Matthias Edler

Tuning the threshold voltage in organic thin-film transistors by local channel doping using photoreactive interfacial layers.

In recent years, organic thin-film transistors (OTFTs) have attracted a great deal of attention due to their potential applications in low cost sensors,1 memory cards,2 and integrated circuits.3 Great efforts are under way to design OTFTs with high performance, high stability, high reproducibility, and low cost.4 Two of the most crucial device parameters are the charge carrier mobility and the threshold voltage (VTh). Concerning the mobility, the main goals for most applications is its maximization.5 For VTh, the situation is more complex: for example, for integrated circuits it would be desirable to tune VTh over a broad range,6 e.g., for inverter applications. In silicon technology, complementary circuits that consist of p-channel and n-channel transistors are typically used.7 There have been many attempts to adapt this technology to OTFTs and fabricate organic complementary inverters.2,8,9 They, however, suffer from poor n-type transistor performance and/or air instability of n-type semiconductor materials. An alternative approach is the adaptation of unipolar depletion-load inverters enabling simplified processing, even if they do not provide the low power consumption and the simple circuit design intrinsic to complementary logic.10,11 Depletion-load inverters consist of an enhancement-mode driver transistor and a depletion-mode load transistor and can be realized using only p-type OTFTs. So far there have been attempts to achieve this target by using a level shifter12,13 or a dual gate structure.14 The main objective is to find a reproducible method to realize driver and load transistors with equivalent device characteristics (in particular mobilities), but different VTh values.

Water-Developable Poly(2-oxazoline)-Based Negative Photoresists

Copoly(2-oxazoline)-based photoresists are prepared from pEtOx(80) Bu(=) Ox(20) and pPhOx(80) Dc(=) Ox(20) , respectively, a tetrathiol, and a photosensitive initiator. It is possible to prepare copoly(2-oxazoline)s bearing unsaturated side chains in a microwave reactor on a decagram scale in reaction times of 100 min or shorter. UV irradiation of dried polymer films through a quartz mask induces the thiol-ene reaction in the illuminated areas. Subsequent development of the polymer films in halogen-free solvents reproduces the negative pattern of the mask with a resolution of 2 μm. The pEtOx(80) Bu(=) Ox(20) -derived photoresists can also be developed in water.

UV-induced modulation of the conductivity of polyaniline: towards a photo-patternable charge injection layer for structured organic light emitting diodes

In macromolecular electronics the organic material should offer—besides the desired electronic properties—the possibility of lateral patterning. Here, we report on a novel polyaniline derivative bearing photosensitive N-formamide groups. UV illumination of this polymeric material leads to a decarbonylation reaction resulting in polyaniline which can be subsequently protonated to yield the conductive emeraldine salt. Due to the fact that the conductivity depends on the conversion of the photoreaction, a selective adjustment of the conductivity by means of UV light is feasible. These photo-induced conductivity changes were corroborated in thin films by conductive AFM measurements. Further, we demonstrate the versatility of this polymer with respect to a patterned modification of the conductivity as well as its application as a photo-patternable charge injection layer for structured OLEDs. This makes the new polymer an interesting candidate for electrodes and interconnects in various organic electronic devices.

Lignocellulose Nanofiber-Reinforced Polystyrene Produced from Composite Microspheres Obtained in Suspension Polymerization Shows Superior Mechanical Performance

A facile approach to obtaining cellulose nanofiber-reinforced polystyrene with greatly improved mechanical performance compared to unreinforced polystyrene is presented. Cellulose nanofibers were obtained by mechanical fibrillation of partially delignified wood (MFLC) and compared to nanofibers obtained from bleached pulp. Residual hemicellulose and lignin imparted amphiphilic surface chemical character to MFLC, which enabled the stabilization of emulsions of styrene in water. Upon suspension polymerization of styrene from the emulsion, polystyrene microspheres coated in MFLC were obtained. When processed into polymer sheets by hot-pressing, improved bending strength and superior impact toughness was observed for the polystyrene-MFLC composite compared to the un-reinforced polystyrene.

Photochemical control of the carrier mobility in pentacene-based organic thin-film transistors

We use a photoreactive interfacial layer of poly(endo,exo-bis(2-nitrobenzyl) bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate) (PBHND) between the gate dielectric and the pentacene active layer to tune the effective hole mobility in organic thin film transistors over approximately one order of magnitude. This can be explained by the photoconversion of the PBHND layer changing its surface energy and thus strongly impacting pentacene growth. A linear relationship between the mobility and the grain size is observed.

