Adrian Mertens

Illumination angle and layer thickness influence on the photo current generation in organic solar cells: A combined simulative and experimental study

In most future organic photovoltaic applications, such as fixed roof installations, facade or clothing integration, the solar cells will face the sun under varying angles. By a combined simulative and experimental study, we investigate the mutual interdependencies of the angle of light incidence, the absorber layer thickness and the photon harvesting efficiency within a typical organic photovoltaic device. For thin absorber layers, we find a steady decrease of the effective photocurrent towards increasing angles. For 90-140 nm thick absorber layers, however, we observe an effective photocurrent enhancement, exhibiting a maximum yield at angles of incidence of about 50°. Both effects mainly originate from the angle-dependent spatial broadening of the optical interference pattern inside the solar cell and a shift of the absorption maximum away from the metal electrode.

Lab-on-Chip, Surface-Enhanced Raman Analysis by Aerosol Jet Printing and Roll-to-Roll Hot Embossing

Surface-enhanced Raman spectroscopy (SERS) combines the high specificity of Raman scattering with high sensitivity due to an enhancement of the electromagnetic field by metallic nanostructures. However, the tyical fabrication methods of SERS substrates suffer from low throughput and therefore high costs. Furthermore, point-of-care applications require the investigation of liquid solutions and thus the integration of the SERS substrate in a microfluidic chip. We present a roll-to-roll fabrication approach for microfluidics with integrated, highly efficient, surface-enhanced Raman scattering structures. Microfluidic channels are formed using roll-to-roll hot embossing in polystyrene foil. Aerosol jet printing of a gold nanoparticle ink is utilized to manufacture highly efficient, homogeneous, and reproducible SERS structures. The modified channels are sealed with a solvent-free, roll-to-roll, thermal bonding process. In continuous flow measurements, these chips overcome time-consuming incubation protocols and the poor reproducibility of SERS experiments often caused by inhomogeneous drying of the analyte. In the present study, we explore the influence of the printing process on the homogeneity and the enhancement of the SERS structures. The feasibility of aerosol-jet-modified microfluidic channels for highly sensitive SERS detection is demonstrated by using solutions with different concentrations of Rhodamine 6G and adenosine. The printed areas provide homogeneous enhancement factors of ~4 × 106. Our work shows a way towards the low-cost production of tailor-made, SERS-enabled, label-free, lab-on- chip systems for bioanalysis.

Understanding the angle-independent photon harvesting in organic homo-tandem solar cells.

The effective device photo current of organic tandem solar cells is independent of the angle of light incidence up to 65°. This feature renders these devices particularly suitable for stationary applications where they receive mainly indirect light. In a combined experimental and simulative study, we develop a fundamental understanding of the causal absorption and charge generation mechanisms in organic homo-tandem solar cells. A 3-terminal tandem device architecture is used to measure the optoelectronic properties of both subcells individually. The analysis of the angle dependent external quantum efficiencies of the subcells and the tandem device reveal an internal balancing of the wavelength dependent subcell currents elucidating the low sensitivity of the tandem device properties on the angle of incidence.

Roll-to-roll production of a microfluidic platform and its functionalization by means of digital printing technologies for gas and fluid sensors (Conference Presentation)

The individualized functionalization of mass-produced microstructures is still challenging for the process technology. Here, a rroll-to-roll based process hot embossing is presented for the production of microfluidic structures by means of hot embossing is presented. The resulting microfluidic channels are functionalized modified with different materials. Thereby, digital printing technologies such as aAerosoljet or inkjet are used. This approach allows for mass production of microfluidic channels and their the individualized individual functionalizationfunctionalization of mass produced microfluidic channels. The encapsulation of the channels also takes placeis realized in an R2R-based thermal bonding process without adding any solvent or adhesive. Taking account ofUsing this approach, several sensor systems for gas and / or fluid detection could be demonstrated. Surface -eEnhanced Raman Scattering scattering (SERS) with amplification enhancement factors of up to 107 [1] is demonstrated by printing gold nanoparticles into the microfluidic channel. We evaluate the printed SERS structures using solutions of rhodamine 6G and adenosine as exemplary analytes. Furthermore, these channels could be functionalized with different fluorescent organic semiconductors. Their fluorescence intensity is quenched in the presence of a nitroaromatic compounds. By using different materials simultaneously, we are able to measure a fingerprint like pattern of different analytes, which we evaluated by means ofusing pattern recognition algorithms. This method can be used both in the gas phase (electronic nose) and in fluids (electronic tongue) for the detection of nitroaromatic compounds [2,3]. With the opto-electronic nose, we were able to reach detections limits below 1ppb. [1] A. Habermehl et al, Sensors 17, 2401 (2017). [2] N. Bolse et al, Flexible and Printed Electronics 2, 024001 (2017) [3] N. Bolse et al, ACS Omega 2 (10), 6500-6505 (2017)

Comparing roll-to-roll and laser-assisted hot embossing for micro- and nanofabrication

We demonstrate the suitability of two cost efficient technologies, namely roll-to-roll hot embossing and laser-assisted hot embossing, to fabricate arrays of structures in the microscale down to the sub-100 nm range. We therefore employ polymers with a relatively moderate glass transition temperature, e.g., cyclic olefin copolymer (COC) and polystyrene (PS). We compare the two replication processes regarding their precision and cost using different 1D and 2D nanostructure gratings and microfluidic channels. All nickel shims used for the replication are fabricated in combination of electron beam or UV lithography and nickel electroforming. The replicated structures are used in different applications. The nanopillar arrays are coated with gold and integrated in the hot embossed microfluidic channels for lab-on-a-chip (LoC) surface-enhanced Raman analysis. We evaluate the as-fabricated 2D nanopillar arrays for surface-enhanced Raman spectroscopy (SERS) using solutions of rhodamine 6G as exemplary analytes. The influence of the geometrical parameters like diameter and pitch of the polymer structures as well as the influence of the gold layer thickness are discussed. 1D-gratings will be used as resonators for organic distributed feedback (DFB) lasers. Both elements, the SERS chips and the organic DFB lasers as tunable excitation source can be combined in the future to form one Raman-on-Chip optofluidic platform for sensitive detection of low-concentrated analytes in water.

