ABSTRACT
From Chapter 2 we learned that if the organic light-emitting diode (OLED) has only fluorescent emitters, only a quarter of the electrically generated excitons (singlets) will be harvested for light emission. This sets the limit on the internal quantum efficiency of the device to ~25%. To boost the performance toward 100% internal quantum efficiency, it is necessary to harvest the remaining 75% of the triplet excitons generated electrically in the device. In this chapter, we will discuss a number of advanced designs that can achieve this goal. First we will present a design involving triplet exciton harvesting and efficient energy transfer using phosphorescent emitters only. Additionally, it is known that triplet excitons in the host tend to spur various quenching processes either with themselves or with charged species, thereby accelerating device degradation. It is therefore beneficial to convert the bulk of the generated triplets into singlets in the device during operation by means of triplet to singlet up-conversion through reverse intersystem crossing (RISC). This is possible with the use of thermally activated delayed fluorescence (TADF) dopants and hosts to harvest triplet excitons and convert to singlets that can either be radiatively emitted directly or perform efficient energy transfer to other lower-energy dopants. Alternatively, this can be achieved using exciplex-forming cohost systems that not
only converts triplets into singlets but also conveniently provides exciton confinement in the emissive layer (EML).
