Kurt Hingerl

Angle dependence of external and internal quantum efficiencies in bulk-heterojunction organic solar cells

The realization of highly efficient organic solar cells requires the understanding and the optimization of the light path in the photoactive layer. We present in this article our approach to measure and model the optical properties of our bulk-heterojunction devices, and to control them in order to enhance the photovoltaic performances. We report our recent observations on the dependence of the external quantum efficiency (EQE) on the incidence angle of the light, and our results on the determination of internal quantum efficiency based on EQE measurement and optical modeling cross-checked by reflection measurements. We investigate poly(3-hexylthiophene): 1-(3-methoxy-carbonyl) propyl-1-phenyl[6,6]C61 based solar cells with two different thicknesses of the active layer (170 and 880nm), and show that in the thin ones the absorption is enhanced for oblique incident radiation.

Spectroscopic ellipsometry and polarimetry for materials and systems analysis at the nanometer scale: state-of-the-art, potential, and perspectives

This paper discusses the fundamentals, applications, potential, limitations, and future perspectives of polarized light reflection techniques for the characterization of materials and related systems and devices at the nanoscale. These techniques include spectroscopic ellipsometry, polarimetry, and reflectance anisotropy. We give an overview of the various ellipsometry strategies for the measurement and analysis of nanometric films, metal nanoparticles and nanowires, semiconductor nanocrystals, and submicron periodic structures. We show that ellipsometry is capable of more than the determination of thickness and optical properties, and it can be exploited to gain information about process control, geometry factors, anisotropy, defects, and quantum confinement effects of nanostructures.

Design of efficient organic tandem cells: On the interplay between molecular absorption and layer sequence

We have carried out detailed optical simulations of tandem solar cells based on the following organic semiconductors: poly(3-hexylthiophene) (P3HT), poly[2,6-(4,4-bis-(2-ethylhexyl)-4H- cyclopenta[2,1-b;3,4-b′]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT), 1-(3- methoxycarbonyl) propyl-1-phenyl[6,6] C61 (PC60BM), and 1-(3-methoxycarbonyl) propyl-1- phenyl[6,6] C71 (PC70BM). We demonstrate that out of the many possible combinations of the component materials, one specific combination emerges as the best to reduce the spectral overlap of the two bulk heterojunction blends and thereby to ensure an optimized short-circuit current density (Jsc). Furthermore, the calculations allow us to predict the maximum Jsc achievable in tandem cells based on P3HT and PCPDTBT. Finally, we show that the efficient tandem cell realized and described recently by Kim et al. [Science 317, 222 (2007)] ensures balanced absorption in the top and bottoms cells.

Controlling electromagnetic fields with graded photonic crystals in metamaterial regime.

Engineering of a refractive index profile is a powerful method for controlling electromagnetic fields. In this paper, we investigate possible realization of isotropic gradient refractive index media at optical frequencies using two-dimensional graded photonic crystals. They consist of dielectric rods with spatially varying radii and can be homogenized in broad frequency range within the lowest band. Here they operate in metamaterial regime, that is, the graded photonic crystals are described with spatially varying effective refractive index so they can be regarded as low-loss and broadband graded dielectric metamaterials. Homogenization of graded photonic crystals is done with Maxwell-Garnett effective medium theory. Based on this theory, the analytical formulas are given for calculations of the rods radii which makes the implementation straightforward. The frequency range where homogenization is valid and where graded photonic crystal based devices work properly is discussed in detail. Numerical simulations of the graded photonic crystal based Luneburg lens and electromagnetic beam bend show that the homogenization based on Maxwell-Garnett theory gives very good results for implementation of devices intended to steer and focus electromagnetic fields.

Realization, characterization, and optical modeling of inverted bulk-heterojunction organic solar cells

Inverted bulk-heterojunction organic solar cells (OSCs) using solution-processed layers possess significant advantages compared to the usual noninverted devices. To investigate the full potential of this type of OSC, we have carried out some optical modeling by rigorous coupled wave analysis. The influence of the thickness of several different layers in the device has been quantified, as well as the maximum possible number of photons absorbed in the poly(3-hexyltiophene):[6,6]-phenyl-C61-butyric acid methyl ester active layer for both conventional and inverted structures. It appears that the thickness of the hole injecting layer placed in front of the metallic mirror can influence the electromagnetic field distribution in the OSC, but no additional beneficial optical spacer effect is observed. The thickness of the electron injecting layer deposited on the semitransparent electrode also has a negligible influence on the photons absorbed in the active layer for the inverted structure.

