Jana Zaumseil

General observation of n-type field-effect behaviour in organic semiconductors

Organic semiconductors have been the subject of active research for over a decade now, with applications emerging in light-emitting displays and printable electronic circuits. One characteristic feature of these materials is the strong trapping of electrons but not holes: organic field-effect transistors (FETs) typically show p-type, but not n-type, conduction even with the appropriate low-work-function electrodes, except for a few special high-electron-affinity or low-bandgap organic semiconductors. Here we demonstrate that the use of an appropriate hydroxyl-free gate dielectric—such as a divinyltetramethylsiloxane-bis(benzocyclobutene) derivative (BCB; ref. 6)—can yield n-channel FET conduction in most conjugated polymers. The FET electron mobilities thus obtained reveal that electrons are considerably more mobile in these materials than previously thought. Electron mobilities of the order of 10-3 to 10-2 cm2 V-1 s-1 have been measured in a number of polyfluorene copolymers and in a dialkyl-substituted poly(p-phenylenevinylene), all in the unaligned state. We further show that the reason why n-type behaviour has previously been so elusive is the trapping of electrons at the semiconductor–dielectric interface by hydroxyl groups, present in the form of silanols in the case of the commonly used SiO2 dielectric. These findings should therefore open up new opportunities for organic complementary metal-oxide semiconductor (CMOS) circuits, in which both p-type and n-type behaviours are harnessed.

Contact resistance in organic transistors that use source and drain electrodes formed by soft contact lamination

Soft contact lamination of source/drain electrodes supported by gold-coated high-resolution rubber stamps against organic semiconductor films can yield high-performance organic transistors. This article presents a detailed study of the electrical properties of these devices, with an emphasis on the nature of the laminated contacts with the p- and n-type semiconductors pentacene and copper hexadecafluorophthalocyanine, respectively. The analysis uses models developed for characterizing amorphous silicon transistors. The results demonstrate that the parasitic resistances related to the laminated contacts and their coupling to the transistor channel are considerably lower than those associated with conventional contacts formed by evaporation of gold electrodes directly on top of the organic semiconductors. These and other attractive features of transistors built by soft contact lamination suggest that they may be important for basic and applied studies in plastic electronics and nanoelectronic systems based ...

Dual electron donor/electron acceptor character of a conjugated polymer in efficient photovoltaic diodes

The authors report efficient photovoltaic diodes which use poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl) (F8TBT) both as electron acceptor, in blends with poly(3-hexylthiophene), and as hole acceptor, in blends with (6,6)-phenyl C61-butyric acid methyl ester. In both cases external quantum efficiencies of over 25% are achieved, with a power conversion efficiency of 1.8% under simulated sunlight for optimized F8TBT/poly(3-hexylthiophene) devices. The ambipolar nature of F8TBT is also demonstrated by the operation of light-emitting F8TBT transistors. The equivalent p- and n-type operation in this conjugated polymer represent an important extension of the range of useful n-type materials which may be developed.

Expanding the chemical versatility of colloidal nanocrystals capped with molecular metal chalcogenide ligands.

We developed different strategies toward the synthesis of colloidal nanocrystals stabilized with molecular metal chalcogenide complexes (MCCs). Negatively charged MCCs, such as SnS(4)(4-), Sn(2)S(6)(4-), SnTe(4)(4-), AsS(3)(3-), MoS(4)(2-), can quantitatively replace the organic ligands at the nanocrystal surface and stabilize nanocrystal solutions in different polar media. We showed that all-inorganic nanocrystals composed of metals, semiconductors, or magnetic materials and capped with various MCC ligands can be synthesized using convenient and inexpensive chemicals and environmentally benign solvents such as water, formamide, or dimethylsulfoxide. The development of mild synthetic routes was found to be crucial for the design of highly luminescent all-inorganic nanocrystals, such as CdSe/ZnS and PbS capped with Sn(2)S(6)(4-) MCCs, respectively. We also prepared conductive and luminescent layer-by-layer assemblies from inorganically capped colloidal nanocrystals and polyelectrolytes. In close-packed films of 5-nm Au nanocrystals stabilized with Na(2)Sn(2)S(6) we observed very high electrical conductivities (>1000 S cm(-1)).

Nanoscale organic transistors that use source/drain electrodes supported by high resolution rubber stamps

Soft contact lamination and metal-coated elastomeric stamps provide the basis for a convenient and noninvasive approach to establishing high resolution electrical contacts to electroactive organic materials. The features of relief on the stamps define, with nanometer resolution, the geometry and separation of electrically independent electrodes that are formed by uniform, blanket evaporation of a thin metal film onto the stamp. Placing this coated stamp on a flat substrate leads to “wetting” and atomic scale contact that establishes efficient electrical connections. When the substrate supports an organic semiconductor, a gate dielectric and a gate, this soft lamination process yields high performance top contact transistors with source/drain electrodes on the stamp. We use this approach to investigate charge transport through pentacene in transistor structures with channel lengths that span more than three decades: from 250 μm to ∼150 nm. We also report some preliminary measurements on charge transport through organic monolayers using the same laminated transistor structures.

