Excitons retrapping

For completeness, we show an example for the temperature dependence of the EL spectra from polymer blend LEDs in Fig. 2.33. Only a very small F8BTexciton contribution to the EL spectrum is visible at any temperature. This is due to exciton retrapping and will be discussed in detail in Section 2.4. Despite this, the... 

Morphology-dependent Exciton Retrapping at Polymer Heterojunctions... 

Fig. 2.46 III ustration of exciton retrapping at PFB F8BT heterojunctions. An exciplex state that was formed at the heterojunction via barrier-free charge capture (Section 2.2) or relaxation of an interfacial geminate pair (Section 2.3) can either decay radiatively and emit (1) or transfer endothermically to the bulk...

We conclude that the experimental data agrees well with a simple kinetic model of exciton retrapping. This confirms that retrapping is responsible for reduced exciton emission in polymer blends. 

Highly efficient LED operation is only achieved through efficient exciton EL (Section 2.2), and exciton retrapping has hence to be reduced. In blend LEDs, this could be achieved via large phases and sharp phase interfaces to provide uninterrupted escape paths for the excitons. On the other hand, this morphology potentially promotes large leakage currents, which is detrimental to efficient operation. Bilayer or vertically phase-separated blends could provide a solution to circumvent this trade-off. 

In order to quantify the retrapping phenomenon, we determine the ratio r of the exciton and the exciplex contributions to the delayed emission spectra in fig. 2.49. We assume the peak intensities of the spectra (at —630 nm) to be a mea-... 

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 retrapping efficiency directly and only relates to the exciton diffusion length and the nanoscale morphology of the film, and we suggest this as a probe of these fundamental qualities. 

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