There are several ways how to prepare electroluminescent materials that emit Red, Green and Blue light. First, and the most direct method is to synthesize different electrolumophores where each emits the required wavelength. This, however, may not be too easy, because the emission properties of organometallic species are controlled by a number of factors, including electronic properties of the ligand and metal ion, coordination bond lengths and angles, coordination number that affects the energy, mixing, and splitting of electronic states involved in the emission. And even if one succeeds in synthesizing the electroluminescent pigments with good emission intensity/color characteristics, these still may not be ompatible in terms of their electric properties, namely turn-on voltage.

 

The necessity to generate three electrolumophores with similar optical and electrical properties may be circumvented by using blends of electroluminescent pigment with other photoluminescent materials that play the role of dopants. Here is how it works: one electroluminescent pigment with good emitting characteristics is excited by applied voltage and emits light. The emitted light or its energy may be absorbed by second chromophore (dopant), which is not electroluminescent, but only photoluminescent. This second chromophore is excited by the light/energy from the electrolumophore and emits light of different color than that of the initial electrolumophore. These mixtures of electrolumophore with other photoluminescent materials are called blends or doped materials.

Particularly advantageous are blends of electrolumophores with compounds capable to act as acceptors in energy transfer (ET). The energy transfer is, however, effective only in case when electrolumophore molecules (donor of energy) and the dopant (acceptor of energy) are very close to each other (the distance factor). One way, how to ensure that the donor electrolumophore and the dopant acceptor molecules are very close to each other is to use high concentration of dopant. Obviously, this is not so easy to do, because on a molecular level, this means that both electrolumophore and dopant have to be able to form ideal mixtures with uniform (favorable) electrolumophore-dopant distances. If the electrolumophore-dopant distances are not uniform and favorable for energy transfer, the resulting device has poor luminance and color quality.

In summary, the OLED preparation via blending/doping poses a stringent requirement on compatibility of the components (electrolumophore/dopant), including miscibility, solubility in the same solvent, etc.

 

The problem of electrolumophore/dopant compatibility may be circumvented by preparing nanocomposite materials that provide cavities of specific sizes for selective accommodation of both electrolumophore and dopant. The electrolumophore-dopant distance is defined by the structure of the nanomaterial. Likewise, it is conceivable that the electrolumophore may be used directly to form the nanomaterial. This approach simplifies the potential problem with selective encapsulation of the electrolumophore and dopant within the nanomaterial structure.

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