W. Schabel

Highly efficient polymer solar cells cast from non-halogenated xylene/anisaldehyde solution

Several high performance polymer:fullerene bulk-heterojunction photo-active layers, deposited from the non-halogenated solvents o-xylene or anisole in combination with the eco-compatible additive p-anisaldehyde, are investigated. The respective solar cells yield excellent power conversion efficiencies up to 9.5%, outperforming reference devices deposited from the commonly used halogenated chlorobenzene/1,8-diiodooctane solvent/additive combination. The impact of the processing solvent on the bulk-heterojunction properties is exemplified on solar cells comprising benzodithiophene-thienothiophene co-polymers and functionalized fullerenes (PTB7:PC71BM). The additive p-anisaldehyde improves film formation, enhances polymer order, reduces fullerene agglomeration and shows high volatility, thereby positively affecting layer deposition, improving charge carrier extraction and reducing drying time, the latter being crucial for future large area roll-to-roll device fabrication.

In situ monitoring the drying kinetics of knife coated polymer-fullerene films for organic solar cells

The efficiency of polymer based bulk heterojunction (BHJ) solar cells mainly depends on the film morphology of the absorption layer and the interface properties between the stacked layers. A comparative study using atomic force microscopy(AFM) and optical in situthin film drying measurements is performed. The strong impact of distinct drying scenarios on the polymer:fullerene BHJ layer morphology is investigated by AFM. The AFM images show a systematic dependency of structure sizes at the surface on drying kinetics. In addition thin film optical measurements for the determination of thin film drying kinetics and parameters are performed using a dedicated experimental setup. The data are used as the input for a quantitative simulation of the drying process. The film thickness decreases linearly during drying while the solvent mass fraction decreases moderately over a wide range until it drops rapidly. Subsequently the remaining solvent fraction evaporates considerably slower. Our work gives a fundamental understanding of the film formation kinetics and prerequisites for the systematic optimization of the film morphology in solution processed organic photovoltaic devices.

Delamination behavior of lithium-ion battery anodes: Influence of drying temperature during electrode processing

ABSTRACT One essential process step during electrode processing for lithium-ion batteries is the drying of the wet particulate electrode coating. The electrode film solidifies during evaporation of the solvent and a porous film is formed. In this study, we focus on the influence of drying temperature on the internal electrode structure of the dry film. Anode slurries that consist of graphite and an aqueous binder system were coated and subsequently dried. To assure defined and controllable drying conditions, a laboratory set-up with a temperature-controlled substrate carrier and an impingement dryer was used. To facilitate a scale-up to continuously passed dryers, the choice of experimental temperatures was based on a calculation of steady-state temperatures that result from gas temperatures that are commonly applied in industrial drying processes. The delamination behavior of the differently dried electrodes was investigated by means of a 90° peel test. The results show a strong dependency of electrode adhesion on drying temperature. A lower adhesion force at higher temperatures hints at a variation in binder content at the interface between the copper substrate and the coating layer. The formation of a consolidation layer at the air-film interface during drying is identified as a possible explanation and a criterion for consolidation layer formation is suggested.

Slot die coating of lithium-ion battery electrodes: investigations on edge effect issues for stripe and pattern coatings

An important step in the production of lithium-ion batteries is the coating of electrodes onto conducting foils. The most frequently used coating method in industry is slot die coating. This process allows the reproducible preparation of thin functional films at high velocities. A phenomenon that is often neglected in scientific studies and has attracted little attention, compared to film stability, is the inevitable presence of edge effects. Film elevations appear at the coating edges and at the start of each single patch during intermittent coating. These superelevations will cause problems at the downstream cell-assembling steps. In this study the influence of dynamic and geometric coating parameters on the shaping of coating edge effects was investigated. A quantitative measurement technique for edge profiles was developed and implemented. Film stretching has been identified in literature as a main reason for edge effects. We could show that varying the coating speed and the gap ratio did not lead to ideal edges, but affected the shape of the coating edges. The elevation height appeared to be independent from the varied process parameters, in the range of investigation. Also the gradient at the very edge of the film was not affected by these parameters. Only the edge width showed a disproportionately increasing trend towards higher applied gap ratios. The results indicate that the approach of film stretching is not sufficient to fully describe the cause of superelevations in lithium-ion battery coatings.

Measurements of Concentration Profiles in Polymeric Solvent Coatings by Means of an Inverse Confocal Micro Raman Spectrometer—Initial Results

Abstract Concerning quality and drying time of a process the key issue is the progression of the concentration profiles within the film during drying. This contribution reports on the development of a new measuring technique, an Inverse-Confocal-Micro-Raman-Spectrometer, for online measurements of concentration profiles during drying in thin (5–200 µm) polymeric solvent coatings. The spatial and temporal resolution of the measurements is up to one micron and one second. A calibration method has been developed by taking Raman spectra of different, precisely concentrated samples in sealed quartz glass cells and by calculating the ratio of the intensities of the characteristic Raman peaks. The accuracy of this basic calibration method is very good over a wide concentration range. Initial investigations were carried out on the polymer solvent systems: polyvinyl acetate(PVAc)–toluene, PVAc–methanol, PVAc–benzene, and PVAc–methanol-toluene, PVAc–methanol-benzene.

