The Energy Agenda: Exciplex blend small-molecule OLEDs; high performance fluorescent devices from E-type triplet harvesting

能源议程要求更高效的显示和照明技术，以实现英国政府的目标。Monkman和Bryce教授提出了一种高效有机LED(OLED)的新范例，只使用有机荧光发射器，而不是目前非常流行的有机金属磷光发射器。磷光发射器有很大的局限性。蓝色发光磷光化合物稳定性差，寿命短，还没有产生显示器和固态照明应用所需的深蓝色发光。这种有意产生激基复合体的想法与偶然的“混合”激基复合体发射层或输运层和发射层之间界面状态的激基复合体贡献有很大不同。做这项研究的原因有两个：首先，激基络合物的电子交换能量可以是零的，从而产生几乎相等的单态和三态能量。这意味着，三重态激基复合体有效地(热地)将系统间交叉反向返回到单重态流形，从而提供了一种方法，该方法最高可获得100%由电荷复合形成的三重态激基复合态，即它们可以与磷光发射体一样有效。其次，使用非常简单的器件结构，包括空穴传输层(施主)、混合施主-受主发射层和电子传输层(受主)，可以实现将电荷直接注入激基复合体，从而获得非常低的开启电压(约2.5V)，这也提供了非常高的功率效率(lm/W)。更换蓝色磷光发射体尤为重要，因为它们在器件制造中使用的真空沉积过程中会发生化学降解；它们的工作寿命很短，不会发出对高质量显示器和照明都至关重要的深蓝色光。我们已经展示了一个非常有前途的深蓝色激基复合体系统：该器件发射波长为450 nm，在2.8V电压下的效率为2.7%，使用的是现成的材料。如果施主的光致发光产额能从30%提高到90%，我们可以预期激基络合物发射的效率将增加4倍：这将产生10%的效率，功率效率接近30lm/W。改善施主的电荷迁移率应该会进一步提高效率。颜色调节也相对简单，因为对给体或受体组分进行化学修饰将改变决定激基复合体能量的分子能级。非常简单的激基复合体器件结构非常适合工业应用，因为减少器件中的层数大大提高了制造产量并降低了成本。在项目期间，我们将专注于单色器件，目标是生产红色、绿色和蓝色磷光系统的替代品，然后转移到制造初始的白色发光器件，以进一步试验生产非常简单的白色器件结构的新想法。新的exciplex器件可以提供磷光系统的真正替代方案，并带来更高的功率效率、更长的使用寿命和更简单的器件架构。

The energy agenda demands more efficient display and lighting technologies to meet the UK government's targets. Professors Monkman and Bryce propose a new paradigm for highly efficient organic LEDs (OLEDs) using only organic fluorescent emitters, rather than organometallic phosphorescent emitters that are currently very fashionable. Phosphorescent emitters have major limitations. Blue-emitting phosphorescent complexes especially suffer from poor stability and short lifetimes, and they have not yet produced the deep-blue emission required for both display and solid-state lighting applications. Our new devices will have as their emitters fully blended organic molecules which form exciplexes. An exciplex is a special type of electronically excited state that can occur at the interface of electron donating and electron accepting molecules. Exciplexes can emit light with high efficiency if the donor part also has high photoluminescence quantum yield. This idea of intentionally generating exciplexes is very different from accidental 'mixed' exciplex emission layers or exciplex contributions from interfacial states between transport and emission layers. The reason for doing this research is two-fold: firstly, exciplexes can have vanishingly small electron exchange energies resulting in nearly equivalent singlet and triplet energies. This means that it is efficient for triplet exciplexes to (thermally) reverse intersystem crossing back to the singlet manifold, thereby giving a method of 'harvesting' up to 100% of triplet exciplex states formed from charge recombination, i.e. they can be as efficient as a phosphorescent emitter. Secondly, using a very simple device structure comprising a hole transport layer (the donor), a blend donor-acceptor emission layer, and an electron transport layer (the acceptor) one achieves direct injection of charges into the exciplex which gives very low turn-on voltages (ca. 2.5 V) which also gives very high power efficiencies (lm/W). These features are very important for both mobile, battery-operated display devices and for lighting applications. Thus, exciplex based devices offer a step change in OLED design. It is particularly important to replace phosphorescent blue emitters, as they chemically degrade during the vacuum deposition process used in device fabrication; they have short working lifetimes and do not emit deep-blue light which is essential for both high quality displays and lighting. We have demonstrated an initial deep blue exciplex system which is very promising: the device emits at 450 nm and has an efficiency of 2.7% at 2.8 V, using 'off-the-shelf' materials. There is considerable scope for improved design of materials. If the photoluminescence yield of the donor could be increased from 30% to 90% we could expect the exciplex emission to increase 4-fold in efficiency: this would yield a 10% efficient deep-blue device with a power efficiency approaching 30 lm/W. Improving the charge mobility of the donor should increase the efficiency still further. Colour tuning is also relatively simple as chemically modifying the donor or acceptor components will alter the molecular energy levels which dictate the exciplex energy.The very simple exciplex device structure is perfect for industrial application as reducing the numbers of layers in a device greatly improves manufacturing yield and lowers the cost. During the project we will concentrate on monochrome devices with the goal of producing alternatives to red, green and blue phosphorescent systems, and then move to fabricate initial white-emitting devices to trial further new ideas to produce very simple white device structures.New exciplex devices could provide a real alternative to phosphorescent systems and bring with them higher power efficiencies, longer lifetimes and far simpler device architectures. Doing this research in the UK will maintain our world capability and strength in OLED and OLED-lighting research and development.