Reduction In Optical Absorption Research Articles

Functionalizing plasmonic nanoparticles through adding a shell to improve electronic properties of c-Si thin-film solar cells

Embedding plasmonic nanoparticles (p-NPs) inside the solar cell’s active layer is capable of enhancing active layer optical absorption; however, such inclusion has some detrimental effects on the electronic properties of the solar cells. In addition, p-NPs are highly catalytic, their presence enables other non-radiative decay channels besides generation of electron–hole pairs, and the electrons usually get absorbed by these p-NPs. This results in the unfavorable fact that the potential enhancement in the carrier generation rate and the generated current is no longer in line with the enhancement in the optical absorption. In this paper, we propose to functionalize p-NPs by adding a dielectric or semiconductor shell, to passivate the p-NP without deteriorating scattering and/or plasmonic effects. Ag@SiO2 and Ag@TiO2 core–shell p-NPs have been intensely studied using extensive computational electromagnetic simulations to model the spectral response of the active layer’s optical absorption as well as electronic properties as a function of both shell composition and thickness. It is shown that a 5 nm TiO2 shell is apt to optically passivate the p-NP without any reduction in optical absorption, while improving the short circuit current density (J sc ) of the thin-film solar cell by 33.3%.

Visible-Light-Induced Oxidation of Poly(3-hexylthiophene-2,5-diyl) Thin Films on ZnO Surfaces under Humid Conditions: Study of Light Wavelength Dependence

The oxidation behaviors of poly(3-hexylthiophene-2,5-diyl) (P3HT) thin films (∼60 nm thickness)/ZnO irradiated with three different wavelengths of visible light (blue, green, and red) in a humid atmosphere were studied using X-ray photoelectron spectroscopy (XPS) and UV–vis spectroscopy. The formation of sulfoxide states upon visible-light irradiation was observed in the S 2p core-level XPS spectra, and more importantly, this oxidation became pronounced with decreasing wavelength of incident light. Photo-oxidation of P3HT films also resulted in a reduction in optical absorption. In contrast to the XPS results, changes in the UV–vis absorption spectrum were rather insensitive to the wavelength of incident visible light. The wavelength dependency of the photo-oxidation of P3HT films seen in the XPS spectra is attributed to the more pronounced photoinduced oxidation of locally disordered thiophene rings on the surfaces of P3HT films under irradiation with shorter-wavelength visible light. The population of local-disordered sites that increases the optical transition gap compared to that of the well-ordered bulk P3HT film decreases from the top surface to the interior of P3HT films due to stronger interchain interactions in the interior portion of the films. Therefore, changes in the optical absorbance seen in the UV–vis absorption spectra of the entire P3HT film upon photoinduced oxidation are less sensitive to the wavelength of incident light in the visible regime.

Enhancement of the refractive index of sputtered zinc oxide thin films through doping with Fe2O3

Certain optical applications, such as antireflective coatings on solar cells, require transparent films with high refractive indices (>2). Zinc oxide (ZnO) is a transparent semiconductor with exceptional optical properties. However, its refractive index in the transparent dispersionless region of the visible spectrum is lower than 2. In this study, we enhanced the refractive index of sputtered ZnO thin films through doping with iron oxide (Fe2O3), where ZnO targets were doped by 0.5–2.0 wt% of Fe2O3. The films were polycrystalline with smooth surfaces. Differential transmittance spectra were employed to derive the optical band gaps of the films, which showed a minor variation by ±0.03 eV due to doping. The refractive index was extracted from the transmittance spectra using a Cauchy equation and was further fitted by a single-oscillator model. The optimum refractive index was obtained for the films prepared from the target doped with 1.5 wt% of Fe2O3. Enhancement of the refractive index was accompanied by a reduction of optical absorption in the doped films.

SiO2/TiO2 distributed Bragg reflector near 1.5 μm fabricated by e-beam evaporation

The authors report on the fabrication and characterization of SiO2/TiO2 distributed Bragg reflector (DBR) mirrors operating at the eye safe and optical communication wavelength window, λ = 1.5 μm. Our experimental results demonstrated that SiO2/TiO2 DBR mirrors with reflectivity exceeding 95% at λ = 1.5 μm can be achieved using e-beam evaporation in conjunction with postdeposition thermal annealing process in ambient air. It was found that the postdeposition annealing process transformed the crystal structure of the as-deposited TixOy to TiO2, leading to a significant reduction in optical absorption. Erbium doped III-nitride semiconductors incorporating DBR mirrors at 1.5 μm emission may open up many novel applications, including infrared emitters, optical amplifiers, and high power infrared lasers.

