Dirk Beckmann

Rational optimization of benzo[2,1-b;3,4-b']dithiophene-containing polymers for organic field-effect transistors.

Adv. Mater. 2010, 22, 83–86 2010 WILEY-VCH Verlag Gmb Thin-film transistors based on organic semiconductors have attracted a lot of attention in academic and industrial research in recent years. First commercial applications are about to be launched into new products which cannot be realized with conventional, expensive, and energy-demanding silicon-based electronics. Technologies using organic materials promise large area devices, which are light weight, flexible, and come at low cost. Experts predict a huge demand for devices such as radio frequency identification tags (RFIDs) for single item tagging or other integrated circuits. The search for new materials which meet the requirements for these applications is, therefore, of great importance. These crucial requirements are (i) good solution processability especially on flexible polymer substrates, (ii) short annealing times at low temperatures to enable high-speed processing such as mass printing, (iii) high operational stability at ambient conditions to avoid the necessity of costly barrier layers, (iv) high charge-carrier mobilities, and (v) a facile and economically feasible synthesis which can be scaled up for production. Semiconducting polymers show a microcrystalline or amorphous superstructure simplifying the fabrication of homogeneous films over large area, which makes this class very promising. The charge-carrier mobilities are usually not as high as for small molecules but still sufficient for most applications. Thiophene-containing polymers have proven to be most effective. This paper presents the incorporation of benzo[2,1-b; 3,4-b0]dithiophene into a polythiophene as active material in a field-effect transistor (FET). The structure motif of a 2,5 connected thiophene, which is found, for example, in the well-known poly-3-hexylthiophene (P3HT), is modified by a benzo annellation. This reduces the ionization potential of the polymer and is thus expected to improve air stability under operation. The extended p system should, furthermore, help in forming highly ordered films which are needed to transport charge-carriers efficiently. The first approach toward a polymer-containing benzo[2,1-b;3,4-b0]dithiophene is the iron(III) chloride mediated oxidative polymerization of benzodithiophene having solubilizing alkyl chains attached to the benzo unit. In this way polymer P1 (Scheme 1) is obtained with a molecular weight of Mn1⁄4 18 kgmol 1 as determined by size exclusion chromatography in 1,2,4-trichlorobenzene at 135 8C in reference to polystyrene as standard. Good solubility in dichlorobenzene enables the polymer to be tested as the active material in an FET. The polymer is spin cast from 1,2-dichlorobenzene solution onto a silicon wafer and dried at 100 8C for 5min. Finally, top-contact gold electrodes are deposited by thermal evaporation. Good device characteristics can be recorded, showing low charge-carrier mobility in the 10 4 cm V 1 s 1 region. The organization in the thin film is probed with X-ray reflection diffraction revealing a low organization of the macromolecules. Even better insight into the organization behavior is gained by X-ray scattering of an extruded polymer fiber. In addition to the low degree of order, a p–p distance of 0.43 nm is detected, a large value in comparison to highperformance polymers reported in the literature and unfavorable for high mobility. It seems that polymer P1 is too stiff to obtain high order; more structural flexibility along the backbone is needed. For this reason, additional thiophene units bearing alkyl chains are incorporated into the backbone yielding polymer P2, a general concept known in the literature. By optimization of the polymerization conditions, a comparable molecular weight as for polymer P1 could be obtained (Mn1⁄4 22 kg mol ), minimizing field-effect mobility differences due to different polymer chain lengths. FETs are prepared following exactly the same procedure as for P1 to ensure maximum comparability. Average charge-carrier mobility in the low 10 3 cm V 1 s 1 region is measured for polymer P2. In fact, higher order and decreased p–p distance (0.39 nm) in comparison to polymer P1 are determined by fiber X-ray scattering. Obviously, the gain in backbone flexibility proves to be favorable, but the electronic performance is still not satisfactory. As polymer P2 possesses high solubility in organic solvents, fewer alkyl chains seemed possible to still guarantee the necessary solubility for processing. Therefore, polymer P3 is synthesized having no alkyl chains at the additional thiophene units to avoid ‘‘overalkylation’’ which may hinder close contact of adjacent polymer chains. Since using dodecyl chains as for polymer P1 renders an insoluble polymer, longer hexadecyl

Thiadiazoloquinoxaline–Acetylene Containing Polymers as Semiconductors in Ambipolar Field Effect Transistors

Two conjugated copolymers, PPTQT and PTTQT, were developed based on thiadiazoloquinoxalines connected via ethynylene π-spacer to thiophene units. PPTQT showed maximum hole and electron mobility of 0.028 and 0.042 cm(2)/V s, respectively, being the first example of an ambipolar semiconducting material bearing triple bonds in the polymer backbone.

An Extended Polythiophene with Interesting Core Synthesis

conjugated polymers base-induced cyclization polythiophenes R . R I EG E R , D . BE CK M A N N , W. P I S U L A , M . K A S T L E R, K . M Ü L L E N * ( M A X P L A N C K I N S T I T U T E F O R P O L Y M E R R E S E A R C H , M A I N Z , G E RM A N Y ) Tetrathiahexacene as Building Block for Solution-Processable Semiconducting Polymers: Exploring the Monomer Size Limit Macromolecules 2010, 43, 6264-6267.

Curvature as design concept for semiconducting benzodithiophene-containing polymers in organic field-effect transistors

Organic semiconductors are expected to play an important role in low-cost and energy-efficient electronic products, especially transistors. Our concept to improve the active materials is to introduce curvature into the polymer backbone. The beneficial contribution of the curvature is shown by a series of five isomeric polymers which demonstrate that an intermediate degree of curvature is the optimum compromise between good solubility and high order in the film. In this way, a polythiophene is developed that is easily synthesized, well soluble, exhibits high charge-carrier mobility using mild annealing conditions, and is stable under operation. When processed on a flexible substrate, it shows average mobilities as high as 0.5 cm2V-1s-1 .