Absorption Of Thin Films Research Articles

Determination of the fundamental absorption and optical bandgap of dielectric thin films from single optical transmittance measurements.

In this work, we propose a method to retrieve the thickness and optical constants of dielectric thin films from single optical transmittance measurements. The method is based on the envelope method and requires a simple dispersion model for the real part of the refractive index with few fitting parameters, while the absorption coefficient can be determined without the aid of a dispersion model. The wavelength-dependent optical constants can be obtained even from spectra that exhibit few interference fringes. We have tested the method with simulated and real transmittance data from thin films in the spectral range covering the fundamental absorption. In order to assess the method's reliability to retrieve the optical constants and optical bandgap, a comparison is performed with the method by Chambouleyron, known as the Pointwise Unconstrained Minimization Approach, and a fit using the Cody-Lorentz dispersion model. We evaluate the methods' capability to retrieve the fundamental absorption and optical bandgap, and their compromise with film thickness accuracy. Finally, the methods are tested and contrasted using optical transmittance of three different semiconductor material thin films.

Engineering of morphological, optical, structural, photocatalytic and catalytic properties of nanostructured CuO thin films fabricated by reactive DC magnetron sputtering

Nanostructured thin films of CuO were deposited on silica glass substrates using reactive DC magnetron sputtering technique. Microstructural, morphological, optical, catalytic and photocatalytic properties of the prepared CuO thin films were examined using FESEM, AFM, Rutherford backscattering spectrometry, XRD, XPS, UV–Vis absorption and PL spectroscopy. FESEM showed nanostructures in the thin films, which were confirmed to be of monoclinic CuO by XRD analysis. Substrate temperature variation (40 °C, 100 °C and 300 °C) was found to significantly alter the optical, morphological, photocatalytic and structural properties of the CuO nanostructured thin film coatings. FESEM and AFM analyses showed decrease in size of nanostructures and surface roughness increase with increase in substrate temperature. Increase in UV–Vis absorbance and PL intensity of CuO thin films with decrease in crystallite size were noticed as the substrate temperature was increased. The prepared nanostructured CuO thin films exhibited highly enhanced photocatalytic activities and degraded dyes (MB and MO) in water in just 40 min under solar exposure and catalytic transformation of 4-nitrophenol (4-NP) took place in just 15 min. The developed CuO nanostructured thin film coatings are very promising for large scale, practical and advanced catalytic reduction of toxic 4-NP and photocatalytic applications in solar driven water purification.

Uracil-doped DNA thin solid films: a new way to control optical dispersion of DNA film using a RNA constituent.

Among five nucleobases, adenine (A), guanine (G), cytosine (C), thymine (T) and uracil (U), uracil is a key distinctive constituent existing only in ribonucleic acid (RNA). RNA shares the common A, G, and C with deoxyribonucleic acid (DNA) made of A-T, G-C hydrogen bonding. We explored a new attempt to combine uracil (U) with DNA, successfully realizing U-doped DNA thin solid films for the first time. Impacts of uracil on optical properties of the films were thoroughly investigated. The method was based on optimal spin-coating of an aqueous solution of DNA and uracil over silicon or silica substrates. Optical absorption of both aqueous solution and U-doped DNA thin solid films was characterized in a wide spectral range covering UV-visible-IR. Immobilization of uracil within DNA thin solid films was experimentally confirmed by FTIR spectroscopy studies. By using an ellipsometer, we measured the refractive indices of the films and discovered that U-doping was a very effective means to control optical dispersion DNA thin solid film. We further investigated thermo-optic behavior to find impacts of U-doping in DNA films. Detailed thin film processes and optical characterizations are discussed.

Polarization dependent multiphoton absorption in ZnO thin films

We present a simple non-destructive approach for studying the polarization dependence of nonlinear absorption processes in semiconductors. The method is based on measuring the yield of the near UV photoluminescence as a function of polarization and intensity of femtosecond laser pulses. In particular, we investigated the polarization dependence of three photon laser absorption in intrinsic and Al-doped ZnO thin films. Both specimen show stronger emission for linearly polarized excitation compared to circular polarization. The ratios for the three-photon absorption coefficients are about 1.8 and independent of the doping. It is shown that Al-doped films have lower threshold for stimulated emission in comparison to the intrinsic films.