Effects of annealing on the structure of ion beam sputtered amorphous tantalum oxide: Oxygen-17 NMR spectra and relaxation times

Unlike well-studied network-structured oxide glasses such as silicates and borates, the structures of amorphous films of heavy transition metal oxides such as tantalum oxide (Ta2O5) are poorly known, in spite of their importance in applications such as the multilayer mirror coatings in optical interferometers, such as the Laser Interferometer Gravitational Wave Observatory (LIGO). 17O MAS NMR can provide new and unique structural information about these materials, but requires isotopic enrichment. For optical coatings formed by ion-beam sputtering (IBS), annealing at temperature up to 600°C is known to improve properties such as optical absorption and thermal noise. In this paper we report NMR results on tantala films deposited by IBS in an 17O-enriched environment, which allows the structure to be examined by NMR for the first time without an added step of isotope exchange at elevated temperature. Subsequent annealing at temperatures from 300 to 600°C shows only subtle changes in the proportions of two- and three-coordinated oxygen anions and in bond distances, which may be related to minor structural relaxation and property changes. In contrast, the nuclear spin-relaxation rates increase dramatically on annealing in oxygen gas, suggesting the progressive removal of defects in the as-grown films that contain paramagnetic, unpaired electron spins. EPR spectra qualitatively confirm this observation. The most likely candidate for this effect is the oxidation of lower-valent tantalum cations (e.g. Ta4+) and thus the removal of a slight oxygen deficit present in the as-deposited films. This may lead to reduction in optical absorption and possibly to improvements in other physical properties during annealing.

Further improvement in the light output power of InGaN-based light emitting diodes by reflective current blocking design

In this study, the fabrication and characterization of InGaN/GaN multiple-quantum-well light-emitting diodes (LEDs) with further improvement by the design of a reflective current blocking layer (CBL) were described, and these are demonstrated to be an inexpensive and feasible way for improving the performance of LEDs. With the reflective CBL, not only was the injected current forced to spread outside instead of flowing directly downward under a p-pad, but the light generated from the active region could also be extracted outside of the LED by reflection under the p-pad. At 20 mA, as compared to the conventional LED, the light output power of the LEDs with the normal and reflective CBL can be increased by 15.7% and 25.8%, respectively. We found that the forward voltages of the LEDs with CBL structure were both about 3.7 V at 20 mA, which was slightly higher than that of the conventional LED (3.6 V). In our experiment, the further increase in the light output power of the reflective CBL LED could be attributed to more current injection into the light-emitting active region outside of the p-pad by the CBL and a reduction in optical absorption at the p-pad with more extraction by the reflective design.

Capped semiconductor nanocrystals for device applications

For device purpose, our main aim is to synthesise material which is chemically and thermally stable, as well as enhancement in luminescence properties followed with matching lattice parameters. This can be achieved by precisely controlling the size of semiconductor nanocrystals which can create an opportunity for producing functional materials with new properties. Here we showed advantages of using both organic and inorganic capping agents. We reported two synthesis routes, one will lead to nanocomposites and other to Core/Shell nanostructures. Our mechanism consists of two stages: core nanoparticle formation and shell growth. Gibbs free energy of hydration of Zn+2 gives more clarity for shell growth over core rather than ion displacement from core. Colloidal films comprising of nanocrystalline CdS/ZnS were fabricated by the dip coating method. A blue shift in energy level at the nanoscale is demonstrated by optical absorption. Electron microscopy studies with an SEM and TEM show a particle size of 10 nm and diffraction patterns show a crystalline nature. Absence of lattice mismatching is one of the important parameter for device fabrication, which is confirmed by Raman spectroscopy. Overall reduction in optical absorption due to blue shift is expected to result in higher performance, especially in short-circuit currents in CdS/CdTe solar cells.

Improvement in the Light Output Power of GaN-Based Light-Emitting Diodes by Natural-Cluster Silicon Dioxide Nanoparticles as the Current-Blocking Layer

In this study, the fabrication and characterization of InGaN-GaN multiple-quantum-well light-emitting diodes (LEDs) with silicon dioxide (SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) nanoparticles as the current-blocking layer (CBL) are described. The performance was improved by introducing SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoparticles, not deposited by commonly used plasma-enhanced chemical vapor deposition, as the CBL beneath the p-pad. The injected current was forced to spread outside instead of flowing directly downward. At 20 mA, the light output power of the LED with a CBL was increased 15.7% as compared to that of the conventional LED. The forward voltage of the LED with the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoparticle CBL was 3.34 V at 20 mA, which was slightly higher than that of the conventional LED (3.29 V). The increase in the light output power can be attributed to the injection of additional current into the light-emitting active layer of the LED by the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> nanoparticles CBL and thus a reduction in optical absorption at the p-pad.

Selective high resistivity region formation by a Ni catalyst on GaN light-emitting diodes

The activation of Mg-doped GaN using a thin Ni film has been investigated. The results show that the Ni film significantly enhances the hydrogen desorption from the GaN film, which could improve the hole concentration and the resistivity of a p-GaN film. By means of a selective high resistivity region (SHRR) with no Ni catalyst involved, the current blocking layer is thus simply developed beneath the p-pad electrode of the GaN-based light-emitting diodes (LEDs). The light-output intensity for the LED chip with a SHRR is significantly increased as compared with the conventional LED chip. This improvement can be explained by both the additional current injection into the effective active layer of the LED by the SHRR structure and the reduction in optical absorption under the p-pad electrode.

