M. Theander

Substituted polythiophenes designed for optoelectronic devices and conductors

Polythiophenes are a very versatile class of conjugated polymers. Substituted polythiophenes can be tailored for various applications by designing the side groups to give the polymer different desired properties. Our work on preparing polythiophenes designed to have high stability in the doped state is described. We also discuss our efforts on tuning the colour of the emission from polythiophenes for use in polymer light-emitting diodes. Design criteria for the synthesis of polythiophenes with high luminescence efficiency for use in light-emitting diodes and lasers are also described. Finally, the design of polythiophenes for use in photodiodes is discussed.

A polythiophene microcavity laser

We report photopumped lasing in a microcavity device with a polythiophene layer as the emitter. These microcavity devices are built by joining two polymer coated dielectric mirrors at elevated temperature. When photopumping the film, a lasing threshold is observed at 120 nJ/cm(2). Comparative studies with fast pump-probe spectroscopy of thin polythiophene films and the same polymer in photopumped lasing studies, indicate that the gain coefficient is 80 +/- 20 cm(-1), and that the exciton concentration is 2 X 10(17) cm(-3) at the lasing transition, well below the exciton-exciton recombination level

Ultrafast photogeneration of inter-chain charge pairs in polythiophene films

Photoexcitation dynamics in films of polythiophenes with different side groups were studied by transient absorption spectroscopy using 80 fs pulses. We show that inter-chain charge pairs (CP) are generated with similar to 20% efficiency in a polymer with short inter-chain distance (similar to 4 Angstrom) within 100 fs after photoexcitation. We discuss two mechanisms for inter-chain charge separation: (i) electron or hole transfer from an initially excited intra-chain singlet state or (ii) optical excitation of mixed exciton-charge transfer states, which quickly evolve to inter-chain CPs

Conformational disorder of a substituted polythiophene in solution revealed by excitation transfer

Anisotropy and isotropic transient absorption decays of a polythiophene derivative in dilute solution have been measured and compared to calculated decays on Monte Carlo simulated polymer chains to obtain knowledge about the geometrical conformation of the polymer. The polymer chain conformations are generated by a partially correlated self-avoiding random walk. Excitation migration along the isolated chain is modeled assuming a hopping mechanism. This approach can be used to determine conformational disorder and possible realizations of the polymer chains in solution as well as the trapping time and migration length of the excitation.

Synthesis of regioregular phenyl substituted polythiophenes with FeCl3

Abstract The versatility of the regioregular polymerization of substituted 3-phenylthiophenes with FeCl3 was investigated. Regioregular and soluble polythiophenes with alkyl, alkoxy or diethylene glycol sidechains on the phenyl ring have been prepared. Some of the polymers have relatively low bandgaps and some have high photoluminescence efficiencies.

Energy transfer in a conjugated polymer with reduced inter-chain coupling

Energy transfer in spin-coated films and solutions of poly(dioctylphenylthiophene) with bulky side chains has been studied by femtosecond pump-probe spectroscopy, polarisation anisotropy and excitation annihilation. A red shift of stimulated emission spectra with time and a gradual decay of anisotropy are observed in film and solution and attributed to excitation migration towards lower-energy sites. At high-excitation intensities singlet-singlet annihilation with time-independent rate dominates the decay of excited-state population in the film for short times. It is concluded that energy migration in film, which results in effective excitation trapping and nonradiative decay, is a three-dimensional process, while in solution a one-dimensional intra-chain energy migration is much more limited.

Lasing in a microcavity with an oriented liquid-crystalline polyfluorene copolymer as active layer

Lasing in a microcavity with an oriented liquid-crystalline polyfluorene copolymer as active layer

Luminescence quenching by inter-chain aggregates in substituted polythiophenes

Time-resolved photo luminescence spectra measured in solid films of two polythiophene derivatives with different chain packing allow to distinguish emission of intra-chain excitations from the luminescence of inter-chain aggregates. Aggregate luminescence is red shifted by about 0.1 eV relative to intra-chain emission and shows vibronic coupling to the C=C bond stretch with the Huang-Phys factor of S approximate to 1.5, which is twice bigger than that of the intra-chain emission. Combining time resolved luminescence data with femtosecond transient absorption, we show that the dynamic quenching of the luminescence in films with dense chain packing is mainly due to excitation energy transfer to aggregates. The radiative lifetime of the lowest excited state of the aggregate with the optical gap of 1.84-1.9 eV is estimated to be about 20 ns. The aggregate contribution to the total luminescence in the polythiophene films with dense chain packing is about 50% and does not change significantly with temperature

Optical emission from confined poly(thiophene) chains

We discuss the stages of the luminescence processes in substituted and soluble poly(thiophenes), using results from determinations of absolute quantum yields of photoluminescence, fast pump-probe experiments and microcavity devices. Enhancement of the quantum yield of photoluminescence, with poly(thiophenes) in the solid state, call be obtained by dispersing the conjugated chain in a molecularly dispersed polymer blend, or by adding side chains designed for forcing the conjugated main chains apart. At the wavelength of stimulated emission, we observe a narrow bandwidth emission in the high-Q one-dimensional microcavity devices prepared by sandwiching two dielectric mirrors around a thin polymer film, A strong enhancement of the power efficiency is observed at a pump power of 1 mu W/cm(2), but does not conclusively show lasing characteristics

New monomers for polythiophenes

Two series of chalcogen substituted thiophene monomers have been synthesised; 3-(4-alkylchalcogenophenyl)thiophenes 1a-c and 3-(4-alkylphenylchalcogeno)thiophenes 2a-d. Polymerisations of examples of these with iron(III)chloride gave regiorandom polymers. Light-emitting diodes with low efficiency could be fabricated from poly-1a and poly-1b.