Gabriele Nelles

Tandem dye-sensitized solar cell for improved power conversion efficiencies

The power conversion efficiency of dye-sensitized solar cells crucially depends on the ability of the sensitizer to absorb light in a broad range of the solar spectrum. In order to increase this range of absorption, a tandem structure with two different sensitizer dyes in two different compartments of the cell, respectively, was realized. Overall power conversion efficiencies as high as η=10.5% and short circuit current densities of JSC=21.1 mA/cm2 were achieved under air mass 1.5 illumination with red dye and black dye in the upper and lower compartment of the cell, respectively.

Upconversion with ultrabroad excitation band: Simultaneous use of two sensitizers

The authors demonstrate the ability to combine sensitizers effectively working with single emitter in order to increase the excitation window for noncoherent upconversion. They show effective upconversion of the red part of the sun spectrum realized by ultralow excitation intensity (as low as 1Wcm−2) and ultrabroad excitation spectrum (Δλ∼80nm).

TEMPLATED CRYSTALLISATION OF CALCIUM AND STRONTIUM CARBONATES ON CENTRED RECTANGULAR SELF-ASSEMBLED MONOLAYER SUBSTRATES

SrCO3crystals display patterns of templating when grown on tailored self-assembled monolayers (right). As SrCO3 is isostructural with aragonite, comparison of the crystallisation of SrCO3 in the aragonite/strontianite modification with the crystallisation of CaCO3 in all three modifications might yield some insights into which factors are important for crystal growth.

On the origin of increased open circuit voltage of dye-sensitized solar cells using 4-tert-butyl pyridine as additive to the electrolyte

A combination of electrical measurements and surface analysis techniques was used to investigate the electronic structure of the TiO2/electrolyte interface and its change upon addition of 4-tert-butyl pyridine to the electrolyte. A reduced interface defect density and a resulting reduction of charge carrier recombination were found to be the main mechanism for an increased open circuit voltage of dye-sensitized solar cells with 4-tert-butyl pyridine in the electrolyte. The reduction of interface defect states was traced back to specific binding of 4-tert-butyl pyridine at defect sites on the TiO2 surface.A combination of electrical measurements and surface analysis techniques was used to investigate the electronic structure of the TiO2/electrolyte interface and its change upon addition of 4-tert-butyl pyridine to the electrolyte. A reduced interface defect density and a resulting reduction of charge carrier recombination were found to be the main mechanism for an increased open circuit voltage of dye-sensitized solar cells with 4-tert-butyl pyridine in the electrolyte. The reduction of interface defect states was traced back to specific binding of 4-tert-butyl pyridine at defect sites on the TiO2 surface.

Two pathways for photon upconversion in model organic compound systems

We have studied the phenomenon of photon upconversion in systems of two model compounds as highly efficient blue emitters sensitized with metallated macrocycle molecules in thin films. The bimolecular upconversion process in these systems is based on the presence of a metastable triplet excited state of the macrocycles giving rise to dramatically different photophysical characteristics relative to the other known methods for photon upconversion such as two-photon absorption, parametric processes, second harmonic generation, and sequential multiphoton absorption. The chosen blue emitter molecules have suitably positioned triplet levels: in the case of an oligofluorine—essentially higher and in the case of diphenylanthracene—lower than the sensitizer porphyrin platinum triplet level and thus two excitation pathways for photon upconversion were observed and investigated.

Impact of micropatterned surfaces on neuronal polarity.

Experimental control over cellular polarity in a neuronal network is a promising tool to study synapse formation and network behavior. We aimed to exploit a mechanism described by Stenger et al. [J. Neurosci. Methods 82 (1998) 167] to manipulate the direction of axonal versus dendritic outgrowth on a micropattern. The group had used laser ablation to create patterns of aminated silanes for cell attachment on a background of repellent fluorinated silanes. The pattern offered continuous adhesive pathways for axonal and interrupted pathways for dendritic outgrowth. By microcontact printing, we created similar patterns containing continuous and interrupted pathways consisting of extracellular matrix proteins on a background of polystyrene. Neuronal polarity was determined on the functional level through double patch clamp measurements, detecting synapses and their orientation. Although our pattern reproduced the properties that were assumed to be critical for the described effect, namely contrasting pathways of different adhesiveness, we failed to reproduce the above results. It is indicated that other qualities of alternative pathways than mere differences in adhesiveness are required to orient neuronal polarity in vitro. We suggest that the effect observed by Stenger et al. has to be attributed to less universal characteristics of the micropattern, e.g. to the specific chemical groups that were utilized.

Upconversion photoluminescence in poly(ladder-type-pentaphenylene) doped with metal (II)-octaethyl porphyrins

We report on the optimization of the upconversion photoluminescence in films of conjugated polymers doped with platinum porphyrin. The upconversion emission was observed to take place at pump intensities as low as 0.5kW∕cm2. Comparison between the photoluminescence integral intensity of polyfluorene and ladder-type pentaphenylene polymer shows a five-fold increase of the efficiency of the upconversion process for the latter. The higher upconversion efficiency can be partially attributed to the reduced reabsorption of the photoluminescence in the case of the ladder-type pentaphenylene matrix.

Spectroscopic and surface-analytical characterization of self-assembled layers on Au

Abstract Future devices for electronic, photonic or other “intelligent” application involving (bio-) organic materials require nano-fabrication, -manipulation, -patterning and -functionalization techniques. Supramolecular assemblies, aggregates, small molecules and ions have to be controlled with regard to their structure, order and dynamic behaviour down to the molecular or even atomic level. This contribution summarizes some of our activities aiming at a better understanding of the physical and chemical properties of functionalized and patterned surfaces. We focus on structure/order-property/function relations in such complex systems as interfaces and thin film architectures. Optical techniques (surface plasmon-spectroscopy) as well as surface analytical techniques (cyclic voltammetry and contact angle investigations) are introduced and demonstrated as powerful tools for the characterization of these interfaces and thin films. Examples will be given covering self-assembly monolayers and molecular recognition—as well as complexation-reactions.

Monothiols derived from glycols as agents for stabilizing gold colloids in water: synthesis, self-assembly and use as crystallization templates

Tetraethylene glycol (HO-(C 2 H 4 O) 4 -H) can be monofunctionalized by replacing one of the terminal hydroxy groups with the thiol SH group. The resulting molecule can be self-assembled on gold (111) surfaces. More importantly, this molecule allows the simple one-step preparation of protected, water-soluble gold colloids within a single aqueous phase. Attempts are made to use such protected water-soluble colloids as nucleating ‘seeds’ around which calcium carbonate can be crystallized.

Guest-host interactions with immobilized cyclodextrins

In previous papers we have demonstrated the immobilization of thiol-functionalized cyclodextrins on gold surfaces. We have studied the film properties and kinetics of the formation of covalently bound monolayers. In this paper we focus on the guest-host interactions with immobilized cyclodextrins. Four guest molecules are taken into account: the redox couple ferro-/ferricenecarboxylic acid, 4-tert-butylbenzoic acid, cyclohexanol and the electro-optically active dye methyl orange.