Tough and degradable photopolymers derived from alkyne monomers for 3D printing of biomedical materials

This contribution deals with the synthesis and exploration of alkyne carbonate derivatives as biocompatible building blocks in the thiol–yne photopolymerisation reaction with the aim to facilitate the fabrication of tailor made medical devices by UV based additive manufacturing technologies. It turned out that the investigated alkyne carbonates offer curing rates similar to comparable acrylates, while providing much higher conversion and lower monomer cytotoxicity. Curing the synthesized building blocks in combination with the commercially available thiol pentaerythritol tetra(3-mercaptopropionate) (PETMP) leads to networks that degrade in aqueous alkaline and acidic media in a surface erosion manner. Additionally, a selective adjustment of the degradability is feasible by the choice and content of thiol monomers. Notably, monomers containing a tricyclo[5.2.1.02,6]decane-4,8-dimethanol backbone provide decent thermo-mechanical properties and appropriate impact strengths similar to polylactic acid (PLA). Most importantly, selected thiol–yne formulations were printed successfully with an accuracy of 40 × 40 μm, which seems to be sufficiently high to prints medical devices in appropriate resolution.

Wavelength selective refractive index modulation in a ROMP derived polymer bearing phenyl- and ortho-nitrobenzyl ester groups

Patterning and tuning of the refractive index in polymers by means of UV-light is of great interest for optical applications such as polymeric waveguides or optical data storage devices. In this contribution, we report on the synthesis of a polynorbornene based copolymer bearing ortho-nitrobenzyl and phenyl ester groups capable of undergoing the photo-induced cleavage reaction and in a subsequent step optionally the photo-Fries rearrangement upon irradiation with UV-light. The photoreaction of these aromatic ester groups was investigated by means of FTIR, UV-VIS and spectroscopic ellipsometry. Due to the difference in UV absorption of the photoreactive units, the o-nitrobenzyl ester can be selectively excited by UV-light above 300 nm, while a subsequent illumination with 254 nm induces the photo-Fries rearrangement of the remaining phenyl ester groups. The structural changes in the chemical composition upon UV illumination lead to significant changes in the refractive index of the polymeric materials. Whilst the photo-Fries rearrangement of the phenyl ester groups causes a remarkable increase in the refractive index, it was found that the photo-induced cleavage reaction of ortho-nitrobenzyl ester moieties causes a decrease of the refractive index. This fact enables a selective tailoring of the refractive index by the choice of the applied wavelength. In addition, a two-step irradiation procedure using a sequence of different wavelengths provides the possibility of erasing and even inverting the index contrast.

Exploring Network Formation of Tough and Biocompatible Thiol-yne Based Photopolymers.

This work deals with the in-depth investigation of thiol-yne based network formation and its effect on thermomechanical properties and impact strength. The results show that the bifunctional alkyne monomer di(but-1-yne-4-yl)carbonate (DBC) provides significantly lower cytotoxicity than the comparable acrylate, 1,4-butanediol diacrylate (BDA). Real-time near infrared photorheology measurements reveal that gel formation is shifted to higher conversions for DBC/thiol resins leading to lower shrinkage stress and higher overall monomer conversion than BDA. Glass transition temperature (Tg ), shrinkage stress, as well as network density determined by double quantum solid state NMR, increase proportionally with the thiol functionality. Most importantly, highly cross-linked DBC/dipentaerythritol hexa(3-mercaptopropionate) networks (Tg ≈ 61 °C) provide a 5.3 times higher impact strength than BDA, which is explained by the unique network homogeneity of thiol-yne photopolymers.

Surface chemical functionalization of cellulose nanocrystals by 3-aminopropyltriethoxysilane

Surface functionalization of cellulose nanocrystals (CNCs) is valuable option to tailor properties as well as increase opportunities for their application. In this study, the surface of CNCs was functionalized with 3-aminopropyltriethoxysilane (APTES), without using hazardous solvents and by a direct, simple and straightforward method. APTES was firstly hydrolyzed in water and then adsorbed onto CNC through hydrogen bonds, finally the chain hydrocarbon was covalently linked to the surface of CNC through SiOC bonds which formed via the condensation reaction between hydroxyl and silanol groups. The chemical modification of the CNCs surface was confirmed by ATR-IR and NMR spectroscopy. Experiments conducted by AFM and XRD showed no significant change in the CNC dimensions and crystalline structure as a result of the modification. The EDX and XPS results confirmed the exsistence of APTES onto the CNCs. Silylated CNC exhibited good thermal stability and a greater amount of residual char was formed at 500 °C compared to non-chemically modified CNC. Thus, The silylation of CNCs may offer applications in composite manufacturing, where these nanoparticles have limited dispersibility in hydrophobic polymer matrices, and as nano-adsorbers due to the presence of amino groups attached on the surface.

Microwave‐Assisted Syntheses in Recyclable Ionic Liquids: Photoresists Based on Renewable Resources

The copoly(2-oxazoline) pNonOx80-stat-pDc=Ox20 can be synthesized from the cationic ring-opening copolymerization of 2-nonyl-2-oxazoline NonOx and 2-dec-9′-enyl-2-oxazoline Dc=Ox in the ionic liquid n-hexyl methylimidazolium tetrafluoroborate under microwave irradiation in 250 g/batch quantities. The polymer precipitates upon cooling, enabling easy recovery of the polymer and the ionic liquid. Both monomers can be obtained from fatty acids from renewable resources. pNonOx80-stat-pDc=Ox20 can be used as polymer in a photoresist (resolution of 1 μm) based on UV-induced thiol–ene reactions.