We utilize photonic crystals to simulate enhanced light-trapping in a-Si:H thin film solar cells A one dimensional photonic crystal or distributed Bragg reflector with alternating dielectric layers acts as low loss backreflector A two dimensional photonic crystal between the absorber layer and the Bragg reflector diffracts light at oblique angles within the absorber The photonic crystal geometry is optimized to obtain maximum absorption The photonic crystal provides lossless diffraction of photons, increasing the photon path length within the absorber layer The simulation predicts significantly enhanced photon harvesting between 600 and 775nm below the band edge, and an absorption increase by more than a factor of 10 near the band edge The optical path length ratio can exceed the classical limit predicted for randomly roughened scattering surfaces at most wavelengths near the band edge The optical modeling is performed with a rigorous scattering matrix approach where Maxwell’s equations are solved 

Photonic crystal enhanced light-trapping in thin film solar cells

In this paper, we report a successful realization and integration of a gold two-dimensional hole array (2DHA) structure with semiconductor InAs quantum dot (QD). We show experimentally that a properly designed 2DHA-QD photodetector can facilitate a strong plasmonic-QD interaction, leading to a 130% absolute enhancement of infrared photoresponse at the plasmonic resonance. Our study indicates two key mechanisms for the performance improvement. One is an optimized 2DHA design that permits an efficient coupling of light from the far-field to a localized plasmonic mode. The other is the close spatial matching of the QD layers to the wave function extent of the plasmonic mode. Furthermore, the processing of our 2DHA is amenable to large scale fabrication and, more importantly, does not degrade the noise current characteristics of the photodetector. We believe that this demonstration would bring the performance of QD-based infrared detectors to a level suitable for emerging surveillance and medical diagnostic applications.

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A Surface Plasmon Enhanced Infrared Photodetector Based on InAs Quantum Dots

To harness the full spectrum of solar energy, Fresnel reflection at the surface of a solar cell must be eliminated over the entire solar spectrum and at all angles. Here, we show that a multilayer nanostructure having a graded-index profile, as predicted by theory [J. Opt. Soc. Am. 66, 515 (1976); Appl. Opt. 46, 6533 (2007)], can accomplish a near-perfect transmission of all-color of sunlight. An ultralow total reflectance of 1%-6% has been achieved over a broad spectrum, λ=400to1600 nm, and a wide range of angles of incidence, θ=0°-60°. The measured angle- and wavelength-averaged total reflectance of 3.79% is the smallest ever reported in the literature, to our knowledge.

Realization of a near-perfect antireflection coating for silicon solar energy utilization

Photonic crystal heterostructures, like their semiconductor quantum electronic counterparts, generate complex function from simple, well-understood building blocks. They have led to compact photonic crystal-based waveguides and record-quality-factor resonant cavities. Here the progress on the experimental realization of photonic crystal heterostructure devices, and on the development of convenient, intuitive, and computationally efficient models of devices that unite multiple finite-sized photonic crystal media to engineer photon localization and guidance is summarized.

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Photonic crystal heterostructures and interfaces

The literature describing scattering matrices for semi- analytical methods almost exclusively contains ine-cient formulations and formulations that deviate from long-standing convention in terms of how the scattering parameters are deflned. This paper presents a novel and highly improved formulation of scattering matrices that is consistent with convention, more e-cient to implement, and more versatile than what has been otherwise presented in the literature. Semi-analytical methods represent a device as a stack of layers that are uniform in the longitudinal direction. Scattering matrices are calculated for each layer and are combined into a single overall scattering matrix that describes propagation through the entire device. Free space gaps with zero thickness are inserted between the layers and the scattering matrices are made to relate flelds which exist outside of the layers, but directly on their boundaries. This framework produces symmetric scattering matrices so only two parameters need to be calculated and stored instead of four. It also enables the scattering matrices to be arbitrarily interchanged and reused to describe longitudinally periodic devices more e-ciently. Numerical results are presented that show speed and e-ciency can be increased by more than an order of magnitude using the improved formulation.

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Improved formulation of scattering matrices for semi-analytical methods that is consistent with convention

Transfer-matrix methods adopting a plane-wave basis have been routinely used to calculate the scattering of electromagnetic waves by general multilayer gratings and photonic crystal slabs. In this paper we show that this technique, when combined with Bloch's theorem, can be extended to solve the photonic band structure for 2D and 3D photonic crystal structures. Three different eigensolution schemes to solve the traditional band diagrams along high-symmetry lines in the first Brillouin zone of the crystal are discussed. Optimal rules for the Fourier expansion over the dielectric function and electromagnetic fields with discontinuities occurring at the boundary of different material domains have been employed to accelerate the convergence of numerical computation. Application of this method to an important class of 3D layer-by-layer photonic crystals reveals the superior convergency of this different approach over the conventional plane-wave expansion method.

Lan-Lan Lin

Papers

Photonic band structures solved by a plane-wave-based transfer-matrix method.