Absorption Of Thin Films Research Articles

Salisbury screen absorbers using epsilon-near-zero substrate

As a planar resonant structure, Salisbury screen offers a cost-effective way of manipulating electromagnetic waves for both fundamental studies and practical applications in optoelectronics. In this paper, we demonstrate Salisbury screen absorbers using epsilon-near-zero substrate, which reduces the spacer thickness below typical one quarter wavelength limit. Three-layered thin-film absorbers made of SiC substrate, ZnSe spacer layer and top NiCr film are designed and fabricated, which exhibit near-perfect absorption at 11.72 μm with spacer thickness of about half of a quarter-wavelength. For ideal zero-index material without optical loss, our proposed thin-film absorber simplifies to a two-layered structure even without the spacer layer in theory. These results suggest that epsilon-near-zero materials provide an alternative approach in developing compact planar absorbing structures without involving lithographic patterning.

Fabrication, Structural, Optical, Electrical Properties and Influence of Complexing Agents on Ternary CuZnS2 Thin Film

In the present work, copper zinc sulphide (CuZnS2) thin films with and without complexing agents using glass plate as substrate were prepared. Chemical bath deposition method was employed to deposit the thin films. Powder X-ray diffraction (PXRD) patterns of the prepared films indicate the crystalline nature of CuZnS2 with cubic phases. The SEM and AFM images illustrate that the deposited films were highly influenced on the polyhedral morphology by the complexing agents. The influence of complexing agents on absorbance and band gap of the CuZnS2 thin films were characterized using UV-Vis absorption studies. Hall effect measurements indicate the CuZnS2 thin film without surfactant belongs to p-type semiconductor and become n-type after adding the complexing agents EDTA and Leishman stain. From the I-V curve, all the samples having slow conducting nature was found for changing the voltage with the current from -32 nA to +30 nA using solar stimulator.

8-Hydroxyquinolino cadmium chloride hydrate for generating nanosecond and picosecond pulses in erbium-doped fiber laser cavity

In this paper, we experimentally proposed and demonstrated the application of thin-film saturable absorber (SA) based on 8-Hydroxyquinolino cadmium chloride hydrate (8-HQCdCl2H2O) material as both Q-switcher and mode-locker in the cavity of erbium-doped fiber laser (EDFL). The 8-HQCdCl2H2O was embedded in polyvinyl alcohol (PVA) to fabricate the thin-film, which has then employed in the Q-switched and mode-locked EDFL cavity for passively generating nanosecond and picosecond pulses, respectively. At the maximum pump power of 167 mW, the Q-switched laser-produced pulse train with a pulse width of 726 ns, a repetition rate of 150 kHz, and a signal-to-noise ratio (SNR) of 72 dB. We also demonstrated the generation of mode-locked picosecond pulses by adding 200 m long single-mode fiber (SMF) into the ring EDFL cavity. The mode-locked laser-generated pulses have a pulse width of 1.83 ps, a repetition rate of 1 MHz, and the SNR of 77 dB for the electrical spectrum.

Wide-Bandgap Metal Halide Perovskites for Tandem Solar Cells.

Metal halide perovskite solar cells (PSCs) have become the most promising new-generation solar cell technology. To date, perovskites also represent the only polycrystalline thin-film absorber technology that has enabled >20% efficiency for wide-bandgap solar cells, making wide-bandgap PSCs uniquely positioned to enable high-efficiency and low-cost tandem solar cell technologies by coupling wide-bandgap perovskites with low-bandgap absorbers. In this Focus Review, we highlight recent research progress on developing wide-bandgap PSCs, including the key mechanisms associated with efficiency loss and instability as well as strategies for overcoming these challenges. We also discuss recent accomplishments and research trends on using wide-bandgap PSCs in perovskite-based tandem configurations, including perovskite/perovskite, perovskite/Si, perovskite/CIGS, and other emerging tandem technologies.

Electrical and optical properties linked to laser damage behavior in conductive thin film materials

Epsilon-near-zero-materials (ENZ-materials) and their unique properties are key to the successful integration and miniaturization of optical components. Novel concepts, which promise significant progress in this field of research, such as optical switches and thin film electro-optical modulators, are possible when the electrical and optical properties of ENZ-materials are carefully exploited. To achieve a greater understanding of these properties, in this paper the electrical conductivity, optical transmittance, as well as absorption of thin indium tin oxide films, are investigated and linked to their laser-induced damage threshold in the ultra-short pulse regime. To the best of the authors’ knowledge, this is the first concise study linking the electrical properties of indium tin oxide to its properties regarding high-power laser applications.

