Publications

We employ photoluminescence (PL) spectroscopy on individual nanoscale aggregates of the conjugated polymer poly(3-hexylthiophene), P3HT, at room temperature (RT) and at low temperature (LT) (1.5 K), to unravel different levels of structural and electronic disorder within P3HT nanoparticles. The aggregates are prepared by self-assembly of the block copolymer P3HT-<i>block</i>-poly(ethylene glycol) (P3HT-<i>b</i>-PEG) into micelles, with the P3HT aggregates constituting the micelles' core. Irrespective of temperature, we find from the intensity ratio between the 0-1 and 0-0 peaks in the PL spectra that the P3HT aggregates are of H-type nature, as expected from π-stacked conjugated thiophene backbones. Moreover, the distributions of the PL peak ratios demonstrate a large variation of disorder between micelles (inter-aggregate disorder) and within individual aggregates (intra-aggregate disorder). Upon cooling from RT to LT, the PL spectra red-shift by 550 cm^-1, and the energy of the (effective) carbon-bond stretch mode is reduced by 100 cm^-1. These spectral changes indicate that the P3HT backbone in the P3HT-<i>b</i>-PEG copolymer does not fully planarize before aggregation at RT and that upon cooling, partial planarization occurs. This intra-chain torsional disorder is ultimately responsible for the intra- and inter-aggregate disorder. These findings are supported by temperature-dependent absorption spectra on thin P3HT films. The interplay between intra-chain, intra-aggregate, and inter-aggregate disorder is key for the bulk photophysical properties of nanoparticles based on conjugated polymers, for example, in hierarchical (super-) structures. Ultimately, these properties determine the usefulness of such structures in hybrid organic-inorganic materials, for example, in (bio-)sensing and optoelectronics applications.

In article number 1700314, Mukundan Thelakkat and co-workers focus on the correlation of structures, morphology and chain orientation as a function of molecular weight, dispersity, intramolecular and intermolecular interactions and processing techniques of conjugated polymers and their photovoltaic blends. The tools for elucidating fundamental information of structure formation and orientation consist of optical spectroscopy and scattering techniques.

A novel organic photorefractive composite system is presented consisting of the amorphous multifunctional low-molar mass compound DR1-DCTA, doped with C 60 as sensitizer and diisooctyl phthalate (DOP) as plasticizer. DR1-DCTA, a carbazole substituted triphenylamine with a covalently attached nonlinear optical azo chromophore unit has been synthesized in a 4 step reaction. The compound forms a stable glass. Its thermal, optical and electrochemical properties are discussed. Holographic experiments (2BC, DFWM) of the photorefractive composite exhibit a fast response time in the range of only 30 ms and a maximum gain coefficient of 90 cm –1 .

The photovoltaic properties of a series of EDOT-containing DPP polymers with optical band gap as low as 1.13 eV are reported.

In recent years much attention has been given to design multistate molecular components with functionalities that cover the range from simple switches to logic gates [1-3]. In this regard photochromic molecules, i.e., molecules that can be interconverted between two bistable forms by light, have played an important role. Promising candidates that fulfill obvious demands such as high photochemical/ photophysical stability and high fatigue resistance are compounds of the family of diarylethenes [2,3]. However, a serious drawback of this class of molecules is a low fluorescence quantum yield. Therefore we adapted the strategy developed by Irie and coworkers [2,3], to chemically synthesize complex tailor-made triads consisting of a photochromic dithienylcyclopentene (DCP) unit covalently linked to two peryline bisimide (PBI) molecules that are known as strong fluorophores, see fig.1 inset top left. This facilitates the combination of high fatigue resistance and high fluorescence quantum yield. Illumination with light in the UV spectral region induces a ring-closure reaction of the DCP and leads to a state with suppressed fluorescence from the PBIs, whereas light in the VIS spectral region yields a ring opening reaction of the DCP and restores the fluorescence from the PBI units. This allowed us to verify functionalities like optical gating and amplifying, yet where the electrons have been replaced by photons as signal carriers [4-6], see fig.1.

Abstract Organic mixed ionic‐electronic conductors with tunable doping, low threshold voltages, and air stability are crucial for bioelectronic applications. A homopolymer based on an alkoxy thiophene monomer and its copolymer with a thiophene carrying ethylene glycol side chains are synthesized and converted to self‐doped conjugated polyelectrolytes, P3HOTS ‐ TMA + , and P3HOTS ‐ TMA + ‐co‐P3MEEET. The self‐doping occurs during the conversion to polyelectrolytes. Both polyelectrolytes show high electrical conductivity without any external dopants. UV–Vis–NIR spectroscopy and spectroelectrochemistry confirm excellent air stability of the doped state. In an organic electrochemical transistor (OECT), the P3HOTS ‐ TMA + operates in depletion mode, while P3HOTS ‐ TMA + ‐co‐P3MEEET exhibits accumulation mode of operation with low threshold voltage, both showing fast response times. On the other hand, the non‐doped homopolymer, P3MEEET, shows a high negative threshold voltage in accumulation mode. Thus, copolymerization with the self‐dopable monomer changes the mode of operation as well as the threshold voltage substantially. Ultraviolet photoelectron spectroscopy reveals a considerable reduction of the hole injection barrier for the self‐doped system P3HOTS ‐ TMA + . Mott‐Schottky analysis shows reduction in charge carrier concentration in the copolymer compared to the homopolymer. Thus, the copolymerization strategy with a self‐dopable monomer is an efficient tool for tuning the degree of doping leading to low threshold voltage in OECTs.

We demonstrate that the often observed double peak emission in lead halide perovskites is caused by an extensive self-absorption effect.

We present the successful fabrication of CH₃NH₃PbI₃ perovskite layers by the aerosol deposition method (ADM). The layers show high structural purity and compactness, thus making them suitable for application in perovskite-based optoelectronic devices. By using the aerosol deposition method we are able to decouple material synthesis from layer processing. Our results therefore allow for enhanced and easy control over the fabrication of perovskite-based devices, further paving the way for their commercialization.