Obviously a key material for successful OLED operation, the LEM must be amenable to a high-quality deposition technique such as vacuum deposition. It also requires the capability to transport both holes and electrons to enable the recombination of these carriers. Moreover, it must effectively allow for the creation of excitons and their decay to generate photons and it must remain stable at the electric fields needed to transport the holes and electrons and the migration of molecules must be minimized for device stability.
In OLED operation, electrons injected from the cathode and holes injected from the anode combine to form molecular excitons, which were discussed in Chapter 3. The spin of the electron and hole are generally random, which means that the exciton population will occur with a 25% chance of being a singlet exciton and a 75% chance of being a triplet exciton. The singlet exciton can recombine by a dipole emission process and fluorescence will occur; however, the triplet exciton is forbidden to emit dipole radiation. The maximum quantum yield is therefore expected to be 25%.
It is common for mixtures of two or more molecules to be used as light emitting materials in order for the material to provide the various required functions. In such mixtures stability is important and a solid solution of the component molecular materials is usually preferred; the segregation of the components must be avoided. This is commonly referred to as molecular doping of one molecular material by another molecular material.
The emission colour of the OLED is ultimately determined by the LEM, and in many cases molecules are modified by changing side-groups, for example, to achieve a specific desired emission colour. In particular, since full-colour displays require efficient red, green and blue light emission, the achievement of LEM emission with suitable colour coordinates for each of the three colours is the goal. In this case a set of three LEMs having the appropriate stability and efficiency and colour coordinate values is required.
A common process used in light emitting materials is the host-guest energy transfer process. Here, a host molecule is excited and it can either directly produce radiation or it can transfer its energy to a guest molecule. There are advantages to this energy transfer process which are as follows:
(a) The transfer process can create emission at wavelengths needed for a variety of emission colours without changing the host material.
(b) The 25% theoretical quantum yield can be overcome.
In Section 3.7 energy transfer processes were introduced. The Dexter process involves the transfer of an electron between host and guest molecules and is operational over short-range separations (10-20 A) only and falls off exponentially. The Forster process is a dipole-dipole interaction process that transfers energy over longer distances of 50 to 100A. It falls off with the sixth power of the separation R between the two molecules. Finally energy transfer can occur through photon emission from a host molecule followed by photon absorption by a guest molecule. These mechanisms were introduced in Chapter 3.
The host-guest transfer is illustrated in Figure 6.21. SH and Sf refer to the singlet ground states of the host and guest respectively. SH and Sf refer to excited singlet states of the host and guest respectively. T1H and Tf refer to the excited triplet states of the host and guest respectively. When a host molecule electron is excited to its SH level it could radiate and emit a photon by a dipole-allowed process and fluoresce. Alternatively the molecule could lose energy by collisions and the electron could return to the ground state and transfer its energy to heat, which is a non-radiative process. In addition the molecule may transfer energy to another molecule by Forster, Dexter or radiative energy transfer, as shown in Figure 6.21. Once transferred, the energy can be radiated from the guest molecule by a dipole-allowed fluorescence process, or it may radiate by phosphorescence. Phosphorescent guest materials are described in Section 6.14. These dopants are particularly interesting since they offer the possibility to improve the quantum yield of the emission process beyond the 25% limit imposed by the 1:3 singlet:triplet ratio.
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The transfer of energy from host to guest molecules may be a virtually complete transfer or it may involve only a fraction of the excited host molecules. As a result the measured emission spectrum often contains features characteristic of both host and guest emission. This is not desirable when saturated colour coordinates are required for red, green or yellow emission. White OLED emission is desirable for lighting or for monochrome displays, however, and a combined guest-host emission process leads to a broader emission spectrum, which can approximate a desired white spectrum. An important requirement for any energy transfer process is that the host energy is high enough to excite the guest molecule to an excited state. This means that the excited state of the host should be higher in energy than that of the guest.
We can now examine the requirements as well as examples of both host and guest materials.
