The power conversion efficiency of an ultrathin CuIn 1−ξ Ga ξ Se2 (CIGS) solar cell was maximized using a coupled optoelectronic model to determine the optimal bandgap grading of the nonhomogeneous CIGS layer in the thickness direction. The bandgap of the CIGS layer was either sinusoidally or linearly graded, and the solar cell was modeled to have a metallic backreflector corrugated periodically along a fixed direction in the plane. The model predicts that specially tailored bandgap grading can significantly improve the efficiency, with much smaller improvements due to the periodic corrugations. An efficiency of 27.7% with the conventional 2200-nm-thick CIGS layer is predicted with sinusoidal bandgap grading, in comparison to 22% efficiency obtained experimentally with homogeneous bandgap. Furthermore, the inclusion of sinusoidal grading increases the predicted efficiency to 22.89% with just a 600-nm-thick CIGS layer. These high efficiencies arise due to a large electron-hole-pair generation rate in the narrow-bandgap regions and the elevation of the open-circuit voltage due to a wider bandgap in the region toward the front surface of the CIGS layer. Thus, bandgap nonhomogeneity, in conjunction with periodic corrugation of the backreflector, can be effective in realizing ultrathin CIGS solar cells that can help overcome the scarcity of indium.

A design tool was formulated for optimizing the efficiency of inorganic, thin-film, photovoltaic solar cells. The solar cell can have multiple semiconductor layers in addition to antireflection coatings, passivation layers, and buffer layers. The solar cell is backed by a metallic grating which is periodic along a fixed direction. The rigorous coupled-wave approach is used to calculate the electron-hole-pair generation rate. The hybridizable discontinuous Galerkin method is used to solve the drift-diffusion equations that govern charge-carrier transport in the semiconductor layers. The chief output is the solar-cell efficiency which is maximized using the differential evolution algorithm to determine the optimal dimensions and bandgaps of the semiconductor layers.

A study of ray trajectories was undertaken for the Tamm medium which represents the spacetime of a cosmic spinning string, under the geometric-optics approximation. Our numerical studies revealed that: (i) rays never cross the string's boundary; (ii) the Tamm medium supports evanescent waves in regions of phase space that correspond to those regions of the string's spacetime which could support closed timelike curves; and (iii) a spinning string can be slightly visible while a non-spinning string is almost perfectly invisible.

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The rigorous coupled wave approach (RCWA) was implemented to investigate optical absorption in a triple-p-i-n-junction amorphous-silicon solar cell with a 2D metallic periodically corrugated backreflector (PCBR). Both total and useful absorptances were computed against the free-space wavelength λ0 for both s-and p-polarized polarization states. The useful absorptance in each of the three p-i-n junctions was also computed for normal as well as oblique incidence. Furthermore, two canonical boundary-value problems were solved for the prediction of guided-wave modes (GWMs): surface-plasmon-polariton waves and waveguide modes. Use of the doubly periodic PCBR enhanced both useful and total absorptances in comparison to a planar backreflector. The predicted GWMs were correlated with the peaks of the total and useful absorptances. The excitation of GWMs was mostly confined to λ0 < 700 nm and enhanced absorption. As excitation of certain GWMs could be correlated with the total absorptance but not with the useful absorptance, the useful absorptance should be studied while devising light-trapping strategies. arXiv:1806.02854v1 [physics.app-ph] 7 Jun 2018In solar-cell research, often the excitation of GWMs is correlated with the total absorptanceĀ tot of the device [18,23], which however is not a good measure of useful photonic absorption in a solar cell, as photons absorbed in the metallic portions of a solar cell are not available for conversion into electric current. Therefore, the chief objective for the work reported in this paper was to determine the spectrums of both the total absorptanceĀ tot and the useful absorptanceĀ sc [31] in a tandem solar cell with a 2D PCBR exposed to either normally or obliquely incident linearly polarized light. The solar cell was taken to comprise three p-in solar cells made of a-Si alloys [32] that can be fabricated using plasma-enhanced chemical-vapor deposition over planar and patterned substrates. A top layer of aluminum-doped zinc oxide (AZO) was incorporated to provide a transparent electrode. Also, an AZO layer was taken to be sandwiched between the 2D PCBR and the stack of nine semiconductor layers in order to avoid the deterioration of the electrical properties of the a-Si alloy closest to the metal [33], which was chosen to be silver [34]. The total absorptance and the useful absorptance calculated using the rigorous coupled-wave approach (RCWA) [35,36,24] were correlated against the predicted excitations of GWMs.The plan of this paper is as follows. Section 2 is divided into four parts. Section 2.1 presents the boundary-value problem that can be solved to determine the optical electromagnetic fields everywhere in a device comprising a stratified, isotropic dielectric material atop a 2D PCBR, when the device is illuminated by a plane wave. The formulations for useful and total absorptances are discussed in Sec. 2.2. Section 2.3 provides brief descriptions of the underlying canonical problems to predict the excitation of SPP waves and WGMs. Excitation of GWMs is discussed in Sec. ...