Characteristics of nanocrystalline CdS films fabricated by sonochemical, microwave and solution growth methods for solar cell applications

Nanocrystalline CdS was synthesized by sonochemical and microwave methods. Characterization of nano-CdS was carried out by optical absorption, X-ray diffraction, TEM, SEM and EDX. A hyperbolic band model was used to calculate a shift in energy over that of the band gap of bulk CdS. Colloidal films comprising of nanocrystalline CdS were fabricated by the dip coating method. A comparison of optical absorption of CdS films on indium tin oxide (ITO)/glass by sonochemical and microwave methods was made with solution grown CdS films on ITO/glass. A blue shift in energy level at the nanoscale is demonstrated by optical absorption, and X-ray diffraction of nano-CdS shows a cubic structure. Electron microscopy studies with an FE-SEM and TEM show a particle size of 15 nm and diffraction patterns show a crystalline nature. Overall reduction in optical absorption due to blue shift is expected to result in higher performance, especially in short-circuit currents in CdS/CdTe solar cells.

Growth of subgrain-free Tl 3AsSe 3 crystals for efficient harmonic conversion to mid-infrared frequency

Minute crystal lattice misorientations, termed “subgrains”, degrade the optical homogeneity of Tl 3AsSe 3 (TAS) crystals. Three origins of the subgrain formation were identified: variations in the seeding process, thermal stresses, and local adhesion of molten TAS to its growth ampoule. Systematic experiments and improved diagnostic techniques led to defect control and to the growth of TAS crystals free from subgrains along their entire crystal length. Key results were (1) the achievement of high optical quality, (2) demonstration of 57% efficient second harmonic generation from CO 2 laser radiation and (3) a significant reduction in optical absorption.

Reduction of Optical Absorption in Bi-Substituted Garnet Film by Annealing

LPE film was grown using Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> single flux to realize the high Faraday rotation and low absorption provided by high density Bi-substitution required of material used for 0.8 μm optical isolators. The melt from Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> single flux does not contain bivalent ions such as Pb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> , so Pt <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> taken into the film or oxygen vacancies occurring within the film generate Fe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> , and increase optical absorption at 0.8 μm. Generation of Fe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> was prevented by adding bivalent ions, like Ca <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> , in the melt, and optical absorption declined. This paper reports the results of investigations in which in addition to adding bivalent ions, annealing was used successfully to reduce Fe <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> and optical absorption. Investigation of the relation between the effect of oxygen partial pressure during annealing and annealing temperature established that the equilibrium oxygen partial pressure and reaction rate differ with the electrical type of the film.

Magneto¿Opticial Properties of Bi¿Substituted GdIG Films Grown by LPE

Reduction of optical absorption as the amount of Bi in Bi-substituted films is increased is discussed. The amount of Bi in the films grown was controlled by adjusting the supercooling temperature of the solvent, the ratio of PbO to Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> in the solvent, the substrate rotation speed and other parameters. Variation in Bi content with respect to Bi-YIG was found at substrate rotation speeds of 84 rpm and 170 rpm. The measurements of the absorption characteristics of Bi-GdIG and Bi-GdGaIG films grown on a GGG substrate indicated that they included a Bi quantity of approximately 0.8 with respect to the Bi-GdIG. A figure of merit of 6.7 deg/dB was obtained for a 21.2 μm thick Bi-GdIG film at a wavelength of 0.76 μm.

Self-consistent detailed balance treatment of the solar cell

The Shockley-Queisser detailed balance calculation of solar-cell behaviour is not quite exact, but if the correct expression is used in a straightforward way, then the dark current shows an unphysical infinity when the terminal voltage approaches the energy gap value from below. To eliminate this, it is necessary to allow for the reduction in optical absorption which occurs at the same time, turning to emission when qV&gt;Eg. If this is done, there is a reduction in photocurrent and an increase in dark current, however there is no discontinuity in behaviour when qV=Eg. When qV&gt;Eg, the lasing condition holds, so that the calculation shows clearly the transition of diode behaviour from solar cell to laser.

Reduction of optical absorption at a wavelength of around 0.8 ?m in LPE garnet (TmBiCa)3(FeGaPt)5O12

Liquid-phase epitaxy (LPE) garnet films (TmBiCa)3(FeGaPt)5O12 have been grown using only Bi2O3 as the flux so that the film containing Bi gives high specific Faraday rotation. The film does not contain Pb, which may affect optical absorption. The optical absorption coefficientα at 810 nm has been effectively reduced by doping Ca in the melt. Our data show that a minimum level ofα and of the anisotropy constantKu and also the maximum of the electrical resistivityϱ are achieved when Ca2+ replaces Fe2+. Fe2+ results from Pt4+ incorporation in the film due to a Bi2O3 flux attack on a Pt crucible. Using a compensated film,α of 58 cm−1 and a figure of merit of 9deg/dB were obtained.