Analysis of Schottky Contact Formation in Coplanar Au/ZnO/Al Nanogap Radio Frequency Diodes Processed from Solution at Low Temperature.

Much work has been carried out in recent years in fabricating and studying the Schottky contact formed between various metals and the n-type wide bandgap semiconductor zinc oxide (ZnO). In spite of significant progress, reliable formation of such technologically interesting contacts remains a challenge. Here, we report on solution-processed ZnO Schottky diodes based on a coplanar Al/ZnO/Au nanogap architecture and study the nature of the rectifying contact formed at the ZnO/Au interface. Resultant diodes exhibit excellent operating characteristics, including low-operating voltages (±2.5 V) and exceptionally high current rectification ratios of >10(6) that can be independently tuned via scaling of the nanogap's width. The barrier height for electron injection responsible for the rectifying behavior is studied using current-voltage-temperature and capacitance-voltage measurements (C-V) yielding values in the range of 0.54-0.89 eV. C-V measurements also show that electron traps present at the Au/ZnO interface appear to become less significant at higher frequencies, hence making the diodes particularly attractive for high-frequency applications. Finally, an alternative method for calculating the Richardson constant is presented yielding a value of 38.9 A cm(-2) K(-2), which is close to the theoretically predicted value of 32 A cm(-2) K(-2). The implications of the obtained results for the use of these coplanar Schottky diodes in radio frequency applications is discussed.

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.

Hybrid Modulation-Doping of Solution-Processed Ultrathin Layers of ZnO Using Molecular Dopants.

An alternative doping approach that exploits the use of organic donor/acceptor molecules for the effective tuning of the free electron concentration in quasi-2D ZnO transistor channel layers is reported. The method relies on the deposition of molecular dopants/formulations directly onto the ultrathin ZnO channels. Through careful choice of materials combinations, electron transfer from the dopant molecule to ZnO and vice versa is demonstrated.

High-Entropy Mixtures of Pristine Fullerenes for Solution-Processed Transistors and Solar Cells.

The solubility of pristine fullerenes can be enhanced by mixing C60 and C70 due to the associated increase in configurational entropy. This "entropic dissolution" allows the preparation of field-effect transistors with an electron mobility of 1 cm(2) V(-1) s(-1) and polymer solar cells with a highly reproducible power-conversion efficiency of 6%, as well as a thermally stable active layer.

An Air-Stable DPP-thieno-TTF Copolymer for Single-Material Solar Cell Devices and Field Effect Transistors.

Following an approach developed in our group to incorporate tetrathiafulvalene (TTF) units into conjugated polymeric systems, we have studied a low band gap polymer incorporating TTF as a donor component. This polymer is based on a fused thieno-TTF unit that enables the direct incorporation of the TTF unit into the polymer, and a second comonomer based on the diketopyrrolopyrrole (DPP) molecule. These units represent a donor-acceptor copolymer system, p(DPP-TTF), showing strong absorption in the UV-visible region of the spectrum. An optimized p(DPP-TTF) polymer organic field effect transistor and a single material organic solar cell device showed excellent performance with a hole mobility of up to 5.3 × 10(-2) cm(2)/(V s) and a power conversion efficiency (PCE) of 0.3%, respectively. Bulk heterojunction organic photovoltaic devices of p(DPP-TTF) blended with phenyl-C71-butyric acid methyl ester (PC71BM) exhibited a PCE of 1.8%.

Conjugated polymer-porphyrin complexes for organic electronics.

We present the synthesis of novel conjugated polymer-porphyrin complexes for use in organic electronics. Linear and star-shaped platinated porphyrins were attached to regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) arms to investigate whether porphyrin stacking and increased dimensionality can be used to control polymer morphology. The novel materials display similar optical properties to P3HT, but give higher mobilities when used in organic field-effect transistors. Atomic force microscopy measurements show that incorporation of only a small amount of porphyrin into the conjugated polymer backbone leads to increased aggregation. These materials demonstrate that polymer morphology and performance can be tuned and enhanced effectively through the use of conjugatively linked porphyrins.

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.

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.

Thiophene fluorination to enhance photovoltaic performance in low band gap donor-acceptor polymers.

We report the first synthesis of a tetrafluorinated 4,7-bis(3,4-difluorothiophen-2-yl)-2,1,3-benzothiadiazole monomer and its polymerisation with dithieno[3,2-b:2',3'-d]germole by Stille coupling to afford a low band gap polymer with a high ionisation potential. Direct comparison to the non-fluorinated analogue demonstrates that fluorination results in an increase in ionisation potential with no change in optical band gap, and enhanced aggregation over the non-fluorinated polymer. These desirable properties result in a significant enhancement in OPV device performance in blends with PC(71)BM.

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.