Sebastian Westenhoff

Exciton migration in a polythiophene: Probing the spatial and energy domain by line-dipole Forster-type energy transfer

We study exciton migration in low molecular weight poly[3-(2,5-dioctylphenyl)thiophene] in dilute solution by means of ultrafast spectroscopy and Monte Carlo simulations of resonance energy transfer using the line-dipole Forster approach. The model includes the build-up of polymer chains, site-selective exciton generation, and diffusion through incoherent energy transfer. Time-resolved, ensemble-averaged experimental data are reproduced, namely photoluminescence spectral migration and stimulated emission anisotropy decays measured by streak camera and femtosecond transient absorption spectroscopy under site-selective excitation conditions. Importantly, the relatively simple line-dipole Forster-type approach beyond the point-dipole approximation reproduces both experiments quantitatively. Since explicit chain conformations are used in the model, the simulations yield a descriptive microscopic picture of exciton migration. The effective conjugation length (l(seg)=2.9 nm, 7.4 monomer units) and the disorder of the chains (Omega=0.8) are yielded as the only fitting parameters. We find an extra component that is not covered by our fits in anisotropy decays at early times for high excitation energies. This is interpreted within the context that the effective conjugation is limited by conformational disorder.

Conformational disorder of conjugated polymers

Conformational disorder of conjugated polymers is an important issue to be understood and quantified. In this paper we present a new method to assess the chain conformation of conjugated polymers based on measurements of intrachain energy transfer. The chain conformation is modeled on the basis of monomer-monomer interactions, such as torsion, bending, and stretching of the connecting bond. The latter two potentials are assumed to be harmonic, while the torsional potential was calculated by density functional theory using B3-LYP functional with the SVP basis set. The energy transfer dynamics of excitons on these chains are quantitatively simulated using Forster-type line-dipole energy transfer. This allows us to compare the simulated ground state conformation of single polymer chains to ultrafast depolarization experiments of poly [3-(2,5-dioctylphenyl)thiophene] in solution. We identify torsional rotation as the main contributor to conformational disorder and find that this disorder is mainly controlled by the energy difference between syn and anti bonds.

Dynamics of Excited States and Charge Photogeneration in Organic Semiconductor Materials

Organic semiconductor materials are attracting increasing interest for application as active materialsin large-area displays, light-emitting diodes, transistors, photodetectors, solar cells, etc. Elementaryprocesses like excitation energy transfer, charge photogeneration, charge transport, and photoluminescencequenching are at the basis of the function of the materials. In order to design new materials and optimizetheir function, a detailed knowledge of the various light-induced processes is required. In this contributionwe discuss how ultrafast spectroscopy can be used to study excited-state relaxation and quenching, energytransfer, and charge generation in neat conjugated polymers as well as in polymer–C60 blends.

Charge transfer and recombination at conjugated polymer-semiconductor nanoparticle interfaces

We study the processes of charge transfer and recombination at the interface between semiconductor nanoparticles and conjugated polymers. These processes are crucial in determining the performance of photovoltaic devices based on these materials. Using femtosecond transient absorption we are able to follow the charge separation on picosecond timescales in blends of spherical CdSe nanocrystals with a poly(p-phenylenevinylene) derivative. Charge separation occurs on timescales of greater than 15 ps, indicating that it is limited by the diffusion of excitons to the nanoparticle interface. We also use time-resolved photoluminescence and quasi-steady-state photoinduced absorption measurements to study the vertical structure in films containing conjugated polymers and semiconductor tetrapods. Finally, we demonstrate that use of slow-evaporating solvents allows the formation of fibrilar structures in poly(3-hexylthiophene) films, and that this is correlated with improved performance in photovoltaic devices containing poly(3-hexylthiophene) and CdSe nanorods.

Charge Generation in Inorganic/Organic Photovoltaic Blends

We apply ultrafast spectroscopy to investigate charge generation kinetics in blends of semiconducting polymers and cadmium selenide nanocrystals with photovoltaic applications. The rate is limited by diffusion of the primary excitons in the polymer network.