Publication date: 15th December 2025
Y-family acceptors have propelled organic solar cell (OSC) power conversion efficiencies to beyond 21%, yet the reason for the high efficiency is a matter of ongoing debate. Here, we investigated OSCs based on either Y6 or ZY-4Cl as acceptor, both having very similar ionization energies, combined with one of 11 different electron donor materials, spanning an ionization energy offset (ΔIE) range from 0.2 to 0.9 eV. We find that near-unity quantum (charge generation) efficiency is obtained in D:Y6 blends at IE offsets as low as ~0.3 eV, while D:ZY-4Cl blends require double the ΔIE, i.e., at least ~0.6 eV. Density functional theory (DFT) calculations attribute this substantial difference to a lower interfacial charge-transfer (CT) energy in D:Y6 blends. We rule out intrinsic charge generation in Y6 or exceptionally long exciton delocalization in Y6 aggregates as the main origin of efficient charge separation. Instead, the stronger electron-withdrawing acceptor group in Y6 appears to reduce the interfacial CT energy more than it reduces the local exciton energy, a trend that holds also for seven other Y-family derivatives, explaining their high efficiency.
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