Role of Morphology and Förster Resonance Energy Transfer in Ternary Blend Organic Solar Cells

Organic solar cells (OSCs) fabricated from ternary blend thin film absorbers are designed to maximize the range of absorption in the solar spectrum and thus increase the short-circuit current densi...

Structural, spectroscopic and electrical investigations of novel organic thin films bearing push-pull azo – Phenol dye for UV photodetection applications

In the current study, novel thin films of a phenol-based push-pull azo dye, 2-acetyl-4-(4-chloro-phenylazo) phenol (ACAP), with tunable optical and electronic properties were designed, synthesized and characterized for UV photodetection applications. The crystalline structure and morphological features of the thermally evaporated ACAP thin films are investigated. The fabricated thin films exhibit an amorphous-like structure with low-intensity crystalline regions of average crystallite size of about 29.51 nm and a smooth surface with nanostructured sheets formation. The optical transmittance, reflectance, and absorption of ACAP thin films are measured in the spectral range UV–vis-NIR. A significant high UV absorption extending from 190 nm to 385 nm is observed with semi-transparency nature in the visible region. Furthermore, a good agreement is obtained between the estimated value of the direct energy gap that is obtained experimentally (3.62 eV) and that calculated from the theoretical DFT approach (3.74 eV). The dispersion behavior is analyzed in terms of the single oscillator model and is employed to estimate the dispersion parameters. Finally, an organic/inorganic heterojunction device based on Au/ACAP/n-Si/Al for UV photodetection is successfully fabricated. The current–voltage relations of the manufactured photodetector showed significant stability and sensitivity to the incident UV illumination. The fabricated UV photodetector exhibits responsivity ~25.7 mA/W, specific detectivity ~2 × 109 Jones, efficiency ~16.74%, a fast and reproducible ON/OFF switching behavior with 480 ms and 218 ms rise and fall time, respectively.

Ultraviolet light exposure enhances the protective ability of synthetic melanin

Study shows refractive index and light absorption of melanin thin films increase after intense UV radiation exposure.

Climbing Jacob’s ladder: A density functional theory case study for Ag2ZnSnSe4 and Cu2ZnSnSe4

Recent advances in the development of exchange and correlation functionals to be employed in density functional theory calculations combined with the availability of ever more powerful high-performance computing facilities, let predictive computational materials science become reality. In order to assess the quality of calculated material properties, Jacob’s ladder provides an informal classification, where exchange and correlation functionals of similar capabilities are placed at the same rung of this ladder, while improved and more accurate ones are placed at higher rungs. Climbing Jacob’s ladder, i.e. employing more accurate exchange and correlation functionals, increases the quality of the results and the computational demands, and provides some guidance as to what accuracies and computational costs to expect from specific calculations. This is particular important for materials whose electronic ground state properties are incorrectly described, e.g. small band gap semiconductors, and materials, where system sizes for subsequent investigations, like defect properties or band offsets in heterostructures, become prohibitively large for more accurate exchange and correlation functionals.Here, we provide a systematic density functional theory study on the ground state properties of Ag2ZnSnSe4 and Cu2ZnSnSe4 for the lowest four rungs of Jacob’s ladder. Cu2ZnSnSe4, and in particular its alloys with Ag, is a promising candidate material for future thin-film solar cell absorber layers. In the present work, the obtained material properties are compared to available experimental data, allowing to benchmark the accuracy of the employed exchange and correlation functionals. We also provide a comparative study for subsequent quasiparticle calculations. Therein, the influence of differently obtained eigenvalues and orbitals as starting points are critically assessed with respect to available experimental data. Our results show that, structural properties based on the SCAN functional show overall best agreement with available experimental data, whereas additional hybrid functional calculations are necessary for satisfying results on electronic and optical properties.

Defect depth-profiling in kesterite absorber by means of chemical etching and surface analysis

A method to probe the depth morphology, defect profile and possible secondary phases in a thin film semiconductor is presented, taking a standard Kesterite film as an example. Using a top-down approach based on a previously reported controlled Methanol-Br2 chemical etching, well-defined slabs of a state of the art Kesterite absorber are fabricated. The analysis of their morphology both by Scanning Electron Microscopy and 3D optical Profilometry reveals the extent of a previously reported poor film morphology toward the back interface, and we are able to determine that more than 50% of a standard absorber is disconnected from the substrate. More importantly, these etched films are subsequently analyzed by surface sensitive techniques such as X-ray Photoelectron Spectroscopy and UV-Raman analysis. An accurate composition profile is established, and for the first time, a direct observation of the defects’ nature and their depth profiling in Kesterite is made possible. While VCu are found with a constant amount throughout the absorber, indicating a homogenous carrier concentration, a prevalence of the ZnSn defect is observed with a steep gradient toward the back interface, associated with an increase in the SnSe2 secondary phase. With bulk defects being often pointed out as the intrinsic limitation of this material, this result highlights what possibly is the main impediment of Kesterite solar cells, and a critical point to address in the design of future devices. Beyond the case of Kesterite absorbers, the method presented here offers a combination of simplicity, tunability and versatility making a straightforward transfer to other emerging thin film absorbers feasible, and it could possibly be an important tool in their future performance assessment and comparison.

