Gordon J. Hedley

Determining the optimum morphology in high-performance polymer-fullerene organic photovoltaic cells

The morphology of bulk heterojunction organic photovoltaic cells controls many of the performance characteristics of devices. However, measuring this morphology is challenging because of the small length-scales and low contrast between organic materials. Here we use nanoscale photocurrent mapping, ultrafast fluorescence and exciton diffusion to observe the detailed morphology of a high-performance blend of PTB7:PC71BM. We show that optimized blends consist of elongated fullerene-rich and polymer-rich fibre-like domains, which are 10–50 nm wide and 200–400 nm long. These elongated domains provide a concentration gradient for directional charge diffusion that helps in the extraction of charge pairs with 80% efficiency. In contrast, blends with agglomerated fullerene domains show a much lower efficiency of charge extraction of ~45%, which is attributed to poor electron and hole transport. Our results show that the formation of narrow and elongated domains is desirable for efficient bulk heterojunction solar cells.

Light Harvesting for Organic Photovoltaics

The field of organic photovoltaics has developed rapidly over the last 2 decades, and small solar cells with power conversion efficiencies of 13% have been demonstrated. Light absorbed in the organic layers forms tightly bound excitons that are split into free electrons and holes using heterojunctions of electron donor and acceptor materials, which are then extracted at electrodes to give useful electrical power. This review gives a concise description of the fundamental processes in photovoltaic devices, with the main emphasis on the characterization of energy transfer and its role in dictating device architecture, including multilayer planar heterojunctions, and on the factors that impact free carrier generation from dissociated excitons. We briefly discuss harvesting of triplet excitons, which now attracts substantial interest when used in conjunction with singlet fission. Finally, we introduce the techniques used by researchers for characterization and engineering of bulk heterojunctions to realize large photocurrents, and examine the formed morphology in three prototypical blends.

Ultrafast Intersystem Crossing in a Red Phosphorescent Iridium Complex

Femtosecond photoluminescence (PL) and transient absorption (TA) studies have been carried out on the red phosphorescent metal complex tris(1-phenylisoquinoline)iridium(III) [Ir(piq)(3)] following excitation of the metal-ligand charge transfer singlet state. Rapid decay of the PL observed at 270 meV above the phosphorescence peak and TA dynamics are indicative of intersystem crossing, which occurs with a time constant of 70 fs. PL decays at 140 meV above the phosphorescence peak are biexponential with time constants of 95 fs and 3 ps, attributed to intramolecular vibrational redistribution (IVR) and vibrational cooling. The larger Ir(piq)(3) ligands facilitate faster dissipation of excess energy by IVR than the smaller Ir(ppy)(3) core.

Direct observation of intersystem crossing in a thermally activated delayed fluorescence copper complex in the solid state.

An intersystem crossing time of 27 ps is measured in a copper complex that shows thermally activated delayed fluorescence. Intersystem crossing in thermally activated delayed fluorescence (TADF) materials is an important process that controls the rate at which singlet states convert to triplets; however, measuring this directly in TADF materials is difficult. TADF is a significant emerging technology that enables the harvesting of triplets as well as singlet excited states for emission in organic light emitting diodes. We have observed the picosecond time-resolved photoluminescence of a highly luminescent, neutral copper(I) complex in the solid state that shows TADF. The time constant of intersystem crossing is measured to be 27 picoseconds. Subsequent overall reverse intersystem crossing is slow, leading to population equilibration and TADF with an average lifetime of 11.5 microseconds. These first measurements of intersystem crossing in the solid state in this class of mononuclear copper(I) complexes give a better understanding of the excited-state processes and mechanisms that ensure efficient TADF.

Conformational Effects on the Dynamics of Internal Conversion in Boron Dipyrromethene Dyes in Solution

Internal conversion between the S2 and S1 excited singlet states is rapid in the target dyes (see picture) but shows a clear sensitivity to rotational flexibility of the meso‐phenyl ring. Steric crowding by methyl groups at the 4,7‐positions leads to a curved S2 potential‐energy surface punctured with nonlocal pinholes coupled to the S1 surface. Removal of these groups flattens the potential‐energy surface, promoting barrierless crossing to the S1 surface.

Dynamics of fluorescence depolarisation in star-shaped oligofluorene-truxene molecules.

