Publications

The first synthesis of a semiconductor hybrid diblock copolymer comprised of p-type dendritic and n-type linear blocks by nitroxide mediated radical polymerization (NMRP) is reported. A triphenylamine (TPA) bearing second generation polyether dendron [G2]-OH has been functionalized with an alkoxyamine and, subsequently, perylene bisimide acrylate (PerAcr) was polymerized to obtain a hybrid block copolymer, [G2]-b-PPerAcr. The hybrid block copolymer structure is supported by (1) H NMR and size exclusion chromatography. Furthermore, the novel materials were studied by UV-vis absorption spectrometry, photoluminescence, cyclic voltammetry, differential scanning calorimetry and thermogravimetry analysis.

Photochromic molecules can be reversibly converted between two bistable conformations by light, and are considered as promising building blocks in novel macromolecular structures for sensing and imaging techniques. We have studied individual molecular triads consisting of two strong fluorophores (perylene bisimide) that are covalently linked via a photochromic unit (dithienylcyclopentene) and distinguished between deliberate switching and spontaneous blinking. It was verified that the probability for observing deliberate light-induced switching of a single triad (rather than stochastic blinking) amounts to 0.8 ± 0.1. In a few exceptional cases this probability can exceed 0.95. These numbers are sufficiently large for application in sensitive biosensing, and super-resolution imaging. This opens the possibility to develop devices that can be controlled by an external optical stimulus on a truly molecular length scale.

Abstract The effects of the morphology of anatase TiO 2 substrates on the adsorption nature of Ru‐bipyridyl complexes with carboxylate groups as anchoring moieties have been investigated with infrared absorption spectroscopy and electron microscopy. The investigated dyes were the recently synthesized [Ru(dcbpyH 2 ) 2 (bpy‐TPA 2 )](PF 6 ) 2 , Ru‐TPA 2 , and the well‐known (Bu 4 N) 2 [Ru(dcbpyH) 2 (NCS) 2 ], N719, complexes. As substrates, TiO 2 paste films and pristine anatase nanopowder were used. Scanning electron microscopy images show big differences in the grain size, shape, and dispersity of the titania nanoparticles. The attenuated total reflection Fourier transform infrared spectra suggest that the coordination of both complexes on the TiO 2 paste films occurs via bidentate or bridging linkage. The adsorption of the dyes on pristine anatase nanopowder appears to be via bridging or bidentate chelate linkage for the dye N719 as well, however, the Ru‐TPA 2 complex appears to bind via unidentate or ester‐like linkage or by direct physisorpion via H‐bonding.

Boron-dipyrrin dyes, through rational design, yield promising new materials. With strong electron-donor functionalities and anchoring groups for attachment to nanocrystalline TiO(2), these dyes proved useful as sensitizers in dye-sensitized solar cells. Their applicability in a solid-state electrolyte regime offers additional opportunities for practical applications.

Excess PbI₂ in MAPbI₃ thin films has no direct impact on the electronic structure but changes the temperature dependence of the exciton binding energy and the phase transition behaviour.

Conjugated polymers, especially their second generation with a donor-acceptor alternating structure, have promising properties. These are suitable for two emerging fields, thermoelectrics and bioelectronics, if appropriate structural designs are implemented. This review aims to give a perspective for the potential and challenges of novel conjugated polymers in such applications. In particular, the aspects of synthetic design and the consequences of modifications of the chemical structure on the charge transport in selected second-generation conjugated polymers are reviewed. By understanding the effects of structural motifs on the overall material properties, polymers can be specifically tailored for the respective application. The basics of charge transport measurements are briefly summarized, as the charge transport plays an important role for thermoelectrics as well as for bioelectronics. In particular, the correlation between the reported charge carrier mobility values and the structural design of the polymers is reviewed. Examples of the application of second-generation conducting polymers in thermoelectrics and bioelectronics are shown to demonstrate the current state of research. Finally, the prospect of a purposeful design of new materials for these two emerging fields is discussed.

We present the synthesis of a new amphiphilic poly(3-hexylthiophene)-block-poly(tetrabutylammonium-4-styrenesulfonate) (P3HT-b-PSS–/TBA+) block copolymer via polymer–polymer click reaction. The P3HT-alkyne block has a molecular weight of 17.6 kg/mol (SEC)/11.3 kg/mol (MALDI-ToF), and the block copolymer has a high purity due to the complete removal of both precursor blocks by a column chromatographic approach. The fluorinated styrenesulfonate polymer P(SS-CH2-CF3)-N3 is synthesized by reversible addition–fragmentation transfer (RAFT) polymerization, and a specific RAFT agent is used to avoid labile ester linkages. SEC with THF as solvent overestimates the molecular weight (MW) of P(SS-CH2-CF3)-N3, but 1H NMR spectroscopy allows for the exact calculation of the molecular weight. We show that the precursor block copolymer P3HT-b-P(SS-CH2-CF3) forms wormlike micelles by self-assembly with a crystalline P3HT core (diameter <30 nm) in polar solvents. In these BCP micelles the P(SS-CH2-CF3) precursor in the corona can be deprotected, and P3HT-b-PSS–/TBA+ micelles are formed. Additionally, the compartmentalized in situ synthesis of highly crystalline TiO2 (anatase) nanocrystals (<10 nm) stabilized within the PSS corona of the micelles is demonstrated. The resulting P3HT–TiO2 nanocomposites with well-defined nanoscale morphology are water-processable, and they are investigated in detail by cryo-TEM and UV–vis spectroscopy.

