John M. Lupton

Light‐Emitting Diodes with Semiconductor Nanocrystals

Colloidal semiconductor nanocrystals are promising luminophores for creating a new generation of electroluminescence devices. Research on semiconductor nanocrystal based light-emitting diodes (LEDs) has made remarkable advances in just one decade: the external quantum efficiency has improved by over two orders of magnitude and highly saturated color emission is now the norm. Although the device efficiencies are still more than an order of magnitude lower than those of the purely organic LEDs there are potential advantages associated with nanocrystal-based devices, such as a spectrally pure emission color, which will certainly merit future research. Further developments of nanocrystal-based LEDs will be improving material stability, understanding and controlling chemical and physical phenomena at the interfaces, and optimizing charge injection and charge transport.

Bragg scattering from periodically microstructured light emitting diodes

We present a simple method of generating a periodic wavelength scale structure in the optically active layer of a light emitting diode. This is achieved by solution deposition of a light emitting polymer on top of a corrugated substrate. The periodic structure allows waveguide modes normally trapped both in the substrate and in the thin polymer film to be Bragg scattered out of the structure, thus leading to a doubling of efficiency. This scattering process gives rise to a polarization of the emission spectrum as well as angular dispersion effects.

Energy transfer with semiconductor nanocrystals

Forster (or fluorescence) resonant energy transfer (FRET) is a powerful spectroscopic technique to study interactions, conformational and distance changes, in hybrid nanosystems. Semiconductor nanocrystals, also known as colloidal quantum dots, are highly efficient fluorophores with a strong band-gap luminescence tuneable by size as a result of the quantum confinement effect. Starting from a short summary on the FRET formalism and on the basic properties of semiconductor nanocrystals, this Feature Article provides an overview of the major classes of hybrid FRET systems with semiconductor nanocrystals as at least one component. Systems under consideration include thin solid films containing differently sized semiconductor nanocrystals, solution-based complexes of differently sized semiconductor nanocrystals, nanocrystal-based bioconjugates, and hybrid structures of semiconductor and gold nanoparticles. We focus in particular on the directional energy transfer in layer-by-layer assembled multilayers of differently sized CdTe semiconductor nanocrystals and on the energy transfer from individual rod-like semiconductor CdSe/CdS nanoantennae to single dye molecules, which can be efficiently controlled by external electric fields leading to the realisation of the FRET optical switch.

The role of particle morphology in interfacial energy transfer in CdSe/CdS heterostructure nanocrystals.

An Upside of Asymmetry Advances in synthetic techniques have enabled the preparation of nanometer-scale semiconductors in a wide range of precise shapes and sizes, including core-shell morphologies that layer several different materials in the same particle. Such two-in-one motifs are promising for light-harvesting applications because they allow optically induced charge separation across the internal interface. Borys et al. (p. 1371) studied a series of rod-shaped cadmium sulfide–cadmium selenide hybrid particles using single-particle–resolved optical spectroscopy and found that smooth versus bulbous geometries produced distinct emission spectra. Further analysis of more complex, tetrapodal particles (with four arms aligned tetrahedrally) suggested that nonuniform geometries facilitate interfacial charge transfer by reducing the likelihood of electronic band misalignment. Single-particle spectroscopy suggests that non-uniform geometries favor efficient charge separation for light harvesting. Nanoscale semiconductor heterostructures such as tetrapods can be used to mimic light-harvesting processes. We used single-particle light-harvesting action spectroscopy to probe the impact of particle morphology on energy transfer and carrier relaxation across a heterojunction. The generic form of an action spectrum [in our experiments, photoluminescence excitation (PLE) under absorption in CdS and emission from CdSe in nanocrystal tetrapods, rods, and spheres] was controlled by the physical shape and resulting morphological variation in the quantum confinement parameters of the nanoparticle. A correlation between single-particle PLE and physical shape as determined by scanning electron microscopy was demonstrated. Such an analysis links local structural non-uniformities such as CdS bulbs forming around the CdSe core in CdSe/CdS nanorods to a lower probability of manifesting excitation energy–dependent emission spectra, which in turn is probably related to band alignment and electron delocalization at the heterojunction interface.

On-chain defect emission in electroluminescent polyfluorenes

We present time-resolved photoluminescence measurements on a range of poly- and oligofluorenes with different molecular weights in both dilute solution and thin films. The commonly observed parasitic broad green emission band, which has previously been attributed to an excimer, is identified in all solution and film samples and assigned to an on-chain emissive defect. By comparison of the luminescence decay in the solid state at different temperatures it is shown that, at room temperature, intramolecular relaxation is faster in these polyphenylenes than intermolecular exciton diffusion.

