Ralf B. Wehrspohn

Photonic crystal gas sensors

The bandstructure of photonic crystals offers intriguing possibilities for the manipulation of electromagnetic waves. During the last years, research has mainly focussed on the application of these photonic crystal properties in the telecom area. We suggest utilization of photonic crystals for sensor applications such as qualitative and quantitative gas and liquid analysis. Taking advantage of the low group velocity and certain mode distributions for some k-points in the bandstructure of a photonic crystal should enable the realization of very compact sensor devices. We show different device configurations of a photonic crystal based on macroporous silicon that fulfill the demands to serve as a compact gas sensor.

Photonic Crystal Devices with Multiple Dyes by Consecutive Local Infiltration of Single Pores

[ ∗] P. W Nolte , . Dr. D. Pergande , Dr. S. L. Schweizer , Prof. R. B. Wehrspohn Martin-Luther-Univesity Halle-Wittenberg Heinrich Damerow Str. 4, 06120 Halle (Germany) E-mail: peter.nolte@physik.uni-halle.de M. Geuss , B. Makowski , Prof. C. Weder Adolphe Merkle Institute and Fribourg Center for NanomaterialsUniversity of Fribourg P.O. Box 209, CH-1723 Marly (Switzerland) B. Makowski , Prof. C. Weder Case Western Reserve University Department of Macromolecular Science and Engineering 2100 Adelbert Rd., Cleveland, OH 44107–7202 (USA) R. Salzer , Prof. R. B. Wehrspohn Fraunhofer Institute for Mechanics of Materials Walter-Hulse-Strase 1, 06120 Halle (Germany) P. Mack Institut fur Nanotechnologie Karlsruhe Institute of Technology (KIT) Hermann-von-Helmholtz-Platz 176344 Eggenstein-Leopoldshafen (Germany) Dr. D. Hermann , Prof. K. Busch Institut fur Theoretische Festkorperphysik and DFG-Center for Functional Nanostructures (CFN) Karlsruhe Institute of Technology (KIT) Wolfgang-Gaede-Strasse 1, 76131 Karlsruhe (Germany) M. Geuss , Prof. M. Steinhart Max Planck Institute of Microstructure Physics Weinberg 2, 06114 Halle (Germany) Prof. M. Steinhart Institute for ChemistryUniversity of Osnabruck 49069 Osnabruck (Germany)

Directional selectivity and light-trapping in solar cells

The Yablonovitch limit for light trapping in solar cells with Lambertian surfaces can be increased using angle selective absorbers thereby exploiting the limited incidence angle of solar radiation. We simulate the efficiency gain or loss caused by an angular and energy selective filter on top of the absorber, compared to a Lambertian and a flat absorber. Additionally, we introduce two possible implementations of such a filter, a Rugate stack and inverted opal layers.

Tuning 2D photonic crystals

We demonstrate three ways in which the optical band-gap of 2-D macroporous silicon photonic crystals can be tuned. In the first method the temperature dependence of the refractive index of an infiltrated nematic liquid crystal is used to tune the high frequency edge of the photonic band gap by up to 70 nm for H-polarized radiation as the temperature is increased from 35 to 59°C. In a second technique we have optically pumped the silicon backbone using 150 fs, 800 nm pulses, injecting high density electron hole pairs. Through the induced changes to the dielectric constant via the Drude contribution we have observed shifts upt to 30 nm of the high frequency edge of the E-polarized band-gap. Finally, we show that below-band-gap radiation at 2.0 and 1.7 μm can induce changes to the optical properties of silicon via the Kerr effect and tune the band edges of the 2-D macroporous silicon photonic crystal.

