Optimal morphology, H-aggregation, and ternary blend excited state disruption in equilibrated squaraine-based all small molecule solar-cells

•Control of OPV thin film morphology and aggregation via second squaraine surfactant.•Despite prediction, no evidence of inherently poor OPV efficiency from H-aggregates.•Broad and transient population of aggregated states drives down OPV efficiency.•OPV commercial ability when optimal morphology parallels thermal equilibrium state. Organic photovoltaic (OPV) devices, while demonstrating high efficiency in the lab, will often degrade substantially with use in the field due to changes in active layer morphology associated with long-term demixing away from a kinetically stable pristine device. With Squaraine-Fullerene blends, the formation of H-aggregates through annealing would theoretically provide an efficiency loss through expected changes to energy transfer, or exciton diffusion behaviors. Here, by combining different amounts of two Squaraines in a ternary blend with a fixed amount of fullerene acceptor, the film morphology is varied by Squaraine intermixing leading to control over the extent of dimer H-aggregate (DHA) and charge transfer H-aggregate (CT-HA) formation. Morphological changes are inferred through absorbance spectroscopy, by known differences between the spectra of the aggregate and the monomer, and with kelvin probe force microscopy. The secondary Squaraine is shown to act as a form of surfactant, enhancing mixing between the primary Squaraine and the fullerene; we present data that suggests that morphology changes from annealing may be accessed through changing material combinations. Devices made from varying Squaraine-Squaraine-Fullerene blend ratios are measured for efficiency, thereby providing a measure of the relative importance of both morphology and H-aggregation on that efficiency. These data reveal that H-aggregates are not inherently bad for device performance but, rather, it is the co-existence of DHAs and CT-HAs which is problematic, reducing power conversion efficiency by upwards of 15%. This raises concern for other OPV devices as the natural annealing of field operation induces aggregation, changing morphology, and a similar co-existence of incompatible species.