Reducing Trap Density in Organic Solar Cells via Extending the Fused Ring Donor Unit of an A–D–A‐Type Nonfullerene Acceptor for Over 17% Efficiency

Abstract The high trap density (generally 10¹⁶ to 10¹⁸ cm⁻³) in thin films of organic semiconductors is the primary reason for the inferior charge‐carrier mobility and large nonradiative recombination energy loss (ΔE_nr) in organic solar cells (OSCs), limiting improvement in power conversion efficiencies (PCEs). In this study, the trap density in OSCs is efficiently reduced via extending the donor core of nonfullerene acceptors (NFAs) from a heptacyclic unit to a nonacyclic unit. TTPIC‐4F with a nonacyclic unit has stronger intramolecular and intermolecular interactions, affording higher crystallinity in thin films relative to its counterpart BTPIC‐4F. Thus, the D18:TTPIC‐4F‐based device achieves a lower trap density of 4.02 × 10¹⁵ cm⁻³, comparable to some typical high‐performance inorganic/hybrid semiconductors, with higher mobility and inhibited charge‐carrier recombination in devices. Therefore, the D18:TTPIC‐4F‐based OSC exhibits an impressive PCE of 17.1% with a low ΔE_nr of 0.208 eV, which is the best known value for A–D–A‐type NFAs. Therefore, extending the donor core of NFAs is an efficient method for suppressing trap states in OSCs for high PCEs.