Large Optical Absorption Coefficient Research Articles

Effects of the sulfurization temperature on sol gel-processed Cu2ZnSnS4 thin films

Abstract As a promising and alternative solar absorber material, the copper–zinc–tin–sulfide compound (Cu 2 ZnSnS 4 ) has been drawing attention in recent years for the production of cheap thin-film solar cells owing to the high natural abundance and non-toxicity of all the constituents, a tunable direct-band-gap energy and a large optical absorption coefficient. In addition, to overcome the problem of expensive vacuum-based methods, solution-based approaches are being developed for Cu 2 ZnSnS 4 deposition. In this study, we have produced Cu 2 ZnSnS 4 thin films via the sol–gel technique and subsequent sulfurization. The effects of the sulfurization temperature on the structural, morphological, compositional and optical properties of the films were investigated. X-ray diffraction and Raman spectroscopy analyses confirmed the formation of phase-pure CZTS films. The crystallinity of the films increased with an increasing sulfurization temperature. From the surface images and the results of the composition analysis, it was found that the films are uniform, composed of homogenously distributed grains and have compositions with Cu deficit. The values of the optical absorption coefficients for the films were found to be 10 4 cm −1 based on absorbance spectroscopy. The optical band-gap values were estimated to be between 1.32 and 2.27 eV depending on the sulfurization temperature.

Improved photoelectrochemical performance of Cu(In,Ga)Se2 thin films prepared by pulsed electrodeposition

Solar cells based on polycrystalline Cu(In,Ga)Se2 absorber layers have yielded the highest conversion efficiency among all the thin-film technologies. CIGS thin-films possess large optical absorption coefficient (≈105 cm−1) and a suitable bandgap of ≈ 1.20 eV for an ideal stoichiometry of CuIn0.7Ga0.3Se2. In the present study, Direct Current (DC) and Pulsed Current (PC) electrodeposition techniques are employed to obtain the near ideal stoichiometric CIGS thin-films on a Mo foil using a two electrode system at a constant potential. Deposited films are annealed at 550 °C under Ar atmosphere. Characterization of the annealed CIGS films is performed using SEM-energy dispersive X-ray spectroscopy, X-ray diffraction, Raman spectroscopy, and photoelectrochemistry to study the morphology, stoichiometry, phase constitution, and the photoelectrochemical response. PC deposition offered suitable manipulation of various parameters, which has helped in obtaining a better quality stoichiometric single phase chalcopyrite structured CIGS thin films with the elimination of unwanted secondary phases like Cu2−xSe. An improved photoelectrochemical performance, characteristic of a p-type semiconductor, is observed for the PC deposited CIGS films.

Preparation of Cu2ZnSnS4 film by sulfurizing solution deposited precursors

Cu2ZnSnS4 (CZTS) film was obtained by sulfurizing (Cu, Sn)S/ZnS structured precursor that was prepared by a combination of the successive ionic layer adsorption and reaction method and the chemical bath deposition method. Pure Cu2ZnSnS4 phase was obtained when the precursor was annealed at 500°C for 2h. The structure, composition, morphology and optical properties of the CZTS film were studied. The results show that the CZTS film has a relatively compact morphology, and the composition of the film is close to the stoichiometric ratio of Cu2ZnSnS4. The film presents a large optical absorption coefficient (larger than 104cm−1 when the photo energy is beyond 1.47eV). The optical band-gap of the film is around 1.56eV. This method is considered a potential way to prepare high quality CZTS film.

Influences of Hydrogen Passivation on Near-Infrared Light Detection of n-Type β-FeSi2/p-Type Si Heterojunction Photodiodes

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Band gap reduction and dielectric function of Ga1−xZnxN1−xOx and In1−xZnxN1−xOx alloys

