Organometallic Emitters for OLEDs: Triplet Harvesting, Singlet Harvesting, Case Structures, and Trends
During the last years, organotransition metal compounds have attracted much attention due to outstanding emission properties. For example, colors over the whole visible range are realizable and high quantum yields at relatively short emission decay times are obtainable. Accordingly, these compounds have successfully been used as emitters in organic light-emitting diodes (OLEDs) [1–11] and in many other applications that are connected with light emission [12–23]. In most cases, the emitting state is a triplet (T1 state), while the ground state S0 is a singlet. Rarely, for example, for selected Cu(I) complexes, the emitting state at ambient temperature is the lowest excited singlet state S1 [24–28]. The knowledge about the states' properties is still much less common or even less developed than for purely organic compounds [29–31]. Thus, it seems to be justified to focus on the properties of these metal complexes. From detailed spectroscopic investigations, new insights can be extracted that lead to valuable stimulations also for the development of new emitter materials.
In particular, a transition between the triplet state and the singlet ground state is formally forbidden, but can become distinctly allowed by spin–orbit coupling (SOC) induced by the heavy metal central ion, such as Pt(II) or Ir(III). ...