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Short channel organic thin film transistors in bottom-gate, bottom contact configuration use typically gold metallization for the source and drain contacts because this metal can easily be cleaned from photoresist residuals by oxygen plasma or ultraviolet-ozone and allows also surface modification by self-assembled monolayers (e.g. thiols). Alternative low-cost bottom contact metallization for high performance short-channel organic thin film transistors are scarce because of the incompatibility of the bottom contact material with the cleaning step. In this work a new process flow, involving a temporary thin aluminum protection layer, is presented. Short channel (3.4 μm) pentacene transistors with lithographical defined and thiol modified silver source/drain bottom contacts (25 nm thick, on a 2 nm titanium adhesion layer) prepared according to this process achieved a saturation mobility of 0.316 cm2/(V.s), and this at a metal cost below 1% of the standard 30 nm thick gold metallization.
Thin film growth by high vacuum evaporation of the n-type organic semiconductor 5, 5″′-diperfluorohexylcarbonyl-2,2′:5′,2″:5″,2″′-quaterthiophene (DFHCO-4T) on poly-(α-methylstyrene)-coated n++-Si/SiO2 substrates is investigated at various deposition fluxes and substrate temperatures. Film characterization by atomic force microscopy reveals typical Stransky-Krastanov growth. Transistors with Au source-drain top contacts and optimized DFHCO-4T deposition conditions attain an apparent saturation mobility of 4.6 cm2/Vs, whereas this parameter is 100× lower for similar transistors with LiF/Al top contacts. We explain this lower performance by the formation of a thin interfacial layer with poor injection properties resulting from a redox reaction between Al and DFHCO-4T.
Memory cells based on Cu+ and Ag+ metal-organic charge-transfer complexes, as for example CuTCNQ (where TCNQ denotes 7,7',8,8'-tetracyanoquinodimethane), are well known for their bistable resistive electrical switching since 1979. The switching mechanism however remained unclear for very long time. In this contribution we describe the different views (bulk vs. interfacial switching), give evidence for interfacial switching in the case of CuTCNQ, and present a model allowing explaining the bipolar resistive electrical switching by an interfacial effect, even for experiments considered until now as proof for bulk switching. The proposed switching mechanism is based on bridging of an ion-permeable layer (or gap) by conductive Cu channels, which are formed and dissolved by an electrochemical reaction implying monovalent Cu+ cations, originating from a solid ionic conductor (as for example CuTCNQ). The model was furthermore generalized to other memory systems consisting of a permeable layer and a solid ionic conductor, including also inorganic solid ionic conductors as for example Ag2S.
The performance of organic solar cells based on the blend of regioregular poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) is strongly influenced by the morphology of the active layer on the nanoscale level. X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM) measurements show that ordering of P3HT plays a key role in optimizing the photovoltaic performance. It is demonstrated that the natural tendency of regioregular P3HT to crystallize is disturbed by the addition of PCBM. The crystallinity of the photo-active blend is typically restored by an annealing procedure resulting in improved device performance, characterized by a spectral broadening of the optical absorption.
The morphological changes upon annealing of the P3HT:PCBM blends are accompanied by electrical changes as shown in charge carrier mobility measurements. Space-charge limited current measurements have been performed in hole-only devices with various P3HT:PCBM blend ratios. The mobility before and after annealing is compared and from temperature dependent measurements the width of the density of states distribution (DOS) is determined. The hole mobility in pristine P3HT remains practically unaffected by the annealing treatment. The as-produced P3HT:PCBM blends on the other hand, with a more disordered P3HT phase, have a much lower hole mobility. Annealing is capable of increasing the P3HT ordering with as a result an orders of magnitude larger hole mobility, approaching the value found in pristine P3HT. The DOS bandwidths are affected similarly. In the as-produced blend films a value of 100 meV is found, larger than in the annealed films, there reaching a value around 70 meV similar as in pristine P3HT. Variation of the processing solvent demonstrated however that an optimized morphology and charge transport situation can also be obtained without an additional annealing step. It is shown that in that case the as-produced active layer has already a favorable crystalline morphology. We argue that the high boiling point of the solvent plays an important role in this by influencing the evaporation speed during deposition of the photo-active blend. Further proof is delivered that indeed slowing down the evaporation speed can beneficially influence the solar cell performance. Power conversion efficiency over 4% has been achieved in this way.
