Organic photovoltaics (OPV) are close to reaching a landmark 20% device efficiency. One of the proposed reasons that OPVs have yet to attain this milestone is their propensity toward triplet formation. However, the observation of significant triplet populations in high-performing blends suggests that the influence of triplet states may not be so simple. I will discuss two systems in which triplets are, respectively, a hindrance and a help.

Small molecule donor DRCN5T can achieve OPV efficiencies of over 10%, but it generates an unusually high population of triplets. These triplets are primarily formed in amorphous regions via back recombination from a charge transfer state, and also undergo triplet-charge annihilation. 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.

Efficient charge photogeneration in conjugated polymers typically requires the presence of a second component to act as electron acceptor. A novel low band-gap conjugated polymer (PCPDT-sFCN) with a donor / orthogonal acceptor motif is examined, where the role of the orthogonal acceptor is to spatially isolate the LUMO from the HOMO. This allows for negligible exchange energy between electrons in these orbitals and minimises the energy gap between singlet and triplet charge transfer states. We employ ultrafast and microsecond transient absorption spectroscopy to demonstrate that, even in the absence of a separate electron acceptor, PCPDT-sFCN shows efficient charge photogeneration in both pristine solution and film. This efficient charge generation is a result of an isoenergetic singlet/triplet charge transfer state equilibrium acting as a reservoir for charge carrier formation. Remarkably, the free charges that form via the charge transfer state are extraordinarily long-lived with millisecond lifetimes, due to the stabilisation imparted by the spatial separation of PCPDT-sFCN’s donor and orthogonal acceptor motifs. The efficient generation of long-lived charge carriers in a pristine polymer paves the way for single-material applications such as organic photovoltaics and photodetectors.