap for indoor photovoltaics (IPVs). The resulting Se photovoltaics exhibit an efficiency of 20.1 % under 1000-lux indoor illumination, outperforming market-dominant amorphous silicon and all types of lead-free perovskite IPVs. Unencapsulated Se devices show no efficiency degradation after 20,000 hours of storage in ambient atmosphere.

Synergic Effect of N and Se Facilitates Photoelectric Performance in Co-Hyperdoped Silicon.

Femtosecond-laser-fabricated black silicon has been widely used in the fields of solar cells, photodetectors, semiconductor devices, optical coatings, and quantum computing. However, the responsive spectral range limits its application in the near- to mid-infrared wavelengths. To further increase the optical responsivity in longer wavelengths, in this work, silicon (Si) was co-hyperdoped with nitrogen (N) and selenium (Se) through the deposition of Se films on Si followed by femtosecond (fs)-laser irradiation in an atmosphere of NF3. The optical and crystalline properties of the Si:N/Se were found to be influenced by the precursor Se film and laser fluence. The resulting photodetector, a product of this innovative approach, exhibited an impressive responsivity of 24.8 A/W at 840 nm and 19.8 A/W at 1060 nm, surpassing photodetectors made from Si:N, Si:S, and Si:S/Se (the latter two fabricated in SF6). These findings underscore the co-hyperdoping method's potential in significantly improving optoelectronic device performance.

Investigation of Physical Properties of Nanostructured Selenium-Based Compound Semiconductors

The study of physical properties of compound semiconductors plays a vital role in the device fabrication point of view. Different investigations suggest that one can tune these properties by impurity dopants, irradiation techniques, adopting different synthesis techniques, etc. In this study, the physical properties of nanostructured thin films of Se–S compound were tuned by different Hg dopant concentrations. X-ray diffraction spectra clarify the preferred crystallite growth occurs in the cubic phase of the corresponding Bravi’s crystal system. The particle size calculated from Bragg’s reflection spectra by using Scherrer’s analytical formula illustrates an enhancement of crystallite size in highly concentrated Hg-doped sample. Similar consequences are found in surface morphological micrographs. The optical band gap reduced from 1.65[Formula: see text]eV to 1.54[Formula: see text]eV on increasing the doping concentration of Hg, according to the computed optical parameters using optical transmission spectra in the wavelength range 200–1100[Formula: see text]nm. The thin-film refractive index dispersion data fits well in accordance with the single oscillator model. Investigation of electrical properties tells that there is significant improvement in electrical conductivity in highly concentrated Hg-doped samples. Therefore, the apparent tuning as observed in the physical properties of the investigated material may have practical uses in industry for a variety of optoelectronic devices.

Controllable synthesis of Ag2Se binary thin-film via electrochemical atomic layer epitaxy (ECALE) and its characterization

Herein, synthesis of Ag2Se nanofilms was attempted for the first time at room temperature using electrochemical atomic layer epitaxy (ECALE) in underpotential deposition (UPD) basis. Spectrochemical/electrochemical measurements verified a 2-D growth for the Ag–Se films when the deposition potentials were gradually adjusted to negative (−1 mV/cycle). Electrochemical impedance spectroscopy (EIS) measurements illustrated less interfacial charge transfer resistance at the nanofilm coated surface. Scanning electron microscope (SEM) indicated that the film was composed of crystallites with size of around 110 nm. Energy dispersive X-ray spectroscopy (EDS) analyzes verified the 2:1 stoichiometric ratio for the compound and X-ray diffraction (XRD) analysis confirmed an orthorhombic β-Ag2Se crystalline phase. Ultraviolet–visible (UV–Vis) absorption measurements were performed to estimate the band gap of Ag2Se thin-film. Contact angle (CA) measurements demonstrated a decreased surface wettability. The proposed ECALE approach offered a low-cost and a facile method for the synthesis of epitaxial Ag2Se nanofilms that could be controlled at the atomic level.

