A Review on the Emitting Species in Conjugated Polymers for Photo- and Electro-luminescence

In conjugated polymers, fluorescence from a single polymer chain, which is contrasted with measurements on an ensemble or bulk collection of molecules, can provide important information about conformation of single polymer chain and energy transfer between different conjugation length segments within single polymer chain. On the other hand, the presence of intrachain or interchain interactions plays an important role on their photoluminescence (PL) and electroluminescence (EL) properties In addition to non-interacting isolated chain, there are five interacting emitting species, namely photo-excited dimers including excimer and exciplex, electrical-excited dimers including electromer and electroplex, and aggregate. Excimer, exciplex, and aggregate can lead to a generation of new peak with featureless structure on the longer wavelength side in the fluorescence spectrum and usually cause a lowering in PL quantum efficiency These three emitting species can be observed in both PL and EL spectra In addition, electromer and electroplex only appear in EL spectrum but not in PL spectrum. The presence of electromer and electroplex also leads to a new peak at longer wavelength side. Another non-interacting ground-state emitting species, beta phase, consists of a chain segment with a more coplanar conformation and therefore results in a longer conjugation and clear vibronic transitions.