Single-molecule spectroscopy for plastic electronics: materials analysis from the bottom-up.

pi-conjugated polymers find a range of applications in electronic devices. These materials are generally highly disordered in terms of chain length and chain conformation, besides being influenced by a variety of chemical and physical defects. Although this characteristic can be of benefit in certain device applications, disorder severely complicates materials analysis. Accurate analytical techniques are, however, crucial to optimising synthetic procedures and assessing overall material purity. Fortunately, single-molecule spectroscopic techniques have emerged as an unlikely but uniquely powerful approach to unraveling intrinsic material properties from the bottom up. Building on the success of such techniques in the life sciences, single-molecule spectroscopy is finding increasing applicability in materials science, effectively enabling the dissection of the bulk down to the level of the individual molecular constituent. This article reviews recent progress in single molecule spectroscopy of conjugated polymers as used in organic electronics.

Control of charge transport and intermolecular interaction in organic light-emitting diodes by dendrimer generation

A novel family of conjugated dendrimers is used as model compounds to explore the effect of intermolecular interactions on photophysical and transport properties. The Figure shows the third generation of the dendrimers. The dendrimer generation controls the degree of chromophore interaction, which leads to a unique correlation between the chemical structure of the molecules and the macroscopic device properties (see also inside front cover).

Spin Rabi flopping in the photocurrent of a polymer light-emitting diode

Electron spin is fundamental in electrical and optical properties of organic electronic devices. Despite recent interest in spin mixing and spin transport in organic semiconductors, the actual spin coherence times in these materials have remained elusive. Measurements of spin coherence provide impartial insight into spin relaxation mechanisms, which is significant in view of recent models of spin-dependent transport and recombination involving high levels of spin mixing. We demonstrate coherent manipulation of spins in an organic light-emitting diode (OLED), using nanosecond pulsed electrically detected electron spin resonance to drive singlet-triplet spin Rabi oscillations. By measuring the change in photovoltaic response due to spin-dependent recombination, we demonstrate spin control of electronic transport and thus directly observe spin coherence over 0.5 s. This surprisingly slow spin dephasing underlines that spin mixing is not responsible for magnetoresistance in OLEDs. The long coherence times and the spin manipulation demonstrated are crucially important for expanding the impact of organic spintronics.

How chromophore shape determines the spectroscopy of phenylene-vinylenes: origin of spectral broadening in the absence of aggregation.

Single oligo(phenylene-vinylene) molecules constitute model systems of chromophores in disordered conjugated polymers and can elucidate how the actual conformation of an individual chromophore, rather than that of an overall polymer chain, controls its photophysics. Single oligomers and polymer chains display the same range of spectral properties. Even heptamers support pi-electron conjugation across approximately 80 degrees curvature, as revealed by the polarization anisotropy in excitation and supported by quantum chemical calculations. As the chain becomes more deformed, the spectral linewidth at low temperatures, often interpreted as a sign of aggregation, increases up to 30-fold due to a reduction in photophysical stability of the molecule and an increase in random spectral fluctuations. The conclusions aid the interpretation of results from single-chain Stark spectroscopy in which large static dipoles were only observed in the case of narrow transition lines. These narrow transitions originate from extended chromophores in which the dipoles induced by backbone substituents do not cancel out. Chromophores in conjugated polymers are often thought of as individual linear transition dipoles, the sum of which make up the polymers optical properties. Our results demonstrate that, at least for phenylene-vinylenes, it is the actual shape of the individual chromophore rather than the overall chromophoric arrangement and form of the polymer chain that dominates the spectroscopic properties.

Control of Electrophosphorescence in Conjugated Dendrimer Light‐Emitting Diodes

We present a novel platinum porphyrin based phosphorescent dendrimer for use as a triplet harvesting dopant in organic light-emitting diodes. Two types of dendritic host materials are used. Through the choice of a common branching architecture around the emissive chromophore unit of both guest and host materials, we are able to achieve excellent miscibility. The relative contribution of guest to host emission is found to depend strongly on the energy level offsets of the two blend materials, indicating strong trapping processes. Under pulsed operation, we observe a striking dependence of the emission spectrum on pulse period, independent of the host material used. This spectral modification is attributed to the quenching of triplet excitations at high excitation densities. We find excellent agreement between our measured data and a model based on bimolecular recombination.