Daniel Lockau

Nanophotonic light trapping in 3-dimensional thin-film silicon architectures

Emerging low cost and large area periodic texturing methods promote the fabrication of complex absorber structures for thin film silicon solar cells. We present a comprehensive numerical analysis of a 2 μm square periodic polycrystalline silicon absorber architecture designed in our laboratories. Simulations are performed on the basis of a precise finite element reconstruction of the experimentally realized silicon structure. In contrast to many other publications, superstrate light trapping effects are included in our model. Excellent agreement to measured absorptance spectra is obtained. For the inclusion of the absorber into a standard single junction cell layout, we show that light trapping close to the Yablonovitch limit can be realized, but is usually strongly damped by parasitic absorption.

Large-area 2D periodic crystalline silicon nanodome arrays on nanoimprinted glass exhibiting photonic band structure effects

Two-dimensional silicon nanodome arrays are prepared on large areas up to 50 cm² exhibiting photonic band structure effects in the near-infrared and visible wavelength region by downscaling a recently developed fabrication method based on nanoimprint-patterned glass, high-rate electron-beam evaporation of silicon, self-organized solid phase crystallization and wet-chemical etching. The silicon nanodomes, arranged in square lattice geometry with 300 nm lattice constant, are optically characterized by angular resolved reflection measurements, allowing the partial determination of the photonic band structure. This experimentally determined band structure agrees well with the outcome of three-dimensional optical finite-element simulations. A 16% photonic bandgap is predicted for an optimized geometry of the silicon nanodome arrays. By variation of the duration of the selective etching step, the geometry as well as the optical properties of the periodic silicon nanodome arrays can be controlled systematically.

Implications of TCO Topography on Intermediate Reflector Design for a-Si/μc-Si Tandem Solar Cells—Experiments and Rigorous Optical Simulations

The influence of the transparent conducting oxide (TCO) topography was studied on the performance of a silicon oxide intermediate reflector layer (IRL) in a-Si/μc-Si tandem cells, both experimentally and by 3-D optical simulations. Therefore, cells with varying IRL thickness were deposited on three different types of TCOs. Clear differences were observed regarding the performance of the IRL as well as its ideal thickness, both experimentally and in the simulations. Optical modeling suggests that a small autocorrelation length is essential for a good performance. Design rules for both the TCO topography and the IRL thickness can be derived from this interplay.

Rigorous optical simulation of light management in crystalline silicon thin film solar cells with rough interface textures

We apply a hybrid finite element / transfer matrix solver to calculate generation rate spectra of thin film silicon solar cells with textured interfaces. Our focus lies on interfaces with statistical rough textures. Due to limited computational domain size the treatment of such textures requires a Monte Carlo sampling of texture representations to obtain a statistical average of integral target quantities. This contribution discusses our choice of synthetic rough interface generation, the Monte Carlo sampling and the need for an incorporation of the cells substrate into optical simulation when illumination of the cell happens through the substrate. We present results of the numerical characterization and generation rates for a single junction cell layout.

Optical modelling of incoherent substrate light-trapping in silicon thin film multi-junction solar cells with finite elements and domain decomposition

In many experimentally realized applications, e.g. photonic crystals, solar cells and light-emitting diodes, nanophotonic systems are coupled to a thick substrate layer, which in certain cases has to be included as a part of the optical system. The finite element method (FEM) yields rigorous, high accuracy solutions of full 3D vectorial Maxwells equations1 and allows for great flexibility and accuracy in the geometrical modelling. Time-harmonic FEM solvers have been combined with Fourier methods in domain decomposition algorithms to compute coherent solutions of these coupled system.2, 3 The basic idea of a domain decomposition approach lies in a decomposition of the domain into smaller subdomains, separate calculations of the solutions and coupling of these solutions on adjacent subdomains. In experiments light sources are often not perfectly monochromatic and hence a comparision to simulation results might only be justified if the simulation results, which include interference patterns in the substrate, are spectrally averaged. In this contribution we present a scattering matrix domain decomposition algorithm for Maxwells equations based on FEM. We study its convergence and advantages in the context of optical simulations of silicon thin film multi-junction solar cells. This allows for substrate lighttrapping to be included in optical simulations and leads to a more realistic estimation of light path enhancement factors in thin-film devices near the band edge.

FEM-based optical modeling of silicon thin-film tandem solar cells with randomly textured interfaces in 3D

Light trapping techniques are one of the key research areas in thin film silicon photovoltaics. Since the 1980s randomly rough textured front transparent oxides (TCOs) have been the methods of choice as light trapping strategies for thin-film devices. Light-trapping efficiency can be optimized by means of optical simulations of nano-structured solar cells. We present a FEM based simulator for 3D rigorous optical modeling of amorphous silicon / microcrystalline silicon tandem thin-film solar cells with randomly textured layer interfaces. We focus strongly on an error analysis study for the presented simulator to demonstrate the numerical convergence of the method and investigate grid and finite element degree refinement strategies in order to obtain reliable simulation results.

A comparison of scattering and non-scattering anti-reflection designs for back contacted polycrystalline thin film silicon solar cells in superstrate configuration

A new generation of polycrystalline silicon thin film solar cells is currently being developed in laboratories, employing a combination of novel laser or electron beam based liquid phase crystallization (LPC) techniques and single side contacting systems. The lateral grain size of these polycrystalline cells is in the millimeter range at an absorber thickness of up to 10 μm. In this contribution we present a comparative simulation study of several 1D, 2D and 3D nano-optical designs for the substrate / illumination side interface to the several micrometer thick back contacted LPC silicon absorber material. The compared geometries comprise multilayer coatings, gratings with step and continuous profiles as well as combinations thereof. Using the transfer matrix method and a finite element method implementation to rigorously solve Maxwell’s equations, we discuss anti-reflection and scattering properties of the different front interface designs in view of the angular distribution of incident light.

3D optical modeling of thin-film a-Si/μc-Si tandem solar cells with random textured interfaces using FEM

We present a FEM based simulator for 3D rigorous optical modeling of a-Si/μc-Si tandem thin-film solar cells with randomly textured layer interfaces. Our focus lies on a detailed analysis of the numerical error.

Correlation between structural and opto-electronic characteristics of crystalline Si microhole arrays for photonic light management

By employing electron paramagnetic resonance spectroscopy, transmission electron microscopy, and optical measurements, we systematically correlate the structural and optical properties with the deep-level defect characteristics of various tailored periodic Si microhole arrays, which are manufactured in an easily scalable and versatile process on nanoimprinted sol-gel coated glass. While tapered microhole arrays in a structured base layer are characterized by partly nanocrystalline features, poor electronic quality with a defect concentration of 1017 cm−3 and a high optical sub-band gap absorption, planar polycrystalline Si layers perforated with periodic arrays of tapered microholes are composed of a compact crystalline structure and a defect concentration in the low 1016 cm−3 regime. The low defect concentration is equivalent to the one in planar state-of-the-art solid phase crystallized Si films and correlates with a low optical sub-band gap absorption. By complementing the experimental characterization...

3D finite-element simulations of enhanced light transmission through arrays of holes in metal films

Light transmission through a 2D-periodic array of small rectangular apertures in a film of highly conductive material is simulated using a finite-element method. It is demonstrated that well converged results are obtained using higher-order finite-elements. The influence of the array periodicity and of corner roundings on transmission properties is investigated.