Jakob Heier

Origin of the Kink in Current-Density Versus Voltage Curves and Efficiency Enhancement of Polymer-C

Current-voltage (J-V ) curves of photovoltaic devices can reveal important microscopic phenomena when parameterization is properly related to physical processes. Here, we identify a pronounced effect of thermal annealing on the organic-cathode metal interface and show that this interface is related to the origin of the kink often observed in J-V curves close to the open circuit. We propose that isolated metal nanoclusters that form upon cathode evaporation lead to defect states close to the interface and change the electric field distribution in the device. We express this scenario with a modified equivalent circuit and consistently fit J- V curves as a function of the annealing process. The developed model is general in the sense that any physical process that leads to the change in electric potential as described in this paper will possibly lead to a kink in the J- V curves. Knowing the origin of the kink allowed us to largely increase the device efficiency of the archetypal material combination Poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) -C. We fabricated solar cells with an efficiency of 1.85% under 100 mW/cm AM1.5 illumination by using a deliberately designed interpenetrating bilayer film morphology, aluminium as cathode and thermal annealing. This is so far the highest reported efficiency for this particular combination of materials.

_{\bf 60}

We investigate the transfer of a chemical pattern on a substrate into a symmetric diblock copolymer thin film of poly(styrene-2-vinylpyridine) (PS-PVP). The substrates have patterns of self-assembled monolayers (SAMs) produced by microcontact printing H3C-terminated (H3C-) SAM stripes alternating with HO-terminated (HO-) SAM stripes. The PS-PVP lamellae over the H3C-SAM have a defect structure that attracts excess PS-PVP that would normally form islands on a uniform HO-SAM stripe. We seek to understand the process that limits our ability to accommodate all excess polymers on top of the H3C-SAM. In the early stages of annealing, waves of thickness develop from the H3C/HO-SAM boundary and propagate into the film over the HO-SAM. For very short annealing times, the wavelength λ of these thickness waves is constant at any given time for all grating periodicities. Large amplitude patterns develop when λ=2d/(2n−1), where d is the width of the HO-SAM stripe and n is an integer ⩾1. Such patterns suggest construct...

Heterojunction Solar Cells

Significant progress is being made in the photovoltaic energy conversion using soluble small organic molecules. We report the fabrication of layered heterojunction solar cells with 3% power conversion efficiency consisting of a solution-processed cyanine dye, C60 and doped polyaniline anode layers that match the cyanine energy level and facilitate hole extraction.

Transfer of a chemical substrate pattern into an island-forming diblock copolymer film

A method to exchange the counterion of cyanine dyes to Δ-TRISPHAT(-) and PF6(-) is presented. The influence of these counterions on the photophysical and electrochemical properties of the cyanine dye in solution is discussed, and tendencies in the solid packing are highlighted by X-ray crystal structures. The compounds were applied in semitransparent bilayer organic solar cells together with C60, and a power conversion efficiency of 2.2% was achieved while maintaining a high transparency level in the visible region of 66%.

Improved performance of cyanine solar cells with polyaniline anodes

Thin films involving an oxide heterojunction are increasingly employed as electrodes for solar water splitting in photoelectrochemical cells. Hematite (α-Fe2O3) and tungsten oxide form an attractive heterojunction for this purpose. A major limitation of this strategy is the short charge carrier diffusion length in hematite. Ultra-thin films were implemented to address this low conductivity issue. Nevertheless, such ultrathin films do not absorb light efficiently. The present study explores light trapping strategies to increase the optical path length of photons in hematite. Vesicle suspensions were developed to obtain thin films composed of a microspheroid array with a tungsten oxide core and a nanometer sized hematite overlayer. This bottom-up approach allows a fine control of the spheroid dimensions at the micrometric to the submicrometric scale. Using the finite difference time domain method, light propagation inside the microstructures was quantitatively simulated. The simulation results were coupled to an analysis of the photoelectrochemical response of the films. Experiments and simulation show quantitative agreement and bring important insights into the relationship between the interaction of light with the microstructure and the photoanode performance.

