We suggest an energy selective and diffractive optical element as intermediate layer in thin-film tandem solar cells. By
adjusting the lattice constant of this photonic crystal, we fitted the optical properties to match a silicon tandem pair. Our
device enhances the pathway of incident light within an amorphous silicon top cell in its spectral region of low
absorption. In this spectral overlap region of the tandem-junction's quantum efficiencies, photons are being transferred
towards the amorphous cell, which leads to an increase in the short-circuit current of the limiting top cell. From our
simulations we expect a current increase of 1.44mA/cm<sup>2</sup> for an - amorphous/microcrystalline - silicon tandem cell,
corresponding to improvement of the tandem's absolute efficiency of about 1.3%.
The possibility to create defined structures inside a synthetic opal is a key step towards applications in optics, where control of the propagation of light inside a photonic crystal is necessary. Here we report different methods for realizing defined embedded defects in opaline structures. Monodisperse colloids are synthesized by surfactant free emulsion polymerization of the acid labile monomer <i>t</i>-butyl-methacrylate (<i>t</i>BMA). The P<i>t</i>BMA colloids can be filled with sensitizer and photo acid generator and it is possible to crystallize them into photosensitive polymer opals. One method for the introduction of defects is a multilayer build-up of photo-labile (filled with photo acid generator) and photo-stable (not filled with photo acid generator) polymer beads. Irradiation through a mask with UV-light followed by baking and development with aqueous base allows subsequent patterning of the opaline film. Alternatively defects can be directly produced in the depth of an opal by two photon lithography. For this method a photo stable opal is infiltrated with ORMOCER, which is then polymerized. After removing the PtBMA opal an inverse opal structure is obtained. The holes are then filled with a resin and polymerization takes places at defined places via two photon lithography.