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

Spatial layout planning in sub-surface rail station design for effective fire evacuation

The London Underground network is a crucial part of the transportation system in one of only four ‘Alpha’ world cities. The other three – Paris, New York and Tokyo – also have such sub-surface railway transport systems that may benefit from this shape grammar station design process in a future research proposal. In London’s case, the passenger flow rates are the underlining factor in sizing infrastructure where passengers have access – it is therefore this criterion that provides the basis for the shape grammar formulation for the largest, oldest and one of the most complex underground systems in the world. The research aims to improve passenger fire evacuation times, with due cognisance of the growth of numbers using the system, and its present susceptibility to terrorist attacks taken into account. The proposed shape grammar approach will provide for generation of spatial layouts, based upon visual rules of shape recognition, replacement / union, their connectivity and spatial relationships. The paper concentrates on definition and implementation of novel shape grammar design rules that incorporate station planning design knowledge, and in particular also discusses designers’ fire risk assessment approach and related knowledge that is also needed to produce credible station design solutions. Development, to date, of the proposed artificially intelligent CAD environment is also described along with parallel theoretical research. The proposed CAD interface provides familiarity to the designer and avoids incompatibility issues regarding drawing exchange format between various software systems. The shape grammar layouts produced will be tested in SIMULEX, a commercially available evacuation package, and be compared against ‘traditionally’ designed layouts to demonstrate improvements of preliminary ‘reference’ designs, which follow the standard London Underground design process as a later stage of this research

Cyclodehydrogenation of poly(perylene) to poly(quaterrylene): Toward poly(peri-naphthalene)

Cyclodehydrogenation of soluble polyperylenes gives polymers containing predominantly quaterrylene units by spectral analysis. Evidence for larger rylene units is also seen. These polymers represent important intermediates toward a poly(peri- naphthalene) (PPN). Reactions on model compounds suggest that the insolubility of polymers with larger rylene units is predominantly due to the rigidity of the polymer backbone

Efficient light harvesting in a photovoltaic diode composed of a semiconductor conjugated copolymer blend

We explore charge generation and recombination kinetics quantitatively in a model polymeric semiconductor ternary system. Resonance energy transfer from a polyfluorene copolymer backbone to perylene-substituted side chains selectively excites the dye. In a blend with an alkoxy-substituted poly-p- phenylenevinylene, photoinduced charge transfer is observed. A comparison of picosecond time scale and quasisteady-state charge generation efficiencies allows measurement of the fraction of charges that survive rapid geminate recombination. This simple procedure allows the unraveling of the role played by recombination kinetics in the device. (C) 2002 American Institute of Physics

Interfacial charge transfer anisotropy in polycrystalline lead iodide perovskite films

Solar cells based on organic-inorganic lead iodide perovskite (CH3NH3PbI3) exhibit remarkably high power conversion efficiency (PCE). One of the key issues in solution-processed films is that often the polycrystalline domain orientation is not well-defined, which makes it difficult to predict energy alignment and charge transfer efficiency. Here we combine ab initio calculations and photoelectron spectroscopy to unravel the electronic structure and charge redistribution at the interface between different surfaces of CH3NH3PbI3 and typical organic hole acceptor Spiro-OMeTAD and electron acceptor PCBM. We find that both hole and electron interfacial transfer depend strongly on the CH3NH3PbI3 surface orientation: while the (001) and (110) surfaces tend to favor hole injection to Spiro-OMeTAD, the (100) surface facilitates electron transfer to PCBM due to surface delocalized charges and hole/electron accumulation at the CH3NH3PbI3/organic interfaces. Molecular dynamic simulations indicate that this is due to strong orbital interactions under thermal fluctuations at room temperature, suggesting the possibility to further improve charge separation and extraction in perovskite-based solar cells by controlling perovskite film crystallization and surface orientation