A NOVEL METHOD FOR PRODUCING NANOSTRUCTURED CdSe THIN FILM

In this study, we produced Cadmium selenide (CdSe) crystalline thin film on commercial glass substrates via chemical bath deposition. Transmittance, absorbance, refractive index and optical bandgap of thin films were examined by UV/vis spectrum. XRD revealed a hexagonal form. The pH level of the baths in which CdSe thin films were deposited varied and optical and structural properties of the resulting thin films were analyzed. SEM analysis was used for surface analysis. Some features of the films were supposed to change with pH and these properties were investigated by testing different pH levels, which were 8, 9, 10 and 11. The variation of the optical bandgap changed between 1.60 and 1.75[Formula: see text]eV, according to the pH of deposition bath. Film thickness varied from 60[Formula: see text]nm to 93[Formula: see text]nm, with the variation of deposition bath’s pH. Moreover, it has been found that refractive index values were also changed with film thickness; these were calculated as 2.28, 2.19, 2.22 and 2.36 for 93.27, 60.97, 61.09 and 60.18[Formula: see text]nm, respectively. Dielectric constant also varied with refractive index, taking values 0.85, 0.75, 0.78, 0.93 for refractive indexes 2.28, 2.19, 2.22 and 2.36, respectively.

Fabrication of Cu2ZnSnS4 Thin Films from Ball-Milled Nanoparticle inks under Various Annealing Temperatures

Cu2ZnSnS4 (CZTS) has been recognized as a promising thin-film absorber material of chalcopyrite-related solar cells. A two-stage method for fabricating CZTS films using CZTS nanoparticles was developed. Nanocrystal inks fabricated by a ball-milling method was utilized to °C deposit CZTS precursors by spin-coating approach. The CZTS precursors were annealed in the sulfur atmosphere under different annealing temperatures ranging from 550 °C to 650 °C. Influences of annealing temperature on grain growth, composition, crystallinity, and photovoltaic properties of CZTS films were characterized. With the increase of annealing temperature, grain growth was enhanced, while the sulfur atomic ratio fist increased then decreased. The crystallinity of the films was significantly improved after the annealing, and the obvious peak of the secondary phase of ZnS, were observed from the X-ray diffraction results, when the annealing temperature increased to 625 °C. However, the secondary phase was not detected from the surface Raman spectrum. Through comparing the Raman spectrum of different areas of the CZTS film, secondary phases of ZnS and SnS were observed, indicating the decomposition of CZTS films, due to the high temperature. The highest conversion efficiency of 7.5% was obtained when the annealing temperature was 600 °C.

Strain Mapping of CdTe Grains in Photovoltaic Devices

Strain within grains and at grain boundaries (GBs) in polycrystalline thin-film absorber layers limits the overall performance because of higher defect concentrations and band fluctuations. However, the nanoscale strain distribution in operational devices is not easily accessible using standard methods. X-ray nanodiffraction offers the unique possibility to evaluate the strain or lattice spacing at nanoscale resolution. Furthermore, the combination of nanodiffraction with additional techniques in the framework of multimodal scanning X-ray microscopy enables the direct correlation of the strain with material and device parameters such as the elemental distribution or local performance. This approach is applied for the investigation of the strain distribution in CdTe grains in fully operational photovoltaic solar cells. It is found that the lattice spacing in the (111) direction remains fairly constant in the grain cores but systematically decreases at the GBs. The lower strain at GBs is accompanied by an increase of the total tilt. These observations are both compatible with the inhomogeneous incorporation of smaller atoms into the lattice, and local stress induced by neighboring grains.

