Ultrafast holography and transient-absorption spectroscopy in charge-transfer polymers

Charge-transfer polymers are a new class of nonlinear optical materials which can be used for generating femtosecond holographic gratings. Using semiconducting polymers sensitized with varying concentrations of C{sub 60}, holographic gratings were recorded by individual ultrafast laser pulses; the diffraction efficiency and time decay of the gratings were measured using non-degenerate four-wave mixing. Using a figure of merit for dynamic data processing, the temporal diffraction efficiency, this new class of materials exhibits between two and 12 orders of magnitude higher response than previous reports. The charge transfer range at polymer/C{sub 60} interfaces was further studied using transient absorption spectroscopy. The fact that charge-transfer occurs in the picosecond-time scale in bilayer structures (thickness 200 {angstrom}) implies that diffusion of localized excitations to the interface is not the dominant mechanism; the charge transfer range is a significant fraction of the film thickness. From analysis of the excited state decay curves, we estimate the charge transfer range to be 80 {angstrom} and interpret that range as resulting from quantum delocalization of the photoexcitations

A composite light-harvesting layer from photoactive polymer and halide perovskite for planar heterojunction solar cells

A new route for fabrication of photoactive materials in organic-inorganic hybrid solar cells is presented in this report. Photoactive materials by blending a semiconductive conjugated polymer with an organolead halide perovskite were fabricated for the first time. The composite active layer was then used to make planar heterojunction solar cells with the PCBM film as the electron-acceptor. Photovoltaic performance of solar cells was investigated by J-V curves and external quantum efficiency spectra. We demonstrated that the incorporation of the conjugated photoactive polymer into organolead halide perovskites did not only contribute to the generation of charges, but also enhance stability of solar cells by providing a barrier protection to halide perovskites. It is expected that versatile of conjugated semi-conductive polymers and halide perovskites in photoactive properties enables to create various combinations, forming composites with advantages offered by both types of photoactive materials

Exciton bimolecular annihilation dynamics in supramolecular nanostructures of conjugated oligomers

We present femtosecond transient absorption measurements on $\pi$-conjugated supramolecular assemblies in a high pump fluence regime. Oligo(\emph{p}-phenylenevinylene) monofunctionalized with ureido-\emph{s}-triazine (MOPV) self-assembles into chiral stacks in dodecane solution below 75$^{\circ}$C at a concentration of $4\times 10^{-4}$ M. We observe exciton bimolecular annihilation in MOPV stacks at high excitation fluence, indicated by the fluence-dependent decay of $1^1$B$_{u}$-exciton spectral signatures, and by the sub-linear fluence dependence of time- and wavelength-integrated photoluminescence (PL) intensity. These two characteristics are much less pronounced in MOPV solution where the phase equilibrium is shifted significantly away from supramolecular assembly, slightly below the transition temperature. A mesoscopic rate-equation model is applied to extract the bimolecular annihilation rate constant from the excitation fluence dependence of transient absorption and PL signals. The results demonstrate that the bimolecular annihilation rate is very high with a square-root dependence in time. The exciton annihilation results from a combination of fast exciton diffusion and resonance energy transfer. The supramolecular nanostructures studied here have electronic properties that are intermediate between molecular aggregates and polymeric semiconductors

The synthesis of novel 3-substituted pyrrole monomers possessing chiral side groups: A study of their chemical polymerisation and the assessment of their chiral discrimination properties

Novel 3-substituted pyrrole monomers possessing chiral side groups were synthesised. These compounds were successfully polymerised with ferric salts and the polymer materials fabricated into sensors. The sensors were found to elicit differential changes in electrical resistance and mass, when exposed to different chiral enantiomers in the vapour phase. © 2003 Elsevier Science B.V. All rights reserved

Femtosecond electron-transfer holography in C{sub 60}/polymer blends

Holographic recording has recently been demonstrated in conducting polymer/C{sub 60} blends. Results are presented that demonstrate an improved signal-to-noise ratio is obtained when holographic detection is used to observe the dynamics of photo-induced absorption

Subpicosecond photoinduced electron transfer from conjugated polymers to functionalized fullerenes

We report time-resolved excited state absorption measurements which demonstrate subpicosecond photoinduced electron transfer using soluble derivatives of poly(p-phenylene vinylene) as donors blended with a functionalized fullerene (methanofullerene) as acceptor. The subpicosecond photoinduced absorption spectra of the polymer/methanofullerene blends show that electron transfer from the donor to the acceptor occurs within a picosecond of photoexcitation of the conjugated polymer. Precise determination of the electron transfer dynamics was obtained by monitoring the dichroic ratio. The charge separated state is metastable and persists into the millisecond time domain, yielding an asymmetry of 10 orders of magnitude between the forward and reverse electron transfer times. The increased miscibility of the functionalized methanofullerene with the conjugated polymer is important for preparation of films with sufficiently high acceptor concentrations for practical devices based on photoinduced charge separation.