Carrier Recombination and Emission Process

Neutral excited states, typically singlet excitons, are produced by charge recombination, and emission is due to emissive transitions from neutral excited states to ground states. No experimental evidence on direct emission due to a recombination of positive and negative charges in OLED devices has been provided yet. The previous theoretical consideration38,45 revealed that carrier recombination creates both singlet (.S ) and triplet excitons (7j) at a ratio of 1 to 3  

In addition, we should mention the exciplex formation between adjoining molecules, which is frequent and poses a serious problem for obtaining pure EL colors and high efficiency in organic multilayer solid devices 46 

In most cases, because the exciplex formation results in a low-fluorescence efficiency with long-wavelength emission, we need to prevent it in order to achieve high luminance. 

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