Synergistic Defect Management for Boosting the Efficiency of Cu(In,Ga)Se2 Solar Cells

In this study, a feasible strategy is proposed for directly depositing high-quality Cu(In,Ga)Se2 (CIGS) films using Na-doped targets in a selenium-free atmosphere to boost the power conversion efficiency (PCE) of CIGS solar cells. Introducing a small amount of sodium dopant effectively promoted the textured growth of CIGS crystals in the prepared films, resulting in larger grain sizes and a smoother interface. The higher MoSe2 content at the CIGS/Mo interface increased the carrier lifetime in the films. In addition, sodium doping increased the proportion of Se atoms on the film surface and reduced the concentration of defects caused by the direct sputtering of the films in the selenium-free atmosphere. Therefore, the separation and transportation of photo-generated carriers in the devices were effectively enhanced. Using the optimized parameters, a record-high PCE of 17.26% was achieved for the 7.5% Na-doped devices, which represents an improvement of nearly 63%.

Analysis of the mechanism for enhanced crystalline quality of wide-bandgap Cu(In,Ga)Se2 films by pre-deposited Ag precursor

Wide-bandgap Cu(In,Ga)Se2 (CIGS) solar cells show prospective utility for both single-junction and tandem solar cell applications. However, poor bulk absorber quality and interface recombination limiting the comprehensive utilization of its...

Phase composition and thermoelectric properties of Cu–Ag–Se films with different Ag content

In this study, a series of Cu–Ag–Se films were prepared on Si (100) substrate by magnetron sputtering. The phase composition, micro-structure and thermoelectric properties were studied. The results showed that the phase composition of deposited Cu–Ag–Se films varied from β-Cu2-xSe and a small amount of CuAgSe phase to majority β-Cu2-xSe and minority CuAgSe phase, minority β-Cu2-xSe and majority CuAgSe phase, CuAgSe and a small amount of β-Cu2-xSe phase, Ag2Se and a small amount of CuAgSe +β-Cu2-xSe phase as Ag content increased from 0.4 to 17.0, 20.8, 29.5, 41.4 at.%, respectively. In the deposited Cu–Ag–Se films, the carrier concentration first decreased and then increased and the mobility displayed an opposite trend with increasing CuAgSe content (decreasing β-Cu2-xSe content). The room temperature electrical conductivity and absolute value of room temperature Seebeck coefficient gradually increased with increasing CuAgSe content for the deposited Cu–Ag–Se films. The power factor of 29.5 at.% Ag samples (CuAgSe phase films) at whole measured temperature was the highest in all samples due to high Seebeck coefficient and high electrical conductivity, resulting in the highest PF value of 2.05 mW m−1 K−2 at 62 °C. However, due to its high thermal conductivity of 10.93 W m−1K−1, the room temperature ZT value of this sample was only 0.05 even if it possessed a higher PF value. Ag2Se phase films possessed the lowest PF value due to their lower Seebeck coefficient even if they possessed higher electrical conductivity.

Highly Sensitive Magnetic Field Sensors Based on a D-Type Fiber With Bi₂O₂ Se Film

Highly Sensitive Magnetic Field Sensors Based on a D-Type Fiber With Bi₂O₂ Se Film

One‐Step DcMS and HiPIMS Sputtered CIGS Films from a Quaternary Target

Using a quaternary compound target, Cu(In,Ga)Se2 films are prepared using one‐step, selenization‐free direct current magneton sputtering (DcMS) and high power impulse magnetron sputtering (HiPIMS) methods. This study investigates how the sputtering power affects the composition, microstructure, morphology, and electrical characteristics of the films. Film crystallinity is found to be affected by the sputtering power utilized. The films deposited at 0.25 kW are amorphous, whereas those formed at 0.5–1 kW display a chalcopyrite structure with a (112)–preferred orientation. With increased sputtering power, the films’ crystal quality improves, displaying a homogeneous and compact morphology free of peeling and cracking. Elemental measurement of the CIGS films reveals that, depending on the deposition method, the film composition deviates from that of the target. The electrical properties of the deposited films vary with increasing sputtering power.

