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，以满足英国政府的目标。Monkman和Bryce教授提出了一种仅使用有机荧光发射体的高效有机LED（OLED）的新范式，而不是目前非常流行的有机金属磷光发射体。磷光体发射体有很大的局限性。发蓝光的磷光配合物尤其受到稳定性差和寿命短的困扰，并且它们尚未产生显示器和固态照明应用所需的深蓝色发射。我们的新设备将作为它们的发射器完全混合有机分子形成激基复合物。激基复合物是一种特殊类型的电子激发态，可以发生在电子供体和电子受体分子的界面处。如果供体部分也具有高光致发光量子产率，则激基复合物可以以高效率发光。这种有意产生激基复合物的想法与意外的“混合”激基复合物发射层或来自传输层和发射层之间界面态的激基复合物贡献非常不同。进行这项研究的原因有两个：首先，激基复合物可以具有非常小的电子交换能，导致几乎相等的单重态和三重态能量。这意味着三重态激基复合物有效地（热）逆转系统间穿越回到单重态流形，从而给出了一种“收获”高达100%的由电荷复合形成的三重态激基复合物状态的方法，即它们可以与磷光发光体一样有效。其次，使用包括空穴传输层（供体）、共混供体-受体发射层和电子传输层（受体）的非常简单的器件结构，实现了将电荷直接注入激基复合物中，这给出了非常低的开启电压（约100 V）。2.5 V），这也给出了非常高的功率效率（lm/W）。这些特征对于移动的电池驱动的显示设备和照明应用都非常重要。因此，基于激基复合物的器件在OLED设计中提供了阶跃变化。更换蓝色磷光发光体尤为重要，因为它们在设备制造中使用的真空沉积过程中会发生化学降解;它们的工作寿命短，并且不会发出高质量显示器和照明所必需的深蓝色光。我们已经展示了一个初始的深蓝色激基复合物系统，这是非常有前途的：该设备在450 nm处发射，并在2.8 V的效率为2.7%，使用“现成的”材料。材料的设计有相当大的改进余地。如果供体的光致发光产率可以从30%增加到90%，我们可以预期激基复合物发射效率增加4倍：这将产生10%效率的深蓝色器件，功率效率接近30 lm/W。提高供体的电荷迁移率应进一步提高效率。颜色调节也相对简单，因为化学修饰供体或受体组分将改变决定激基复合物能量的分子能级。非常简单的激基复合物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。在这个项目中，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，然后制造初始的白色发光器件，以试验进一步的新想法，生产非常简单的白色器件结构。新的激基复合物器件可以提供磷光系统的真实的替代品，并带来更高的功率效率，更长的寿命和更简单的器件结构。在英国进行这项研究将保持我们在OLED和OLED照明研发方面的全球能力和实力。

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.