Introduction
1.7 Organic luminescent materials
seconds. The crossing of an electron from singlet to triplet state is forbidden in principle due to their different spin multiplicity. However, it can be possible if the spin orbit coupling between the orbital magnetic moment and the spin magnetic moment is large enough.
The fourth possible de-excitation pathway is the transition between the excited triplet state T1 to the ground state S0. This is termed as phosphorescence. Usually this is a forbidden process and can only be observed with materials with heavy atoms that possess high spin orbit coupling. In most of the cases the non-radiative de-excitation from the triplet state T1, is predominant over radiative de-excitation called phosphorescence. Phosphorescence is thus very low with a time scale of 10-4 to 10-1 seconds.
Delayed Fluorescence is another possible de-excitation processes through which a molecule can come back to ground state. This can be either thermally activated or due to the triplet-triplet annihilation. In case of thermally activated delayed fluorescence reverse intersystem crossing can happen from T1 to S1 if there is a very small energy difference between them provided that the lifetime of the molecule in the triplet excited state is long enough. This process is thermally activated and therefore the fluorescence increases with the increase in temperature. On the other hand, if the concentration of molecule at the triplet excited state is more, there can be a collision between two molecules in the T1 state. As a result, one of the molecules can gain enough energy to go to the higher triplet excited state and then return to the S1 state, thus leading to a delayed fluorescence emission.
development of large-area light emitting displays. Since then the OLED research community has been divided into two groups, each favouring a specific fabrication method and class of materials resulting from that choice. The highest efficiencies of OLEDs so far have been achieved with small molecule based OLEDs are fabricated via evaporation under high vacuum. On the other hand, in polymer light-emitting diodes (PLEDs, the active layer consists of a semiconducting polymer. These materials cannot be evaporated, but instead they can be tuned to be soluble, which enables solution processing techniques.
1.7.1 Small organic materials
In general, small organic materials are organic molecules that possess low molecular weight. They include organometallic chelates, fluorescent and phosphorescent dyes.
i) Organometallic chelates
Fig. 1.7 Some of the commonly used organometallic chelates.
In metal chelates, light is obtained due to the ligand centred π-π* transitions. The d-d* transition can however interfere with the luminescence of such ligands. Therefore, the choice of the metal ions for these class of organic materials are limited to those metals that either do not contain d- electrons or have filled d- shell. Li3+, Be3+, Mg2+, Zn2+, Al3+ and
N
O N
N O O Al
N O
N O
Mg
N O
N O
Zn N N O
O Be
O N O Li
B3+ are some of the most common metal ions used for this purpose. Tris(8- hydroxyquinoline) aluminium (ALq3) is by far the most commonly used metal chelate in OLED applications and is one of the most efficient luminescent material. Some of the commonly used organometallic chelates are shown in Fig. 1.7.
ii) Fluorescent dyes
Fluorescent dyes are the most commonly used emissive materials in small molecule based OLEDs and emit light by the radiative decay of the singlet excited states. The large availability of the fluorescent dyes makes them an interesting choice for OLED application and till date numerous dyes have been utilized as emissive materials. Some of the widely used fluorescent dyes are shown in Fig. 1.8. Since no emission is obtained from the triplet excited states, the efficiency of devices with these dyes are limited to 25%.
Fig. 1.8 Some of the commonly used fluorescent dyes.
O O
N Et Et
N S
N O
O N Et Et O
CN CN
N
N S N S S
O
CN CN
N
iii) Phosphorescent dyes
Phosphorescent dyes are generally triplet light emitter and utilizes both singlet and triplet excited states to produce light. The presence of heavy metal ions such as iridium, platinum etc in these emitters induces strong spin orbit coupling that eliminates the spin forbidden nature of the triplet states and allows them to decay radiatively. Due to the utilization of singlet as well triplet excited states, these materials possess very high efficiency and the efficiency of devices with these dyes can be 100%. However, confining of the triplet excitons in the emissive layer is a major challenge in this type of devices.
Some of the commonly used phosphorescent dyes are shown in Fig. 1.9.
Fig. 1.9 Some of the commonly used phosphorescent dyes.
1.7.2 Polymers
Polymers are also used as emissive materials in OLEDs and possess several advantages over small molecules. They have higher glass transition temperature, better thermal stability, excellent film forming property and due to their longer conjugation length, they can be processed by low cost techniques such as spin coating, inkjet printing etc. The most commonly used polymers in OLED application are the π conjugated polymers. Another significant advantage of polymer emissive material is that their color
tunability and also their charge transport property can be modified by incorporating suitable electron donating or accepting moieties in either side chain or main chain. Some of the widely used polymer emissive materials are shown in Fig. 1.10.
Fig. 1.10 Some of the commonly used polymers.