High-Quality, InN-Based, Saturable Absorbers for Ultrafast Laser Development

New fabrication methods are strongly demanded for the development of thin-film saturable absorbers with improved optical properties (absorption band, modulation depth, nonlinear optical response). In this sense, we investigate the performance of indium nitride (InN) epitaxial layers with low residual carrier concentration (<1018 cm−3), which results in improved performance at telecom wavelengths (1560 nm). These materials have demonstrated a huge modulation depth of 23% and a saturation fluence of 830 µJ/cm2, and a large saturable absorption around −3 × 104 cm/GW has been observed, attaining an enhanced, nonlinear change in transmittance. We have studied the use of such InN layers as semiconductor saturable absorber mirrors (SESAMs) for an erbium (Er)-doped fiber laser to perform mode-locking generation at 1560 nm. We demonstrate highly stable, ultrashort (134 fs) pulses with an energy of up to 5.6 nJ.

Benzodithiophene-based wide-bandgap small-molecule donors for organic photovoltaics with large open-circuit voltages

Abstract We have developed new small-molecule electron donors, SMBDT-S and SMBDT-SF, based on benzo[1,2-b:4,5-b’]dithiophene (BDT) with thiophene side chains as a core unit and rhodanine end-capping units to enhance the open-circuit voltage (Voc), which is one of the factors limiting performance of small-molecule organic photovoltaics (OPVs). The SMBDT-S and SMBDT-SF donor molecules exhibited excellent thermal stability and high crystallinity. SMBDT-S was found to have a wide bandgap of 1.85 eV with the highest occupied molecular orbital (HOMO) energy level of − 5.56 eV. The fluorination in SMBDT-SF led to the same bandgap of 1.86 eV with an even lower-lying HOMO energy level of − 5.72 eV. Both SMBDT-S and SMBDT-SF have large thin-film absorption coefficients of (1.4–1.5) × 105 cm−1. Bulk heterojunction OPV devices based on pairing SMBDT-S and SMBDT-SF respectively with PC71BM electron acceptor showed Voc as high as 1.18 eV, which is among the highest Voc reported for all-small-molecule solar cells. These results demonstrate that introducing fluorine atoms and alkylthio side chains are effective strategies to downshift the HOMO energy level and enhance the Voc in OPVs.

Highly stable ethanol-based Cu2ZnSnS4 (CZTS) low-cost thin film absorber: Effect of solution aging

Aging is an important factor that can affect the grain size, surface morphology and optical band gap of the thin films. Especially, it is thought that these important parameters will have a strong impact on the solar cell energy conversion ratio in Cu2ZnSnS4 (CZTS) absorber based photovoltaic devices. With this aim the presented study reports the effect of the aging time on the structural, morphological and optical characteristics of p-type quaternary CZTS absorber materials, produced by using a solution assisted dip-coating process on the glass substrate and annealed at 515 °C without using any dangerous environment (such as sulphurization or selenization). The kesterite crystal structure was determined by X-ray diffraction (XRD) analyses for all the CZTS samples. To support the observed kesterite crystal structure of the CZTS, the main vibration modes and chemical composition of the samples were investigated by Raman and XPS tools. Besides, the comprehensive surface analyses were conducted to better understanding of the morphology variation within the CZTS thin films depending on the aging time of the CZTS precursor solution by using SEM and AFM tools. Generally, large grain size and solution stability of absorber material have important impacts on the solar cell energy conversion solar cells. Furthermore, the band gap of the CZTS thin film shifts toward the optimal energy value (from 1.82 eV to 1.56 eV), observed from the optical studies. This last result is highly attractive for the CZTS based solar cell applications.

