Flexible photonics based on polymers
Optical Whispering-Gallery-Resonators with several micrometers in dimension can be used to confine light of a certain wavelength. Such structures are promising building blocks in lots of photonic devices.
Tunable polymeric whispering-gallery-resonators
The tunability of the resonance wavelength is crucial for numerous applications of optical micro-resonators like filters or switches.
One current focus of our research is the realization of tunable photonic building blocks, which exploit the flexibility of different polymers and elastomers. First key experiments have been already performed. A new type of whispering-gallery resonator, the so-called Split-disk resonator has been developed to achieve the tunability of resonant modes over a wide spectral range. This tunability arises from the reversible deformation of the elastomer substrate which affects the size of the gap between two opposite half-disks. Furthermore, such elastomer substrates have been used to realize photonic molecules from up to three goblet-type resonators with flexible and tunable coupling.
Future experiments will study the fabrication and characterization of larger arrays of coupled resonators as well as of hybrid (gaining/absorbing) photonic molecules.
T. Siegle et al., "Split-disk micro-lasers: Tunable whispering gallery mode cavities", APL Photonics 2, 096103 (2017)
T. Siegle et al., "Photonic molecules with a tunable inter-cavity gap", Light Sci. Appl. 6, e16224 (2017)
Tunable photonics based on liquid-crystal elastomers
Besides using mechanically deformable elastomer substrates, our research group also exploits the properties of liquid-crystal elastomers (LCE) for the realization of tunable photonic building blocks.
At relatively low temperatures liquid-crystal elastomers undergo a transition from a state, where the mesogens show nematic alignment, into a disordered isotropic phase. This transition is accompanied by a fully reversible change in dimension and thus of the optical properties of photonic elements.
LCE can serve as substrate material for whispering-gallery resonators allowing precise tuning of inter-cavity gaps by a change in temperature. As a first step a reversible change of the gap depending on the intensity of laser illumination has been was shown recently. In future experiments we aim at observing tunable coupling between several resonators.
Furthermore resonators purely made from LCEs were produced. With this approach, the temperature dependent nature of the material can be used to tune the resonance wavelength of the resonators. An intended tuning of the resonance wavelength by more than one free spectral range has been demonstrated.
Hybrid plasmonic-photonic whispering-gallery resonators
Another focus of our current research on optical whispering-gallery resonators is on the intentional enhancement of light-matter interaction. A strong light-matter interaction hereby requires high quality factors and small mode volumes.
While the quality factors of dielectric whispering-gallery resonators are very high, their modal volumes are rather large compared to those of plasmonic nanostructures. Thus, a possible strategy to boost light-matter interaction is seen in combining the high quality factors of dielectric resonators with the ability of plasmonic structures to squeeze light down to sub-wavelength dimensions. The advantages of both worlds have been successfully combined in a silver-coated whispering-gallery resonator with wedge-like geometry. Besides purely photonic modes, which are localized more in the interior of the resonator, this newly developed resonator type supports also plasmonic modes, which are localized at the interface between the dielectric and the metal. Of special interest for most applications are so called hybrid modes, which arise from the hybridization of photonic and plasmonic modes and are also supported by this kind of resonators. These modes combine the advantages of photonic and plasmonic modes, e. g., the high quality factors associated with photonic modes and the small mode volumes associated with plasmonic modes.
C. Klusmann et al., "Hybridizing whispering gallery mode and plasmonic resonances in a photonic metadevice for biosensing applications", J. Opt. Soc. Am. 34, D46 - D55 (2017)
C. Klusmann et al., "Identification of Dielectric, Plasmonic, and Hybrid Modes in Metal-Coated Whispering-Gallery-Mode Resonators", ACS Photonics 5, 2365–2373 (2018)
Kesterite solar cells
Solar cells based on kesterite (Cu2ZnSn(S,Se)4, CZTSSe) absorbers are promising candidates for thin-film photovoltaics, for they are using environmentally friendly and cheap materials.
Our current research is focusing on both the fabrication, as well as the characterization of regarding solar cells. Hereby the fabrication is realized on two different approaches. On the one hand co-evaporation is used, a technique which allows for precisely controllable conditions and therefor a high purity of the material, since the deposition takes place in vacuum. On the other hand solution-based doctor-blading is used, a very simple process which makes a high throughput of samples possible.
As part of the current research on kesterites, among other things, we investigate modifications of the absorber material by e.g. the incorporation of germanium, which can substitute tin within the absorber material and also work as a catalyst during the whole fabrication process. Furthermore a fabrication technique was developed, which allows for a precise tuning of the sulfur-selenium-ratio in CZTSSe and therefor also of the absorber materials’ band gap. This technique opens up for a lot of possible applications of kesterite absorbers, like for example tandem solar cells. In addition we are working on alternative buffer materials, which are also used in Cu(In,Ga)(S,Se)_2 (CIGS) research. Up to now CdS is mostly used as buffer material for both kesterite and CIGS solar cells. This is planned to be replaced by a more environmentally friendly material with smaller absorption losses. For a non-destructive investigation of buffer and absorber layers a novel type of modulation spectroscopy is used, which is based on electro reflectance and was developed within our research group. Further methods used for the characterization of kesterite solar cells are photoluminescence, Raman spectroscopy, X-ray diffraction, electron microscopy and studies on absorption and j/V-characteristics.
