Morphology dependent exciton

Morphology-dependent Exciton Retrapping at Polymer Heterojunctions... 

There are several issues involved in formation of exciplexes and excimers that limit the device performance. In the blended emissive layers, the formation of exciplex strongly depends on the concentration, structure and morphology of the film (Mazzeo et al 2002). In such cases fine-tuning of the emission spectrum can be done by concentration variation. However, there is a limit for concentration variation since phase separation or aggregation occurs above an optimum concentration, which in turn, will not allow spatial overlap of the LUMOs of the donor and the acceptor (Mazzeo et al 2002). Another difficulty in exciplex formation is the temperature dependence since at low temperatures the emission by exciton recombination will be predominant while at high temperatures, the exciplex emission will be predominant (Chao and Chen 1998). 

By considering the EL spectra from Section 2.2.3, we have shown that the amount of EL observed from the endothermically generated bulk excitons is dependent on the film morphology, and we introduced the concept of retrapping of excitons. For nm-scale morphologies, excitons are likely to encounter again an interface during their lifetime and get retrapped into an exciplex, which can reduce exciton EL by more than 70%. 

The donor-acceptor components should be the suitable phase size to allow the maximum p-n interface for charge generation. This will allow all excitons transfer to p-n interface and effective charge tfansport in the bicontinuous pathway to the electrodes to minimize the recombination of free charges. The morphology of the active layer depends on the interplay between the material property and device fabrication, such as the solvent used, overall concentration of the blend component, deposition technique (spin coating, ink-jet printing, roll-to-roll, etc.), solvent evaporation rate, and temperature. 

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