Narrowband and flexible perfect absorber based on a thin-film nano-resonator incorporating a dielectric overlay

We developed a flexible perfect absorber based on a thin-film nano-resonator, which consists of metal–dielectric–metal integrated with a dielectric overlay. The proposed perfect absorber exhibits a high quality (Q-)factor of ~ 33 with a narrow bandwidth of ~ 20 nm in the visible band. The resonance condition hinging on the adoption of a dielectric overlay was comprehensively explored by referring to the absorption spectra as a function of the wavelength and thicknesses of the overlay and metal. The results verified that utilizing a thicker metal layer improved the Q-factor and surface smoothness, while the presence of the overlay allowed for a relaxed tolerance during practical fabrication, in favor of high fidelity with the design. The origin of the perfect absorption pertaining to zero reflection was elucidated by referring to the optical admittance. We also explored a suite of perfect absorbers with varying thicknesses. An angle insensitive performance, which is integral to such a flexible optical device, was experimentally identified. Consequently, the proposed thin-film absorber featured an enhanced Q-factor in conjunction with a wide angle of acceptance. It is anticipated that our absorber can facilitate seminal applications encompassing advanced sensors and absorption filtering devices geared for smart camouflage and stealth.

Ba-acceptor doping in ZnSnN2 by reactive RF magnetron sputtering: (002) faceted Ba–ZnSnN2 films

Zn-IV-N2 semiconductors display outstanding optoelectronic behavior and hence developed as an effective thin film absorber materials in photovoltaic devices. However, alkaline-earth metallic dopants can improve the performance of Zn-IV-N2 semiconductors. In view of this, Barium (Ba) acceptor was successfully doped into ZnSnN2 crystal lattice with various dopant concentrations (∼0–7%) by reactive RF magnetron co-sputtering at 450 °C. The orthorhombic structured Ba doped ZnSnN2 (Ba:ZTN) films with preferential orientation along (002) plane were obtained. Hall measurement studies revealed that the type of conduction is changed from the conventional n-type to p-type via Ba doping. The deposited Ba:ZTN films exhibited high hole concentration in the range of 1018-1019 cm−3 with low resistivity of ∼10−1 Ω cm and a reasonable mobility of ∼0.5–5 cm2 V−1 s−1. The doping of Ba at either Zn or Sn site would be responsible for the p-type conductivity in Ba:ZTN films. The variations in the surface properties such as morphology and topography of ZnSnN2 on Ba doping were described by FE-SEM and AFM analysis. Further analysis by UV–Vis–NIR and X-ray photoelectron spectroscopies were also performed in order to reveal the optical performance and chemical bonding, composition of Ba:ZTN thin films. The existence of Ba2+ ions in Ba:ZTN was confirmed by the obtained binding energies from XPS analysis. From the collective results, it is suggested that Ba:ZTN could be employed as effective p-type layer in thin film solar cells.

Improving the Efficiency of SnS Thin Film Solar Cells by Adjusting the Mg/(Mg+Zn) Ratio of Secondary Buffer Layer ZnMgO Thin Film

In the recent years, thin film solar cells (TFSCs) have emerged as a viable replacement for crystalline silicon solar cells and offer a variety of choices, particularly in terms of synthesis processes and substrates (rigid or flexible, metal or insulator). Among the thin-film absorber materials, SnS has great potential for the manufacturing of low-cost TFSCs due to its suitable optical and electrical properties, non-toxic nature, and earth abundancy. However, the efficiency of SnS-based solar cells is found to be in the range of 1 ~ 4 % and remains far below those of CdTe-, CIGS-, and CZTSSe-based TFSCs. Aside from the improvement in the physical properties of absorber layer, enormous efforts have been focused on the development of suitable buffer layer for SnS-based solar cells. Herein, we investigate the device performance of SnS-based TFSCs by introducing double buffer layers, in which CdS is applied as first buffer layer and ZnMgO films is employed as second buffer layer. The effect of the composition ratio (Mg/(Mg+Zn)) of RF sputtered ZnMgO films on the device performance is studied. The structural and optical properties of ZnMgO films with various Mg/(Mg+Zn) ratios are also analyzed systemically. The fabricated SnS-based TFSCs with device structure of SLG/Mo/SnS/CdS/ZnMgO/AZO/Al exhibit a highest cell efficiency of 1.84 % along with open-circuit voltage of 0.302 V, short-circuit current density of 13.55 mA cm−2, and fill factor of 0.45 with an optimum Mg/(Mg + Zn) ratio of 0.02.

Tailoring Multi-Walled Carbon Nanotubes into Graphene Quantum Sheets.

