The Interfacial Energetic Landscape in Non-Fullerene Acceptor Organic Solar Cells and Its Impact on Charge Generation and Recombination

In bulk heterojunction organic solar cells, the energetic landscape at the donor-acceptor interface provides the driving force for charge separation. The mechanism leading to efficient charge separation in fullerene-based blends has been intensively investigated, however with the recent advent of high-efficiency non-fullerene acceptors (NFAs) now surpassing 19% power conversion efficiency, the previous findings have to be revisited for NFA-based systems. In this presentation, I will discuss our latest insights into the photophysical processes governing charge separation, recombination, and energetic (voltage) losses in novel NFA-based systems studied by steady-state and advanced transient spectroscopy techniques. I will address the question, how the interfacial energy offsets control exciton dissociation and charge separation in binary and ternary blends of polymer or small molecular donors with novel NFAs, including photoactive systems using state-of-the-art Y-type acceptors. Generally, it appears that it is primarily the ionization energy (IE) offset that limits the exciton-to-charge transfer (CT) state conversion in many NFA-based systems, while the subsequent separation of the CT state into free charges is barrier-less. Sizeable IE offsets of 0.4-0.5 eV are required to ensure quantitative exciton-to-CT state conversion. The underlying reasons for this observation and the implications for future donor and acceptor material design strategies will be discussed.<br/>Refs.: <i>Nat. Mater.</i> <b>2021</b>, <i>20</i> (3), 378-384; <i>Adv. Energy Mater. </i><b>2021</b>, <i>11 </i>(28), 2100839;<i> Adv. Energy Mater. </i><b>2021</b>, 2102363;