ABSTRACT
To summarize, in an organic light-emitting diode (OLED), the internal quantum efficiency (hIQE) is dependent on two main factors. The first factor is to obtain a balanced flow of electrons and holes into the emission layer. The second factor concerns with the fraction of recombining electron-hole pairs that results in the production of visible photons. Typically, it is convenient to utilize a host that controls charge transport and exciton energy transfer, together with a dopant that receives the excitonic energy and releases it in the form of photons. Exciton-harvesting dopants such as phosphorescent molecules have been the dopant of choice for green and red emissions with adequate efficiency and lifetime. However, the lifetime of blue phosphorescent emitters remains unsatisfactory. Hence, most panel manufacturers utilize a hybrid system of phosphorescent green and red dopants together with a stable blue fluorescent dopant, which yields a system hIQE of around 75%. One inherent issue with the use of phosphorescent molecules is that their excited-state energy stays for a much longer time (microseconds) than in fluorescent molecules (nanoseconds). Such high excitonic energy accumulating in the device can trigger undesirable quenching processes that not only lower the hIQE, but also damage the device, especially the delicate organic-organic and organic-metal heterojunction interfaces. As a result, phosphorescent OLEDs often display more rapid degradation
under high luminance operation. Recently, fluorescent emitters having small singlet-triplet energy-level differences, are found to promote efficient thermally activated up-conversion of triplet to singlet states, leading to delayed fluorescence and a high hIQE of ~100%. However, further development is needed to verify such low-cost, high-efficiency system can exhibit a sufficiently long lifetime. In terms of device architecture, a number of advanced methods have been introduced to improve the hIQE of fluorescent and phosphorescent blue dopants, including the use of exciton harvesting and conversion dopants, as well as the employment of thermally activated delayed fluorescence (TADF) hosts and exciplex-forming cohosts. At the heart of these concepts is the minimization of nonemissive host triplet states via reverse intersystem crossing (RISC) and energy-level matching between host donors and dopant acceptors. Regardless of the design, the most fundamental requirement is triplet exciton confinement to the emissive layer (EML), and facile host energy transfer to dopants, which are preferably capable of harvesting both triplets and singlets to contribute to radiative emission. Currently, although the efficiency of these concepts show great promise, the lifetime remains to be proven with the goal of out-performing state-of-the-art blue fluorescent and phosphorescent OLEDs. The light out-coupling efficiency in an OLED is defined by the ratio of visible photons emitted from the panel to the photons actually generated in the EML. The major loss in light out-coupling is the trapping of photons in the two electrodes, the transparent substrate, and the organic layers. Due to the refractive index mismatch between the organic materials, the substrate, and air, there exists a small cone of incidence where light can be out-coupled. Despite this challenge, a number of light out-coupling enhancement techniques can be used to improve the light extraction efficiency. These include exploiting microcavity effect so that the desired wavelength of light is emitted predominantly in the normal (forward) directions of the panel, bending the light toward the normal using microlens arrays or periodic/aperiodic nanostructures, employing exotic transparent conducting electrodes, and incorporating scattering centers or rough interfaces such that light makes numerous attempts to escape at different angles. While most
of these techniques are effective at improving the performance in the laboratory setting, only a few methods shown are compatible with inexpensive manufacturing at a large scale. It is worth emphasizing that once a cost-effective solution for light out-coupling is attained, the lifetime of the product will also be naturally extended due to the requirement of less driving current to achieve a desired brightness. In the short term, most competitive OLED products will be made primarily by vacuum deposition techniques since solution processing still yields devices with unsatisfactory efficiency and lifetime. As shown in Table 11.2, even with a hybrid of vacuum deposition and solution processing, the performance is still only half of those fabricated entirely by vacuum deposition. The inabil-ity to make reliable and efficient OLEDs by solution processing will certainly prevent a rapid cost reduction. Roll-to-roll processing on flexible substrates is another attractive, low-cost approach that is currently being pursued by Konica Minolta. The challenge here lies in the development of a reliable barrier technology that prevents water and oxygen penetration through the flexible plastic substrates and covers. Eventually, vacuum processing on flexible substrates may yield many more intriguing products, which are on the short-term roadmap of major electronic display companies such as Samsung Display and LG Display. After observing a remarkable technological progress over the past two decades, OLED technology is steadily becoming the ultimate display technology of the coming decades. As an efficient, design-friendly broadband light source, OLED will likely provide a niche application initially before eventually excelling in the exploding solid-state lighting market.
