F. Buss

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.

Effect of soluble polymer binder on particle distribution in a drying particulate coating.

Soluble polymer is frequently added to inorganic particle suspensions to provide mechanical strength and adhesiveness to particulate coatings. To engineer coating microstructure, it is essential to understand how drying conditions and dispersion composition influence particle and polymer distribution in a drying coating. Here, a 1D model revealing the transient concentration profiles of particles and soluble polymer in a drying suspension is proposed. Sedimentation, evaporation and diffusion govern particle movement with the presence of soluble polymer influencing the evaporation rate and solution viscosity. Results are summarized in drying regime maps that predict particle accumulation at the free surface or near the substrate as conditions vary. Calculations and experiments based on a model system of poly(vinyl alcohol) (PVA), silica particles and water reveal that the addition of PVA slows the sedimentation and diffusion of the particles during drying such that accumulation of particles at the free surface is more likely.

In situ Studies of Morphology Formation in Solution-Processed Polymer–Fullerene Blends

Control of the blend nanomorphology in bulk heterojunctions (BHJs) is still a challenge that demands more fundamental knowledge of the mechanism of phase separation and crystallization during solvent drying. In this review we show that in situ studies using combined laser reflectometry and grazing-incidence wide-angle X-ray scattering provide a fundamental understanding on how the nanomorphology develops dynamically during film drying. We identify influencing parameters for controlled film formation in order to obtain optimized solar cell performance. We review here our results on BHJs of poly(3-hexylthiophene)–[6,6]-phenyl-C61-butyric acid methyl ester and poly{[4,40-bis(2-ethylhexyl)dithieno(3,2-b;20,30-d)silole]-2,6-diyl-alt-(2,1,3 benzothidiazole)-4,7-diyl} with [6,6]-phenyl-C71-butyric acid methyl ester.

By Orders of Magnitudes Decelerated Solvent Transport in Polymer Nanolayers

The fabrication of organic electronic devices by liquid deposition features a number of advantages compared to vacuum deposition such as reduced material loss, better suitability for polymer deposition and the increased feasibility of scale-up using continuous roll-to-roll coating processes for reaching commercial scale. In this context, mass transport in polymer nanolayers is of high interest regarding the drying, solvent residues, solvent vapor annealing and layer intermixing. In this talk the decreased mass transport in polymer nanolayers is addressed experimentally and by proposing a numerically underpinned diffusion coefficient model.