What type of organic compounds are used in OLED technology?

Question

I know that there are organic compounds that exhibit electroluminescence, and are those types of materials difficult to synthesize?

Answer 1

The short answer is a variety of pi-conjugated materials.

A huge variety of compounds, both small molecules, oligomers, polymers, and organometallic species have been used. Key requirements for the emissive material is a high efficiency emission process, either fluorescence or phosphorescence.

Some of the first compounds were simply conjugated aromatic molecules and dyes. Martin Pope at NYU observed luminescence from anthracene (e.g. Wikipedia) in the 60s.

A wide variety of conjugated dyes have been tried with varying success.

In general, there are 2-3 different types of compounds used in an OLED:

• Hole-transport materials: Easily oxidized and conduct positive charges (holes).
• Electron-transport materials: Easily reduced and conduct negative charges (electrons).
• Emissive materials: Holes and electrons recombine and emit light.

In many cases, these are made in a sandwich device: ITO anode with hole transport layer on top, emissive layer, then electron transport layer, then cathode. (I'm leaving out various interface agents, "blocking layers," adhesion layers, etc.)

You're asking about the emissive component. Basically, you want materials that will efficiently emit (fluoresce or phosphoresce) at different wavelengths / colors.

The biggest real breakthrough, IMHO was from $\ce{Alq3}$ which is an aluminum organometallic compound with remarkably bright emission. But emits green, so for displays, you need other things.

I've been out of the field for a while, but here are some examples. Chemical companies like lumtec sell various components.

• Polyfluorenes: different copolymers have tailored emission colors. Usually the "bridge" carbon is substituted with two alkyl substituents for solubility and processability (and to avoid aggregation).

In OLEDs, polyfluorenes are desirable because they are the only family of conjugated polymers that can emit colors spanning the entire visible range with high efficiency and low operating voltage. Furthermore, polyfluorenes are relatively soluble in most solvents, making them ideal for general applications[...]

• Various vinyl carbazoles: Similar reasons to the polyfluorenes.

• Phosphorescent metal complexes (e.g., iridium): If you work out statistics, you get a lot of electron-hole combinations that are triplets, so phosphorescent emission can be very efficient, particularly with transition metals with a high spin-orbit coupling.

I think most leading emissive materials still largely fall into those categories. Small molecule purely organic targets exist, but aren't as efficient.

A bigger question is usually how you intend to deposit the compounds. Vapor deposition (i.e., evaporation or sputtering) can be done with almost anything that sublimates, but if you want to do drop-casting, spin-coating, or inkjet printing, you'll want soluble compounds.