Transient Absorption Reveals the Key Role of a Cascaded Energy Landscape in High-Performance OPV

In this work, a newly synthesized small-molecule acceptor, DM-F, was investigated using femtosecond transient absorption (fs-TA) spectroscopy. It was designed to enhance J-aggregation by replacing the donor core of BTP-eC9-4F with a three-dimensional (3D) central group. UV–visible absorption and photoluminescence measurements confirm that the 3D core promotes stronger J-aggregation in the neat DM-F film, which is generally favorable for organic photovoltaic (OPV) applications. The fs-TA measurements of the D18:BTP-eC9-4F blend revealed a red shift of the acceptor’s ground-state bleach with time delay, which is a signature of the downhill energy transfer process arising from the cascaded energy landscape. But this feature was not observed in D18:DM-F. Such a cascaded energy landscape is widely recognized as critical for achieving high power conversion efficiency (PCE) in OPV. The absence of this downhill energy transfer process is a key reason D18:DM-F exhibits a lower PCE than D18:BTP-eC9-4F, despite enhanced J-aggregation in DM-F. This study highlights and reaffirms the essential role of a cascaded energy landscape in enabling high-performance nonfullerene OPV.