Efficiency of OLED

The diode efficiency can be improved by improving both rjop and t]. As the external medium is generally air, the optical efficiency depends on the refractive index of the active layer, which is almost constant for usual polymers or organic materials. Therefore, can only be improved by an appropriate design of the diode surface to avoid subsequent reflection of the emitted light. On the contrary, rj depends only on the quality of the materials, which are involved in the injection, transport, and recombination processes. The use of nanocomposites can favor these processes by improving the internal efficiency of diodes as follows. 

In the same way, hole injection from the anode side into the HTL can also be improved by replacing the PEDOT PSS by a composite material made of PEDOT  

PSS mixed with NCs [11-13]. The limiting factor of the use of composites is a possible alteration of the light emission by quenching of created photons due to the NCs when their concentration becomes high enough. For the HTL layer, the incorporation of NCs in the PEDOTPSS film may alter the light transmission to the external medium, and thus decrease the overall efficiency. 

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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. 

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. 

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. 

It is not necessary to use only dyes to take advantage of the energy transfer blends of two polymers can also be used as host-guest systems (Lee et al 2002). The guest molecules can be florescent or phosphorescent in nature. However, phosphorescent dyes based on Ir and Pt complexes have provided significantly higher efficiency of OLEDs because of their ability to emit from both singlet and triplet excitons of the host molecule (Kamata et al 2002),... 

Fig. 9.10 Study of UV-curable polynorbonenes (a) Chemical structures of the precursor polymers (b) efFect of UV-crosslinking of the polynorbonene-based HTL on the external quantum efficiency of OLEDs of the structure ITO/HTL/AlQ3/Mg.

In the following paragraphs, the effects that can affect the luminance and the global efficiency of OLEDs will be discussed briefly. 

The record-efficiency of OLEDs is based on small molecules. Their characteristics are often unpublished and protected by patents. Novaled reports on 16% and 19% external quantum efficiencies for red and green phosphorescent OLED, respectively, whereas Universal Display produced a 20% white OLED. 

The luminous efficiency of OLEDs can be improved Imther by use of phosphorescent emissive materials. Incorporated into OLED devices starting in the 1990s, phosphorescent dopants have a potential to achieve 100% internal quantum efficiency. We will describe materials and architecture developments of phosphorescent OLED devices in Section 14.4. Finally, we discuss the future outlook of the OLED technology in Section 14.5. 

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. 

The interface between the organic layer and metallic anode of an OLED is crucial to the stability and the performance of the device. The a-septithiophene (a-7T) has been used as the buffer layer at the interface between the ITO electrode and the hole transport layer (HTL). The insertion of a-7T layer lowered the operating voltage and improved the external power efficiency. Moreover EL emission was not saturated up to 1600mA/cm. The Maximum EL intensity was over 17000cd/m and the maximum external power efficiency at 2000cd/m is 6.41m/W and that at lOOcd/m is 9.341m/W. The EL intensity and external power efficiency of OLEDs depend on the thickness of the a-7T layer. 

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... 

The development of phosphorescent iridium organometallic complexes was another major breakthrough [8]. Such iridium complexes are triplet emitters and enhance the theoretical quantum efficiency of OLEDs from 25% for fluorescent... 

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. 

Figure 9.18 Efficiency of OLED s formed with coumarin 6 (C6) [see Figure 9.19] and poly(3-n-butyl-p-pyridyl vinylene) (Bu-PPyV) dyes in a polyvinyl carbazole (PVK) hole transport layer. The remainder of the device included an electron transport layer and ITO and Mg-AI electrodes. With permission after Shoustikov, Andrei A. You, Yuijian Thompson, Mark E. Electrolumineseenee eolor tuning by dye doping in organic light-emitting diodes. lEEEJ. Sel. Topics in Quantum Electronics 1998 4 3-13. Copyright [1998] IEEE...

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