Significance of the double-layer capacitor effect in polar rubbery dielectrics and exceptionally stable low-voltage high transconductance organic transistors.

Both high gain and transconductance at low operating voltages are essential for practical applications of organic field-effect transistors (OFETs). Here, we describe the significance of the double-layer capacitance effect in polar rubbery dielectrics, even when present in a very low ion concentration and conductivity. We observed that this effect can greatly enhance the OFET transconductance when driven at low voltages. Specifically, when the polar elastomer poly(vinylidene fluoride-co-hexafluoropropylene) (e-PVDF-HFP) was used as the dielectric layer, despite a thickness of several micrometers, we obtained a transconductance per channel width 30 times higher than that measured for the same organic semiconductors fabricated on a semicrystalline PVDF-HFP with a similar thickness. After a series of detailed experimental investigations, we attribute the above observation to the double-layer capacitance effect, even though the ionic conductivity is as low as 10(-10) S/cm. Different from previously reported OFETs with double-layer capacitance effects, our devices showed unprecedented high bias-stress stability in air and even in water.

Experimental evidence that short-range intermolecular aggregation is sufficient for efficient charge transport in conjugated polymers.

Efficiency, current throughput, and speed of electronic devices are to a great extent dictated by charge carrier mobility. The classic approach to impart high carrier mobility to polymeric semiconductors has often relied on the assumption that extensive order and crystallinity are needed. Recently, however, this assumption has been challenged, because high mobility has been reported for semiconducting polymers that exhibit a surprisingly low degree of order. Here, we show that semiconducting polymers can be confined into weakly ordered fibers within an inert polymer matrix without affecting their charge transport properties. In these conditions, the semiconducting polymer chains are inhibited from attaining long-range order in the π-stacking or alkyl-stacking directions, as demonstrated from the absence of significant X-ray diffraction intensity corresponding to these crystallographic directions, yet still remain extended along the backbone direction and aggregate on a local length scale. As a result, the polymer films maintain high mobility even at very low concentrations. Our findings provide a simple picture that clarifies the role of local order and connectivity of domains.

The Effect of Processing Additives on Energetic Disorder in Highly Efficient Organic Photovoltaics: A Case Study on PBDTTT-C-T:PC71 BM.

Energetic disorder, an important parameter affecting the performance of organic photovoltaics, is significantly decreased upon the addition of processing additives in a highly efficient benzodithiophene-based copolymer blend (PBDTTT-C-T:PC71 BM). Wide-angle and small-angle X-ray scattering measurements suggest that the origin of this reduced energetic disorder is due to increased aggregation and a larger average fullerene domain size together with purer phases.

A new tetracyclic lactam building block for thick, broad-bandgap photovoltaics.

A new tetracyclic lactam building block for polymer semiconductors is reported that was designed to combine the many favorable properties that larger fused and/or amide-containing building blocks can induce, including improved solid-state packing, high charge carrier mobility, and improved charge separation. Copolymerization with thiophene resulted in a semicrystalline conjugated polymer, PTNT, with a broad bandgap of 2.2 eV. Grazing incidence wide-angle X-ray scattering of PTNT thin films revealed a strong tendency for face-on π-stacking of the polymer backbone, which was retained in PTNT:fullerene blends. Corresponding solar cells featured a high open-circuit voltage of 0.9 V, a fill factor around 0.6, and a power conversion efficiency as high as 5% for >200 nm thick active layers, regardless of variations in blend stoichiometry and nanostructure. Moreover, efficiencies of >4% could be retained when thick active layers of ∼400 nm were employed. Overall, these values are the highest reported for a conjugated polymer with such a broad bandgap and are unprecedented in materials for tandem and particularly ternary blend photovoltaics. Hence, the newly developed tetracyclic lactam unit has significant potential as a conjugated building block in future organic electronic materials.

The role of regioregularity, crystallinity, and chain orientation on electron transport in a high-mobility n-type copolymer.

