Green fabrication of white-emitting organic diodes based on blue and orange nanoparticles

Light emission by white organic light-emitting diodes (OLEDs) from polymer blends typically relies on partial Förster resonance energy transfer (FRET) between mixed blue and red chromophores upon electrical excitation. The fact that the chromophores are cast from a common solvent opens the possibility to prepare the emitting layer in one single processing step. However, since the FRET is typically very efficient, only a small fraction of the low band gap chromophore is required, which poses challenges in precisely tuning the blend stoichiometry and provides a cause for charge trapping. Furthermore, since the solvents are environmentally hazardous, production is non-sustainable. In this work, we demonstrate that these drawbacks can be lifted by casting the active components with a nanoparticle architecture. We apply poly(9,9-dioctylfluorenyl-2,7-diyl) (PFO) and poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenvinylen] (MEH-PPV) as blue and red emitters, from aqueous nanoparticle suspensions, as our emitting materials. Nearly pure white light emitting devices are robustly obtained using a 1:1 particle blend ratio. The key is geometric control of energy transfer, as the confinement provided by nanoparticles introduces a spatial separation between the bulk of the PFO and the MEH-PPV beyond the Förster radius, giving rise to the simultaneous emission by both polymers and allowing for FRET only at particle-particle interfaces. Our nanoparticle-based route allows white-light emission in a water-processed sustainable method and eliminates dopant-precision stoichiometric errors obtained in the known polymers blend strategies.