Martin Grell

X-ray diffraction study of the structure of thin polyfluorene films

The molecular arrangement in thin films of poly(9,9-dioctylfluorene) and poly(9,9-dihexylfluorene) deposited on silicon substrates has been investigated with grazing incidence X-ray diffraction. In particular, the effect of the interface on the molecular orientation is highlighted. Both materials display a periodicity normal to the surface arising from stacked sheets of fluorene chains in both the crystalline and liquid crystalline phases. For the crystalline phase, a periodicity in the plane of the surface of 4.15 A is observed corresponding to half the fluorene ring repeat distance along the backbone, consistent with interdigitating side-chains. For crystalline films deposited onto rubbed polyimide films, strong orientation effects are observed. In the liquid-crystalline phase, this strong in-plane ordering of backbones is lost. Poly(9,9-dihexylfluorene) exhibits an additional degree of ordering in the plane of the interface, which is likely to arise from hexagonal ordering of the backbone chains.

Electron transporting water-gated thin film transistors

We demonstrate an electron-transporting water-gated thin film transistor, using thermally converted precursor-route zinc-oxide (ZnO) intrinsic semiconductors with hexamethyldisilazene (HMDS) hydrophobic surface modification. Water gated HMDS-ZnO thin film transistors (TFT) display low threshold and high electron mobility. ZnO films constitute an attractive alternative to organic semiconductors for TFT transducers in sensor applications for waterborne analytes. Despite the use of an electrolyte as gate medium, the gate geometry (shape of gate electrode and distance between gate electrode and TFT channel) is relevant for optimum performance of water-gated TFTs.

Growth of nanocrystals and thin films at the water–oil interface

The use of the water–oil interface provides significant advantages in the synthesis of inorganic nanostructures. Employing the water–toluene interface, luminescent CdS nanocrystals have been obtained at a relatively modest temperature of 35°C. The diameters of the particulates can be varied between 1.0 and 5.0 nm. In addition, we have devised a new method for transferring thin films at the water–toluene interface onto solid substrates. Using this method, thin films consisting of Au and Ag nanocrystals spread over very large areas (square centimetres) are obtained in a single step. These films are directly usable as ingredients of functional devices. We show this by constructing a working amine sensor based on films of Au nanocrystals. The materials obtained have been characterized by X-ray diffraction, scanning and transmission electron microscopy, absorption and emission spectroscopy and charge transport measurements.

An ionic liquid-gated polymer thin film transistor with exceptionally low “on” resistance

We report the ionic liquid (IL) gating of a solution processed semiconducting polymer, poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT). IL gating relies on the poor solubility of PBTTT, which requires hot chlorinated benzenes for solution processing. PBTTT, thus, resists dissolution even in IL, which otherwise rapidly dissolves semiconducting polymers. The resulting organic thin film transistors (OTFTs) display low threshold, very high carrier mobility (>3 cm2/Vs), and deliver high currents (in the order of 1 mA) at low operational voltages. Such OTFTs are interesting both practically, for the addressing of current-driven devices (e.g., organic LEDs), and for the study of charge transport in semiconducting polymers at very high carrier density.

Organic solvents as gate media for thin-film transistors

Here, we show that some organic solvents can act as electric double layer (EDL) gate media for thin film transistors (TFTs), in a similar way to solid electrolytes, ionic liquids, and water. The ability to act as gate medium is a property of the solvent, not a property of the semiconductor used in the TFT. Only solvents that are fully miscible with water can act as a gate media. However, the ability to undergo autoprotolysis is not an essential property of a gate medium. The most likely explanation for the ability of water-miscible solvents to act as EDL gate media is the inevitable presence of trace amounts of dissolved salts. Because of its large electrochemical window, and aprotic character, acetonitrile is particularly attractive as an alternative gate medium.

A water-gated organic thin film transistor as a sensor for water-borne amines

The p-type semiconducting polymer Poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) displays innate sensitivity to water-borne amines. We demonstrate this with the help of water-gated PBTTT thin film transistors (TFTs). When octylamine is added to the gating water, TFTs respond with a significantly reduced saturated drain current. Underlying TFT drift is minimised by initial conditioning, and remaining drift can be accounted for by normalising current response to the current level under purge immediately before exposure. Normalised current response vs. amine concentration is reproducible between different transistors, and can be modelled by a Langmuir surface adsorption isotherm, which suggests physisorption of analyte at the PBTTT surface, rather than bulk penetration. Same PBTTT transistors do not respond to 1- octanol, confirming the specific affinity between amines and thiophene- based organic semiconductors.

