Zihuan Xia

Broadband absorption enhancement in elliptical silicon nanowire arrays for photovoltaic applications

Semiconducting nanowire arrays have emerged as a promising route toward achieving high efficiencies in solar cells. Here we propose a perpendicular elliptical silicon nanowire (PEE-SiNW) array for broadband light absorption in thin film silicon solar cells. Simulation results reveal that light absorption enhancement is originated from the split of the principal modes as well as the excitation of high order modes caused by the asymmetry of the elliptical nanowires and the enhanced mode coupling between adjacent elliptical nanowires attained by the appropriate arrangement of nanowires. An ultimate efficiency of 29.1% is achieved for the optimal PEE-SiNW array, which is 16.4% higher than that of the circular SiNW array with the same fill fraction.

Misaligned conformal gratings enhanced light trapping in thin film silicon solar cells

The effect of the relative lateral displacement between the front and back sinusoidal textured layers of a conformal grating solar cell on light trapping was investigated. For various amount of relative lateral displacements and thicknesses of the active layer, the external quantum efficiency (EQE) of the misaligned solar cell structures and their EQE enhancement relative to the aligned structure were studied. For both aligned and misaligned solar cell structures, the electric field distribution at the wavelength corresponding to the EQE peaks was analyzed, and the corresponding guided modes were identified. Additional modes were observed in the misaligned grating structures. A 25.1 times enhancement of the EQE at the wavelength of 950 nm and an average of 2.2 times enhancement in the wavelength range from 700 to 900 nm were observed. For the misaligned grating structure with the phase shift β = π/4 and the active layer thickness D(Si) = 230 nm, a maximum short circuit current density J(sc) enhancement of 34% was achieved for normal incidence, and a short circuit current enhancement of more than 15% was obtained for the incident angle between -15° and + 15°.

Efficient broadband light absorption in elliptical nanohole arrays for photovoltaic application.

We propose a perpendicular elliptical silicon nanohole (PE-SiNH) array for light absorption in thin film silicon solar cells. Our analysis shows that this architecture is capable of increasing the absorption of a thin film silicon solar cell by 11.3% in comparison to that of the optimal circular SiNH array. The process of breaking the mirror symmetries is responsible for the increase of the coupled modes. The PE-SiNH structures show additional near-zero spatial Fourier components compared with the circular SiNH structure, which helps to couple more incident light into slow Bloch modes. The mode interaction between adjacent elliptical nanoholes is in favor of the coupling of the incident light into channeling modes and, therefore, enhances light absorption in the short wavelength region.

Thin film silicon solar cells with non-simple integral period ratio between front and back gratings

We propose an ultra-thin amorphous silicon solar cell with dual-interface gratings of non-simple integral period ratios. The non-simple integral period ratio structure (NIPRS) shows a broad absorption band compared to the simple integral period ratio structure (SIPRS). Both the front and back gratings couple light into different optical modes in wavelength ranges that do not overlap. An integrated absorption of 67.2% is achieved when the thickness of the active layer is 100 nm.

Preparation and characterization of tungsten oxide thin films with high electrochromic performance

Tungsten oxide thin films were prepared by depositing WO3 onto glass substrates coated with ITO using reactive evaporation process at ambient temperature and 200°C respectively. The thin films were grown at different deposition rate. Chronoamperometry was carried out and spectral measurements were performed in situ. Results showed that the thin films prepared at low deposition rates possess higher coloration efficiency (CE), and the thin films grown at ambient temperature have high CE than those grown at 200°C. The origin of the differences in coloration efficiency of the thin films were analyzed and discussed based on the electrochromic mechanism of amorphous tungsten oxide films. The samples morphology was characterized by atom force microscopy (AFM).

Light absorption enhancement in elliptical nanohole array for photovoltaic application

We propose an elliptical silicon nanohole (SiNH) array for broadband light absorption in thin film silicon solar cells. Our analysis shows that this architecture is capable of increasing the ultimate efficiency of a thin film silicon solar cell by 17.6 % in comparison to that of the circular SiNH array with the same fill fraction. Lattice symmetry breaking and extension of the irreducible Brillouin zone are responsible for the enhancement of the absorption.

Wavelength tunable properties of the perpendicular dual-grating guided-mode resonance filters

Perpendicular dual-grating (PDG) guided-mode resonance filters were constructed by placing two identical one-dimensional waveguide gratings close to and their grooves perpendicular to each other with a nano air gap between them. Multilayer waveguide theory was used to estimate the split of the resonant reflection peaks corresponding to the TE and TM modes, and the rigorous coupled wave analysis (RCWA) was used to investigate the resonant wavelength, the linewidth of the resonant peaks, and electric field intensity distribution in the filter structures. The filters present identical spectral characteristics for normally incident wave with arbitrary polarization. The TM01 and the TM01 modes are found displaying the greatest wavelength shift for the air gap variation between 0 and 100 nm, and 100 nm and 1000 nm，respectively. The coupling between the TE and TM modes is much greater in the g/w/a/w/g structure than that in the w/g/a/g/w structure, since there is no space between the two waveguide layers in the former. The resonant peaks of the TM01 mode for the one-dimensional PDG g/w/a/w/g structure exhibit narrower width compared with those for the two-dimensional g/w/a/w/g structure. In addtion, the horizontal shift between the two gratings does not influence the measured spectra, although it will certainly have great effect on the resonant peak width if the measurement were carried out by the guided-mode filters where the two gratings are two dimensional.

Multimode resonance bandpass filter with twin wideband channels

A bandpass filter with twin wideband channels in a single-layer guided-mode resonance grating is presented. Strong refractive-index modulation is used to support the excitation of multimode resonances TE1,0, TE1,1, TE2,0, TE1,2, and TE2,1, which are excited by the first and second diffraction orders, relate asymmetrical line shapes and broad low-transmission bands, where TE is the transverse electric. Taking advantage of narrow linewidth and sharp edge line shape in the spectra of TE2,v (v is the mode), a bandpass filter with form factors of 0.61 and 0.7 for long- and short-wave channels is presented to demonstrate this concept.

The influence of micron-sized nodules on the electric-field districution in thin-film polarizers

The influence of micron-sized nodules on the electric-field enhancement in the HfO2/SiO2 thin-film polarizers with non-quarter- wave layers at 1053nm and 56° is studied using the finite-difference time-domain electromagnetic modeling. The theoretical results show that the electric-field enhancements in HfO2 material are greater at s polarization than those at ppolarization. Nodular defect originating from the large, shallow seed leads to the highest electric-field enhancement while that containing the small, deep seed leads to the lowest electric-field enhancement. The TFP coating designed with the electric-field peaks located in the SiO2 layers has no obvious advantage in decreasing the laser-induced damage than that designed with the electric-field peaks located in the HfO2 layers, once they have the similar nodular defects in them.

Optimization of triangle grating structures for light trapping in thin film silicon solar cells

The influence of different triangular grating shapes on the absorption performance of the thin-film silicon solar cell is discussed in this paper. A finite difference time domain method is used to make the numerical simulation. By studying the shape of the triangular grating, the designs of structure are optimized to achieve higher short circuit currents and quantum efficiencies. We find that the blaze grating structure is the ideal initial texture for thin-film solar cells. Compared with the triangular grating, the short circuit current of the blaze grating is improved by 7.09% for the entire spectrum. The short circuit current for the short wavelength is increased by 13.5% whereas the short circuit current in the red and infrared part of spectrum is increased by 86.5%.