The ultimate efficiency of polymer light-emitting diodes is limited by the fraction of charges recombining in the molecular singlet manifold. We address the question of whether this fraction can principally exceed the fundamental limit set down by spin statistics, which requires the possibility of spin changes during exciton formation. Sensitized phosphorescence at 4-300 K enables a direct quantification of spin conversion in coulombically bound electron-hole pairs, the precursors to exciton formation. These are stabilized in external electric fields over times relevant to carrier transport, capture and recombination in devices. No interconversion of exciton intermediates between singlet and triplet configurations is observed. Static magnetic fields are equally unable to induce spin mixing in electroluminescence. Our observations imply substantial exchange splitting at all times during carrier capture. Prior statements regarding increased singlet yields above 25% merely on the basis of higher singlet than triplet formation rates should therefore be re-examined.

Composites based on conjugated polymers and strongly luminescent semiconductor nanocrystals (NCs) are considered to be promising materials for optoelectronic applications such as large area light-emitting devices (LEDs). A number of LEDs for the visible spectral range utilizing CdSe [1][2][3][4] or CdTe [5,6] NCs have been reported. If extended to the near-infrared (NIR) region, they could potentially provide active optical components compatible with existing telecommunication technology utilizing the low-loss fiber windows at 1.3 and 1.55 microns. [7] Currently available molecular and polymer dyes generally emit at wavelengths considerably shorter than 1 mm, thus posing an intrinsic limit to the development of all-organic NIR LEDs and other optoelectronic devices. The first examples of the NCbased NIR LEDs were demonstrated very recently utilizing core-shell InAs/ZnSe, [8] PbS [9] and PbSe [10] NCs. Among conjugated polymers, poly(para-phenylene vinylene) derivates were the most widely used so far for the fabrication of both visible and NIR emitting NC-based composites, owing to their holeconducting properties and the possibility of processing both from organics [1,2,8,9] and from aqueous solutions.[3] Herein, we demonstrate, for the first time, NIR LEDs based on a composite of HgTe NCs and a highly ordered ladder-type conjugated polymer. HgTe NCs can be easily produced by room-temperature aqueous synthesis [11][12][13] providing a material with strong fluorescence, ranging from 40 to 50 % quantum efficiency at room temperature, whose emission wavelength is tunable between 900 nm and 2000 nm by changing the size of the NCs. The combination with a wide band-gap hole-transporting conjugated polymer is readily demonstrated and allows the fabrication of composite films by spin casting techniques, which are well established for processing of organic polymers into large area thin films.

Optical losses in waveguides comprising metallic contacts are thought to be a major hurdle to the realization of organic laser diodes. We demonstrate here that careful tuning of the waveguide mode in flexible distributed feedback lasers can allow lasing action to occur in organic thin films in the presence of contacting electrodes with virtually no difference when compared to metal free devices. A metallic electrode is most suited as the bottom contact between the polymer and the substrate as it reduces mode leakage into the substrate and enhances modal gain. In contrast, a thin transparent electrode such as a metal oxide is preferable for the top electrode, where confinement is not a problem

We demonstrate that the amplified spontaneous emission (ASE) threshold in multilayer waveguide structures suitable for the use in future organic injection lasers can be drastically reduced by inserting a crosslinked hole transport layer (HTL) between a highly conductive indium tin oxide (ITO) electrode and the polymer emission layer. While no ASE is observed when the active layer material is directly spincoated onto the ITO electrode, it can be completely restored upon insertion of a 300-nm-thick HTL. This observation is attributed to reduced attenuation of the waveguided mode enabling the ASE process and is theoretically confirmed by calculations of the mode intensity fraction propagating in the absorptive ITO electrode.

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