AbstractThe band gap reductions, dielectric functions and absorption coefficients of the Ga1−xZnxN1−xOx and In1−xZnxN1−xOx (x = 0.00, 0.25, 0.50, 0.75, and 1.00) alloys were calculated, employing the partial self‐consistent GW approximation. As a comparison, the local density approximation (LDA) and the Heyd–Scueria–Ernzerhof (HSE) hybrid functional were also used to calculate the gap reduction. Both Ga1−xZnxN1−xOx and In1−xZnxN1−xOx alloys show strong band gap bowing. As a result, the band gap energy in Ga1−xZnxN1−xOx is reduced by Eg(GaN) − Eg (Ga1−xZnxN1−xOx) = 1.61, 2.01 and 1.91 eV for x = 0.25, 0.50, and 0.75, respectively. This allows optoelectronic devices based on GaN and ZnO with more efficient absorption or emission of light in the visible light range. The calculated dielectric functions and absorption spectra demonstrate that the band gap reduction enhances the optical absorption around the 2.5 eV region. Interestingly, the In1−xZnxN1−xOx alloy with x = 0.25 has the large optical absorption coefficient in the energy region 0.69–6.0 eV, and the alloy has very good absorption at 2–3 eV.

A Review on Progress of Amorphous and Microcrystalline Silicon Thin-Film Solar Cells

This paper describes the continuous progress of amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon based thin film solar cells and discusses its present scenarios based on patents and open reviews. The efficiency of hydrogenated amorphous silicon (a-Si:H) thin film solar cells has gradually improved from 2.4% to 15.2% (stable 13.0%). The major advantages of a-Si:H solar cells are the multijunction device capability, bandgap tailoring, easy fabrication, large scale production and high optical absorption coefficient. But the important factor for higher stabilized efficiencies is light induced effect on a-Si:H based solar cells was first reported by Staebler and Wronski. The highest recorded stabilized efficiency in micromorph cell is 10.18%. Though a lot of improvements have been done yet it needs more investigation. However, the improvement of open circuit voltage (Voc) is reviewed that is done with wide bangdgap window layers (a-SiC:H, μc-SiO:H, nc-Si:H etc) and inserting buffer layers at p/i and i/n interfaces in a-Si:H solar cells. An achievement of higher short circuit current (Jsc) by light trapping scheme is also reviewed. The material search for good optical and electrical properties and the device design technology is an immense need for further improvements of a- Si:H solar cell performances which are also being discussed with analysis of most relevant patents.

Nonequilibrium motion of a metal surface exposed to submicrosecond laser pulses

The results of experimental studies and theoretical analysis of the displacement of a metal mirror surface upon heating by submicrosecond laser pulses (t 0 = 15 × 10−9 s) are reported. The dynamics of the surface displacement in the irradiation zone was monitored by a Michelson interferometer with photoelectron count of fringes. A substantial delay of the motion of the exposed surface relative to a heating pulse was observed in experiments. It is shown that under thermal perturbations with high temperature gradients corresponding to large optical absorption coefficients, the contribution of nonequilibrium processes to the thermomechanical response of metals becomes decisive even for a submicrosecond duration of heating. The allowance for nonequilibrium processes in theoretical analysis leads to the generalized Duhamel law with thermal memory, which can be used for describing the observed effect correctly. The resultant characteristic time of a transient process (∼(2–6) × 10−8 s) considerably exceeds the available estimates of the phonon-phonon interaction time and is due to the effect of nonequilibrium processes on the scale of structural elements, which is considerably larger than the atomic scale.

A dual-beam photothermal reflection based system for thermal diffusivity measurement of optically dense liquids

A dual-beam photothermal reflection based system capable to measure thermal diffusivities of optically dense liquids has been designed and implemented. The large optical absorption coefficient of these liquids inhibits the possibility to use conventional transmission instruments for direct thermal diffusivity measurements. To overcome this problem, a front heating front detection photothermal reflection system has been proposed. This method expands the range of application and simplifies the experimental procedure of traditional photothermal methods, allowing precise measurement of thermal diffusivity of a variety of liquids. Measurements of the change in thermal diffusivity with the concentration of asphaltene in toluene solutions are described to test the applicability of this technique for reliable measurements of thermal diffusivities of optically dense liquids.