NIR-Absorbing Heptamethine Dyes with Tailor-Made Counterions for Application in Light to Energy Conversion

We present a theoretical model and experimental data describing the response of a thin liquid polymer film to a heterogeneous electric field. The theory predicts two different regimes separated by a distinct boundary: in one regime the film is characterized by steady-state low amplitude surface modulations following the periodicity of the electric field, in the other regime the film breaks up into pillars centered around the region of highest field strength. The theoretical analysis describes the film destabilization in terms of two dimensionless variables, which fully describe the low amplitude limit. The corresponding experimental system was realized with a photo-structured epoxy resin covering the top electrode and thin polystyrene films that were destabilized by the applied electric field and characterized by AFM after quenching. Theoretical and experimental results allow us to determine the destabilization conditions and to identify the experimental variables that characterize film destabilization. The theoretical framework provides a tool for the experimentalist to predict in detail the structures generated by film destabilization, thereby enabling a new way of ‘soft-lithography’.

Photonic light trapping in self-organized all-oxide microspheroids impacts photoelectrochemical water splitting

The importance of highly ordered surfaces, containing adsorptive surface states, is discussed for J-aggregation by self-assembly. Such nucleating surfaces are nanometer-sized edges and corners of cubic AgBr microcrystals, or surface iodide-clusters located along edges and corners of AgBr:I microcrystals. Of particular interest are dendrimers, monoatomic steps on terraced silver halide microcrystals and fullerene derivatives as nucleating surfaces. Molecular organisation into J-aggregates by self-assembly was realized using aprotic, apolar solvents for fullerenes, and polar solvents for dendrimers and monoatomic surface steps. By using dendrimers as nucleating agents in mesopores of metal oxide nanoparticle coatings, size-controlled and stable J-aggregates with high optical densities and strong fluorescence were obtained reproducibly. Such films may be useful for sensors, opto-electronics, lighting and photovoltaics.

Pattern formation in thin polymer films by spatially modulated electric fields

The controlled fabrication of submicrometer phase-separated morphologies of semiconducting organic materials is attracting considerable interest, for example, in emerging thin-film optoelectronic device applications. For thin films of spin-coated blends of PCBM ([6,6]-phenyl-C 61-butyric acid methyl ester) and cationic cyanine dyes, we used atomic force microscopy scans to infer the structure formation mechanism: The solutions separate into transient bilayers, which further spinodally destabilize because of long-range molecular interactions. A thin layer ruptures earlier than a thick layer, and the earlier instability determines the morphology. Consequently, the resulting morphology type mainly depends on the ratio of the layer thicknesses, whereas the periodicity of structures is determined by the absolute film thickness. These findings allow control of the feature sizes, and nodular domains with diameters well below 50 nm were produced. Films prepared with dyes possessing a mobile counterion were always unstable. To rationalize the findings, we developed a thermodynamic model showing that electrostatic forces induced by the mobile counterions act as destabilizing pressure.

J-aggregation of cyanine dyes by self-assembly

Studying and understanding the conditions under which organic semiconductors can be engineered to form aligned single crystals in thin films is of primary importance owing to their unique orientation-dependent optoelectronic properties. Efforts to reach this goal by self-assembly from solution-processed films have been rewarded only with limited success. In this article we present a new method to grow single crystalline thin films via solvent annealing. We identify solvate crystal growth in combination with a specific film dewetting morphology as key to successful fabrication of single crystals. Furthermore, these 2D single crystals can align on chemically patterned substrates to minimize their interfacial energy. We explore in situ the conditions for crystal formation and alignment.

Nanoscale structuring of semiconducting molecular blend films in the presence of mobile counterions

Small organic semiconducting molecules assembling into supramolecular J- and H- aggregates have attracted much attention due to outstanding optoelectronic properties. However, their easy and reproducible fabrication is not yet sufficiently developed for industrial applications, except for silver halide photography. Here we present a method based on aggregate precipitation during the phase separation and dewetting of the evaporating dye precursor solution. The smaller the precursor droplets, the more pronounced the J-aggregation. The aggregates cause the films to resonantly scatter incoming light. Because the dye aggregate extinction resonances have narrowest bandwidths, a wavelength selectivity is observed that exceeds the selectivity of localized surface plasmon resonances. The aggregation mechanism can be easily applied to periodically structured substrates, making the method appealing for photonic applications. We demonstrate this point with a 2D grating, where the narrow absorption range of the aggregates leads to wavelength specific (one color only) scattering.