A computational study for understanding the effect of various parameters on the performance of three different CI(G)Se-based thin film solar cells

The copper indium selenium/copper indium gallium selenium (CI(G)Se)-based thin film solar cells (TFSCs) have been fascinating in the photovoltaic market due to their potential to attain high efficiency of solar cells and modules at relatively little cost. This research work introduces the simulation of the CI(G)Se TFSCs through SCAPS software. Some parameters like thickness, bandgap, and carrier concentration of semiconducting materials used in the CI(G)Se TFSCs were optimized to get the solar cell efficiency as high as possible. The optimized efficiencies for CISe, CIGSe, and CIGSe bilayer TFSCs were 22.81, 27.32, and 27.99%, respectively. It is seen from the results that the device’s performance using two absorber layers in CI(G)Se TFSCs was comparatively higher than using a single absorber layer. This outcome is mainly due to the better absorption of photons in CI(G)Se TFSCs containing two absorber layers. The SCAPS software also analyzed the experimentally obtained parameters of CI(G)Se and CdS thin films and noticed more than 20% efficiency, showing potential parameters to use in commercial applications. The effect of defects found in the CI(G)Se absorber layer, CdS buffer layer, and CdS/CI(G)Se interface on the solar cell parameters are primarily studied here. It is observed that the increasing defects in the CI(G)Se TFSCs could impact negatively the device’s performance by recombining the generated charge carriers. Better results were found for CI(G)Se TFSCs at a lower work temperature (by reducing the saturation current, and at a smaller series resistance value as well as a larger shunt resistance value (by reducing the alternative paths for generated charge carriers). Therefore, understanding these results could provide complete information on the optimization process that can serve as a guide to achieve highly efficient electronic devices experimentally.

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.

Remarkable average thermoelectric performance of the highly oriented Bi(Te, Se)-based thin films and devices

Bi(Te, Se)-based compounds have attracted lots of attention for nearly two centuries as one of the most successful commercial thermoelectric (TE) materials due to their high performance at near room temperature. Compared with 3D bulks, 2D thin films are more compatible with modern semiconductor technology and have unique advantages in the construction of micro- and nano-devices. For device applications, high average TE performance over the entire operating temperature range is critical. Herein, highly c-axis-oriented N-type Bi(Te, Se) epitaxial thin films have been successfully prepared using the pulsed laser deposition technology by adjusting the deposition temperature. The film deposited at ∼260 °C demonstrate a remarkable average power factor (PFave) of ∼24.4 μW·cm−1·K−2 over the temperature range of 305–470 K, higher than most of the state-of-the-art Bi(Te, Se)-based films. Moreover, the estimated average zT value of the film is as high as ∼0.81. We then constructed thin-film TE devices by using the above oriented Bi(Te, Se) films, and the maximum output power density of the device can reach up to ∼30.1 W/m2 under the temperature difference of 40 K. Predictably, the outstanding average TE performance of the highly oriented Bi(Te, Se) thin films will have an excellent panorama of applications in semiconductor cooling and power generation.

Pair density wave state in a monolayer high-Tc iron-based superconductor.

The pair density wave (PDW) is an extraordinary superconducting state in which Cooper pairs carry non-zero momentum1,2. Evidence for the existence of intrinsic PDW order in high-temperature (high-Tc) cuprate superconductors3,4 and kagome superconductors5 has emerged recently. However, the PDW order in iron-based high-Tc superconductors has not been observed experimentally. Here, using scanning tunnelling microscopy and spectroscopy, we report the discovery of the PDW state in monolayer iron-based high-Tc Fe(Te,Se) films grown on SrTiO3(001) substrates. The PDW state with a period of λ ≈ 3.6aFe (aFe is the distance between neighbouring Fe atoms) is observed at the domain walls by the spatial electronic modulations of the local density of states, the superconducting gap and the π-phase shift boundaries of the PDW around the vortices of the intertwined charge density wave order. The discovery of the PDW state in the monolayer Fe(Te,Se) film provides a low-dimensional platform to study the interplay between the correlated electronic states and unconventional Cooper pairing in high-Tc superconductors.

The Discovery of a High-Mobility Two-Dimensional Bismuth Oxyselenide Semiconductor and Its Application in Nonvolatile Neuromorphic Devices.