CuTCNQ (TCNQ=7,7,8,8-tetracyanoquinodimethane) is a resistive switching charge-transfer complex which can be used for organic nonvolatile memories. In this contribution we report on a thorough investigation of the electrical switching of CuTCNQ memories. Our memories currently achieve an endurance of up to 10000 write/erase cycles with a clear distinction between ON and OFF reading currents. ON and OFF threshold voltages follow a Gaussian distribution. Temperature dependent measurements of CuTCNQ based organic memories show a semiconductor like behavior for the ON state. The retention time of the ON state exceeded 60 hours at room temperature. Electrical switching of CuTCNQ memories in air was virtually not affected by temperatures up to 80°C, but becomes erratic at 120°C. The CuTCNQ material itself already starts to decompose around 200°C in presence of oxygen as shown by thermogravimetric analysis.
In this work, we report on high-performance low voltage pentacene Organic Thin-Film Transistors (OTFT's) and circuits. Inverters and ring oscillators have been designed and fabricated. At 15 V supply voltage, we have observed invertors showing a voltage gain of 9 and an output swing of more than 13 V. As for the ring oscillators, oscillations started at supply voltages as low as 8.5 V. At a supply voltage of only 15 V, a stage delay time of 3.3 νs is calculated from experimental results.
We believe that these results show for the first time a high speed ring oscillator at relatively low supply voltages. The required supply voltages can be obtained by rectification using an organic (pentacene) diode. These results may have an important impact on the realization of RF-ID tags: by integrating our circuits with an organic diode, the fabrication of organic RF-ID tags comes closer.
Screen-printing is studied as deposition technique for conjugated material based layers. Photovoltaics based on the principle of bulk donor-acceptor heterojunction are tested using a blend of poly(2-methoxy-5-(2'-ethyl-hexyloxy)-1, 4-phenylene vinylene) (MEH-PPV) mixed with the C60-derivative (6, 6)-phenyl C61-butyric acid methyl ester (PCBM). First, different solution concentrations of the donor MEH-PPV material and of the blend are subjected to rheology measurements. Addition of the acceptor (PCBM) to a donor material based solution induces a decrease of the solution viscosity. However, the overall flow behaviour of the blend remains similar to that of the MEH-PPV based solution. Secondly, it is shown that specific printer settings have to be used to obtain active layers that are suitable for opto-electronic applications. Finally, devices with an overall energy conversion efficiency of 1.25% under standardized simulated solar illumination (AM1.5G; 100mW/cm2) have been obtained showing that screen-printing can be a suitable technique for the deposition of the active layer of polymer solar cells.
We will demonstrate a novel approach to the problem of patterning organic small molecule layers, which is compatible with printing techniques. By locally changing the chemistry of the dielectric surface by means of Self-Assembly Monolayers (SAM), we predefine active and passive areas of the circuit. The subsequent growth of the organic film by sublimation takes place under growth conditions which cause the formation of a high-mobility, well- connected two-dimensional (2D) grains in the predefined active area contrasting with ill- connected, low-mobility 3D-grains in the passive area. We will explain this patterned growth in terms of a microscopic theory of nucleation of organic semiconductor on inert substrates and present experimental results.
Doping of a disordered organic semiconductor gives rise to additional energy disorder due to the Coulomb interaction between randomly distributed dopant ions and carriers localized in intrinsic hopping sites. Although the carrier density increases with increasing doping level the additional energy disorder can significantly reduce the carrier hopping mobility. At higher doping levels the filling of deep states takes over, which leads to steeply increasing mobility at high dopant concentrations.
We describe and demonstrate a micromachined shadowmask that allows the realization of intra-grain and oligo-grain top-contact organic thin film transistors (OTFTs). First experimental results of OTFT's show that for small channel lengths, grain boundary barriers indeed appear to dominate the output characteristics of OTFTs.
A model of carrier photogeneration in doped conjugated polymers is formulated. The model suggests that dissociation of a relaxed exciton into a Coulombically bound geminate pair (GP) occurs at a charge transfer center that consists of a polymer chain and a nearby electron acceptor. Further dissociation of the GP into free carriers is facilitated either by the kinetic energy of the zero-point oscillation of the on-chain carrier at low dopant concentrations or by the interfacial potential barrier for geminate pair recombination in polymer/electron acceptor blends.
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