Stephan Rossbauer

Low band gap selenophene – diketopyrrolopyrrole polymers exhibiting high and balanced ambipolar performance in bottom-gate transistors

We report the synthesis of a selenophene–diketopyrrolopyrrole monomer and its co-polymerisation with selenophene and thieno[3,2-b]thiophene monomers by Stille coupling. The resulting low band gap polymers exhibit ambipolar charge transport in organic field effect transistors. High and balanced electron and hole mobilities in excess of 0.1 cm2 V−1 s−1 were observed in bottom-gate, bottom-contact devices, suggesting that selenophene inclusion is a promising strategy for the development of ambipolar organic semiconductors.

Fused dithienogermolodithiophene low band gap polymers for high-performance organic solar cells without processing additives.

We report the synthesis of a novel ladder-type fused ring donor, dithienogermolodithiophene, in which two thieno[3,2-b]thiophene units are held coplanar by a bridging dialkyl germanium. Polymerization of this extended monomer with N-octylthienopyrrolodione by Stille polycondensation afforded a polymer, pDTTG-TPD, with an optical band gap of 1.75 eV combined with a high ionization potential. Bulk heterojunction solar cells based upon pDTTG-TPD:PC(71)BM blends afforded efficiencies up to 7.2% without the need for thermal annealing or processing additives.

Air‐Stable and High‐Mobility n‐Channel Organic Transistors Based on Small‐Molecule/Polymer Semiconducting Blends

Use of a carefully designed small-molecule organic semiconductor based on an oxidized diketopyrrolopyrrole core enables the fabrication by solution processing of electron-transporting (n-channel) blend-based organic thin-film transistors with high electron mobility (0.5 cm(2)/Vs) and high operating stability even when the devices are exposed to ambient air for prolonged periods of time.

BPTs: thiophene-flanked benzodipyrrolidone conjugated polymers for ambipolar organic transistors

A series of novel thiophene-flanked benzodipyrrolidone (BPT)-based alternating copolymers are synthesised, their optical and electrical properties evaluated. The BPT unit promotes a conjugated, planar polymer backbone, with a low bandgap, primarily due to low lying LUMO energy levels. Copolymerisation with thiophene exhibits well balanced ambipolar organic field-effect transistor performance, with electron and hole mobilities 0.1 and 0.2 cm(2) V(-1) s(-1), respectively.

Dihydropyrroloindoledione-based copolymers for organic electronics

A series of four dihydropyrroloindoledione-based organic semi-conducting polymers are examined for performance in transistor and photovoltaic cell devices. The dihydropyrroloindoledione unit was alternately copolymerized with phenyl, thiophene and bithiophene comonomers, and the resultant polymers exhibit broad absorption, low-bandgaps and deep energy levels, with charge carrier mobilities approaching 0.1 cm2 V−1 s−1. Solar cells processed in a printing friendly solvent (m-xylene) exhibited >2% PCE with a high fill-factor of 0.62 and Voc of 0.75 V.

Low band gap dithienogermolodithiophene copolymers with tunable acceptors and side-chains for organic solar cells

We report the synthesis and characterisation of five new donor–acceptor type co-polymers based on a fused dithienogermolodithiophene unit for use in photovoltaic devices. The influence of three electron deficient co-monomers, as well as the length and variety of the solubilising side-chains, on the physical and optoelectronic properties of the polymers is reported. The number and variety of alkyl side-chains is found to have a significant impact on the polymer aggregation and film morphology, with larger and more bulky side-chains leading to improved solubility and molecular weight. The influence of these properties upon the performance of bulk heterojunction solar cells is shown.

Solution-processed ZnO nanoparticle-based transistors via a room-temperature photochemical conversion process

We report on zinc oxide (ZnO) nanoparticle-based transistors in which the solution-processed semiconducting layers are post-treated using UV light irradiation instead of high temperature annealing. Field-effect transistors based on these ZnO nanoparticle films exhibit n-channel characteristics with electron mobilities on the order of ∼10−3 cm2 V−1s−1—a value comparable to that obtained from devices thermally annealed at 250 °C. Because of the photochemical nature of the process, the sample temperature during UV irradiation remains low and close to room temperature. This enables facile fabrication of ZnO transistors onto inexpensive, temperature-sensitive substrate materials such as plastic.

Radio Frequency Coplanar ZnO Schottky Nanodiodes Processed from Solution on Plastic Substrates

Coplanar radio frequency Schottky diodes based on solution-processed C60 and ZnO semiconductors are fabricated via adhesion-lithography. The development of a unique asymmetric nanogap electrode architecture results in devices with a high current rectification ratio (10(3) -10(6) ), low operating voltage (<3 V), and cut-off frequencies of >400 MHz. Device fabrication is scalable and can be performed at low temperatures even on plastic substrates with very high yield.

Incorporation of benzocarborane into conjugated polymer systems: synthesis, characterisation and optoelectronic properties

We present the novel 1,4-difunctionalisation of benzocarborane with organometallic groups suitable for cross-coupling and its subsequent insertion for the first time into conjugated polymer backbones. Copolymers with solubilised cyclopentadithiophene and diketopyrrolopyrrole derivatives were prepared by Stille polymerisation in good molecular weight. The physical, material and optoelectronic properties of the resulting polymers were investigated, demonstrating that benzocarborane acts similarly to a stabilised, electron-deficient cis-diene linker. We also report the first polymer field effect transistors incorporating a benzocarborane in the backbone.

Solution-processed dye-sensitized ZnO phototransistors with extremely high photoresponsivity

We report the fabrication of light-sensing thin-film transistors based on solution processed films of ZnO, as the channel material, functionalized with an organic dye as the light sensitizer. Due to the presence of the dye, the hybrid devices show exceptionally high photosensitivity to green light of 106 and a maximum photoresponsivity on the order of 104 A/W. The high performance is argued to be the result of the grain barrier limited nature of electron transport across the polycrystalline ZnO film and its dependence on charge carrier density upon illumination with green light. In addition to the excellent photoresponsivity and signal gain, the hybrid ZnO-dye photoactive layer exhibits high optical transparency. The unique combination of simple device fabrication and distinctive physical characteristics, such as optical transparency, renders the technology attractive for application in large-area transparent photodetectors.