Triplet-charge annihilation in a small molecule donor: acceptor blend as a major loss mechanism in organic photovoltaics

Abstract

Organic photovoltaics (OPV) are close to reaching a landmark 20% device efficiency.[1] One of the proposed reasons that OPVs have yet to attain this milestone is their propensity toward triplet formation.

In this talk,[2] the small molecule donor, DRCN5T, is studied using a variety of spectroscopy techniques, and blended with both fullerene and non-fullerene acceptors. Specifically, picosecond and microsecond transient absorption and Raman spectroscopies are focused on. Despite DRCN5T's ability to achieve OPV efficiencies of over 10%,[3] it generates an unusually high population of triplets. These triplets are primarily formed in amorphous regions via back recombination from a charge transfer state. As such, triplets have a dual role in DRCN5T device efficiency suppression: they both hinder free charge carrier formation and annihilate those free charges that do form.

Using microsecond transient absorption spectroscopy under oxygen conditions, this triplet-charge annihilation (TCA) is directly observed as a general phenomenon in a variety of DRCN5T: fullerene and non-fullerene blends. Since TCA is usually inferred rather than directly observed, it is demonstrated that this technique is a reliable method to establish the presence of TCA.