Matthias Kroll

Extremely low surface recombination velocities in black silicon passivated by atomic layer deposition

We investigate the optical and opto-electronic properties of black silicon (b-Si) nanostructures passivated with Al2O3. The b-Si nanostructures significantly improve the absorption of silicon due to superior anti-reflection and light trapping properties. By coating the b-Si nanostructures with a conformal layer of Al2O3 by atomic layer deposition, the surface recombination velocity can be effectively reduced. We show that control of plasma-induced subsurface damage is equally important to achieve low interface recombination. Surface recombination velocities of Seff<13 cm/s have been measured for an optimized structure which, like the polished reference, exhibits lifetimes in the millisecond range.

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

Conformal Transparent Conducting Oxides on Black Silicon

higher than those of state-of-the-art homojunction silicon solar cells. One reason is the interfacial electronic properties of the b-Si layer. In the literature, typically a thermally grown oxide is used to passivate the statistically structured surface of b-Si and metal contacts are fi red into the oxide. Because of the enhanced surface area of the b-Si surface, electronic transport is strongly infl uenced by surface-recombination and complex fi eld distributions inside the nanostructured needles. In this communication, we address the question: is a perfectly conformal coating with a transparent conducting oxide (TCO) possible on a b-Si surface and how does the TCO affect the optical properties of the

Black silicon for solar cell applications

We present experimental results and rigorous numerical simulations on the optical properties of Black Silicon surfaces and their implications for solar cell applications. The Black Silicon is fabricated by reactive ion etching of crystalline silicon with SF6 and O2. This produces a surface consisting of sharp randomly distributed needle like features with a characteristic lateral spacing of about a few hundreds of nanometers and a wide range of aspect ratios depending on the process parameters. Due to the very low reflectance over a broad spectral range and a pronounced light trapping effect at the silicon absorption edge such Black Silicon surface textures are beneficial for photon management in photovoltaic applications. We demonstrate that those light trapping properties prevail upon functionalization of the Black Silicon with dielectric coatings, necessary to construct a photovoltaic system. The experimental investigations are accompanied by rigorous numerical simulations based on three dimensional models of the Black Silicon structures. Those simulations allow insights into the light trapping mechanism and the influence of the substrate thickness onto the optical performance of the Black Silicon. Finally we use an analytical solar cell model to relate the optical properties of Black Silicon to the maximum photo current and solar cell efficiency in dependence of the solar cell thickness. The results are compared to standard light trapping schemes and implications especially for thin solar cells are discussed.

Resonant metasurfaces at oblique incidence: interplay of order and disorder

Understanding the impact of order and disorder is of fundamental importance to perceive and to appreciate the functionality of modern photonic metasurfaces. Metasurfaces with disordered and amorphous inner arrangements promise to mitigate problems that arise for their counterparts with strictly periodic lattices of elementary unit cells such as, e.g., spatial dispersion, and allows the use of fabrication techniques that are suitable for large scale and cheap fabrication of metasurfaces. In this study, we analytically, numerically and experimentally investigate metasurfaces with different lattice arrangements and uncover the influence of lattice disorder on their electromagnetic properties. The considered metasurfaces are composed of metal-dielectric-metal elements that sustain both electric and magnetic resonances. Emphasis is placed on understanding the effect of the transition of the lattice symmetry from a periodic to an amorphous state and on studying oblique illumination. For this scenario, we develop a powerful analytical model that yields, for the first time, an adequate description of the scattering properties of amorphous metasurfaces, paving the way for their integration into future applications.

Optical modeling of needle like silicon surfaces produced by an ICP-RIE process

We present results of rigorous optical modeling of reactive ion etched crystalline silicon surfaces, so called Black Silicon, for different etching parameters and compare them to experimental data. Reactive ion etching of crystalline silicon with SF6 and O2 can produce a surface consisting of sharp randomly distributed needle like features with a characteristic lateral spacing of about a few hundreds of nanometers and a wide range of aspect ratios depending on the process parameters. Due to the very low reflectance over a broad spectral range such surface textures can be beneficial for photon management in photovoltaic applications. To gain a detailed understanding of the optical properties of Black Silicon surfaces we recovered the full three dimensional geometry of differently etched samples. With these data we calculated the optical response using the finite differences time domain method. From the calculations we will give insight into the magnitude of resonant phenomena within the Black Silicon and the resulting near field enhancement. Furthermore we will present carrier generation profiles which quantify the effect of absorption enhancement due to the nanostructured surface. We also investigate the angular forward scattering distribution into the silicon substrate and the resulting path length enhancement which is crucial for the near band edge absorption especially in thin solar cells.

Black silicon photovoltaics

The challenge of future solar cell technologies is the combination of highly efficient cell concepts and low cost fabrication processes. A promising concept for high efficiencies is the usage of nanostructured silicon, so-called black silicon. Due to its unique surface geometry the optical path of the incoming light through the silicon substrate is enhanced to nearly perfect light trapping. Combined with the semiconductor-insulator-semiconductor (SIS) solar cell concept it is possible to fabricate a low cost device by using conventional sputtering technologies. Therefore, a thin insulator is coated on the nanostructured silicon surface, followed by the deposition of a transparent conductive oxide (TCO), e.g. indium tin oxide (ITO) or aluminum doped zinc oxide (AZO). In such systems the TCO induces a heterojunction, hence, high temperature diffusion processes are not necessary. The optical and geometrical properties of different nanostructured silicon surfaces will be presented. Furthermore, the influence of the used TCO materials will be discussed and the solar cell performance under AM1.5G illumination of unstructured and structured SIS devices is shown.

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.

Properties of periodic metasurfaces versus amorphous arrangements at oblique incidence

Electromagnetic properties of resonant metasurfaces are studied in transition from a periodic to an amorphous arrangement of their constituents. Emphasis is put to investigate the response at oblique incidence and for TM polarized light. Our metasurfaces possess both electric and magnetic resonances which makes them to be canonical samples that reflect properties of many metasurfaces. The suggested analytical model explains many if not to say all far-field optical properties that can be observed in resonant metasurfaces. For example, the resonance broadening and damping for the electric mode which contrasts with the persistence of these properties for the magnetic mode in transition from crystalline to the amorphous state can be fully explained. All theoretical predictions are verified with complementary numerical calculations and experimental measurements.

Passivation of optically black silicon wafers by atomic layer deposited Al2O3 films

Optically black silicon nanostructures show excellent light trapping properties. Towards the integration of these structures into a solar cell device, the passivation performance of atomic layer deposited thin Al2O3 films is investigated.