J-aggregation of a squaraine dye and its application in organic photovoltaic cells

In this paper, we investigated the aggregation behavior of a squaraine dye in solid films, as well as the effects on its optical and electrical properties. We observed a strong intermolecular interaction in thin films, which can be controlled via processing solvents. Two typical aggregation behaviors, i.e. H- and J-aggregation, can be selectively formed via the proper selection of solvents, resulting in a significant difference in material properties and the resultant photovoltaic performance. In addition to the monomer absorption between 600 nm and 700 nm, the H-aggregation mainly absorbs green light around 500-600 nm, while the J-aggregation shows a strong absorption in the near infrared range up to 850 nm. Therefore in squaraine/fullerene bi-layer hetero-junction photovoltaic cells, we demonstrated a significant contribution of J-aggregation to the photocurrent. Furthermore the J-aggregation films have a deeper ionization potential, most likely due to the intermolecular charge-transfer interaction, and thus gave rise to a larger open circuit voltage than those of the H-aggregation based devices. The J-aggregation formation shall be considered as an effective approach to tune the optical/electrical properties in organic optoelectronic devices.