Aging Effect of a Cu(In,Ga)(S,Se)2 Absorber on the Photovoltaic Performance of Its Cd‐Free Solar Cell Fabricated by an All‐Dry Process: Its Carrier Recombination Analysis

AbstractCd‐free Cu(In,Ga)(S,Se)2 (CIGSSe) solar cells are fabricated by an all‐dry process (a Cd‐free and all‐dry process CIGSSe solar cell) with aged CIGSSe thin film absorbers. The aged CIGSSe thin films are kept in a desiccator cabinet under partial pressure of oxygen of ≈200 Pa for aging time up to 10 months. It is reported for the first time that aged CIGSSe thin film with increased aging time results in significant enhancement of photovoltaic performance of Cd‐free and all‐dry process CIGSSe solar cells, regardless of the alkali treatment. Based on carrier recombination analysis, carrier recombination rates at the interface and in the depletion region of the Cd‐free and all‐dry process CIGSSe solar cells are reduced owing to avoidance of sputtering damage on CIGSSe absorber surface, which is consistent with the strong electron beam‐induced current signal near CIGSSe surface after the increased aging time. It is implied that the interface and near‐surface qualities are clearly improved through the increased aging time, which is attributable to the self‐forming of Inx(O,S)y near CIGSSe surface, which acts as a buffer layer. Ultimately, the 22.0%‐efficient Cd‐free CIGSSe solar cell fabricated by all‐dry process is achieved with the aged Cs‐treated CIGSSe absorber with the aging time of 10 months.

Inorganic and Pb-Free CsBi3I10 Thin Film for Photovoltaic Applications

Computational, thin-film deposition, and characterization approaches have been used to investigate the all-inorganic lead-free CsBi3I10 as a candidate to act as a thin-film photovoltaic absorber. I...

Study on broadband optical constants of yttrium fluoride thin films deposited by electron beam evaporation

In this paper, the broadband optical constants of YF3 thin films prepared by electron beam evaporation were studied (0.2 μm–14 μm), which are consistent with the data reported in other literature. The composite dispersion physical model of the optical constants of YF3 thin films was established by using Cody-Lorentz model and Gaussian oscillator model, which is employed in this study and is universal for YF3 thin films with water defects. The water absorption of the YF3 thin films occurs at 3251.9 cm−1 and 3454.3 cm-1 representing the symmetrical stretching vibration and asymmetric stretching vibration frequency of the HOH bond, and 1640.8 cm−1 representing the shear vibration frequency of the HOH bond. In addition, the Raman spectra of amorphous YF3 thin film shows that the intrinsic vibrational frequencies are 383 cm−1 and 462 cm−1. Based on the electron interband transition and the micro-structural vibration characteristics mentioned above, the broadband optical constants of the YF3 thin film are obtained, which makes up the deficiency of extinction coefficient data of YF3 thin film.

Wideband microwave absorption in thin nanocomposite films induced by a concentration gradient of mixed carbonaceous nanostructures

Absorption in excess of 90% is induced over a 22 GHz bandwidth by the combination of carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs) dispersed in a polymer matrix. This performance is achieved by a stack of polymer films showing a concentration gradient of mixed CNTs and GNPs from layer to layer, having a total thickness of only $$0.76\,\lambda /4.$$ Excellent agreement is observed from 18 to 40 GHz between the predicted and measured reflectivity and the resulting high absorption.

Effect of Various Shapes of Silver Nanoparticles on the Performance of Plasmonic Solar Cells Active Layer

In this work, the enhancement of optical absorption in organic thin film active layer was investigated theoretically. The optical absorption results reveal that the introduction of periodic silver (Ag) nanoparticles (NPs) into the thin film active layer influences the absorption spectra. In this study, copper phthalocyanine (CuPc) as one of the organic solar cell materials has been used. Finite Different Time Domain technique has been used to demonstrate the role of the three different shapes of the silver nanoparticles (SNPs). It has been found that the NPs can result in broadband optical absorption enhancement in the wavelength range (350-550) nm due to the surface plasmonic resonance (SPR) phenomena. Moreover, the investigation has been performed the effect of the few shapes of (Cylinder with radius (R) 20 nm and height (H) 60 nm, radius (R) 30 nm for Sphere NPs, and Ellipse NPs with radius R1, R2 of 20 and 30 nm respectively) on the absorption in the organic thin films.

