Wide-angle and wavelength-independent perfect absorption at metamaterial surfaces

In this paper, the reflection and absorption properties of a metamaterial layer on a metallic substrate is investigated theoretically and numerically. Perfect absorption is achieved for any frequency and for any angle of incidence when specified conditions are satisfied. These conditions are as follows:(i) the real permeability Re μ of the metamaterial is very small as compared with the imaginary part Im μ and (ii) the metamaterial thickness is very thin as compared with the wavelength of the incident radiation. Expressions for reflection and absorption coefficients are derived in detail. In the numerical results, the mentioned coefficients are computed and illustrated as a function of angle of incidence when Re μ, Im μ, and metamaterial thickness change

Simulation of an Asymmetric Transvers Electric TE Metamaterial Absorber

This paper introduces the principle method and simulation of an asymmetric TE (transverse electric) mode absorption in a lossy artificial metamaterial (LHM (left-handed material)). LHM is sandwiched between a lossy substrate and covered by a lossless dielectric cladding. The asymmetry solutions of the eigenvalue equation describe lossy-guided modes with complex-valued propagation constants. The dispersion relations, normalized field and the longitudinal attenuation were numerically solved for a given set of parameters: frequency range; film’s thicknesses; and TE mode order. We found that high order modes, which are guided in thinner films, generally have more loss of power than low-order modes since the mode attenuation along z-axis z α increases to negative values as the mode’s number increases, and the film thickness decreases. Moreover, for LHM, at incident wavelength= 1.9 m μ, refractive index= 2i 3.74+-and at thickness m μ 3.0=, the modes of order (4, 5, 6) attain high positive attenuation which means these modes have larger absorption lengths and they are better absorber than the others. This LHM is appropriate for solar cell applications. For arbitrary LHM, at frequency band of wavelengt (600, 700 to 900 nm), the best absorption is attained at longer wavelengths and for lower order modes at wider films. The obtained results could be useful for the design of future light absorbers

FINITE DIFFERENCE TIME DOMAIN SIMULATION OF LIGHT TRAPPING IN A GaAs COMPLEX STRUCTURE

We theoretically investigate the effects of Gallium Arsenide (GaAs) as an absorbing material in a complex waveguide structure model. Finite Difference Time Domain (FDTD) method is used to discretize the Maxwell’s curl equations for the proposed structure. A comparison between Amorphous Silicon (a-Si) and GaAs as absorbing materials is presented through the computation of the absorption spectra. It has been realized that GaAs is still a promising candidate to be used in the waveguide structure models through maximizing the absorption and minimizing the reflectance in the proposed waveguide structure

Solar cell with multilayer structure based on nanoparticles composite

In this study, a four-layer waveguide structure has been investigated as a solar cell model. In the proposed structure, a nanoparticle composite layer is added to enhance the efficiency of the solar cell due to their ability of controlling the light transmission and reflection. The nanoparticles are taken to be a mixture of Ag and Au embedded in a dielectric media consists of polyacrylic acid laid above a SiNx antireflection coating layer. Both layers are sandwiched between glass substrate and air cladding. The average reflectance for TE and TM fields are calculated using Maple. Results show that the reflectance depends on the ratio of the nanoparticle in the dielectric media, refractive index of SiNx layer and the angle of incidence. Thus, the performance of solar cell has been optimized by tuning and adjusting the above-mentioned parameters

Metallization of solar cells, exciton channel of plasmon photovoltaic effect in perovskite cells

Abstract Metallic nanoparticles are used to improve solar cell efficiency due to plasmon mediated photo-voltaic effect. We present various channels of this phenomenon in semiconductor solar cells with p − n junction and in chemical-type cells with exciton photovoltaic mechanism. Besides of previously known by plasmon strengthening of sun light absorption in metalized solar cells we have described the influence of plasmonic nanoparticles onto internal electricity of cells. The latter case we analyze on the example of hybridized perovskite solar cells regarded as most promising cells of III-rd generation. The explanation of recent experimental achievements with the metallization of perovskite cells is presented in comparison to the metallization of conventional Si-based cells

