**Author details**

Jai Singh

18 Organic Light Emitting Devices

( *me* 

expressed as:

For triplet excitons using || / *<sup>T</sup>*

simplified as follows: Using 3

 and / 2 

*<sup>x</sup> r a*

 

2 1

32

*e Z <sup>R</sup>*

rate in Eq. (48) depends linearly on the emission energy

polymers the rate is obtained as: <sup>5</sup>

with the measured rate for Ir complexes [26].

complexes Eq. (46) produces more favourable results.

the energy band gap of a QD depends on its size as [29]:

622

47 4 0

*x ex*

*c a* 

s *sp*

For different phosphorescent materials only the atomic number of the heavy metal atom and the emitted energy will be different so the rate of spontaneous emission in Eq. (37) can be

<sup>12</sup> <sup>12</sup> *R Z sp* 25.3 ( ) s

For phosphorescent materials like fac-tris (2-phenylpyridine) iridium (Ir(ppy)3) and iridium(III) bis(2-phenyl quinolyl-N,C20) acetylacetonate (PDIr), where Ir has the largest atomic number *Z* = 77, other atomic numbers can be neglected being mainly of carbon. The

same for all iridium doped materials. Thus, for iridium complexes doped in organic

Ir(ppy)3 has been doped for emission energy of 2.4 eV, for orange phosphor Ir(MMQ) [25] and FIrpic [4] have been doped for emission at 2.00 eV. In all these films the rate of spontaneous would be of the same order of magnitude (3 - 4 x105 s-1 ). This agrees quite well

Both rates of spontaneous emission derived in Eq. (37) on the basis of single electron excitation (atomic case) and that obtained in Eq. (46) for an electron-hole pair excitation have been applied to calculate it in organic solids and polymers [9, 27]. Apparently for platinum complexes Eq. (37) gives rates that agree better with experimental results but for iridium

In addition to developing the introduction of the phosphorescent materials to enhance the radiative recombination of triplet excitons, a step progression of HOMO and LUMO of the organic materials to confine the injected carriers within the emission layer has been applied [25]. This enables the injected e and h confined in a thinner space that enhances their

Another approach for meeting the requirement of availing different energy levels for singlet and triplet emissions within the same layer of a WOLED is to incorporate nanostructures, particularly quantum dots (QDs), in the host polymers [28]. As the size of QDs controls their energy band gap, the emission energy can be manipulated by the QD sizes. It is found that

<sup>2</sup>

bulk *Eg Eg C d* eV / (49)

recombination. This scheme has apparently proven to be most efficient so far.

s-1 (

<sup>12</sup> *R* 1.5 10

and g = 2, the rate in Eq. (46) becomes [20]:

(47)

which gives the triplet exciton Bohr radius 0 6 *ex a a*

 in eV (48)

<sup>12</sup> and other quantities are the

<sup>12</sup> in eV). For green phosphor

12 1

*x e m* ; *me* being the free electron mass), the rate in Eq. (47) can be

*School of Engineering and IT, B-purple-12, Faculty of EHSE, Charles Darwin University, Darwin, NT, Australia* 
