The Efficiency of OLEDs

Single-carrier-dominated transport, including a detailed treatment which includes space-charge effects that are prominent in single layer devices, have been developed to provide a satisfactory explanation of the I(V) characteristics in 

Efficiency is a key issue not only for energy-consumption, but also for its effect on the longevity of the devices, since the ability to operate the device at a lower input power at a given luminance decreases ohmic heating and increases the device lifetime. A high power efficiency implies a low IV product for a given luminance. However, much of the analysis of the efficiency in the literature has been devoted to the external quantum efficiency rjEL, i.e., the number of photons emitted through 

Hence for n 1.7, 0.17. However, the recent detailed analysis by Kim et al.38 shows that if the optical interference with the cathode reflector is taken into account, for isotropic and in-plane dipoles A/n2, where A ss 0.75 0.1 and 1.2 0.1, respectively. 

As mentioned in Sec. 1.2 above, electrophosphorescing OLEDs, i.e., devices in which the emission is due to partially-allowed radiative decay of the TEs, have been fabricated and studied,40-42 and values of i]El as high as 12% have been reported 41 

In summary, in evaluating the upper limit of r)el from Eq. (11), the upper limits of the different terms, appear to be f 0.35, y 1, and rST 0.5. Hence fluorescence (as opposed to phosphorescence)-based OLEDs with these values of f, y, and rsj should yield r el 0.15jjpl. 

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Hence, we find that today, the most significant limitation to the efficiency of OLEDs is the internal reflection of about 80% of the emitting light in the glass substrate. In this case, without light extraction enhancement outcoupling, rjext 20% presents a fundamental limit for devices with 100% internal efficiency. 

The efficiency of OLEDs can be defined and measured in a variety of ways. Forrest et at. (2003) have discussed these and recommended a standard approach to enable accurate comparison to made between devices originating from different sources. 

A number of parameters are used in the reporting of the efficiencies of OLEDs, namely quantum efficiency, current efficiency in cdA (qp) or luminous efficiency (qp) in lmW . For the quantum efficiency there are two different parameters, the external quantum efficiency (qext) and the internal quantum efficiency (qmt). The external quantum efficiency qext of an OLED may be expressed as ... 

Indeed, spin statistics mandate that if the rates of reactions (1) and (2) are the same, then the nongeminate polaron pairs generated by carrier injection in OLEDs would yield 3 TEs for every SE. This SE/TE branching ratio is one of the most important factors suppressing the efficiency of OLEDs based on the fluorescent decay of SEs. However, recent studies suggest that in luminescent -conjugated polymers the rate of reaction (1) is higher than that of (2), so the yield of SEs is... 

The analytic theory outlined above provides valuable insight into the factors that determine the efficiency of OLEDs. However, there is no completely analytical solution that includes diffusive transport of carriers, field-dependent mobilities, and specific injection mechanisms. Therefore, numerical simulations have been undertaken in order to provide quantitative solutions to die general case of the bipolar current problem for typical parameters of OLED materials [144-148]. Emphasis was given to die influence of charge injection and transport on OLED performance. I. Campbell et al. [147] found dial, for Richardson-Dushman thermionic emission from a barrier height lower than 0.4 eV, the contact is able to sup-... 

The efficiency of OLEDs is characterized by quantum efficiency, power efficiency and luminous efficiency. Over the past several years, the power (rjp) and external quantum (j/ext) efficiencies of white OLEDs have been steadily improving. 

The luminous efficacy or power efficiency is the lumen output p>er input electrical power of the device. It is measured in lumen per watt (hn/ W) or candela per ampere (cd/ A). It is represented by Jjp. In order to compete with the fluorescent lighting market, the efficiency of OLED sources should be 120 Im/ W or more. To meet the above requirement the OLED sources must have an electrical to optical pxjwer conversion effidency of 34%. For white light with a CRI of 90 the maximum value is 408 Im/W and for a CRI of 100 it is 240 Im/W (Kamtekar 2010). 

One of the measure problems in OLEDs is its low efficiency. Various techniques are used to improve the efficiency of OLED devices. 

Begley, W. J. and Hatwar, T. K. 2006. Novel electron-transporting layer for lowering drive voltage and improving the efficiency of OLED devices. SID Inti. Symp. Dig. Tech. Papers 37 942. 

