Summary

A disadvantage of using organic semiconductors for displays and lighting is their relatively broad emission spectrum due to inhomogeneous broadening. The emission spectrum can be considerably narrowed by blending a blue-emitting organic host with a green or red-emitting dye or sensitizer with narrow linewidth. In polymer-based LEDs (PLEDs) it has been demonstrated that due to efficient energy transfer from the host to the sensitizer already for 1% sensitizer concentration 95% of the blue excitons are transferred to a red dye [1]. For efficient energy transfer the absorption spectrum of the sensitizer has to overlap with the emission spectrum of the host, such that by definition sensitizers have a smaller bandgap as compared to the host. With regard to charge transport these sensitizers will therefore act as defects that will severely trap charge carriers, as schematically shown in Figure 1a. As a result, incorporation of dyes strongly reduces the charge transport in the host, which is detrimental for the PLED performance [2]. Ideally, a solution would be preferred where the electroluminescence of the host is transferred to the dye without the negative contribution of additional charge trapping, as shown in Figure 1b. Here, the sensitizer or defect is engineered in such a way that it is surrounded by an insulating organic shell. To rationalize the design of the engineered sensitizer the two competing processes in hybrid polymer: sensitizer blend LEDs, namely charge trapping and direct energy transfer, are considered. The hopping distance for charge carriers is determined by the wave function overlap of the localized sites and typically amounts to 1.5-2 nm [3]. This distance also governs the charge transfer process from a host polymer into a trap. In contrast, fluorescence resonance energy transfer (FRET) is driven by dipole–dipole interaction between an excited donor molecule and an acceptor molecule. The efficiency of the energy transfer is inversely proportional to the sixth power of the distance between donor and acceptor, and a Förster radius of typically 8 nm has been found [4]. As a result, energy transfer takes place over a longer distance as compared to hopping of charges as occurs with trapping. This difference in characteristic distance between the two competing processes can be utilized to selectively suppress trapping by organic dyes and nanodiamonds while allowing energy transfer.

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[3] Kuik, M.; Wetzelaer, G. A. H.; Nicolai, H. T.; Crăciun, N. I.; de Leeuw, D. M.; Blom, P. W. M. Charge transport and recombination in polymer light-emitting diodes. Adv. Mater. 2014, 26, 512–531.

[4] Halls, J. J. M.; Pichler, K.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Exciton diffusion and dissociation in a poly(p‐phenylenevinylene)/C60 heterojunction photovoltaic cell. Synth. Met. 1996, 77, 277.