In the focus of the spectroscopic characterizations are compositional dependencies of material-specific properties like e.g. the degree of order-disorder within the the Cu-Zn-plane of the kesterite crystal, which shows a direct impact on the band gap of the material. The insights gained within this research essentially contribute to the overall understanding of the material CZTSSe.
Our research is carried out in close cooperation with the Light Technology Institute (LTI) of KIT, Baden-Württemberg's Centre for Solar Energy and Hydrogen (ZSW) in Stuttgart, the Laboratory for Chalcogenide Photovoltaics (LCP) of the University of Oldenburg and the Institute for Chemical Technology and Polymer Chemistry (ITCP) of KIT and is funded by the Federal Ministry of Education and Research (BMBF) as part of the project “FREE-INCA”.
M. Neuwirth et al., "Band-gap tuning of Cu2ZnSn(S,Se)4 solar cell absorbers via defined incorporation of sulfur based on a post-sulfurization process", Sol. Energy Mater. Sol. Cells 182, 158-165 (2018)
M. Lang et al., "Impact of the degree of Cu-Zn order in Cu2ZnSn(S,Se)4 solar cell absorbers on defect states and band tails", Appl. Phys. Lett. 113, 033901 (2018)
M. Lang et al., "Influence of the Cu Content in Cu2ZnSn(S,Se)4 solar cell absorbers on order-disorder related band gap changes", Appl. Phys. Lett. 109, 142103 (2016)
M. Neuwirth et al., "Morphology of multiple-selenized Cu2ZnSn(S,Se)4 absorber layers", phys. stat. solidi (b) 14, 1600163 (2017)
C. Krämmer, "Optoelectronic Characterization of Thin-Film Solar Cells by Electroreflectance and Luminescence Spectroscopy", Dissertation, Karlsruhe Institut für Technologie (2015)
Perovskite solar cells
Thin-film solar cells based on organic-inorganic perovskite absorber layers have been shown to be the fastest growing solar technology in history so far. Starting with a power conversion efficiency of 3% in 2009, perovskite solar cells have achieved an efficiency above 20% nowadays.
The main interest of our research is to investigate the fundamental optoelectronic properties of the perovskite absorber material and solar cells by means of advanced optical spectroscopy. The aim is to gain valuable insight into the material and device physics and their correlation with the overall material properties and device performance. For this, we utilize different spectroscopic techniques, e.g., absorption spectroscopy, steady-state and time-resolved photoluminescence (PL) measurements, and modulation spectroscopy.
For example, modulation spectroscopy (in particular electroreflectance (ER) and electroabsorption (EA)) allows for a precise determination of the bandgap and deeper understanding of the electronic structure. Using this technique, we are able to investigate complete solar cells in a non-destructive way. The basic principle is to measure the influence of a modulated external stimulus, e.g., an applied bias, on the optical properties of the sample, e.g., transmission or reflectance.
By employing these advanced spectroscopic techniques, we are able to investigate fundamental material properties, e.g., the role of excitonic effects on optical transitions or temperature-dependent phase transitions of the crystal structure. Furthermore, we look at different absorber compositions and the implications for the performance and stability of perovskite-based solar cells.
F. Ruf et al., "Excitonic Nature of Optical Transitions in Electroabsorption Spectra of Perovskite Solar Cells", Appl. Phys. Lett. 112, 083902 (2018)
CIGS solar cells
Cu(In,Ga)(S,Se)2 (CIGS) solar cells belong to the established thin-film solar cells reaching record efficiencies of up to 22.9%. One of the big advantages of the absorber material is the tunability of its bandgap energy through the variation of the Ga/(Ga+In) ratio which allows to make better use of the solar spectrum. Due to the absorber’s high absorption coefficient a 2 µm thick layer is sufficient for the electricity generation which leads to numerous new applications for the solar cell. For instance, CIGS thin-film solar cells can be used for façade-integrated photovoltaic systems and for light and flexible solar modules for outdoor activities.
Our research group closely collaborates with project partners from the industry and other research institutes to further optimize the solar cell and therefore increase the power conversion efficiency. For this, our focus lies, on the one hand, on the investigation of the influence of different absorber compositions, and on the other hand, on the study of alternative materials for the cadmium sulfide buffer layer for the reduction of the absorption losses in this layer. An additional reason for the replacement of the buffer layer material is to get rid of the toxic cadmium.
Further CIGS solar cells with different absorber compositions and buffer materials are investigated by means of Kelvin probe force microscopy (KPFM) and Electroreflectance spectroscopy (ER). KPFM enables the measurement of the potential distribution through all the layers of the solar cell and provides information about the influence of the gallium content on the Fermi level of the absorber and the localization and width of the space–charge region. Electroreflectance spectroscopy is a modulation spectroscopy method and applied for the destruction-free determination of the bandgap energies of the absorber and buffer layers and for the consequent analysis of a possible intermixing at the absorber–buffer interface.