Quantum efficiency of ambipolar light-emitting polymer field-effect transistors

The emission characteristics and external quantum efficiencies of ambipolar polymer light-emitting field-effect transistors are investigated as a function of applied voltage, current density, and ratio of hole to electron mobility. Green-emitting poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) with balanced electron and hole mobilities and red-emitting poly((9,9-dioctylfluorene)-2,7- diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl) (F8TBT) with strongly unbalanced hole and electron mobilities as semiconducting and emissive polymers are compared. The current-voltage and light output characteristics of the two types of light-emitting transistors were found to be fundamentally alike independent of mobility ratio. Device modeling allowing for a single (Langevin-type) charge recombination mechanism was able to reproduce the device characteristics for both cases but could not replicate the experimentally observed dependence of external quantum efficiency on current density. The ...

Enhanced ambipolar charge injection with semiconducting polymer/carbon nanotube thin films for light-emitting transistors.

We investigate the influence of small amounts of semiconducting single-walled carbon nanotubes (SWNTs) dispersed in polyfluorenes such as poly(9,9-di-n-octylfluorene-alt-benzothiadiazole (F8BT) and poly(9,9-dioctylfluorene) (F8) on device characteristics of bottom contact/top gate ambipolar light-emitting field-effect transistors (LEFETs) based on these conjugated polymers. We find that the presence of SWNTs within the semiconducting layer at concentrations below the percolation limit significantly increases both hole and electron injection, even for a large band gap semiconductor like F8, without leading to significant luminescence quenching of the conjugated polymer. As a result of the reduced contact resistance and lower threshold voltages, larger ambipolar currents and thus brighter light emission are observed. We examine possible mechanisms of this effect such as energy level alignment, reduced bulk resistance above the contacts, and field-enhanced injection at the nanotube tips. The observed ambipolar injection improvement is applicable to most conjugated polymers in staggered transistor configurations or similar organic electronic devices where injection barriers are an issue.

Polymer-sorted semiconducting carbon nanotube networks for high-performance ambipolar field-effect transistors.

Efficient selection of semiconducting single-walled carbon nanotubes (SWNTs) from as-grown nanotube samples is crucial for their application as printable and flexible semiconductors in field-effect transistors (FETs). In this study, we use atactic poly(9-dodecyl-9-methyl-fluorene) (a-PF-1-12), a polyfluorene derivative with asymmetric side-chains, for the selective dispersion of semiconducting SWNTs with large diameters (>1 nm) from plasma torch-grown SWNTs. Lowering the molecular weight of the dispersing polymer leads to a significant improvement of selectivity. Combining dense semiconducting SWNT networks deposited from an enriched SWNT dispersion with a polymer/metal-oxide hybrid dielectric enables transistors with balanced ambipolar, contact resistance-corrected mobilities of up to 50 cm2·V–1·s–1, low ohmic contact resistance, steep subthreshold swings (0.12–0.14 V/dec) and high on/off ratios (106) even for short channel lengths (<10 μm). These FETs operate at low voltages (<3 V) and show almost no current hysteresis. The resulting ambipolar complementary-like inverters exhibit gains up to 61.

Electroluminescence from Electrolyte-Gated Carbon Nanotube Field-Effect Transistors

We demonstrate near-infrared electroluminescence from ambipolar, electrolyte-gated arrays of highly aligned single-walled carbon nanotubes (SWNT). Using electrolytes instead of traditional oxide dielectrics in carbon nanotube field-effect transistors (FET) facilitates injection and accumulation of high densities of holes and electrons at very low gate voltages with minimal current hysteresis. We observe numerous emission spots, each corresponding to individual nanotubes in the array. The positions of these spots indicate the meeting point of the electron and hole accumulation zones determined by the applied gate and source-drain voltages. The movement of emission spots with gate voltage yields information about relative band gaps, contact resistance, defects, and interaction between carbon nanotubes within the array. Introducing thin layers of HfO(2) and TiO(2) provides a means to modify exciton screening without fundamentally changing the current-voltage characteristics or electroluminescence yield of these devices.

High-mobility ZnO nanorod field-effect transistors by self-alignment and electrolyte-gating.

High mobility, solution-processed field-effect transistors are important building blocks for flexible electronics. Here we demonstrate the alignment of semiconducting, colloidal ZnO nanorods by a simple solvent evaporation technique and achieve high electron mobilities in field-effect transistors at low operating voltages by electrolyte-gating with ionic liquids. The degree of alignment varies with nanorod length, concentration and solvent evaporation rate. We find a strong dependence of electron mobility on the degree of alignment but less on the length of the nanorods. Maximum field-effect mobilities reach up to 9 cm(2) V(-1) s(-1) for optimal alignment. Because of the low process temperature (150 °C), ZnO nanorod thin films are suitable for application on flexible polymer substrates.