The efficiency of light-emitting diodes (LEDs) made from organic semiconductors is determined by the fraction of injected electrons and holes that recombine to form emissive spin-singlet states rather than non-emissive spin-triplet states. If the process by which these states form is spin-independent, the maximum efficiency of organic LEDs will be limited to 25 per cent. But recent reports have indicated fractions of emissive singlet states ranging from 22 to 63 per cent, and the reason for this variation remains unclear. Here we determine the absolute fraction of singlet states generated in a platinum-containing conjugated polymer and its corresponding monomer. The spin-orbit coupling introduced by the platinum atom allows triplet-state emission, so optically and electrically generated luminescence from both singlet and triplet states can be compared directly. We find an average singlet generation fraction of 22 +/- 1 per cent for the monomer, but 57 +/- 4 per cent for the polymer. This suggests that recombination is spin-independent for the monomer, but that a spin-dependent process, favouring singlet formation, is effective in the polymer. We suggest that this process is a consequence of the exchange interaction, which will operate on overlapping electron and hole wavefunctions on the same polymer chain at their capture radius.

Organic semiconductors fabricated as thin-film light-emitting diodes, LEDs, now provide a promising new display technology.[1] Solution-processed semiconductor polymers make possible direct printing (using ink-jet deposition) and allow high-resolution full-color displays to be conveniently manufactured. [2] Multiple-layer deposition, used in vacuum-sublimed molecular semiconductor LEDs, is difficult to achieve by solution processing. We have instead fabricated distributed heterojunction' structures that are formed by de-mixing of two polymers co-deposited from common solution. We have used hole-accepting and electron-accepting derivatives of polyfluorene, and have optimized these structures to achieve high-efficiency diodes (above 19 lm W ±1 for green emission) that operate at very low voltages (100 cd m ±2 at 2.1 V for green emission). This very low voltage operation is achieved because electron±hole capture across the heterojunction is arranged to be a barrier-free process to form an interface state (termed an exciplex) that has significant charge-transfer character and is lower in energy than the charge-separated state. With respect to the bulk exciton, the exciplex is red-shifted (here between 140 and 360 meV) and its radiative lifetime is strongly increased (between 68 and 118 ns at low temperatures). The barrier for thermal excitation of the exciplex to allow it to move away from the heterojunction is small (100± 250 meV), and this process can give efficient bulk exciton emission at room temperature. The heterojunction formed between dissimilar organic semiconductors is generally found to be remarkably free of gap-states and other defects that would otherwise compromise semiconductor device operation. Heterojunction LEDs are designed so that the offsets between conduction and between valence band edges are type II' and electrons and holes accumulate on opposite sides of the heterojunction (Fig. 1). In a non-interacting electron scheme, type II heterojunctions would destabilize an exciton present in either semiconductor, since the exciton state would be higher in energy than the charge-separated state. However, organic semiconductors are low dielectric constant materials (typically having values less than 4) so that the coulomb interaction between electron and hole gives a substantial exciton binding energy (of order 0.5 eV). When this binding energy is larger than the band-edge offsets, excitons are stable at the interface. By selecting semiconductors with larger band-edge offsets, charge separation at the heterojunction can be readily achieved, giving efficient photovoltaic behavior. [3] LEDs made using molecular semiconductors are generally fabricated as multiple-layer heterojunction structures by successive vacuum sublimation steps.[1] However, with solutionprocessed polymers it is possible to make distributed heterojunction' diodes by de-mixing of two polymers spin-coated from common solution. [4] This is an obviously desirable structure for photovoltaic diodes, because it allows excitons photogenerated...

Solution-processed photovoltaic devices based on blends of conjugated polymers and inorganic semiconductor tetrapods show high efficiencies due to the good electron transport perpendicular to the plane of the film. Here, we show that by using a high-boiling-point solvent, 1,2,4-trichlorobenzene, instead of chloroform for spin-coating, we can typically obtain a threefold increase in solar power conversion efficiency in devices based on CdSe tetrapods and the poly(p-phenylenvinylene) derivative OC1C10-PPV. The optimized devices show AM1.5 solar power conversion efficiencies of typically 2.1% with some devices as high as 2.8%. The results can be explained by the occurrence of vertical phase separation which leads to an optimal structure for charge collection. Evidence for this structure is obtained by environmental scanning electron microscopy, photocurrent action spectra measurements, time-resolved photoluminescence, and spectroscopic measurements of exciton dissociation and charge-carrier recombination.

We have studied electrostatic field-induced doping in La0.8Ca0.2MnO3 transistors using electrolyte as a gate dielectric. For positive gate bias, electron doping drives a transition from a ferromagnetic metal to an insulating ground state. The thickness of the electrostatically doped layer depends on bias voltage but can extend to 5 nm requiring a field doping of 2x10;{15} charges per cm;{2} equivalent to 2.5 electrons per unit cell area. In contrast, negative gate voltages enhance the metallic conductivity by 30%.

By comparing the changes in pi-pi* absorption with the transconductance in PEO-LiClO4 electrolyte-gated FETs, we have demonstrated that the high channel currents obtained at low gate voltages result from reversible electrochemical doping of the semiconducting polymer film. At low temperatures, the conductivity of the electrochemically doped poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene), PBTTT-C14, is nonlinear with a crossover from dsigma(T)/dT > 0 to dsigma(T)/dT approximately 0 as a function of the source-drain voltage. High current densities, up to 10(6) A/cm2 at 4.2 K, can be sustained in the electrochemically doped PBTTT-C14 films.