Bea M. W. Langeveld-Voss

Two-dimensional charge transport in self-organized, high-mobility conjugated polymers

Self-organization in many solution-processed, semiconducting conjugated polymers results in complex microstructures, in which ordered microcrystalline domains are embedded in an amorphous matrix. This has important consequences for electrical properties of these materials: charge transport is usually limited by the most difficult hopping processes and is therefore dominated by the disordered matrix, resulting in low charge-carrier mobilities (⩽10-5 cm2 V-1 s-1). Here we use thin-film, field-effect transistor structures to probe the transport properties of the ordered microcrystalline domains in the conjugated polymer poly(3-hexylthiophene), P3HT. Self-organization in P3HT results in a lamella structure with two-dimensional conjugated sheets formed by interchain stacking. We find that, depending on processing conditions, the lamellae can adopt two different orientations—parallel and normal to the substrate—the mobilities of which differ by more than a factor of 100, and can reach values as high as 0.1 cm2 V-1 s-1 (refs 3, 4). Optical spectroscopy of the field-induced charge, combined with the mobility anisotropy, reveals the two-dimensional interchain character of the polaronic charge carriers, which exhibit lower relaxation energies than the corresponding radical cations on isolated one-dimensional chains. The possibility of achieving high mobilities via two-dimensional transport in self-organized conjugated lamellae is important for applications of polymer transistors in logic circuits and active-matrix displays.

On the origin of optical activity in polythiophenes

Abstract Optically active substituents attached as sidechain to π-conjugated polymers provide a particularly interesting and revealing tool to study the nanoscopic and mesoscopic supramolecular organization of π-conjugated polymers in general. Chiral substituents allow the properties of these polymers to be studied with circularly polarized light in both absorption and emission. Here we describe a comprehensive investigation on the structure and optical properties of poly{3,4-bis[( S )-2-methylbutoxy]thiophene} (PBMBT) as an example to elucidate the molecular origin of the chiroptical effects in chiral conjugated polymers. The chiral side chains of PBMBT induce very strong bisignate circular dichroism (CD) effects in the π–π ∗ absorption when the polymer is in an aggregated phase. The anisotropy factor in absorption ( g abs ) is as high as 8×10 −2 . The photoluminescence of PBMBT aggregates is characterized by a very small Stokes shift ( ∗ transition of PBMBT is attributed to originate from a helical packing of predominantly planar chains, rather than from a helical intrachain conformation. As a result a detailed molecular description of the polythiophene morphology is obtained.

Microstructure–mobility correlation in self-organised, conjugated polymer field-effect transistors

Abstract We have investigated the correlation between polymer microstructure and charge carrier mobility in high-mobility, self-organised field-effect transistors of poly-3-hexyl-thiophene (P3HT). Two different preferential orientations of the microcrystalline P3HT domains with respect to the substrate have been identified by X-ray diffraction, for which charge carrier mobilities differ by a factor >100. High mobilities of 0.1 cm 2 /V s reflect efficient interchain transport along the direction of π–π stacking. The results provide a firm basis for exploring supramolecular self-organisation to enhance the charge transport properties of conjugated polymer field-effect devices.

Investigation of Exciton Coupling in Oligothiophenes by Circular Dichroism Spectroscopy

The solid-state photophysical properties of semiconducting polymers result from the interplay between the intrinsic features of the polymer chains and interchain interactions. Exciton coupling, which in principle can be estimated from circular dichroism (CD) spectra, is one of the major interchain interactions affecting the optical properties of semiconducting polymers. Here the authors present a combined experimental and theoretical investigation of the chiroptical properties of oligothiophenes. The results confirm that the CD spectrum provides an independent estimate of the exciton coupling.

Chiroptical properties of highly ordered di[(S)-2-methylbutoxy] substituted polythiophene and poly(p-phenylene vinylene)

Circular dichroism (CD) spectroscopy is used to study the nature of the excited state of optically active di[(S)-2-methylbutoxy] substituted polythiophene (PDMBT) in small aggregates with a high degree of intrachain and interchain order. The observed CD effects indicate oscillator coupling between chains in the excited state. The photoluminescence of the PDMBT aggregates is characterized by a very small Stokes shift (< 0.09 eV) and is circularly polarized but less intense than of free PDMBT chains in solution. The decrease of luminescence efficiency in the aggregated phase can be explained by the exciton coupling observed in CD and results in a non-radiative low-energy state. Similar results are obtained for optically active poly(p-phenylene vinylene).

Convenient Synthesis of 3,4-bis(Alkylthio) Thiophenes

Abstract A variety of symmetrically substituted 3,4-dithioether-thiophenes were prepared by the acid catalyzed nucleophilic substitution of 3,4-di(methoxy)thiophene with the appropriate thiol.

Photoinduced singlet and triplet energy transfer in fullerene-oligothiophene-fullerene triads

Photophysical properties of fullerene-oligothiophene-fullerene (C 60 -nT-C 60 ) triads with n = 3, 6, or 9 thiophene units have been investigated using photoinduced absorption (PIA) and (time-resolved) fluorescence spectroscopy in toluene and compared to mixtures of oligothiophenes (nT) with N-methylfulleropyrrolidine (MP-C 60 ). Photoexcitation of the oligothiophene moiety of the C 60 -nT-C 60 triads in toluene results in a very fast singlet-energy transfer reaction to the fullerene moiety with rates varying between 10 12 and 10 13 s -1 . In the mixtures, an inter-molecular triplet-energy transfer reaction occurs. The preference for intra- and inter-molecular energy instead of electron transfer reactions in these triads in toluene is in full agreement with predictions that can be made for the change in free energy upon charge separation.

Time-resolved microwave measurements of the polarizability of photoexcitations on conjugated polymer chains

Changes as small as 1 ppm in the real and imaginary components of the complex permittivity resulting from flash photoexcitation of dilute solutions of (pi) -bond conjugated oligomers and polymers can be measured with nanosecond time resolution using the time-resolved microwave conductivity technique. The results provide information on the extent of delocalization of singlet and triplet state photoexcitations. Data are presented for oligomeric and polymeric derivatives of phenylene vinylene, paraphenylene and thiophene. For the polymers excess polarizability volumes, (Delta) Vp, of the singlet exciton of 2000, 180 and 570 angstroms3 are found, respectively. (Delta) Vp for the triplet exciton of the polythiophene derivative is found to be only 25 angstroms3.

Odd–even effect in optically active poly(3,4-dialkoxythiophene)

An odd–even effect is observed in the CD spectra of a homologous series of aggregated optically active poly(3,4-dialkoxythiophene)s, suggesting a helical packing of polymer chains into chiral superstructures, similar to that of the cholesteric liquid crystalline phase.

Photoinduced energy and electron transfer in a C60–6T–C60 triad

Photoinduced energy and electron transfer in a fullerene-sexithiophene-fullerene (C60-6T-C60) triad in solvents of different polarity is studied using photoinduced absorption (PIA) and photoluminescence (PL) spectroscopy. The polarity of the solvent has an essential role in the formation and stabilization of a photoinduced charge-separated state. Intramolecular electron transfer is shown to occur in a two-step process, involving singlet-energy transfer prior to charge separation.