Intensity-Modulated Spectroscopy on Loaded Organic Photovoltaic Cells

We configured a generic digital lock-in amplifier as a light intensity-modulated spectrometer for photovoltaic (PV) cells for intensity-modulated spectroscopy (IMS) up to 250 kHz. We performed IMS on a state-of-the-art bulk heterojunction (BHJ) organic PV (OPV) cell and introduced a new mode of IMS, wherein PV cells work under finite load, including maximum power point (MPP). Quantitative analysis supported by equivalent circuit simulations establishes MPP-IMS as favorable alternative to the commonly used intensity-modulated photovoltage/photocurrent spectroscopy (IMVS/IMPS) modes. Using IMS under finite load, we identify a high-frequency feature that is invisible in both IMPS and IMVS. The feature is ageing-related and becomes more prominent after long-term storage. We propose an extended equivalent circuit model that locates the origin of this feature at the BHJ itself and link it to diffusion of indium ions etched from the transparent electrode by the hole extracting PEDOT:PSS. Finally, we introduce a method to determine BHJ capacitance by IMS without absolute calibration of light intensity.

Innate cation sensitivity in a semiconducting polymer

Water-gated organic thin film transistors (OTFTs) using the hole transporting semiconducting polymer, poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT), show an innate response of their threshold voltage to the addition of divalent metal cations to the gating water, without deliberately introducing an ion-sensitive component. A similar threshold response is shown for several divalent cations, but is absent for monovalent cations. Response is absent for transistors using the inorganic semiconductor ZnO, or the similar organic semiconductor poly(3-hexylthiophene) (rrP3HT), instead of PBTTT. We assign innate cation sensitivity to residues of the organometallic Pd(0) complex used as catalyst in PBTTT synthesis which bears strong resemblance to typical metal chelating agents. Organometallic Pd(0) residues are absent from ZnO, and also from rrP3HT which is polymerised with a different type of catalyst. However, when Pd(0) complex is deliberately added to rrP3HT casting solutions, resulting OTFTs also display threshold response to a divalent cation.

A New Precursor Route to Semiconducting Zinc Oxide

We demonstrate a new precursor route toward solution-processed films of the II-VI semiconductor zinc oxide (ZnO). Spray pyrolysis of aqueous solutions of the zinc salt zinc chloride (ZnCl2) onto a substrate heated to at least 250 °C gives films that are insoluble in water, display an absorption edge at 365 nm, and when electrically gated display thin film transistor characteristics similar to ZnO films derived via the established zinc acetate (ZnAc) precursor route; we, therefore, identify it as ZnO. Control experiments attempting spray pyrolysis of aqueous zinc sulfate solutions, and delayed pyrolysis of cold-sprayed and dried ZnCl2 films, do not lead to semiconducting films. Formation of ZnO from an aqueous Zinc salt requires the simultaneous presence of zinc ions, chloride ions, and water, at the time of pyrolysis. We, therefore, suggest that the actual ZnO precursor is the ZnClxH2O(4-x) species that forms when ZnCl2 dissolves in water [The Journal of Chemical Physics 39, 3436 (1963)]. The reported process is easy to upscale for large area ZnO coatings, e.g., on window panes for thermal control, as no organic solvent vapors are released.

A swelling-based chemiresistor for a biogenic odour

Escherichia coli bacteria release 1-decanol as a byproduct of their metabolism. We demonstrate the detection of 1-decanol odour at a partial pressure in the order 100 ppb by the resistance change of a swelling-based sensor, consisting of Langmuir-Schäfer deposited Au core/organic ligand shell nanoparticle films. This is an exceptionally low limit of detection for swelling-based sensors, and relies firstly, in the careful matching of the CSNPs ligands to the targeted odour, and secondly, in the very low volatility of this odour. Sensor response can be substantially increased further when films are cooled below the freezing point of 1-decanol. We observe unexpected quantitative behaviour of our sensors: response is only weakly dependent on the odours partial pressure, and scales differently with temperature than the response of other Au-CSNP odours to more volatile odours. This may be related to their unusually strong thermal resistance drift, the difficulties in delivering very low partial pressure odour atmospheres, and the proximity to the analytes freezing point.