Effect of H- and J-Aggregation on the Photophysical and Voltage Loss of Boron Dipyrromethene Small Molecules in Vacuum-Deposited Organic Solar Cells

An understanding of the factors limiting the open-circuit voltage (V-oc) and related photon energy loss mechanisms is critical to increase the power conversion efficiency (PCE) of small-molecule organic solar cells (OSCs), especially those with near-infrared (NIR) absorbers. In this work, two NIR boron dipyrromethene (BODIPY) molecules are characterized for application in planar (PHJ) and bulk (BHJ) heterojunction OSCs. When two H atoms are substituted by F atoms on the peripheral phenyl rings of the molecules, the molecular aggregation type in the thin film changes from the H-type to J-type. For PHJ devices, the nonradiative voltage loss of 0.35 V in the J-aggregated BODIPY is lower than that of 0.49 V in the H-aggregated device. In BHJ devices with a nonradiative voltage loss of 0.35 V, a PCE of 5.5% is achieved with an external quantum efficiency (EQE) maximum of 68% at 700 nm.