Fluid-dynamic properties and wetting behavior of coating inks for roll-to-roll production of polymer-based solar cells

In the last decade, semiconducting and conducting materials were developed that can be processed by solvent-based deposition to form functional layers or complete electronic devices. These materials are typically synthesized in laboratory scale quantities and tested on small spin-coated substrates, whereas the final goal is to produce them on flexible substrates in a continuous roll-to-roll process. To enable a fast scale up and optimization, fluid-dynamic properties have to be known. Here, we present viscosity and surface tension data for typical material systems, applied in polymer-based solar cells. Materials presented include water-based polymer dispersions (hole-conducting and high-conductive PEDOT:PSS types), solvent-based anorganic nanoparticle dispersions (silver nanoparticle ink, hole-blocking ZnO nanoparticle ink), and dissolved organic molecules and polymers (P3HT:PCBM photoactive blend). Predictive models are proposed to approximate viscosity and surface tension for these materials at various compositions. As well, corona treatment is used to modify the surface energy of P3HT:PCBM and described as a function of web speed and corona power. The importance of material properties is demonstrated by predicting stable conditions for a slot-die coating process. A simple drying simulation highlights the possibility of using property models to investigate wetting problems.

Liquid film coating of small molecule OLEDs

Organic small molecules typically deposited by vacuum deposition are a promising material for efficient, low-cost, area light-emitting diodes (OLEDs). In this article we discuss methods to apply these materials by solvent-based large area coating methods. As a basis for the technical description of the coating and wetting process, we present material properties, such as viscosity and surface tension of SMOLED solutions and polar and disperse part for the surface energy of typical substrates or semiconducting organic layers. Whereas SM content has little effect on the ink properties, impurities were identified as an important factor for the wetting behavior. Based on the material properties and coating experiments with SMOLED solutions, the coating methods of blade and slot die coating are discussed with respect to film thickness and stability. We found that the film thickness of knife-coated films does depend on velocity, temperature, provided fluid volume, and composition. Calibration curves for commercial materials (NMP and Spiro-MeOTAD) are given. The stability of slot die coating was lower than expected from literature. However, homogeneous SMOLED layers could be produced by slot die coating at gap-to-film-thickness ratios of up to 50.

Analytical determination of process windows for bilayer slot die coating

Slot die coating is a film casting process with a highly diverse variety of everyday applications. As a pre-metered process it not only guarantees excellent film uniformity, but is also suitable for simultaneously applied multilayer coatings. Characteristic singularities like the behavior of the liquid–liquid interface and the impact of the additional mid-lip on film uniformity were already investigated before. However, the effect of an altered gap pressure regime on commonly used coating windows has not yet been discussed. In this work, we therefore extended available single-layer coating windows for Newtonian and power-law liquids to the bilayer case. Here, the emphasis was laid on the air entrainment limit. Subsequently, the theoretical results were compared to experimental data. It was found that the onset of air entrainment strongly depends on the top to bottom film thickness ratio for bilayer coatings. A critical film thickness ratio which delivers similar coating limits as those for single-layer coatings was derived and confirmed by experimentally gained results.

Development of a three-stage drying profile based on characteristic drying stages for lithium-ion battery anodes

ABSTRACT Based on the state of current research on battery electrode drying, a custom drying profile design is proposed and validated. It allows for drying time reduction, while the anodes’ mechanical integrity is maintained. In a tripartite process, the deterioration of the active layer adhesion to the substrate that is typically observed for increased drying rates is prevented through adjusting mild drying conditions in an intermediate drying stage. This stage is confined by two high drying rate stages which allow for reduction of the total drying time by about 40% compared to a reference drying process.

Experimental investigation into battery electrode surfaces: The distribution of liquid at the surface and the emptying of pores during drying

HYPOTHESIS Drying constitutes a key step in the production of thin, particulate films as the complex microstructure develops and functional additives redistribute throughout the film, thus shaping the film properties. The onset of pore emptying constitutes a paramount marker in the film formation process as capillary liquid transport emerges. EXPERIMENTS Film shrinkage, development of surface liquid content and liquid depletion in surface pores of thin films are studied by means of a novel experimental approach, using the example of lithium-ion battery anodes. An optical brightener is incorporated into the films, which are produced in a convective slot-nozzle dryer. After various drying times, images of the film, which emit light fluorescently during exposure to UV-A radiation, are captured and analyzed by image processing. Film shrinkage is observed by means of a laser displacement sensor. FINDINGS As soon as the first pores empty, pore fluid is transported convectively. By exerting an external force, i.e. a pressure surge, on the film, the transition from a fully saturated to a partially emptied film is revealed. The significance of drying rate and particle shape are studied. We show that liquid depletion in surface pores can occur both prior and concurrent with the end of film shrinkage.