Christopher J. Collison

Picosecond Time-Resolved Photoluminescence of PPV Derivatives

We report some time-resolved luminescence measurements on the conjugated polymer, MEH-PPV, that show a marked dependence on the intensity of the excitation light. At low laser powers the excited state lifetime is longer than has been reported previously and is consistent with emission from an interchain excitation that has low oscillator strength and hence a long natural radiative lifetime. We rationalise these data with a simple scheme that summarises the excited state kinetics in this and other conjugated, electroluminescent polymers.

INTRA- AND INTER-MOLECULAR PHOTOEXCITATIONS IN A CYANO-SUBSTITUTED POLY(P-PHENYLENEVINYLENE)

Abstract We report a study of time-resolved photoluminescence (PL) in a cyano-substituted poly(p-phenylenevinylene). We have investigated the effect of excitation and emission wavelength as well as solvent on the PL. We observe evidence of luminescence from both intra- and inter-molecular photoexcitations. We find that the relative amounts of these excitations depend strongly on the excitation wavelength and solvent mixture used.

Origins of aggregation quenching in luminescent phenylenevinylene polymers

We measure transient absorption, gain and bleaching of a model conjugated polymer in solution where aggregation of the polymer has been deliberately controlled. Evidence is presented that photoexcitation of aggregated polymer chains leads to creation of interchain non-emissive polaron pairs in large quantum yield. This is shown to be consistent with film behavior and accounts for low luminescence quantum yields in the solid state. It also explains the recent reports of rapidly formed species with characteristic infrared active vibrations. Our data support a picture where polaron pairs mostly recombine to form the ground state directly, in stark contrast to their presumed recombination mechanism in electroluminescent devices where they have been postulated to form only excited singlet and triplet states.

Luminescence efficiency and time dependence in a high electron affinity conjugated polymer

Abstract We report measurements of the efficiency and time dependence of photoluminescence in a high electron affinity cyano-substituted derivative of poly (p-phenylenevinylene). In solution the photoluminescence quantum yield is 0.52 ± 0.05, and the luminescence lifetime is 0.9 ± 0.1 ns. In solid films the luminescence quantum yield is 0.35±0.03 and the luminescence lifetime, of 5.6 ± 0.2 ns, is much longer. These results strongly suggest that the emission from the film is from an inter-chain excitation, and that inter-molecular interactions are an important factor to consider in the design of highly luminescent conjugated polymers.

Aggregation effects on the structure and optical properties of a model PPV oligomer

We report the effects of solvent-induced aggregation on photoluminescence spectra, decay dynamics and quantum yield in trimers of MEH-PPV and compare these to analogous experiments on the high molecular weight polymer. Nuclear magnetic resonance spectra imply that torsional motion is quenched by aggregation leading to a conjugation length increase that explains red shifts in the photoluminescence. We ascribe reduction of fluorescence quantum yield to interchain effects and observe anomalously long-lived luminescence ascribed to geminate recombination of polaron pairs. The decay dynamics suggest that, unlike the polymer case, we are able to time resolve the formation of polaron pairs in the aggregated trimer.

Contribution of Aggregate States and Energetic Disorder to a Squaraine System Targeted for Organic Photovoltaic Devices

Squaraine dyes have significant potential for use in organic photovoltaic devices because their chemical and packing structure tunability leads to a broad solid state panchromaticity. Nevertheless, broadening of the spectrum does not always give rise to increasing power conversion efficiencies. Furthermore, the same processing strategy used to make devices from different squaraines does not lead to the same optimized performance. In this work, by varying the environmental conditions of a set of anilinic squaraines, we demonstrate that spin-cast thin films are made up of a complex set of states, with each state contributing differently to the overall device efficiency. We demonstrate crystallochromy in that small changes in the packing structure give rise to dramatically different absorption spectra. Through a remarkable comparison between squaraines in poly(methyl methacrylate) solid solution and squaraine:PC60BM blends, we also show long-range and orientational disorder broadening, which distorts the ability to correlate qualitative spectroscopic assessment with an understanding of the device mechanism. We conclude that a full quantitative assessment of the populations of each excited state must be carried out in order to make progress toward an improved understanding of each states contribution to charge transfer at the bulk heterojunction interface.

Temperature dependent photoluminescence from a cyano-substituted phenylene vinylene polymer

Abstract We report some steady-state and time-resolved photoluminescence (PL) studies of a cyano-substituted phenlyene vinylene polymer film as a function of temperature. In contrast to simple aromatic molecules, on reducing the temperature the luminescence spectrum drops in intensity, gains structure and shifts to lower energy. While the rate of decay of the PL increases at short wavelengths and remains almost constant at long wavelengths. Using a simple kinetic scheme involving an intra-chain and an inter-chain excitation, we have successfully explained these data in terms of a delayed luminescence model.

Influence of squaraine aggregation on short-circuit current and device efficiency

Linear absorption measurements, current-voltage characteristics, and time-resolved microwave conductivity measurements were utilized to investigate the impact of molecular aggregation of a novel donor material on photovoltaic device efficiency. We report efficiencies of 2.4+/-0.3% and explain the increase in short-circuit current and efficiency as a function of the increased aggregation.

Photoluminescence studies of solution thermochromism in a cyano-subsituted phenylene vinylene derivative

Time-resolved and steady-state photoluminescence (PL) studies of a cyano-substituted poly(phenylene vinylene) (CNPPV) in toluene solution are reported as a function of temperature. On reducing temperature there is no evidence for the formation of aggregates, with both the absorption and emission properties revealing only a moderate increase in the average length of conjugation. Time-resolved anisotropy data reveal a single rotational correlation time of six nanoseconds. These data are consistent with a very low molecular weight polymer sample that behaves in a molecular fashion.

Theory and assignment of intermolecular charge transfer states in squaraines and their impact on efficiency in bulk heterojunction solar cells (Presentation Recording)

Squaraines are targeted for organic photovoltaic devices because of their high extinction coefficients over a broad wavelength range from visible to near infra-red (NIR). Moreover, their side groups can be changed with profound effects upon their ability to crystallize, leading to improvements in charge mobility and exciton diffusion. The broadening in squaraine absorption is often qualitatively attributed to H- and J-aggregates based on the exciton model, proposed by Kasha. However, such assignment is misleading considering that spectral shifts can arise from sources other than excitonic coupling. Our group has shown that packing structure influences the rate of charge transfer; thus a complete and accurate reassessment of the excited states must be completed before the true charge transfer mechanism can be confirmed. In this work, we will show how squaraine H-aggregates can pack in complete vertical stacks or slipped vertical stacks depending upon sidegroups and processing conditions. Hence, we uncover the contribution of an intermolecular charge transfer (IMCT) state through essential states modeling validated by spectroscopic and X-Ray diffraction data. We further show external quantum efficiency data that describe the influence of the IMCT state on the efficiency of our devices. This comprehensive understanding of squaraine aggregates drives the development of more efficient organic photovoltaic devices, leading towards a prescription for derivatives that can be tailored for optimized exciton diffusion, charge transfer, higher mobilities and reduced recombination in small molecule OPV devices.