Electronically functionalized block copolymers, combining covalently linked p-type and n-type blocks, show switching behavior of charge transport in organic field effect transistors (OFETs). The electronically active subunits self-assemble into continuous microdomains in a nanoscale regime, thereby forming percolation channels for holes or electrons or both depending on the composition and processing conditions. Here, we establish a charge transport-morphology relation for donor-acceptor block copolymers with two crystalline blocks. The n-type and p-type blocks self-assemble into two-dimensional lattices of π-π stacks and main chain polymer lamellae, respectively, over a broad composition range. Controlling the crystallization preferences of the two blocks by thermal annealing allows controlling the OFET polarity. Depending on the block ratio, the charge transport can be tuned from p-type to n-type or p-type to ambipolar, respectively. The impact of nanostructured phase separation is further delineated by X-ray diffraction, time-resolved spectroscopy, and scanning electron microscopy studies.

Dye-sensitized hybrid solar cells with a nanoporous TiO<SUB>2</SUB> layer and different organic hole conducting polymers have been investigated. These hole transport materials (HTM) with low glass temperature (T<SUB>g</SUB>) are based on triphenyldiamines (TPD). To enhance the power conversion efficiency, the HTMs were doped. New low T<SUB>g</SUB> TPD-based polymers were designed and synthesized with long alkoxyl groups as sidechains or in the backbone, respectively, to investigate the influence of the soft alkoxyl chains on the penetration behavior into the nanoporous layer which has been studied by SEM. The effect of the penetration depth and wetting of the dye sensitized TiO<SUB>2</SUB> layer by HTM on the solar cell efficiencies have been investigated by I-V- characteristics and steady-state measurements. To improve the penetration, the polymers were heated above T<SUB>g</SUB>. The performance of the cell is decreasing probably due to degradation of the dye during thermal treatment. To enhance the conductivity of the hole transport materials, a Li salt has been used and this doped system was compared to a doped standard Gratzel cell with OMeTAD as HTM. Crystallization on the surface could be seen after storing the standard cell for some weeks.

In this investigation, transparent TiO2 nanotube arrays prepared on a FTO substrate are employed as 1D nanostructures providing elongated direct pathways for electron transport and collection in solid-state dye-sensitized solar cell (SDSC). Donor-antenna (D-A) dyes provide an exciting route for improving the light harvesting efficiency in dye sensitized solar cells owing to their high molar extinction coefficients and the effective spatial separation of charges in the charge-separated state. Hence in this study we fabricated SDSC devices with different thicknesses of transparent TiO2 nanotube array electrodes sensitized with Ru-(II)-donor-antenna dye and spiro-OMeTAD as a hole conductor. At AM 1.5 G, 100 mW/cm2 illumination intensity, a power conversion efficiency of 1.94% was achieved when the TiO2 nanotubes are initially subjected to TiCl4 treatment. Furthermore, a linear increase in the cell current without loss in fill factor is observed for increasing length of TiO2 nanotubes. The structural and morphological characteristics of the transparent TiO2 nanotube arrays as well as the optimal conditions for the fabrication of SDSCs with transparent TiO2 nanotubes on FTO glass are reported.

Abstract Organic electrochemical transistors (OECTs) are gaining attention for their ease of fabrication, flexibility, and biocompatibility, with applications in biosignal sensing, neuromorphic computing, wearable health monitors, environmental monitoring, and bioelectronic interfaces. The interactions between ionic and electronic subcircuits in OECTs raise fundamental questions about the relationship between device design and performance. A major challenge is to meet specific integration, processing, and device performance requirements. While miniaturization of OECTs can improve transconductance and maximum operating frequency, it often compromises cost effectiveness and integratability. This work investigates an OECT architecture that incorporates both a crosslinkable printed aqueous electrolyte and a printed poly(3,4‐ethylenedioxythiophene):ploy(4‐styrenesulfonate) (PEDOT:PSS) top‐gate to achieve efficient gating, higher operating frequencies, and easy integration with low‐cost printing techniques. Improved performance is demonstrated in this top‐gate OECTs over conventional side‐gate structures, achieving sub‐millisecond device operation with channel lengths of 100 µm. This configuration shows practical potential for circuit integration, as demonstrated with a complementary inverter using an ambipolar material.