Structural and Physical Characteristics of Ultrananocrystalline Diamond/Hydrogenated Amorphous Carbon Composite Films Deposited Using a Coaxial Arc Plasma Gun

Ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) films were formed without initial nucleation using a coaxial arc plasma gun. The UNCD crystallite diameters estimated from the X-ray diffraction peaks were approximately 2 nm. The Fourier transform infrared absorption spectrum exhibited an intense sp3-CH peak that might originate from the grain boundaries between UNCD crystallites whose dangling bonds are terminated with hydrogen atoms. A narrow sp3 peak in the photoemission spectrum implied that the film comprises a large number of UNCD crystallites. Large optical absorption coefficients at photon energies larger than 3 eV that might be due to the grain boundaries are specific to the UNCD/a-C:H films.

Hydrogenated nanocrystalline silicon germanium thin films

Hydrogenated nanocrystalline silicon germanium thin films (nc-SiGe:H) is an interesting alternative material to replace hydrogenated nanocrystalline silicon (nc-Si:H) as the narrow bandgap absorber in an a-Si/a-SiGe/nc-SiGe(nc-Si) triple-junction solar cell due to its higher optical absorption in the wavelength range of interest. In this paper, we present results of optical, structural investigations and electrical characterization of nc-SiGe:H thin films made by hot-wire chemical vapor deposition (HWCVD) with a coil-shaped tungsten filament and with a disilane/germane/hydrogen gas mixture. The optical band gaps of a-SiGe:H and nc-SiGe:H thin-films, which are deposited with the same disilane/germane/hydrogen gas mixture ratio of 3.4 :1.7 : 7, are about 1.58 eV and 2.1 eV, respectively. The nc-SiGe:H thin film exhibits a larger optical absorption coefficient of about 2-4 in the 600-900 nm range when compared to nc-Si:H thin film. Therefore, a thinner nc-SiGe:H layer of ∼500 nm thickness may be sufficient for the narrow bandgap absorber in an a-Si based multiple-junction solar cell. We enhanced the transport properties as measured by the photoconductivity frequency mixing technique. These improved alloys do not necessarily show an improvement in the degree of structural heterogeneity on the nanometer scale as measured by small-angle X-ray scattering. Decreasing both the filament temperature and substrate temperature produced a film with relatively low structural heterogeneity while photoluminescence showed an order of magnitude increase in defect density for a similar change in the process.

RETRACTED ARTICLE: Hydrogenated nanocrystalline silicon germanium thin films

Hydrogenated nanocrystalline silicon germanium thin films (nc-SiGe:H) is an interesting alternative material to replace hydrogenated nanocrystalline silicon (nc-Si:H) as the narrow bandgap absorber in an a-Si/a-SiGe/nc-SiGe(nc-Si) triple-junction solar cell due to its higher optical absorption in the wavelength range of interest. In this paper, we present results of optical, structural investigations and electrical characterization of nc-SiGe:H thin films made by hot-wire chemical vapor deposition (HW-CVD) with a coil-shaped tungsten filament and with a disilane/germane/hydrogen gas mixture. The optical band gaps of a-SiGe:H and nc-SiGe:H thin-films, which are deposited with the same disilane/germane/hydrogen gas mixture ratio of 3.4 : 1.7 : 7, are about 1.58 eV and 2.1 eV, respectively. The nc-SiGe:H thin film exhibits a larger optical absorption coefficient of about 2–4 in the 600–900 nm range when compared to nc-Si:H thin film. Therefore, a thinner nc-SiGe:H layer of ∼500 nm thickness may be sufficient for the narrow bandgap absorber in an a-Si based multiple-junction solar cell. We enhanced the transport properties as measured by the photoconductivity frequency mixing technique. These improved alloys do not necessarily show an improvement in the degree of structural heterogeneity on the nanometer scale as measured by smallangle X-ray scattering. Decreasing both the filament temperature and substrate temperature produced a film with relatively low structural heterogeneity while photoluminescence showed an order of magnitude increase in defect density for a similar change in the process.