Nanowires from dirty multi-crystalline Si for hydrogen generation

Silicon nanowires are considered as a promising architecture for solar energy conversion systems. By metal assisted chemical etching of multi-crystalline upgraded metallurgical silicon (UMG-Si), large areas of silicon nanowires (SiNWs) with high quality can be produced on the mother substrates. These areas show a low reflectance comparable to black silicon. More interestingly, we find that various metal impurities inside UMG-Si are removed due to the etching through element analysis. A prototype cell was built to test the photoelectrochemical (PEC) properties of UMG-SiNWs for water splitting. The on-set potential for hydrogen evolution was much reduced, and the photocurrent density showed an increment of 35% in comparison with a ‘dirty’ UMG-Si wafer.

Strategy for tailoring nanospheres for rough interfaces in solar cells (Conference Presentation)

The availability of optimum textures for the purpose of light trapping in solar cells is at stake. Here, we discuss how they can be obtained with a large-area scalable bottom-up approach that utilizes as a template monolayers of densely packed nanospheres from a colloidal solution with tailored size distribution. nTheoretically, we show that the surface textures geometry can be predicted and tuned from a colloidal solution with given nanosphere sizes and relative occurrence probability. With only simple monolayers comprised of two nanosphere size species, we show that one can already obtain a useful scattering pattern relevant for rear scattering light trapping textures. We proceeded to study the application of such textures in thin-film crystalline silicon (c-Si) solar cells. Such monolayers can be tuned to provide diffraction patterns, which form an annulus in Fourier space such that stronger scattering occurs at oblique angles. For such two species nanosphere monolayers, the nanosphere sizes dominantly influence the diffraction efficiency and minimum and maximum scattering angles. The relative occurrence probability of each nanopshere species influences the amount of diffraction states accessible, which translates to how broad the annulus region in Fourier space can be. The simplicity of the monolayer and the behavior of the scattering response allows to easily estimate nanosphere size ranges of interest by considering the radiation condition in c-Si and in air. nIn optimizing the monolayer parameters to obtain optimum rear scattering light trapping textures, we inspect approaches that avoid the severe computational costs, which typically follow the modeling of random scattering geometries. In particular, we investigate the applicability of utilizing the surface textures Power Spectral Density (PSD) and alternatively rigorous diffraction calculations in a semi-infinite c-Si superstrate to deduce net short-circuit current enhancement dependence on the monolayer parameters. The widely used PSD based prediction is shown to significantly deviate in important parameter ranges, where an optimal response can be obtained. This is related to the limitation of the PSD to be used as a predictor for the scattering response at textures with a notable height modulation. In the regime where the PSD fails to be predictive, an excellent prediction on the short-circuit current enhancement can be obtained with minimal computational costs by only examining the diffraction efficiencies in a selected wavelength range where light trapping has its largest impact. We show that the integrated diffraction in the directions of interest at the wavelength of 700 nm is sufficiently representative for the considered 1 μm thin-film c-Si cell and light trapping scheme. Fullwave simulations reveal that the integrated diffraction at 700 nm and the short-circuit current have coinciding trends in their dependency on the nanosphere size distribution.nWe furthermore explore the usage of the nanosphere monolayer template to obtain front surface textures, which provide mainly anti-reflection properties. This is done by considering an inverse pattern of the template to make use of the needle-like structures that emerge from the inverted nanosphere monolayer. The conditions needed for the monolayer parameters in order to ensure broadband suppression of reflection are discussed.

Low Group Velocity Devices in Silicon Photonics

Two possible concepts to slow down the light are discussed: (coupled) cavities in comparison to the concept of low group velocities at flat bands in photonic crystals. Two devices using the second concept are presented.

Periodically arranged point defects in a 2D photonic crystal: the photonic analogue to a doped semiconductor

We present and characterize hexagonal point defects in a two dimensional photonic crystal based on macroporous silicon. These point defects are prepatterned periodically, forming a superstructure within the photonic crystal after electrochemical etching. Spatially resolved, optical investigations related to morphological properties, like defect concentration and pore radius, are compared to bandstructure calculations. The confined defect states are identified and their interaction is evaluated quantitatively.