The development of two-dimensional (2D) electronics is always accompanied by the discovery of 2D semiconductors with high mobility and specific crystal structures, which may bring revolutionary breakthrough on proof-of-concept devices and physics. Here, Bi3O2.5Se2, a 2D bismuth oxyselenide semiconductor with non-neutral layered crystal structure is discovered. Ultrathin Bi3O2.5Se2 films are readily synthesized by chemical vapor deposition, displaying tunable band gaps and high room-temperature field-effect mobility of >220 cm2 V-1 s-1. Moreover, the as-synthesized Bi3O2.5Se2 nanoplates were fabricated into top-gated transistors with a simple device configuration, whose carrier density can be reversibly regulated in the range of 1014 cm-2 just by a facile method of electrostatic doping at room temperature. These features enable it to be functionalized into nonvolatile synaptic transistors with ultralow operating energy consumption (∼0.5 fJ), high repeatability, low operating voltage (0.1 V), and long retention time. Our work extends the family of bismuth oxyselenide 2D semicondutors.

Observation of anion-stoichiometry phase separation in Fe(Te,Se) film

Nanostructure control is of fundamental and technological importance because various properties such as vortex pinning in superconductors are determined by the nanostructure. While the nanostructure in superconducting oxides is well controlled, the nanostructure control of iron-based superconductors is required. The nanostructure has not yet been well controlled even in Fe(Te,Se) with the simplest structure among Fe-based superconductors because the anion may be non-stoichiometric and anionic substitution is possible. In this study, we observed the compositional inhomogeneity originating from phase separation in Fe(Te,Se) films, which were prepared by pulsed laser deposition. The films deposited at lower temperature do not show nanoscale compositional inhomogeneity. On the other hand, the high-temperature deposited films contain an inhomogeneous anion distribution with the size of ∼8 nm due to phase separation. The spacing of the compositional inhomogeneity is ∼16 nm. This structure obtained by phase separation is expected to be an effective pinning center in high magnetic fields after further optimization. Thus, the method of nanostructure control other than nanocomposite formation in oxides is demonstrated for Fe(Te,Se).

Long‐term stable and high responsivity visible-ultraviolet photodetector of Se-doped CdO film on Si by spin coating

Se-doped CdO thin films were prepared on p-Si substrates via spin coating. Morphological, structural and absorption measurements of CdO:Se film was performed. Then, a CdS:Se/p-Si heterojunction was produced by coating CdO:Se film on a p-Si substrate using spin coating method. From the I–V measurements, it has been seen that the device has a very good rectification feature in the dark, at room temperature. In order to investigate the performance of the device under light, a detailed analysis was performed by performing I–V measurements under ultraviolet (UV) light (365 and 395 nm, 10 mW cm−2) and different intensities of visible light (between 10 and 125 mW cm−2) as well as ambient light. It was observed that the CdO:Se/p-Si heterojunction performed well under both illumination conditions. The maximum responsivity and specific detectivity values were obtained as 0.72 and 4.47 A W−1 and 3.31 × 109 and 2.05 × 1010 Jones for visible and UV regions, respectively. It was also seen that the device exhibited very high performance and stability even after 160 days.

Combinatorial screening via high-throughput preparation: Thermoelectric performance optimization for n-type Bi–Te–Se film with high average zT > 1

Thermoelectric materials have drawn extensive interest due to the direct conversion between electricity and heat, however, it is usually a time-consuming process for applying traditional “sequential” methods to grow materials and investigate their properties, especially for thermoelectric films that typically require fine microstructure control. High-throughput experimental approaches can effectively accelerate materials development, but the methods for high-throughput screening of the microstructures require further study. In this work, a combinatorial high-throughput optimization solution of material properties is proposed for the parallel screening and optimizing of composition and microstructure, which involves two distinctive types of high-throughput fabrication approaches for thin films, along with a new portable multiple discrete masks based high-throughput preparation platform. Thus, Bi2Te3–xSex thin film library with 196 throughputs for locating the optimized composition is obtained in one growth cycle. In addition, another thin film library composed of 31 materials with traceable process parameters is built to further investigate the relationship between microstructure, process, and thermoelectric performance. Through high-throughput screening, the Bi2Te2.9Se0.1 film with (00l) orientation is prepared with a peak zT value of 1.303 at 353 K along with a high average zT value of 1.047 in the interval from 313 to 523 K. This method can be also extended to the discovery of other functional thin films with a rapid combinatorial screening of the composition and structure to accelerate material optimization.