Third‐order optical nonlinearity of azobenzene side‐chain polymer thin film

AbstractThis paper reports both the nonlinear refractive index and nonlinear absorption coefficient of an azobenzene side‐chain polymer measured by a single‐beam Z ‐scan technique under 532 nm and 1064 nm excitation. Experimental results at 532 nm excitation show large optical nonlinearity and nonlinear absorption coefficient induced by the trans–cis transition in the dye molecule with γ being –9.46 × 10–12 cm2/W with β = –285.29 cm/GW, respectively. Under 1064 nm excitation, the large nonlinearity induced by the nonresonant electronic effect with γ = 4.57 × 10–12 cm2/W is obtained. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

LASER-INDUCED LATTICE DEFORMATION OF POROUS SILICON REVEALED BY RAMAN AND PHOTOLUMINESCENCE SPECTROSCOPIES

Raman and photoluminescence spectroscopies fo the porous silicon prepared in a hydrothermal solution with addition of titanium have been studied. Only a sharp single band near 520cm-1 appears in the Raman spectrum of the porous silicon when the exciting laser power is low and shifts to the red side with the increase of the laser power. If the laser power is increased to a critical value, the Raman band splits into two bands while the photoluminescence band splits also and the intensity of it increases enormously. We assign the two Raman bands to the lattice deformation-induced non-degeneracy of the LO and TO phonons. The laser-induced lattice deformation may result in the transformation of the porous silicon from a linear to a nonlinear optical material with a large laser-induced nonlinear optical absorption coefficient. It is found that the laser-induced transformation in the porous silicon is a reversible process. This may implicate some new applications of this material.

Selective Area Cell Adhesion on Amorphous Silicon Using Patterned Self-Assembled Monolayers

ABSTRACTThe compatibility of inorganic materials with living tissues and biological compounds is crucial in many areas, including medical diagnostics, biosensors, drug delivery, etc. In this work, we are interested in the interaction of living mammalian cells with semiconductor surfaces for novel thin-film biosensor devices. Amorphous silicon may give advantages over crystalline silicon for some devices because of its large-area, low-temperature compatibility, and its large optical absorption coefficient in the visible spectrum. Amorphous silicon thin films (500Å) deposited on quartz glass were cleaned using ambient UV/O3 treatment, leaving the surface largely OH-terminated and hydrophilic. The hydrophilic surface was then exposed to a vapor of octyltrichlorosilane (CH3(CH2)7SiCl3, OTS). The resulting surface was strongly hydrophobic, with advancing contact angles with water <106°. This organic surface was masked to reserve areas of uncoated hydrophilic substrate, and placed in a cell culture (BHK-21 cells) to observe cell adhesion and proliferation. A high degree of cell attachment was observed on the UV/O3-treated surfaces (~400 cells/mm2) compared to ~450 cells/mm2 on the culture dish control surface, indicating cell proliferation and growth. Little cell adhesion occurred on the hydrophobic organiccoated surface (~40 cells/mm2), and the cells remained round and only minimally attached. On masked surfaces, the organic-free areas showed dense, well-adhered cell growth while the coated areas showed much fewer and rounded cells. On all samples as well as control surfaces, cell death was <1%. These results suggest a means for selectively controlling cell adhesion to thin film electronic device surfaces, through the patterning of hydrophobic surface coatings.

The influence of sample thickness on the determination of large optical absorption coefficients in liquids

The effect of the sample thickness on the measurement of optical absorption coefficients in opaque liquids has been examined in this work both experimentally and theoretically, using the method of photoacoustic spectroscopy and taking into account the decrease of the pulse amplitude ratio with increasing propagation distance. Measurements were performed in water within a narrow wavelength range for various sample thicknesses . The results are in good agreement with data obtained by other methods for samples of up to approximately 4 mm thickness. Furthermore, the conditions for the application of this scheme are outlined.

Pulsed Photothermal Deflection Spectroscopy with Optically Dense Samples

The pulsed photothermal deflection spectroscopy (PTDS) method to determine the large optical absorption coefficient from the transverse PTDS magnitude and to measure spectroscopic data of optically dense sample from the normal-to-transverse magnitude ratio of the PTDS signal is presented and has been examined both theoretically and experimentally. Saturation at high absorption, effects of thermal inhomogeneity of the sample, and energy or power fluctuation of both the excitation and probe beams are avoided by this method. Together with the transverse PTDS technique, the dynamic range of spectroscopic measurement is over 9 to 10 orders of magnitude. This technique can be used with sample which is transparent or semitransparent to the probe beam.