Enhanced thermal stability of organic solar cells comprising ternary D-D-A bulk-heterojunctions

Ternary absorber blends have recently been identified as promising concepts to spectrally broaden the absorption of organic bulk-heterojunction solar cells and hence to improve their power conversion efficiencies. In this work, we demonstrate that D-D-A ternary blends comprising two donor polymers and the acceptor PC61BM can also significantly enhance the thermal stability of the solar cell. Upon harsh thermal stress at 120 °C for 2 h, the ternary solar cells show only a minor relative deterioration of 10%. Whereas the polymer/fullerene blend PTB7-Th:PC61BM is rather unstable under these conditions, its degradation was efficiently suppressed by incorporating the near infrared-absorbing polymer PDTP–DFBT. Spectroscopic ellipsometry investigations and an effective medium analysis of the ternary absorber blend revealed that the domain conformation in presence of PDTP–DFBT remains stable whereas the domain conformation changes in its absence. The ternary PTB7-Th:PDTP–DFBT:PC61BM solar cells yield thermally stable power conversion efficiencies of up to 6%.Organic solar cells: Polymer mixtures enhance the thermal stabilityOrganic solar cells increase their lifetime by adding another polymer component, paving the way towards commercialization.A team led by Alexander Colsmann at Karlsruhe Institute of Technology, Germany conducted systematic spectroscopic investigations and device characterizations to demonstrate that the degradation of PTB7-Th: PC61BM solar cell can be efficiently suppressed by incorporating the near infrared-absorbing polymer PDTP-DFBT. Upon harsh thermal stress at 120 °C for 2 h, the ternary solar cells show only a minor relative deterioration of 10% with a high power conversion efficiency of 6%. This work reveals the importance of a third component to lock the phase conformation of the polymer and fullerene domains. This is a key step for the thermally stable power output thus the commercialization of the organic solar cells.

A low-cost versatile fluorescence quenching detection system for liquid- and vapor-phase sensing

We report on the design and evaluation of a low-cost versatile optical sensor system to detect explosive traces by fluorescence quenching. In comparison to common detection systems, it allows to rapidly sample sensor arrays in various analyte carriers. Moreover, our work enables system development towards low-cost analysis and sensor testing solutions due to the simplicity of the approach. Therefore, we demonstrate the detection of nitroaromatic analytes in toluene, in water and in air by the application of transducing polymers. The results of the system demonstration indicate that the transduction strongly depends on the transducer state and on the analyte carrier.

Plant epidermal structures for enhanced sunlight harvesting in solar cells

Achieving efficient sunlight collection over an entire day is a demanding task for stationary devices. The problem becomes even more complex when the harvested photons have to be absorbed within layers limited to a few hundreds of nanometers (or less) in thickness. This optical challenge can be addressed by a light in-coupling element that operates over a broad angular and spectral range, and that is also capable of trapping the collected light to maximize photon-to-electron conversion efficiency. Solutions developed in the silicon photovoltaic (PV) industry that rely on microscale pyramidal texturization of the absorber for light trapping are not suited to systems made of thin layers. In contrast, experimental nanophotonic approaches employing, for example, planar photonic crystals and diffraction gratings or plasmonic structures have been successfully implemented within or next to the active layer of thin-film solar cells.1, 2 An alternative route consists of integrating a light-harvesting, polymer-based coating onto the planar thin-film stack that does not affect charge-collection properties and could enable fully flexible devices.3 In this context, biomimetic structures—mostly subwavelength and inspired by moth eyes—exhibit broadband omnidirectional anti-reflection (AR) but lack the light-trapping contribution.4 For this reason, we have replicated structures that decorate plant surfaces to produce a light-harvesting layer that combines all the previously mentioned attributes.5 More specifically, we have focused on rose petal epidermal cells, which are densely packed and convexly shaped, and display a height and width of few tens of microns (see Figure 1). Figure 1. Schematic illustration of the replica approach based on plant epidermal cells. Yellow arrows show light in-coupling, which is enhanced in the solar cell as a result of its bioinspired microstructured coating. KIT: Karlsruhe Institute of Technology.

Lichtmanagement in organischen Einzel- und Tandemsolarzellen

Im Rahmen dieser Arbeit wurde die winkelabhangige Absorption organischer Einzel- und Tandemsolarzellen untersucht. Des Weiteren wurden die optischer Konstanten organischer Halbleitermaterialien mittels spektroskopischer Ellipsometrie ermittelt.

Simulation of semi-transparent organic tandem solar cells for solar shading

We present design considerations for semi-transparent organic tandem solar cells that exhibit both good visible transparency and good power conversion efficiencies. The tandem solar cells show excellent color properties, such as the corresponding transparency color temperature and the color rendering index (CRI), that prevail for high angles of incidence of the incoming sunlight. Up to an angle of light incidence of 70°, the devices exhibit a convenient CCT which implies a neutral white and a CRI which is above 96.