Ralph Eckstein

The Compromises of Printing Organic Electronics: A Case Study of Gravure‐Printed Light‐Emitting Electrochemical Cells

Light-emitting electrochemical cells (LECs) are fabricated by gravure printing. The compromise between device performance and printing quality is correlated to the ink formulation and the printing process. It is shown that the rheological properties of the ink formulations of LECs can be tailored without changing the chemical composition of the material blend.

Monolithic hybrid tandem solar cells comprising copper indium gallium diselenide and organic subcells

Combining wide and narrow band gap absorbers in tandem solar cells is a promising approach to improve the energy conversion of sun light. In this work, we present hybrid tandem devices comprising monolithically connected copper indium gallium diselenide (CIGS) bottom cells and polymer top cells. The thin polymer:fullerene bulk heterojunction absorber layers were transferred onto the rough CIGS surface by a soft-contact lamination technique. Sputtered or solution-deposited top cathodes complete the tandem devices with enhanced open circuit voltages.

Impact of processing on the chemical and electronic properties of phenyl-C61-butyric acid methyl ester

For the comparison of solution-processed to evaporated materials in organic optoelectronic devices, phenyl-C61-butyric acid methyl ester (PCBM) has been claimed to be a suitable material. However, we ascertained differences between spin-coated and vacuum sublimed thin films. In this contribution, we thoroughly investigate the effects of thermal evaporation of PCBM in a strongly interdisciplinary approach, applying physical characterization techniques such as photoelectron (PES) and infrared (IR) spectroscopy in combination with further chemical analysis using thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and ultra-performance liquid chromatography-coupled mass spectrometry (UPLC-MS), as well as proton nuclear magnetic resonance spectroscopy (1H-NMR). The methods were applied to thin films prepared by solution-based deposition techniques and by thermal evaporation. Additionally, comparison to an evaporated C60 film and the crucible residues is carried out. Changes in the IR spectrum of the PCBM films already indicate a change in the molecular structure of PCBM. The UPLC chromatogram of the redissolved organic film proves the formation of several molecular species, including bare C60. However, the effect of degradation on the electronic properties was found to be limited, as an almost unchanged ionization potential of 6.1 eV was determined with UPS for both the solution processed as well as the evaporated films. Also bulk-heterojunction (BHJ) solar cells fabricated using pure and thermally treated PCBM showed the same J–V characteristics under illumination.

Surface Lattice Resonances for Enhanced and Directional Electroluminescence at High Current Densities

Hybrid photonic-plasmonic modes in periodic arrays of metallic nanostructures offer a promising trade-off between high-quality cavities and subdiffraction mode confinement. However, their application in electrically driven light-emitting devices is hindered by their sensitivity to the surrounding environment and to charge injecting metallic electrodes in particular. Here, we demonstrate that the planar structure of light-emitting field-effect transistor (LEFET) ensures undisturbed operation of the characteristic modes. We incorporate a square array of gold nanodisks into the charge transporting and emissive layer of a polymer LEFET in order to tailor directionality and emission efficiency via the Purcell effect and variation of the fractional local density of states in particular. Angle- and polarization-resolved spectra confirm that the enhanced electroluminescence correlates with the dispersion curves of the surface lattice resonances supported by these structures. These LEFETs reach current densities on the order of 10 kA/cm2, which may pave the way toward practical optoelectronic devices with tailored emission patterns and potentially electrically pumped plasmonic lasers.

Digitally Printed Dewetting Patterns for Self‐Organized Microelectronics

Self-organization of functional materials induced by low surface-energetic direct printed structures is presented. This study investigates fundamental fluid and substrate interactions and fabricates all-printed small area organic photodetectors with On-Off ratios of ≈10(5) and dark current densities of ≈10(-4) mA cm(-2) , as well as ring oscillators based on n-type organic field-effect transistors showing working frequencies up to 400 Hz.

One-step additive crosslinking of conjugated polyelectrolyte interlayers: improved lifetime and performance of solution-processed OLEDs

We crosslink an amino-functionalized polyfluorene by the solvent additive 1,8-diiodooctane (DIO). DIO remains in the film after drying of the main solvent and chemically binds to the amino-side groups after a low temperature annealing step, rendering the polyfluorene film insoluble in non-polar solvents. We correlate the amount of DIO, the reduction of operational voltage and increase in lifetime of solution-processed OLEDs. We demonstrate a fully solution-processed device using the crosslinked polyfluorene electron injection layer and an inkjet-printed Ag top electrode.

SnO 2 Nanowire-Based Aerosol Jet Printed Electronic Nose as Fire Detector

A smart fire detector preferably reacting before smoke breaks out and providing information about the substance going to start burning, is an unaccomplished hope for fire safety authority since decades. Here, we present an easy method to fabricate, hence cheap, smell detecting electronic nose (e-Nose) which is capable to operate as low cost smart detector for fire-related smells as an example application. Smell sensing in principle is achieved by measuring the resistance pattern of 16 sub-sensor elements combined on a single chip and a subsequent pattern recognition technique using multivariate data analysis. The sensing material of one single sub-sensor is SnO2 nanowires, fabricated in a high temperature condensation process and dispersed on digital aerosol jet printed interdigitated Au structure. Assisted by UV illumination, the basic chip performance was characterized using laboratory gases, such as synthetic air, Isopropanol, CO and Benzene and the detection limit of the e-Nose exposed to Benzene was measured to be 2.2 ppm. It needs only 6.6 mW to activate such sensor for continuous operation. As an application of such system, a smart fire detector was demonstrated, which can not only detect the pre burning smell of several substances, but it can also identify previously taught patterns of burning smell of test substances like, cotton, beech, and PCB.

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.

From printed organic photodiodes to printed image sensors (Conference Presentation)

Organic photodiodes (OPDs) have in recent years reached a level of performance comparable to their inorganic counterparts. Using additives like PMMA, we were able to tune the transparency and viscosity of a P3HT:PCBM photoactive blend while at the same time achieving a two-fold enhancement of the detection speed. Furthermore, we have developed approaches towards the digital realization of image sensors using aerosol jet printing and a direct-printed patterning technique utilizing the self organization of functional inks. These techniques allow for a reproducible deposition of multilayer devices with high registration accuracies and feature sizes down to a few microns. We present a comprehensive electrical and optical characterization of these printed image sensors. The devices exhibit specific detectivities of >1E12 Jones over a broad wavelength range (400-750 nm) and maximum responsivities of 0.25 A/W. An entirely printed matrix image sensor composing of 256 individual pixels with an individual active area of ≈250 μm × 300 μm was fabricated.

Cloaking of metal grid electrodes on Lambertian emitters by free-form refractive surfaces

We discuss invisibility cloaking of metal grid electrodes on Lambertian light emitters by using dielectric free-form surfaces. We show that cloaking can be ideal in geometrical optics for all viewing directions if reflections at the dielectric-air interface are negligible. We also present corresponding white-light proof-of-principle experiments that demonstrate close-to-ideal cloaking for a wide range of viewing angles. Remaining imperfections are analyzed by ray-tracing calculations. The concept can potentially be used to enhance the luminance homogeneity of large-area organic light-emitting diodes.