Thermally stimulated luminescence applied to high-temperature superconductivity research

It was recognized shortly after the discovery of high-temperature superconductors (HTS) that these materials would exhibit thermally stimulated luminescence (TSL) as a manifestation of their granularity and consequent inclusion of insulating material at grain boundaries. The large optical absorption coefficients of these opaque materials implies that TSL emanates from within ∼1 μm of the surface. Therefore, TSL can be used to assess the quality of HTS by detecting the presence of insulating surface impurities, which are, in general, deleterious to technological performance. For example, surface resistance R s is determined by HTS properties within the London penetration depth, which is the region probed by TSL. The presence of insulating surface impurities contributes to the high-frequency losses as manifested by an enhanced R s and is directly correlated with TSL. Therefore, in addition to radiation dosimetry and archaeological dating, TSL can also be applied to HTS research.

Pulsed photoacoustic measurements of large optical absorption coefficients

A method to determine large optical absorption coefficients of liquids from the amplitude ratio of acoustic waves generated under rigid- and free-boundary conditions is presented. This has been examined both experimentally and theoretically, taking into account the attenuation of the pulse amplitude ratio with the increased acoustic path length due to attenuation effects. Although the proposed method was tested in a narrow wavelength interval and close to the limit of precision of the experimental arrangement, the results show quantitative agreement with the literature values.

A numerical method for solving the radiation transport equation in planar and spherical geometry

A computational scheme is proposed for the numerical solution of the stationary radiation transport equation ignoring scattering. It is proved that, at large optical thicknesses, the cosines and absorption coefficients averaged over the one-sided radiation intensities cannot be used in the zeroth approximation and it is necessary to use their first approximation.

Long-wavelength infrared detectors based on strained InAs–Ga1−xInxSb type-II superlattices

We demonstrate that infrared detectors made from strained type-II superlattices consisting of III–V semiconductors can have favorable properties for long-wavelength (λc >10 μm) detection applications. We specifically consider InAs–Ga1−x Inx Sb strained-layer superlattices with x≊0.4 . This is a type-II superlattice where the conduction-band minimum of InAs is lower in energy than the valence-band maximum of Ga1−x Inx Sb. Electrons are localized in the InAs layers while holes are localized in the Ga1−x Inx Sb layers. Generally, in a type-II superlattice such as InAs–GaSb, large absorption coefficients cannot be achieved at the small values of band gap necessary for long-wavelength detection because the localization of electrons and holes in adjacent layers leads to small optical matrix elements. Therefore, small band gaps and large optical matrix elements appear to be two mutually exclusive requirements in the InAs–GaSb superlattice. Here, we show that it is possible to obtain large optical absorption coefficients at the values of band gap required for long-wavelength detection by taking advantage of internal lattice-mismatch induced strains present in InAs–Ga1−x Inx Sb superlattices. This feature eliminates the principal difficulty associated with type-II superlattices as candidate materials for long-wavelength detection applications. In fact, the near-threshold optical absorption coefficient of the InAs–Ga0.6 In0.4 Sb strained-layer superlattice is found to be as large as that of conventional Hg1−x Cdx Te alloys in the λc ≊ 10–12 μm region. Moreover, the electron effective mass of the InAs–Ga0.6 In0.4 Sb strained-layer superlattice is found to be nearly isotropic and much larger (m*super/m ≊0.05 at λc ≊10 μm) than that of the Hg1−x Cdx Te alloy. This value of the electron effective mass is a favorable feature of the InAs–Ga0.6 In0.4 Sb strained-layer superlattice since it is large enough to lead to small diode tunneling currents, yet it is small enough to lead to large mobilities and diffusivities. As a result, leakage currents are expected to be much less of a problem in infrared detector diodes based on InAs–Ga0.6 In0.4 Sb strained-layer superlattices than they are in conventional Hg1−x Cdx Te-based infrared detectors. Overall, InAs–Ga1−x Inx Sb strained-layer superlattices are expected to have superior materials properties for long-wavelength detection than the Hg1−x Cdx Te alloy.