Transformation of carbon nanotubes (CNTs) into sub-10 nm pieces is highly required but remains a great challenge. Herein, we report a robust strategy capable of mechanically tailoring pristine multi-walled carbon nanotubes (MWCNTs) into graphene quantum sheets (M-GQSs) with an extremely high yield of up to 44.6 wt %. The method combines silica-assisted ball-milling and sonication-assisted solvent exfoliation and therefore enables reproducible high-yield production of M-GQSs directly from MWCNTs. Remarkable solvent diversity and extraordinary solvability (up to 7 mg/mL) are demonstrated facilitating the solution processing of the M-GQSs. The M-GQSs are essentially monolayers with intrinsic curvature, which could be determinative to their outstanding performances in both dispersions and thin films. Besides the excitation wavelength-, concentration-, and solvent-dependent photoluminescence in dispersions, the solid-state fluorescence and exceptional nonlinear saturation absorption (NSA) in thin films are demonstrated. Particularly, NSA with relative modulation depth up to 46% and saturation intensity down to 1.53 MW/cm2 are achieved in M-GQS/poly(methyl methacrylate) hybrid thin films with a loading content of merely 0.2 wt %. Our method opens up a new avenue toward conversion and utilization of CNTs.

Effect of acetic acid concentration on optical properties of lead acetate based methylammonium lead iodide perovskite thin film

Abstract High absorption of perovskite thin films is crucial for efficient metal halide perovskite solar cells. In this study, methylammonium lead iodide (CH3NH3PbI3) perovskite thin films are prepared using lead (II) acetate Pb(Ac)2, acetic acid (HAc) and CH3NH3I through solution method. The effect of HAc volume (0 ml–100 ml at step size of 25 ml HAc) on optical properties and parameters of the CH3NH3PbI3 perovskite thin films developed are investigated using UV–Vis spectrophotometry and mathematical correlations. Interestingly, the increase of HAc amount to 75 ml leads to an increase in absorbance, refractive index, extinction co-efficient, dielectric properties and optical conductivity and a decrease in transmittance and optical band gaps of the CH3NH3PbI3 perovskite thin films. The optimized film is obtained at 75 ml HAc at which refractive index, film thickness and optical band gap is 1.45 at 485 nm, 355 nm and 1.47 eV, respectively. The enhanced absorption of the optimized thin film is attributable to an increase in structure ordering at HAc amount equals 75 ml. The fabricated CH3NH3PbI3 perovskite thin films have potential as ideal antireflection coatings for solar cells and optoelectronic applications.

A promising photovoltaic material Cu2MnSn(S,Se)4: Film growth and its application in solar cell

Chalcogenides, such as CdTe, Cu(In,Ga)(S,Se)2 and Cu2ZnSn(S,Se)4, have made significant progress in their thin-film photovoltaic performance, but the toxicity (CdTe) and scarcity (Cu(In,Ga)(S,Se)2) of the constituent elements, as well as the inevitable anti-site disorder (Cu2ZnSn(S,Se)4), limit the further improvement of efficiency. In this work, a new thin-film absorber, Cu2MnSn(S,Se)4, is studied in terms of both film growth and characterization. We prepare the precursor film by the sol-gel method followed by selenization at high temperature. The absorbance of the precursor film is higher than 98% at wavelengths of 200–1300 nm. The crystallinity and the phase transformation of the prepared film can be tailored greatly by modifying the selenization process. When the selenization temperature is 570 °C and the selenization time is 30 min, a highly-crystalline, stannite-structured Cu2MnSn(S,Se)4 film is obtained. A solar cell fabricated with such Cu2MnSn(S,Se)4 film as the absorber layer achieves efficiency of 1.79%, which is the highest efficiency reported to date for this material. This study demonstrates that Cu2MnSn(S,Se)4 is a promising photovoltaic material and has huge potential for the fabrication of low-cost solar cell.

Role of ZnS Particles in the Performance of Cu2ZnSnS4 Thin Film Solar Cells: A Comparative Study by Active Control of Zinc Deposition in Coevaporated Precursors

Formation of ZnS particles in the Cu2ZnSnS4 (CZTS) light absorber layer of CZTS solar cells may impede the photocarrier collection as well as the open circuit voltage. Coevaporation of Cu, Zn, Sn, and S elements can provide a uniform and well‐mixed Cu–Zn–Sn–S precursor at atomic level, yet the annealing/sulfurization process does not preserve such elemental distribution. The Zn element in the formed CZTS absorber thin films from such a uniform precursor distributes unevenly, especially aggregating near the rear interface, and thus deteriorates the film quality. Herein, the movement pathway of the Zn element in the coevaporated precursor during annealing is investigated through energy dispersive X‐ray spectroscopy (EDX) mapping and secondary ion mass spectroscopy (SIMS) profiling. With the understanding of the segregation behavior of the Zn element, an active manipulation of the initial distribution of Zn in the precursor is accordingly designed to effectively alleviate the accumulation of Zn near the rear Mo/CZTS interface and therefore improve the crystalline quality of the absorber. The results suggest that properly controlling the precursor.