Influence of hole shape/size on the growth of site-selective quantum dots

The number of quantum dots which nucleate at a certain place has to be controllable for device integration. It was shown that the number of quantum dots per nucleation site depends on the size of the hole in the substrate, but other dimensions of the nucleation site are vague. We report on the influence of hole shape on site-selectively grown InAs quantum dots (QDs) by molecular beam epitaxy. Dry etching of the GaAs wafers was used because of its high anisotropic etching characteristic. Therefore, it was possible to verify the influence of several hole shape parameters on the subsequent QD growth independently. We show that the nucleation of these QDs depends on several properties of the hole, namely its surface area, aspect ratio of the surface area, and depth. Especially, the aspect ratio shows a big influence on the number of nucleating QDs per site. With knowledge of these dependencies, it is possible to influence the number of QDs per site and also its distribution

Microstructure of non-polar GaN on LiGaO2 grown by plasma-assisted MBE

We have investigated the structure of non-polar GaN, both on the M - and A-plane, grown on LiGaO2 by plasma-assisted molecular beam epitaxy. The epitaxial relationship and the microstructure of the GaN films are investigated by transmission electron microscopy (TEM). The already reported epi-taxial relationship and for M -plane GaN is confirmed. The main defects are threading dislocations and stacking faults in both samples. For the M -plane sample, the density of threading dislocations is around 1 × 1011 cm-2 and the stacking fault density amounts to approximately 2 × 105 cm-1. In the A-plane sample, a threading dislocation density in the same order was found, while the stacking fault density is much lower than in the M -plane sample

Investigation of pre-structured GaAs surfaces for subsequent site-selective InAs quantum dot growth

In this study, we investigated pre-structured (100) GaAs sample surfaces with respect to subsequent site-selective quantum dot growth. Defects occurring in the GaAs buffer layer grown after pre-structuring are attributed to insufficient cleaning of the samples prior to regrowth. Successive cleaning steps were analyzed and optimized. A UV-ozone cleaning is performed at the end of sample preparation in order to get rid of remaining organic contamination

Improvement of performance of InAs quantum dot solar cell by inserting thin AlAs layers

A new measure to enhance the performance of InAs quantum dot solar cell is proposed and measured. One monolayer AlAs is deposited on top of InAs quantum dots (QDs) in multistack solar cells. The devices were fabricated by molecular beam epitaxy. In situ annealing was intended to tune the QD density. A set of four samples were compared: InAs QDs without in situ annealing with and without AlAs cap layer and InAs QDs in situ annealed with and without AlAs cap layer. Atomic force microscopy measurements show that when in situ annealing of QDs without AlAs capping layers is investigated, holes and dashes are present on the device surface, while capping with one monolayer AlAs improves the device surface. On unannealed samples, capping the QDs with one monolayer of AlAs improves the spectral response, the open-circuit voltage and the fill factor. On annealed samples, capping has little effect on the spectral response but reduces the short-circuit current, while increasing the open-circuit voltage, the fill factor and power conversion efficiency

Wide angle antireflection in metal nanoparticles embedded in a dielectric matrix for plasmonic solar cells

The photon density in solar cells is usually optimized through tailored antireflection coatings (ARCs). We develop an analytical model to describe metal hybrid nanoparticles (NPs)-based ARC, where metal NPs are embedded in a standard ARC on a Si-substrate. A point dipole approach is implemented to calculate diffuse reflectance by NPs, while transfer matrix method is used for specular reflectance from front surface. We found that embedding metal NPs in SiN ARC enhances the antireflection property of the former at non-normal angles of incidence (AOI) of light. Electric field distribution patterns of radiation in the substrate by NPs are calculated for various AOI, which support the improvements in the antireflection property. Weighted solar power transmittances from ARCs are calculated, which show that Ag-NPs (radius = 35 nm) embedded in SiN (thickness = 70 nm) performs better than SiN for AOI over 74°, whereas Al-NPs (radius = 35 nm) embedded in SiN (thickness = 70 nm) performs better for AOI over 78°