Triplet states on π-conjugated polymers, oligomers and related materials

Abstract

Organic conjugated polymers and oligomers are continuously growing in interest, with vast applications ranging from OLEDs (where emission from phosphorescent organic light-emitting materials has received particular attention) to photovoltaic cells. For these applications, a deep knowledge on the mechanisms leading to the formation and deactivation of the excited states is mandatory. Relevant developments in recent years have been focused on the characterization and understanding of triplet states, and finding how these can be used to increase light emitting efficiencies in optoelectronic devices by transforming triplet energy into luminescence. Indeed, in electroluminescent devices, more triplet states are generated than singlets and, since phosphorescence in these systems is generally weak, methods, such as delayed fluorescence, either thermally activated (TADF) or involving triplet–triplet annihilation, are showing ways of increasing light emitting efficiencies in polymers or small molecules, while, in certain cases, excited state yields can be doubled through singlet fission, where one excited singlet forms two triplet states. The involvement of triplet states in the efficiency of electroluminescent devices presents the drawback that comes from their long lifetimes, which can lead to a decrease in polymer stability, particularly through sensitized singlet oxygen generation and reactions. A review of all these aspects, essentially covering the 2012–2014 period, is given here.