Martin Meedom Nielsen

High Mobility Ambipolar Charge Transport in Polyselenophene Conjugated Polymers

Adv. Mater. 2010, 22, 2371–2375 2010 WILEY-VCH Verlag G Field-effect transistors (FETs) based on conjugated polymers and small molecules have been of extensive fundamental and practical interest for more than two decades. In terms of fundamental charge transport properties organic semiconductors have been recently shown to be intrinsically ambipolar, i.e., able to accumulate and transport both holes and electrons within the same material under suitable biasing conditions and device configurations. The discovery of the intrinsic ambipolar charge transport properties in common semiconducting polymers was made possible by the understanding of the crucial role played by electronegative trapping groups in the dielectric, such as hydroxyl groups on the surface of a SiO2 gate dielectric. [10] Ambipolar charge transport is not only of fundamental, but also of practical interest as it enables the realization of novel device architectures such as complementary-like voltage inverters with a single organic semiconductor as well as ambipolar light-emitting field-effect transistors (LFETs). Here we report the general observation of ambipolar charge transport characteristics in a series of regioregular polyselenophene-based polymers. Compared to the well-studied polythiophenes, which appear among the most promising solution processable organic semiconductors, polyselenophenes were recently developed as analogue systems providing several advantages over their predecessors. The highest occupied molecular orbital (HOMO) of polythiophenes has little contribution from the sulfur heteroatom, whereas the lowest unoccupied molecular orbital (LUMO) has significant electron density on the heteroatom. Polyselenophenes were initially developed as promising alternatives to polythiophenes for solar cell applications, mainly because of their reduced optical band gaps and their enhanced photostability due to the lower lying LUMO. For FET applications we expect the hole transport to be similar to that of polythiophenes, while the lower lying LUMO in polyselenophenes should result in improved electron transport due to enhanced electron injection from metal electrodes and lower susceptibility of electrons to trap states and oxidation. The regioregular polyselenophenes investigated in this work were: (1) poly(3,300-di-n-alkylterselenophene) (PSSS) of three different alkyl side-chains, namely PSSS-C10, PSSS-C8, and PSSS-C6; and (2) poly(3-octyl)selenophene (P3OS) (Fig. 1). We employed identical top-gate, bottom contact (TGBC) configurations with gold source-drain electrodes for all polymers. For ambipolar FETs the TGBC device configuration offers several advantages over a bottom-gate/bottom-contact (BGBC) configuration: (i) the freedom to select different gate dielectrics to minimize irreversible charge trapping at the semiconductordielectric interface and to act as encapsulation for the FET channel, and (ii) a lower contact resistance due to reduction of current-crowding effects. PSSS is the selenium analogue of the previously reported poly(3,300-dialkylterthiophene) (PTT) with a ‘‘spaced-out’’ distribution of the alkyl side-chains along the polymer backbone. 28] PTT was reported to readily self-assemble into a threedimensional lamellar p-stacking arrangement with an ‘‘edge-on’’

Time-Resolved X-ray Scattering of an Electronically Excited State in Solution. Structure of the 3A2u State of Tetrakis-μ-pyrophosphitodiplatinate(II)

The structure of the (3)A(2u) excited state of tetrakis-mu-pyrophosphitodiplatinate(II) in aqueous solution is investigated by time-resolved X-ray scattering on a time scale from 100 ps to 1 micros after optical pumping. The primary structural parameter, the Pt-Pt distance, is found to be 2.74 A, which is 0.24 A shorter than the ground-state value. The contraction is in excellent agreement with earlier estimates based on spectroscopic data in solution and diffraction data in the crystalline state. As a second structural parameter, the distance between the P planes in the (3)A(2u) excited state was determined to be 2.93 A, i.e., the same as that in the ground state. This result implies that a slight lengthening of the Pt-P bond occurs following excitation.

Structural Tracking of a Bimolecular Reaction in Solution by Time‐Resolved X‐Ray Scattering

Molecular movies: Time-resolved X-ray scattering provides direct structural information on an electronically excited complex while it is formed in the bimolecular reaction between excited octahydrogen[tetrakis-mu-diphosphito-1kappaP:2kappaP-diplatinate](4-) (PtPOP*) and thallium ions. In the exciplex one thallium(I) and two platinum(II) ions are found to be collinear.

Analysis of time-resolved X-ray scattering data from solution-state systems.

As ultrafast time-resolved studies of liquid systems with the laser pump/X-ray scattering probe method have come of age over the past decade, several groups have developed methods for the analysis of such X-ray scattering data. The present article describes a method developed primarily with a focus on determining structural parameters in the excited states of medium-sized molecules (approximately 30 atoms) in solution. The general methodology is set in a maximum-likelihood framework and is introduced through the analysis of the photoactive platinum compound PtPOP, in particular the structure of its lowest triplet excited state ((3)A(2u)). Emphasis is put on structure determination in terms of model comparisons and on the information content of difference scattering signals as well as the related experimental variables. Several suggestions for improving the accuracy of these types of measurements are presented.

Structure of the Buried Metal-Molecule Interface in Organic Thin Film Devices

By use of specular X-ray reflectivity (XR) the structure of a metal-covered organic thin film device is measured with angstrom resolution. The model system is a Langmuir-Blodgett (LB) film, sandwiched between a silicon substrate and a top electrode consisting of 25 A titanium and 100 A aluminum. By comparison of XR data for the five-layer Pb2+ arachidate LB film before and after vapor deposition of the Ti/Al top electrode, a detailed account of the structural damage to the organic film at the buried metal-molecule interface is obtained. We find that the organized structure of the two topmost LB layers (approximately 5 nm) is completely destroyed due to the metal deposition.

Windowless microfluidic platform based on capillary burst valves for high intensity x-ray measurements

We propose and describe a microfluidic system for high intensity x-ray measurements. The required open access to a microfluidic channel is provided by an out-of-plane capillary burst valve (CBV). The functionality of the out-of-plane CBV is characterized with respect to the diameter of the windowless access hole, ranging from 10 to 130 microm. Maximum driving pressures from 22 to 280 mbar corresponding to refresh rates of the exposed sample from 300 Hz to 54 kHz is demonstrated. The microfluidic system is tested at beamline ID09b at the ESRF synchrotron radiation facility in Grenoble, and x-ray scattering measurements are shown to be feasible and to require only very limited amounts of sample, <1 ml/h of measurements without recapturing of sample. With small adjustments of the present chip design, scattering angles up to 30 degrees can be achieved without shadowing effects and integration on-chip mixing and spectroscopy appears straightforward.