Lorenz Steidl

Three‐Dimensional Photonic Crystal Intermediate Reflectors for Enhanced Light‐Trapping in Tandem Solar Cells

Unbalanced currents in serial-connected tandem solar cells are one of the major limitations for cost-effi cient fabrication of third generation solar cells. [ 1 ] By using optimized photon management, we will show for a micromorph solar cell that an embedded 3D photonic crystal acting as an intermediate refl ector can balance the currents. It experimentally enhances the external quantum effi ciency (EQE) for the current-limiting top cell up to factor of 3.6, corresponding to a short-circuit current enhancement of about 25% compared to state-of-the-art textured micromorph solar cells. Our concept of 3D intermediate refl ectors can be transferred to any other multijunction solar cell even on textured substrates. [ 2–4 ] The requirement for current-matched tandem solar cells is that the currents at the maximum power point of all individual cells are the same so that the potentials add up. Even though current-matching is theoretically possible by choosing appropriate absorbers, [ 1 ] limitations in processable materials, deposition time, and materials quality prevent the application of tandem cells on a large scale. Recently, the new research area of photon management in solar cells has emerged. [ 2 ] The paradigm of photon management is not to alter the electronic or geometric properties of the absorbing

Increasing fluorescent concentrator light collection efficiency by restricting the angular emission characteristic of the incorporated luminescent material: the 'Nano-Fluko' concept

Fluorescent concentrators concentrate both diffuse and direct radiation without requiring tracking of the sun. In fluorescent concentrators, luminescent materials embedded in a transparent matrix absorb sunlight and emit radiation with a different wavelength. Total internal reflection traps the emitted light and guides it to solar cells attached to the concentrators edges. The escape cone of total internal reflection, however, limits the light collection efficiency. Spectrally selective photonic structures, which transmit light in the absorption range of the luminescent material and reflect the emitted light, reduce these losses. In this paper, we review different realizations of such structures and show that they increase collection efficiency by 20%. However, light emitted into steep angles in respect to the front surface, which would be lost without the photonic structures, has a very long effective path inside the concentrator until it reaches a solar cell. Therefore it suffers from path length dependent losses. We discuss how emission into the unfavorable directions can be suppressed by integrating the luminescent material into photonic structures, thus reducing these losses. We present possible realizations both for the concentrator design and for the solar cells used in such systems.

Photonic crystal intermediate reflector in micromorph tandem solar cells

Experimental and numerical evidences are presented which show that the efficiency of silicon based tandem solar-cells can be increased by incorporating a three dimensional photonic crystal as an intermediate reflector.

Electrodeposition of ZnO nanorods on opaline replica as hierarchically structured systems

We present a new method to prepare hierarchical structures by using ZnO replica and ZnO-coated PMMA opals as electrodes in an electrodeposition process of ZnO nanorods. Depending on the approach the nanorods can be either grown exclusively on top of the replica or inside the replica structures. Therefore two types of systems are accessible: 3D photonic crystals with a hierarchically structured surface consisting of nanorods and macroporous ZnO structures with an increased surface area.

3D photonic crystals for photon management in solar cells

Summary form only given. Photon management is a key element to optimize the optical and electro-optical performance of solar cells. Photon management concepts and the potential of 3D photonic crystals for photon management in solar cells are discussed.

Emission of Rhodamine B in PMMA opals for luminescentsolar concentrators

In conventional luminescent solar concentrators (LSC) incident light is absorbed by luminophores and emitted isotropically. Most of the emitted light is trapped inside the LSC by total internal reflection and guided to solar cells at the edges. Light emitted towards the surfaces, however, is lost in the escape cone. Furthermore, when the luminophore emits light in its absorption range, light is lost due to reabsorption. To overcome these losses, we embed the luminescent material in photonic structures to influence the emission characteristics. Directional and spectral redistribution of emission is supposed to enhance the light guiding in LSCs and reduce reabsorption losses. For this purpose, we prepared opal films from PMMA colloids, in which Rhodamine B was embedded during the polymerization process. In direction-resolved luminescence measurements a strong dependence of the emitted spectrum on the detection direction was observed. Further, the light collection efficiency of the samples was determined with optical measurements and light guiding due to the intended absorption and emission process was observed. The overall performance, however, suffers from cracks and defects in the photonic crystal.

Inverted-opal photonic crystals for ultra light-trapping in solar cells

We investigated a three dimensional inverted opal having the potential to notably increase light-trapping in solar cells. The 3D photonic crystal top layer is an angle- and direction-selective filter, which decreases the acceptance cone of the solar cell. Numerical optimisation methods are used to verify the optical and electrical properties for a large angluar and energy spectrum for a system consisting of an inverted opal on top of a thin crystalline silicon solar cell. It is numerically shown that an inverted opal grown in the Τ - Xdirection might fulfill the requirement for such a filter. An estimate for the theoretically achievable efficiency for nonconcentrated light is presented that do show an enchanced efficiency near the electronic band edge of the absorber. The fabrication of first opals grown in Τ - Xdirection is presented and discussed with respect to the quality and large scale fabrication.

Micromorph silicon tandem solar cells with fully integrated 3D photonic crystal intermediate reflectors

A 3D photonic intermediate reflector for textured micromorph silicon tandem solar cells has been investigated. In thin-film silicon tandem solar cells consisting of amorphous and microcrystalline silicon with two junctions of a-Si/c-Si, efficiency enhancements can be achieved by increasing the current density in the a-Si top cell providing an optimized current matching at high current densities. For an ideal photon-management between top and bottom cell, a spectrally-selective intermediate reflective layer (IRL) is necessary. We present the first fully-integrated 3D photonic thin-film IRL device incorporated on a planar substrate. Using a ZnO inverted opal structure the external quantum efficiency of the top cell in the spectral region of interest could be enhanced. As an outlook we present the design and the preparation of a 3D self organized photonic crystal structure in a textured micromorph tandem solar cell.

3D photonic crystal intermediate reflectors for enhanced light-trapping in tandem solar cells

The concept of 3D photonic crystals embedded in tandem solar cells as intermediate reflective layers is investigated. Numerical as well as experimental results will be presented.