Explain how triplet harvesting improves the efficiency of OLEDs. 

It can be seen from Eq. (11.5) that the efficiency of OLEDs depends on the number of created photons in the active material, which is a function of the number of transported carriers and the charge balance. As a matter of fact, the motilities of holes and electrons are different in organic materials, holes being more mobile than... 

Kido and Okamoto (2002) published a review article on lanthanide-containing OLEDs. In theory, incorporation of lanthanide complexes in the emitting layer of OLEDs offers two main advantages (i) improved color saturation and (ii) higher efficiency of the OLED. Because of the sharp emission bands of the trivalent lanthanide ions (with a full-width at half maximum of less than 10 run), lanthanide luminescence is highly monochromatic. This results in a much better color saturation than when organic molecirles are used as the emissive material. In this case the band widths of the emission bands are typically around 80 to 100 run. A saturated monochromatic emission is necessary for the development of full-color displays based on OLEDs. Broad emission bands will give dull colors. As mentioned above, the efficiency of OLEDS is limited to 25% by spin statistics. However, when lanthanide complexes are used. 

On the experimental front, Burrows and Forrest 155] have measured the electric field and thickness dependence of the current and radiance from bilayer devices with various HTLs and Alqs as the ETL. The data were analyzed in temis of trap-limited transport in the Alq t layer, with the assumption that the voltage drop across the HTL is negligible. However, this assumption was challenged by Vestweber and Riess [ I56 and Giebcler et al. 1157], who demonstrated that HTL plays an important role in determining the efficiency of bilayer OLEDs. 

Triazines are well-known compounds with high thermal stability and higher EA than 1,3,4-oxadiazoles (PBD) and 1,2,4-triazoles (TAZ, 92). Schmidt et al. studied a series of dimeric 1,3,5-triazine ethers for application as ETMs for OLEDs [150], However, despite their high EA, the efficiency of the OLEDs improved only modestly. One possible explanation is due to their rather poor electron mobilities. 

As an extension of the fluorescent sensitizer concept, Forrest et al. have applied this approach to phosphorescent OLEDs, in which the sensitizer is a phosphorescent molecule such as Ir(ppy)3 [342]. In their system, CBP was used as the host, the green phosphor Ir(ppy)3 as the sensitizer, and the red fluorescent dye DCM2 as the acceptor. Due to the triplet and the singlet state energy transfer processes, the efficiency of such devices is three times higher than that of fluorescent sensitizer-only doped device. The energy transfer processes are shown in Figure 3.21. 

If there is one clear need in the field of OLED materials it continues to be in the area of blue emitters. A blue emissive material with good color coordinates CIE (0.10, <0.10) coupled with long device lifetime (>10,000 h) and high electrical efficiency (>5 cd/A) is the holy grail of materials chemists in this field. A major effort to find such materials continues in many laboratories including our own and the current sets of available materials may be supplanted at any time. However, the current best candidate blue emitters in the SMOLED area compromise many desirable properties — the most troublesome being long lifetime. 

Very recently there has been a report about bridged triarylamine helicenes exhibiting CPL [131], These molecules preferentially emit and absorb CP light without the help of an LC matrix. Currently, there seems to be ongoing work to further increase the efficiency of these types of CPL materials and to develop first devices of polarized OLEDs. 

Another important early advance made by Tang et al. [7] is the use of fluorescent doping, i.e., the addition of a small percentage of an emissive fluorescent material into a host matrix. This can be used to alter the color of emission, in addition to improving the efficiency and the lifetime of devices. The technique of simultaneously vapor depositing the host and the fluorescent dopant material is now widely used in the field of OLEDs. 

One of the most obvious markets for thin-film vapor-deposited organic materials is in flat panel displays [123], a market currently dominated by LCDs. Over the last two decades, a great improvement in the lifetime and efficiency of OLEDs have been achieved. OLED displays can already be found in simple applications such as automobile stereos, mobile phones, and digital cameras. However, to exploit the advantages of the technology fully, it is necessary to pattern the OLEDs to form monochrome, or more preferentially, full-color displays. This section will consider the difficulties involved in addressing such displays (either passively or actively) and the variety of patterning methods that can be used to produce full-color displays. 

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