Singlet-exciton formation, efficiency

The value of y is related to the injection process and depends on electrode materials. InP-LJBD, a calcium electrode is used to increase the electron injection efficiency due to its low work function(12,13). The value of 7 e.h related to not only materials but also device structure. Multi-layer structure is commonly used to increase the value of 7, generating the hole and/or electron accumulation near a light-emitting layer surface(l,14). The maximum value of 7 1 has been suggested to be 25% because of the spin statistics of singlet exciton formation (11). Toin rove g, we have to use highly efficient materials or dye-doped materials(15). 

The efficient formation of singlet excitons from the positive and negative charge carriers, which are injected via the metallic contacts and transported as positive and negative polarons (P+ and P ) in the layer, and the efficient radiative recombination of these singlet excitons formed are crucial processes for the function of efficient electroluminescence devices. 

Despite fission being orders of magnitude slower than the solid state in this diffusion-limited system, fission was equally efficient. The triplet exciton yield determined from the relative extinction coefficients of the triplet and singlet excitons was 197%. Such a high yield indicates that once the molecules collide in solution, fission proceeds rapidly in this exothermic system. Indeed the observed rate of triplet formation in the concentrated solution suggests that fission occurs as fast as the diffusional timescales allow the rise of the triplet exciton absorption had a 500 ps time constant, reaching 30% of the expected diffusion limit for the system. 

It is interesting to compare how this picture of endothermic fission in solution compares to the solid state. Whilst no distinct intermediate has been optically observed in a solid-state system, we consider that the identification of a stabilized triplet-pair state is consistent with other studies of solid state tetracene. The temperature-independence of singlet exciton decay in tetracene has been measured by several groups and has led to proposals of a low-energy dark state that mediates the efficient fission. The spectral differences observed between bound and free triplets may be so pronounced simply due to the conformational freedom of the solution system. It has been speculated that the stabilisation observed in the triplet-pair excimer, relative to free triplet excitons, may occur to a lesser degree in the solid state as well and thus enable the fast formation of bound triplets. 

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