A potential major drawback with organic light-emitting devices, (OLEDs) is the limit of 25% singlet exciton production through spin-dependent charge recombination. Recent device results, however, show that this limit does not hold and far higher efficiencies can be achieved in purely fluorescent-based systems (Wohlgenannt et al. (2001), Dhoot et al. (2002), Lin et al. (2003), Wilson et al. (2001), Cao et al. (1999), Baldo et al. (1999), and Kim et al. (2000)). Thus, the question arises; is recombination spin dependent (Tandon et al. (2003)) or are singlet excitons generated in secondary processes? Direct measurement of the singlet generation rate in working devices of 44% has been shown (Rothe et al. (2006)), which have been verified as being part due to direct singlets formed on recombination and part from triplet fusion, singlets produced during triplet annihilation (Kondakov et al. (2009), King et al. (2011), and Zhang and Forrest (2012)). Here, the various routes by which triplet excitons can generate singlet states are discussed and their relative contributions to the overall electroluminescence yield are given. The materials requirements to obtain maximum singlet production from triplet states are discussed. These triplet contributions can give very high device yields for fluorescent emitters, which in the case of blue devices can be highly advantageous. Further, new devices architectures open up which are simple and have intrinsically low turn on voltages, ideal for large-area OLED lighting applications. 1. Introduction Current state-of-the-art OLED and PLED devices have been optimised for use in displays, having small-area pixels, yielding high efficiency and good individual colour from each pixel. Active matrix displays using these are now found in the latest smart phones (ca. 2012), including Samsung’s fastest selling smart phone to date, the Galaxy S2. The displays employ red phosphorescent emitters but blue and green fluorescent emitters. In the latest generation of high-efficiency (>60？lm/W) organic solid-state lighting (OSSL) panels from Novaled, Osram, and Konica Minolta-Philips, use of all phosphorescent emitters yields very warm white colours, with poor colour temperatures of 2600？K, and poor lifetimes especially for the blue component [1–3]. The phosphorescent emitters lack good saturated colour but more importantly the blue metal organic complexes used are unstable. For the most common blue (aqua) phosphor, FIrpic [4], vacuum deposition causes partial loss of fluorine substituents from the ligands, and partial decomplexation of the
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