Alexander Colsmann

Enhanced Electron Injection into Inverted Polymer Light‐Emitting Diodes by Combined Solution‐Processed Zinc Oxide/Polyethylenimine Interlayers

Inverted device architectures for organic light-emitting diodes (OLEDs) require suitable interfaces or buffer layers to enhance electron injection from highwork-function transparent electrodes. A solution-processable combination of ZnO and PEI is reported, that facilitates electron injection and enables efficient and air-stable inverted devices. Replacing the metal anode by highly conductive polymers enables transparent OLEDs.

Organic tandem solar cells comprising polymer and small-molecule subcells

In this work the authors report monolithic organic tandem solar cells. The front cell is fabricated from a blend of poly(3-hexylthiophene-2,5-diyl) and [6,6]-phenyl C61-butyric acid methyl ester while a copper phthalocyanine/fullerene (CuPc∕C60) bilayer is used for the back cell. An intermediate recombination zone is realized using two doped organic semiconductor layers and a thin noble metal interlayer. The active polymer layer thickness is optimized to ensure matching of the subcell currents. The open circuit voltage Voc=1V nearly reaches the sum of the open circuit voltages of each contributing cell.

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.

Solution processed, white emitting tandem organic light-emitting diodes with inverted device architecture.

Fully solution processed monochromatic and white-light emitting tandem or multi-photon polymer OLEDs with an inverted device architecture have been realized by employing WO3 /PEDOT:PSS/ZnO/PEI charge carrier generation layers. The luminance of the sub-OLEDs adds up in the stacked device indicating multi-photon emission. The white OLEDs exhibit a CRI of 75.

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.

Tungsten Oxide Buffer Layers Fabricated in an Inert Sol‐Gel Process at Room‐Temperature for Blue Organic Light‐Emitting Diodes

WO3 deposition from tungsten ethoxide precursor solutions at room temperature is demonstrated. The W(OEt)6 precursor can be converted under inert conditions and hence avoids sample contamination with oxygen, opening a pathway to more stable devices. The stoichiometry of all WO3 layers and the optoelectronic performance of the respective SMOLEDs well match thermally evaporated WO3 and its corresponding SMOLEDs. The solution processed WO3 hole injection layers enable the fabrication of blue phosphorescent OLEDs with low onset voltage and current efficiencies of up to 14 cd A(-1) .

Eco-Friendly Fabrication of 4% Efficient Organic Solar Cells from Surfactant-Free P3HT:ICBA Nanoparticle Dispersions

Photo-active layers from non-stabilized P3HT:ICBA nanoparticles enable the fabrication of inverted organic solar cells from eco-friendly, alcoholic dispersions. Exhibiting power conversion efficiencies (PCEs) ≈4%, the devices are competitive to state-of-the-art P3HT:ICBA solar cells from chlorinated solvents. Upon thermal annealing, the short circuit current density and consequently the PCE of the inverted solar cells improve radically due to a more intimate contact of the nanoparticles and hence an enhanced charge carrier extraction.

Ferroelectric domains in methylammonium lead iodide perovskite thin-films

We explore the ferroic properties of methylammonium lead iodide perovskite solar cells by piezoresponse force microscopy (PFM). In vertical and horizontal PFM imaging, we find domains of alternating polarization with a width of 90 nm which we identify as polarized ferroelectric domains. High-resolution photo-conductive atomic force micrographs under illumination also show alternating charge carrier extraction patterns which we attribute to the local vertical polarization components within the ferroelectric domains. The correlation of the sample properties with atomic force and kelvin probe force micrographs evidence the ferroelectric nature of the domains.

Inverted organic solar cells comprising a solution-processed cesium fluoride interlayer

We investigate the influence of solution-processed cesium fluoride (CsF) interlayers on the performance of inverted polymer solar cells comprising a blend of poly(3-hexylthiophene-2,5-diyl):[6,6]-phenyl C61-butyric acid methyl ester. The thickness of the CsF layer is optimized in terms of current-voltage characteristics by a variation of the solid content in solution. Capacitance-voltage characteristics reveal a shift of the built-in voltage at the cathode interface by about 0.3 V as compared to devices without a CsF layer, giving rise to an increase in open-circuit voltage by the same value. The vertical distribution of Cs+ and F+ ions is studied by secondary ion mass spectroscopy, indicating a strong diffusion of the alkaline fluoride into the organic layer stack.

N-Fused quinoxalines and benzoquinoxalines as attractive emitters for organic light emitting diodes

We describe the modular synthesis of quinoxaline fluorophores exhibiting strong fluorescence over the whole visible spectrum. Based on their fluorescence quantum yield, frontier orbital energies and solid-state aggregation behavior, several compounds work well in fluorescent organic light emitting diodes (OLEDs) with luminances over 1000 cd m−2 at 8 V driving voltage.