We investigated the correlation between the polymer backbone structural regularity and the charge transport properties of poly{[N,N'-bis(2-octyldodecyl)-1,4,5,8-naphthalenediimide-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} [P(NDI2OD-T2)], a widely studied semiconducting polymer exhibiting high electron mobility and an unconventional micromorphology. To understand the influence of the chemical structure and crystal packing of conventional regioregular P(NDI2OD-T2) [RR-P(NDI2OD-T2)] on the charge transport, the corresponding regioirregular polymer RI-P(NDI2OD-T2) was synthesized. By combining optical, X-ray, and transmission electron microscopy data, we quantitatively characterized the aggregation, crystallization, and backbone orientation of all of the polymer films, which were then correlated to the electron mobilities in electron-only diodes. By carefully selecting the preparation conditions, we were able to obtain RR-P(NDI2OD-T2) films with similar crystalline structure along the three crystallographic axes but with different orientations of the polymer chains with respect to the substrate surface. RI-P(NDI2OD-T2), though exhibiting a rather similar LUMO structure and energy compared with the regioregular counterpart, displayed a very different packing structure characterized by the formation of ordered stacks along the lamellar direction without detectible π-stacking. Vertical electron mobilities were extracted from the space-charge-limited currents in unipolar devices. We demonstrate the anisotropy of the charge transport along the different crystallographic directions and how the mobility depends on π-stacking but is insensitive to the degree or coherence of lamellar stacking. The comparison between the regioregular and regioirregular polymers also shows how the use of large planar functional groups leads to improved charge transport, with mobilities that are less affected by chemical and structural disorder with respect to classic semicrystalline polymers such as poly(3-hexylthiophene).

High mobility N-type transistors based on solution-sheared doped 6,13-bis(triisopropylsilylethynyl)pentacene thin films.

An N-Type organic thin-film transistor (OTFT) based on doped 6,13-Bis(triisopropylsilylethynyl)pentacene is presented. A transition from p-type to n-type occurrs with increasing doping concentrations, and the highest performing n-channel OTFTs are obtained with 50 mol% dopant. X-ray diffraction, scanning Auger microscopy, and secondary ionization mass spectrometry are used to characterize the morphology of the blends. The high performance of the obtained transistors is attributed to the highly crystalline and aligned nature of the doped thin films.

Moderate doping leads to high performance of semiconductor/insulator polymer blend transistors.

Polymer transistors are being intensively developed for next-generation flexible electronics. Blends comprising a small amount of semiconducting polymer mixed into an insulating polymer matrix have simultaneously shown superior performance and environmental stability in organic field-effect transistors compared with the neat semiconductor. Here we show that such blends actually perform very poorly in the undoped state, and that mobility and on/off ratio are improved dramatically upon moderate doping. Structural investigations show that these blend layers feature nanometre-scale semiconductor domains and a vertical composition gradient. This particular morphology enables a quasi three-dimensional spatial distribution of semiconductor pathways within the insulating matrix, in which charge accumulation and depletion via a gate bias is substantially different from neat semiconductor, and where high on-current and low off-current are simultaneously realized in the stable doped state. Adding only 5 wt% of a semiconducting polymer to a polystyrene matrix, we realized an environmentally stable inverter with gain up to 60.

Ultrathin body poly(3-hexylthiophene) transistors with improved short-channel performance.

The microstructure and charge transport properties of binary blends of regioregular (rr) and regiorandom (RRa) poly(3-hexylthiophene) (P3HT) are investigated. X-ray diffraction of the blended films is consistent with a vertically separated structure, with rr-P3HT preferentially crystallizing at the semiconductor/dielectric interface. Thin film transistors made with these blended films preserve high field effect mobility with rr-P3HTcontent as low as 5.6%. In these dilute blends, we estimate that the thickness of rr-P3HT in the channel is only a few nanometers. Significantly, as a result of such an ultrathin active layer at the interface, short channel effects due to bulk currents are eliminated, suggesting a new route to fabricate high-performance, short-channel, and reliable organic electronic devices.

PEDOT:gelatin composites mediate brain endothelial cell adhesion.

Conducting polymers (CPs) are increasingly being used to interface with cells for applications in both bioelectronics and tissue engineering. To facilitate this interaction, cells need to adhere and grow on the CP surface. Extracellular matrix components are usually necessary to support or enhance cell attachment and growth on polymer substrates. Here we show the preparation of PEDOT(TOS):gelatin composites as a new biocompatible substrate for use in tissue engineering. Gelatin, a derivative of the extracellular matrix protein collagen, was incorporated into poly(3,4 ethylenedioxythiophene)-tosylate (PEDOT(TOS)) films via vapour phase polymerisation (VPP) without changing the electrochemical properties of the CP. Further, gelatin, incorporated into the PEDOT(TOS) film, was found to specifically support bovine brain capillary endothelial cell adhesion and growth, indicating that the functionality of the biomolecule was maintained. The biocompatibility of the composite films was demonstrated indicating the significant future potential of biocomposites of this type for use in promoting cell adhesion in electrically active materials for tissue engineering.