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
(Voc) 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.