ABSTRACT
In characterizing organic light-emitting diode (OLED) performance, the most common parameter is the external quantum efficiency (ηEQE), which is defined as the number of photons generated per number of charge carriers injected. It can be described as follows:12 hEQE = hoc hIQE, (2.1) = ghoc he-p, (2.2) = ghoc cfPL, (2.3) where hoc is the optical out-coupling efficiency, γ is a charge carrier balance (CCB) factor, he-p is the exciton to photon conversion efficiency, and hIQE represents the internal quantum efficiency. Here, he-p can also be represented by the product of χ, the fraction of emissive excitons received from the host or directly trapped by
the emitter chosen, and fPL, the luminescence quantum yield of the emitter. Due a significant refractive index mismatch among the different materials used in the device, including the indium tin oxide (ITO) anode, glass substrate, and organic layers, a considerable amount of light is trapped inside the OLED from total internal optical reflections.13 This results in a maximum light out-coupling efficiency of ~0.20-0.30 for standard OLEDs on glass substrates with standard ITO anodes. For a fluorescent emitter, χ is ~0.25. For a phosphorescent emitter or the recently introduced thermally activated delayed fluorescence (TADF) emitter, the maximum χ could reach unity. In the case of a well-optimized device having perfectly matched electron and hole currents, γ could also reach near unity. For an efficient phosphorescent emitter (fPL ≈ 1) such as Ir(ppy)2(acac), together with a highly compatible host such as CBP, he-p could reach unity as well. The bottleneck in raising overall device efficiency is then the optical out-coupling or light extraction. Methods to improve this out-coupling efficiency are outlined in Chapter 9. While ηEQE measures the number of photons extracted to air divided by the number of injected charges, current efficiency, ηCE, and power efficiency (or efficacy), ηPE, are two other useful parameters which are both photometric quantities that also take into consideration the photosensitivity of human eyes. The current efficiency is calculated using a measured luminance L0o in the forward direction together with a measured current density Jmeas passing through the device: hCE measo= LJ 0 [cd/A], (2.4) The power efficiency or efficacy is then computed using the operating voltage at the corresponding current density, V(Jmeas), as follows: h h pPE CE Dmeas= fV J( ) [lm/W], (2.5) with
f I
I d dD = ( )Ú Ú -
q f q f q p
p/ , sin , (2.6)
where fD is the angular distribution of the emitted light intensity I(θ,f) in the forward hemisphere as a function of the azimuthal (θ) and polar (f) angles. I0 denotes the light intensity measured in the forward direction perpendicular to the emitting surface. In general, the emission spectra of OLED may be altered at different angles of view, which will be discussed in detail in Chapter 7. The external quantum efficiency, ηEQE, can be acquired by14 h h pEQE CE Dr ph= ÈÎÍ ˘˚˙f eK E %100 , (2.7) where Eph is the average photon energy of the electroluminescent (EL) spectrum and e is the electron charge. Kr is the luminous efficacy of radiation, which can be calculated by
K V d d
r= ( ) ( )
( ) Ú
Ú F
F
l l l
l l •
[lm/W], (2.8)
where V(λ) is the weighting function that takes into consideration the photosensitivity of human eyes and Φr is the radiant flux. In essence, Kr quantifies lumen per watt for a given spectrum, thereby also representing the theoretical limit in power efficiency of a particular light source, assuming no optical and electrical losses. It is important to note that the angular distribution fD has to be properly measured using an integrating sphere in order to obtain both ηEQEand ηPE accurately. 2.2 Emitter Classifications
For electrically excited organic molecules, the energy may be released radiatively either through a fluorescent or a phosphorescent process. The fundamental mechanisms of fluorescence and phosphorescence are illustrated in Fig. 2.1a. According to spin statistics in quantum mechanics,5 electrically excited excitons in organic molecules are classified as singlet (S) and triplet states (T), with an electronic state density ratio of 1:3. In a fluorescent molecule, the triplet states are nonemissive; hence only a quarter of the total excitons generated may contribute to light emission from its lowest singlet state (S1). Such singlet energy
radiative relaxation happens on a relatively fast time scale of ~10-9 s.5 Conversely, in a phosphorescent molecule, the molecule is attached to a heavy metal atom such as Ir or Pt, which can induce a spin-orbit coupling effect, leading to a rapid exciton energy transfer from the singlet to the triplet state (intersystem crossing [ISC]), as well as allowing for a spin-flip that enables a triplet state to relax to the ground state radiatively.5 The energy relaxation time of the triplet states is in the order of >10-6 s.2 Here, the strength of spin-orbit coupling is critical and it is directly proportional to the fourth power of the atomic number of the metal; hence the heavier the metal, the stronger the spin-orbit coupling and the higher the emission efficiency.15 Essentially, these processes lead to potentially 100% of the electrically generated excitons contributing to light emission. Hence, the use of a phosphorescent emitter yields a fourfold enhancement in light emission efficiency (see Fig. 2.1b) over that of a fluorescent emitter. On the basis of the three orders of magnitude difference in emission decay time scale between fluorescent and phosphorescent emitters, it can be understood that singlet radiative emission leads to much less accumulation, whereas triplets accumulates quickly under high current density.16 This inevitably results in more severe exciton-exciton quenching16 and hence a faster roll-off in efficiency,17 as shown in Fig. 2.1b. Figure 2.2 shows the chemical structures of well-known green, red, and blue phosphorescent emitters.18,19 In terms of the primary colors suitable for commercial applications, green and red phosphorescent emitters are adequate in terms of both efficiency and lifetime for most display applications. It remains a challenge, however, to obtain a stable blue phosphorescent OLED because of the fact that the energy required to excite the blue emitter is close to that of the dissociation energy of the common C-C and C-N chemical bonds in the organic complex.20 In addition, it has recently been found that by pushing the metal-to-ligand charge transfer excited state of these metal-organic molecules into the higher energy blue region, a nonradiative pathway is introduced via the metal d-orbitals, which makes the molecule thermally and photochemically unstable.21 Additionally, to excite the high-energy blue emitters, even-higherenergy (or wider-energy-gap) host materials have to be electrically excited first. This further leads to host molecule instability issues (e.g., aggregation and fragmentation), as will be discussed in Chapter 10.
