Light Intensity Enhancement Inside the Grooves of Metallic Grating

Light absorption inside the grooves of metallic gratings filled with a semiconductor material can be improved by means of the electric field enhancement. To this end, the influence of grating dimensions in the electric field spectral behavior is theoretically investigated. Two conditions of cavity resonance have been analyzed separately: (1) for TE polarization (electric field parallel to the grooves) and (2) for TM polarization (magnetic field parallel to the grooves). When dimensions are chosen according to the first condition, the enhancement of TE fields is found to increase with the height-to-width ratio, and it is accompanied with a decrease in the bandwidth. The same enhancement levels can be achieved for TM fields if the second condition holds, provided that the period-to-width ratio is large enough. The simultaneous enhancement of TE and TM fields, based on a condition of surface resonance excitation, can also be accomplished. In this case, the TM response is very sensitive to changes in groove depth and width

Light trapping by means of electric field enhancement in metallic grantings

The enhancement of the electromagnetic field within the grooves of a metallic diffraction grating is analyzed. A perfect conductor, rectangular-shaped diffraction grating is assumed, therefore, the field enhancement analyzed in this work is due to resonances caused by the structure itself, and not to the so-called surface-plasmon resonances which are observed to be influenced by metal properties as well. The modal approach has been employed for calculations. Basically, the squared amplitude of the electric field within the grooves is calculated as a function of the wavelength of the incident light. Similarly, the behaviour of the structure when exposed to a conical bundle of light is also analyzed. The present results show that a field intensity enhancement of the order of 100 is achievable. However, the spectral width of the resonances decrease with increasing enhancement as is expecte

Low-injection behaviour of a solar cell with a metallic grating back-reflector

Recent studies have dealt with the possibility of increasing light absorption by using the so-called electric field enhancement taking place within the grooves of metallic gratings. In order to evaluate the potential improvements derived from the absorption increase, we employ a simplified model to analyze the low-injection behaviour of a solar cell with a metallic grating back-reflector

Light trapping properties of cylindrical well diffraction gratings in solar cells: Computational calculations

Light trapping using diffraction gratings is a promising approach to increasing absorption in solar cells. In this paper, the computationally calculated absorption enhancement expected from a diffraction grating consisting of a triangular array of cylindrical wells is presented. Angle-extended polychromatic illumination is considered, and special attention is paid to absorption of sub-bandgap photons in an intermediate band solar cell. Results are compared to the absorption enhancement expected from an ideal Lambertian (randomizing) scatterer, which is considered as a baseline. It is found that for cells which absorb very weakly, the diffraction grating provides absorption enhancement above that of the ideal Lambertian scatterer over a wide wavelength range. For cells which absorb more strongly, the grating underperforms the ideal Lambertian scatterer over almost all wavelengths. Finally, the grating period, well height and well radius are optimised. Keywords: Light Trapping, Diffraction Grating, Intermediate Band Solar Cel

Light concentration in the near-field of dielectric spheroidal particles with mesoscopic sizes

This paper presents a numerical study of the light focusing properties of dielectric spheroids with sizes comparable to the illuminating wavelength. An analytical separation-of-variables method is used to determine the electric field distribution inside and in the near-field outside the particles. An optimization algorithm was implemented in the method to determine the particles’ physical parameters that maximize the forward scattered light in the near-field region. It is found that such scatterers can exhibit pronounced electric intensity enhancement (above 100 times the incident intensity) in their close vicinity, or along wide focal regions extending to 10 times the wavelength. The results reveal the potential of wavelength-sized spheroids to manipulate light beyond the limitations of macroscopic geometrical optics. This can be of interest for several applications, such as light management in photovoltaic

Embedment of metal nanoparticles in GaAs and Si for plasmonic absorption enhancement in intermediate band solar cells

The high near-field enhancement occurring in the vicinity of metallic nanoparticles (MNPs) sustaining surface plasmons can only be fully exploited in photovoltaic devices if the MNPs are placed inside their semiconducting material, in the photoactive region. In this work an experimental procedure is studied to embed MNPs in gallium arsenide (GaAs) and silicon (Si), which can be applied to other semiconductor host materials. The approach consists in spin-coating colloidal MNPs dispersed in solution onto the substrate surface. Then a capping layer of the same material as the substrate is deposited on top to embed the MNPs in the semiconductor. The extinction spectra of silver (Ag) and gold (Au) MNPs embedded in GaAs and Si is modeled with Mie theory for comparison with optical measurements. This contribution constitutes the initial step towards the realization of quantum-dot intermediate band solar cells (QD-IBSC) with MN

A numerical study into the influence of quantum dot size on the sub-bandgap interband photocurrent in intermediate band solar cells

A numerical study is presented of the sub-bandgap interband photon absorption in quantum dot intermediate band solar cells. Absorption coefficients and photocurrent densities are calculated for the valence band to intermediate band transitions using a four-band k · p method. It is found that reducing the quantum dot width in the plane perpendicular to the growth direction increases the photocurrent from the valence band to the intermediate-band ground state if the fractional surface coverage of quantum dots is conserved. This provides a path to increase the sub-bandgap photocurrent in intermediate band solar cells

The feasibility of high-efficiency InAs/GaAs quantum dot intermediate band solar cells

In recent years, all the operating principles of intermediate band behaviour have been demonstrated in InAs/GaAs quantum dot (QD) solar cells. Having passed this hurdle, a new stage of research is underway, whose goal is to deliver QD solar cells with efficiencies above those of state-of-the-art single-gap devices. In this work, we demonstrate that this is possible, using the present InAs/GaAs QD system, if the QDs are made to be radiatively dominated, and if absorption enhancements are achieved by a combination of increasing the number of QDs and light trapping. A quantitative prediction is also made of the absorption enhancements required, suggesting that a 30 fold increase in the number of QDs and a light trapping enhancement of 10 are sufficient. Finally, insight is given into the relative merits of absorption enhancement via increasing QD numbers and via light trapping

Radiative thermal escape in intermediate band solar cells

To achieve high efficiency, the intermediate band (IB) solar cell must generate photocurrent from sub-bandgap photons at a voltage higher than that of a single contributing sub-bandgap photon. To achieve the latter, it is necessary that the IB levels be properly isolated from the valence and conduction bands. We prove that this is not the case for IB cells formed with the confined levels of InAs quantum dots (QDs) in GaAs grown so far due to the strong density of internal thermal photons at the transition energies involved. To counteract this, the QD must be smaller

Upper limits to absorption enhancement in thick solar cells using diffraction gratings

The application of diffraction gratings to solar cells is a promising approach to superseding the light trapping limits of conventional Lambertian structures. In this paper a mathematical formalism is derived for calculating the absorption that can be expected in a solar cell equipped with a diffraction grating, which can be applied to any lattice geometry and grating profile. Furthermore, the formalism is used to calculate the upper limit of total absorption that can theoretically be achieved using a diffraction grating. The derived formalism and limits are valid when the solar cell thickness is greater than the coherence length of the illuminating solar spectrum. Comparison is made to the upper limit achievable using an angularly selective Rugate filter, which is also calculated. Both limits are found to be considerably higher than the Lambertian limit within the range of sunlight concentration factors practically employed in photovoltaic systems (1–1000×). The upper limit of absorption using the diffraction grating is shown to be equal to the thermodynamic limit for all absorbances and concentration factors. The limit for the Rugate filter is generally lower, but tends to the thermodynamic limit for lower cell absorbance