Star-shaped molecules are of growing interest as organic optoelectronic materials. Here a detailed study of their photophysics using fluorescence depolarisation is reported. Fluorescence depolarisation dynamics are studied in branched oligofluorene-truxene molecules with a truxene core and well-defined three-fold symmetry, and are compared with linear fluorene oligomers. An initial anisotropy value of 0.4 is observed which shows a two-exponential decay with time constants of 500 fs and 3-8 ps in addition to a long-lived component. The femtosecond component is attributed to exciton localisation on one branch of the molecule and its amplitude reduces when the excitation is tuned to the low energy tail of the absorption spectrum. The picosecond component shows a weak dependence on the excitation wavelength and is similar to the calculated rate of the resonant energy transfer of the localised exciton between the branches. These assignments are supported by density-functional theory calculations which show a disorder-induced splitting of the two degenerate excited states. Exciton localisation is much slower than previously reported in other branched molecules which suggests that efficient light-harvesting systems can be designed using oligofluorenes and truxenes as building blocks.

Vibrational energy flow controls internal conversion in a transition metal complex.

Internal conversion (IC) between excited electronic states is a fundamental photophysical process that is important for understanding protection from UV radiation, energy transfer pathways and electron injection in artificial photosynthetic systems and organic solar cells. We have studied IC between three singlet MLCT states in an iridium complex using femtosecond fluorescence spectroscopy. Very fast IC with a time constant of <20 fs is observed from the highest state and a much slower relaxation to the lowest energy singlet state on a 70 fs time scale. The abrupt slowdown of the relaxation rate occurs when there is >0.6 eV of vibrational energy stored in the complex that has to be dissipated by intramolecular vibrational redistribution before further IC to the lower energy states can occur. These results show that the ability to dissipate vibrational energy can control the relaxation process in this class of materials.

Emergent Properties of an Organic Semiconductor Driven by its Molecular Chirality

Chiral molecules exist as pairs of nonsuperimposable mirror images; a fundamental symmetry property vastly underexplored in organic electronic devices. Here, we show that organic field-effect transistors (OFETs) made from the helically chiral molecule 1-aza[6]helicene can display up to an 80-fold difference in hole mobility, together with differences in thin-film photophysics and morphology, solely depending on whether a single handedness or a 1:1 mixture of left- and right-handed molecules is employed under analogous fabrication conditions. As the molecular properties of either mirror image isomer are identical, these changes must be a result of the different bulk packing induced by chiral composition. Such underlying structures are investigated using crystal structure prediction, a computational methodology rarely applied to molecular materials, and linked to the difference in charge transport. These results illustrate that chirality may be used as a key tuning parameter in future device applications.

Nanoimprinted distributed feedback lasers of solution processed hybrid perovskites

Hybrid perovskite materials have considerable potential for light emitting devices such as LEDs and lasers. We combine solution processed CH3NH3PbI3 perovskite with UV nanoimprinted polymer gratings to fabricate distributed feedback (DFB) lasers. The lead acetate deposition route is shown to be an effective method for fabricating low-loss waveguides (loss coefficient ~6 cm-1) and highly compatible with the polymer grating substrates. The nanoimprinted perovskite exhibited single-mode band-edge lasing, confirmed by angle-dependent transmission measurements. Depending on the excitation pulse duration the lasing threshold shows a value of 110 μJ/cm2 under nanosecond pumping and 4 μJ/cm2 under femtosecond pumping. We demonstrate further that this laser has excellent stability with a lifetime of 108 pulses.

Real-time probing of β-amyloid self-assembly and inhibition using fluorescence self-quenching between neighbouring dyes

The fluorescence response of the Thioflavin-T (ThT) dye and derivatives has become the standard tool for detecting β-amyloid aggregates (Aβ) in solution. However, it is accepted that ThT-based methods suffer from important drawbacks. Some of these are due to the cationic structure of ThT, which limits its application at slightly acidic conditions; whereas some limitations are related to the general use of an extrinsic-dye sensing strategy and its intrinsic requirement for the formation of a sensor-binding site during the aggregation process. Here, we introduce fluorescence-self-quenching (FSQ) between N-terminally tagged peptides as a strategy to overcome some of these limitations. Using a combination of steady-state, picosecond time-resolved fluorescence and transmission electron microscopy, we characterize the fluorescence response of HiLyte fluor 555-labelled Aβ peptides and demonstrate that Aβ self-assembly organizes the covalently attached probes in close proximity to trigger the self-quenching sensing process over a broad range of conditions. Importantly, we prove that N-terminal tagging of β-amyloid peptides does not alter the self-assembly kinetics or the resulting aggregated structures. We also tested the ability of FSQ-based methods to monitor the inhibition of Aβ1-42 aggregation using the small heat-shock protein Hsp20 as a model system. Overall, FSQ-based strategies for amyloid-sensing fill the gap between current morphology-specific protocols using extrinsic dyes, and highly-specialized single-molecule techniques that are difficult to implement in high-throughput analytical determinations. When performed in Förster resonance energy transfer (FRET) format, the method becomes a ratiometric platform to gain insights into amyloid structure and for standardizing in vitro studies of amyloid aggregation.