Selective near-perfect absorbing mirror as a spatial frequency filter for optical image processing

Spatial frequency filtering is a fundamental enabler of information processing methods in biological and technical imaging. Most filtering methods, however, require either bulky and expensive optical equipment or some degree of computational processing. Here, we experimentally demonstrate real-time, on-chip, all-optical spatial frequency filtering using a thin-film perfect absorber structure. We experimentally demonstrate edge enhancement of an amplitude image and conversion of phase gradients to intensity modulation in an image. The device is used to demonstrate enhancement of an image of pond algae.

Effects of different gas flow rates and non-perpendicular incidence angles of argon cold atmospheric-pressure plasma jet on silver thin film treatment

In this study, the influences of variations in the gas flow rate and incidence angles of argon cold atmospheric-pressure plasma jet on the morphology and absorption spectra of silver thin films (60 nm, 80 nm, and 100 nm film thickness) are investigated. To evaluate the surface morphology, atomic force microscopy (AFM) was employed on the silver thin film surface before and after plasma processing. To analyze the effect of plasma treatment on the grain size, the one-dimensional AFM surface profiles of Ag thin films are approximated using a Gaussian function. The absorbance of Ag thin films is measured in wavelength range of 190–1100 nm utilizing UV–Vis absorption spectrometer. Compared to the gas flow rates 0.5 standard litter per minute (SLM) and 2 SLM, surface treatment of Ag thin film with gas flow rate of 1 SLM increased the valley depth, the peak valley height, and the distance between two deepest valleys remarkably. A sequential argon plasma treatment (2-min plasma treatment perpendicular to surface was followed by 2-min plasma processing with non-perpendicular incidence angle of 60°) offers considerable improvement in the uniformity of grains and also changes shape of grains, especially the peak height (about 44 times higher than untreated sample) and area of grains (almost 136 times greater than untreated sample) which can be applicable for optical sensing technology.

Broadband Cavity Enhanced Absorption Spectroscopy of thin films of haemoglobin

In this study, a sensitive technique (Broadband Cavity Enhanced Absorption Spectroscopy (BBCEAS)) is employed for measuring the absorption of thin films deposited onto a glass substrate. A thin film of the biological solution such as Lyophilied Bovine haemoglobin is deposited on glass microscope coverslips. Drop coating method was used to deposit a thin film over the microscope coverslips. The number of passes is calculated: 612 passes for the high reflectivity mirror were obtained. The best measurements are made with the mirror set of reflectivity of (R≥ 0.99) which produced an αmin value of 0.0043 cm-1 and LOD of 1.9×10-7M.

The influence of N2 plasma treatment on the optical bandgap and photoluminescence of orderly nanoporous TiO2 film

Amorphous-like structural orderly nanoporous titanium dioxide thin films were prepared at room temperature by dc reactive magnetron sputtering. It was found that the optical properties of the films depended on the pore diameters and influenced by the post plasma treatment in nitrogen atmosphere, but it seemed that the thickness of the film had no more influence on them. The results showed that the nitrogen plasma treatment was responsible for lowering of the energy bandgap values due to the increase in oxygen vacancy defects of the films, which could be proved by the photoluminescence results. The mechanism for changing of optical bandgaps is discussed in detail in this paper. Absorbance of the amorphous nanoporous TiO2 thin films was calculated using UV-visible NIR spectrophotometer. The morphology of the deposited film was studied by scanning electron microscopy (SEM) and atom-force microscopy (AFM), respectively. The results can be interesting towards the development of nanoporous TiO2 thin films with defined structure and optical properties.

