Elucidating the role of heterojunction in pristine non-fullerene acceptor organic solar cells

Non-fullerene acceptors (NFAs) are rapidly transforming organic solar cell (OSC) performance and stability, yet the operational principles of pristine NFA devices remain underexplored. Here, we reveal that interfacial energetics, rather than bulk properties, dominate charge generation and recombination in pristine NFA-based photoactive layers (PALs). Although recent studies suggest spontaneous charge generation within the NFA bulk, our findings demonstrate that charge generation and extraction predominantly occur at the hole transport layer (HTL)/NFA interface, mimicking bilayer device behavior. Moreover, while CuSCN forms favorable interfaces that sustain long-lived charges and enhance photocurrent, PEDOT:PSS exhibits poor energy level alignment and a high trap density, leading to severe recombination losses via triplet exciton formation. Introducing as little as 2 wt% donor polymer surpasses the PAL percolation threshold, forming donor-acceptor interfaces that enhance photon utilization, reduce injection barriers, and improve charge transport. Our results not only challenge current interpretations of charge generation in pristine NFA devices but also establish new design principles for simplified, scalable single-component OSCs suited for next-generation semitransparent photovoltaics, including building-integrated photovoltaics (BIPV) and agrivoltaics.