Metal Complex Dopants

Metal Complex Dopants

Decreasing triplet lifetime

One of the key advances to the modern OLED was rediscovering heavy metal complexes that had very high phosphorescent quantum yields and short lifetimes. This class of compound has an iridium at the center with both an iridium nitrogen and an iridium carbon bond. It had the characteristics of an ortho-metalated Iridium Carbon bond. It readily forms a bridged dimer. The ppy ligand is a strong sigma donor and pi acceptor. The metal to ligand charge transfer excited state is a triplit with a lifetime of about 1 microsecond.

This led to other orthometalated compounds such as iridium tris ppy sytem first discovered in the 1980s. It has 3 complexing groups around the center. The emission spectrum shows a nice big stokes shift from its absorption band and narrow emission in the green which is where we want some of these OLED to operate.

Ortho-Metallated Ir Complexes

These represent a new class of phosphor. They are related to these tris bby complexes that are known for a number of metals, where you have interactions with two nitrogens and the two bipyramidines. The metal carbon bond turns out to be important and has impact on the redox properties. The Ir(ppy)3 form is much easier to oxidize than the Ru(bby)3 system. The PtOEP was one of the first triplet scavengers that were used in OLEDs. This can be compared with the fac-IR(ppy)3. PtOEP has an emission wavelength farther in to the red which is not a bad thing. Both have a high quantum efficiency for phosphorescence. The PtOEP has a much longer lifetime. The external quantum yields are 1-2% for PtOEP while fac-IR(ppy)3 has an efficiency of 8%. This was exciting.

This is most efficient at this time shows external efficiency is 13% at 1000 candelas which is quite bright. It is a green emitter. These are the relevant quantum efficiency versus current density for this compound above compared to PtOEP below. These were impressive numbers. The quantum efficiency was pretty constant from 1 mA to 100 mA the. This important if you are going run devices at a variable intensity. Eventually you get a drop off at high current densities, you are probably never going to drive displays at these current densities.

Other phosphorescent dopants include iridium compounds with both ir –N and ir-C bonds. As you change the constituents of the other complex agents you go from all the way from red to green to blue. The line widths could be narrower to get purer colors.

Future Directions for OLEDs: Color Purity

Broad emission limits color purity. Graph shows the colors possible by mixing CIE x and y coordinates
Broad emission limits color purity. Graph shows the colors possible by mixing CIE x and y coordinates

The idea of color purity is important. The CIE color chart shows the coordinates that you must hit if you want to get pure red, pure blue or pure green. If you want to combine them you must be in the central region. This is based on the eye’s sensitivity to these colors. If you are in the display business this is something you look at a lot. The PtOEP is a good red. The green could be better and the blue not a pure blue. Subsequent to this other compounds have been developed that hit the CIE color coordinates nicely.

Lanthanides give extremely narrow line shapes for both emission and absorption. Unfortunately these are not very efficient compared to the heavy metal complexes. The color purity is attributed to their f-f transitions.

The efficiency of blue emitters lags behind red and green due to the lack of suitable hosts. Blue emission is harder to get because it requires radical anions and radical cations which are the most positive and the most negative in terms of electrochemical potentials, and the highest excess free energy in the recombination reaction that leads to the excited state. That means you are generating the least stable radical species, the ones that are most likely to react with oxygen and water, etc. There aren’t as many compounds that emit well into the blue. These are silicon based compounds with photo luminescent emission spectrum well into the ultraviolet, that could be used as host materials for phosphorescent blue dopants. These are still being developed.

We can build very thin light sources. There are lots of things that limit efficiency which include the interface between the transparent conductor and the hole transport layer and the cathode interface with the electron transport layer. As these devices have improved in their efficiency we are seeing multilayer devices with 4 or 5 heterojunctions each with their own dopants so you can get full red, green and blue emission.

Here is summary of the kidns of molecules that have used to span the spectrum, just a small sampling.

  • Aluminum quinolate was one of OLED compounds was a green emitter.
  • Eu(TTA)3 is a red emitter
  • Platinum ocoethyl porphyrin PtOEH was a red phosphorescent dopant
  • DCM2 was a laser dye used by the Kodak group to shift the emission into the red region
  • Coumarin -6 is a dopant to create green.
  • Alq2 OPH was used changes slightly to get blues.

White light for room lighting is an important application.

This is a picture of Neal Armstrong at the OLED fabrication line at the Fraunhofer Institute in Dresden. A glass substrate is loaded at the back and the conductive layer is put down. Each vacuum stage allows you to put down a different layer. The next version was going to be about three times this size. Typically you load the sublimation sources with something in range of 5-50 grams so some organic chemists have had to labor long and hard to manufacture that much.

Neal Armstrong at Fraunhofer Institute Fabrication Line
Neal Armstrong at Fraunhofer Institute Fabrication Line

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