Effect of spray volume on the properties of Cu2ZnSnS4 absorber thin film fabricated through nebulizer assisted spray pyrolysis technique

Copper Zinc Tin Sulfide (CZTS) is a growing semiconductor chalcogenide that has the potential to act as an alternative nontoxic absorber for commercial silicon-based devices in the sustainable photovoltaic conversion of solar energy in the near future. A debut attempt was made to economically fabricate this earth-abundant solar absorber thin film through indigenously designed cost-effective nebulizer assisted chemical spray pyrolysis technique without high-temperature annealing. The deposition was carried out at different spray volumes to modify the thickness of the absorber material directly on a bare soda-lime glass substrate and effect of CZTS film thickness on its structural, morphological, optical and electrical properties were studied by using appropriate characterization techniques. The film with a thickness of 740 nm for a precursor spray volume of 12 ml resulted in CZTS film with better energy conversion characteristics.

Towards an optimum saturable absorber for the multi-gigahertz harmonic mode locking of fiber lasers

Ultra-high-pulse-repetition-rate lasers are essential for a number of applications, including, e.g., optical communication and ablation-cooled material processing. Despite several techniques to generate pulses with gigahertz-range repetition rate, incorporating mainly short-length resonators, more widespread applications are still limited by the lack of a robust, simple, and cost-effective solution. Here, we report for the first time, to the best of our knowledge, fully passive harmonic mode locking in an all-polarization-maintaining (PM) fiber laser. The design guarantees a fixed polarization state and stable operation, where the cavity harmonic number is controlled by the pump power only. Self-starting operation is provided by the antimony telluride (Sb2Te3) thin-film saturable absorber (SA), which facilitates multiple pulse operation. The SA acts by means of low modulation depth, low saturation fluence, and an inverse slope in the saturable absorption curve. The optimum features of the SA and limiting factors for high-repetition-rate pulse generation in this regime of operation are discussed. As a result, 2.2 ps pulses with 3 GHz repetition rate are generated at 1560 nm wavelength. The study reports a new approach towards an optimal SA for multi-gigahertz pulse generation in practical, all-PM fiber lasers.

A review on growth optimization of spray pyrolyzed Cu 2 ZnSnS 4 chalcogenide absorber thin film

The progress of solar cell technology in the development of clean and economic quaternary compound copper zinc tin sulfide (CZTS)-based absorber thin films using the spray pyrolysis technique are presented in this review. CZTS (Cu2ZnSnS4) is the only potential competitor for the existing solar thin film absorbing materials owing to its environment-friendly Earth abundant constituents. Even though different nonvacuum thin film technologies have been developed for the large area fabrication of this nontoxic absorber material, spray pyrolysis technique offers more versatility in changing the process parameters which has a direct impact on the cell efficiency. It can be used for depositing a wide variety of materials even with complex composition with good crystallinity, and the method has the advantage of being flexible and straightforward to design and can be quickly adopted for extensive area deposition. A survey on the effects of experimental conditions as well as the nature of precursors on the structural, morphological, electrical, and optical properties on the spray pyrolyzed CZTS thin films is discussed in detail. This analysis certainly could provide a potential to obtain new insights in the fabrication of high-efficiency CZTS-based solar cells and to launch it into the commercial market to satisfy the ever-growing future energy demand.

Modified optical characteristics of TiO2/Au/TiO2 thin composite films

We report the optical characteristics of the TiO2/Au-NPs/TiO2 composite thin films determined from the UV–Vis absorption spectroscopy and spectroscopic ellipsometry. The UV–Vis spectra indicated the enhanced optical absorption in the composite thin films in the range 1.6–3.0 eV. Spectroscopic ellipsometry was employed to determine the optical parameters of the composite films. The experimental results were fitted with hybrid model, which included Drude Lorentz model for Au layer and Urbach tail modified Tauc Lorentz model for TiO2 layers. The photoluminescence spectrum of the composite film clearly showed a weak emission band centered at 500 nm, which was absent in the pure TiO2 films. The extracted optical parameters manifested the generation of hot electrons at the Au/TiO2 interface due to Au surface plasmon (SP) modes, multiple interband transitions in Au in the energy range (1–4 eV) and a long Urbach tail (∼144 meV) in TiO2. These were direct indicators of hot electron scattering from the plasmon modes and their transfer to the TiO2 conduction band. The present study revealed that the plasmon exciton interactions in composite film can modify its optical properties.