Ellipsometry at the nanoscale

Preamble.- Preface.- A Brief History and State of the Art of Ellipsometry.-Advanced Mueller Ellipsometry Instrumentation and Data Analysis.- Data Analysis for Nanomaterials: Effective Medium Approximation, its Limits and Implementations.- Relationship between Surface Morphology and Effective Medium Roughness.- Plasmonics and Effective-Medium Theory.- Thin films of Nanostructured Plasmonic Noble Metals.- Spectroscopic Ellipsometry on Metallic Gratings.- Mueller matrix applied to nanostructures.- Spectroscopic Ellipsometry and Magneto-Optical Kerr Spectroscopy of Magnetic Garnet Thin Films Incorporating Plasmonic Nanoparticles.- Generalized Ellipsometry Characterization of Sculptured Thin Films made by Glancing Angle Deposition.- THz Generalized Ellipsometry characterization of highly-ordered 3-dimensional Nanostructures.- Infrared ellipsometric investigations of free carriers and lattice vibrations in superconducting cuprates.- Real-time Ellipsometry for Probing charge-transfer processes at the nanoscale.- Polarimetric and other Optical Probes for the Solid - Liquid Interface.- Spectroscopic Ellipsometry for functional nano-layers of flexible organic electronic devices.- Spectroscopic Ellipsometry of Nanoscale Materials for Semiconductor Device Applications.- Ellipsometry of semiconductor nanocrystals.- Spectroscopic Ellipsometry for Inline Process Control in the Semiconductor Industry.- Thin film applications in research and industry characterized by spectroscopic ellipsometry.- Ellipsometry and Correlation Measurements.- Nanotechnology: Applications and markets, present and future.

Mechanisms for room temperature direct wafer bonding

Reducing the temperature needed for high strength bonding which was and is driven by the need to reduce effects of coefficient of thermal expansion mismatch, reduce thermal budgets, and increase throughput has led to the development of plasma treatment procedures capable of bonding Si wafers below 300 °C with a bond strength equivalent to Si bulk. Despite being widely used, the physical and chemical mechanisms enabling low temperature wafer bonding have remained poorly understood. We developed an understanding of the beneficial surface modifications by plasma and a model based on short range low temperature diffusion through bonding experiments combined with results from spectroscopic ellipsometry, depth resolving Auger electron spectroscopy, and transmission electron microscopy measurements. We also present experimental results showing that even at room temperature reasonable bond strength can be achieved. We conclude that the gap closing mechanism is therefore a process which balances the lowering of the total energy by minimizing the sum of the free surface energy (maximizing the contact area between the surfaces) and strain energy in the oxide at the bond interface.

Single and multilayer metamaterials fabricated by nanoimprint lithography

We demonstrate for the first time a fast and easy nanoimprint lithography (NIL) based stacking process of negative index structures like fishnet and Swiss-cross metamaterials. The process takes a few seconds, is cheap and produces three-dimensional (3D) negative index materials (NIMs) on a large area which is suitable for mass production. It can be performed on all common substrates even on flexible plastic foils. This work is therefore an important step toward novel and breakthrough applications of NIMs such as cloaking devices, perfect lenses and magnification of objects using NIM prisms. The optical properties of the fabricated samples were measured by means of transmission and reflection spectroscopy. From the measured data we retrieved the effective refractive index which is shown to be negative for a wavelength around 1.8 µm for the fishnet metamaterial while the Swiss-cross metamaterial samples show a distinct resonance at wavelength around 1.4 µm.

Chemical reaction at the ZnSe/GaAs interface detected by Raman spectroscopy

When ZnSe is deposited at temperatures commonly used for epitaxy onto GaAs, the possibility arises that selenium or zinc reacts with the substrate and thin interfacial layers consisting of a gallium selenide or a zinc arsenide are formed. In particular, Ga2Se3, which is thermodynamically the most stable, has been suggested as a likely candidate. In this study we present evidence for the formation of Ga2Se3 using Raman spectroscopy as a fingerprint technique. Ga2Se3 layers were grown on GaAs and the Raman spectra thereof were compared with those of ZnSe/GaAs heterostructures.

Refraction and rightness in photonic crystals

We present a study on relation between the refraction and rightness effects in photonic crystals applied on a 2D square lattice photonic crystal. The plane wave (the band and equifrequency contour analyses) and FDTD calculations for both TM and TE modes revealed all possible refraction and rightness cases in photonic crystal structures in the first three bands. In particular, we show for the first time, a possibility of the left-handed positive refraction. This means that left-handedness does not necessarily imply negative refraction in photonic crystals.