The Role of the Third Component in a Ternary Organic Donor-Acceptor Heterojunction Blend Based on Large-Size Fused-Ring Electron Acceptors

Incorporating large fused-ring electron acceptors as a third component in ternary organic solar cells (OSCs) has emerged as a promising strategy to enhance power conversion efficiency (PCE) and thermal stability. However, the impact of this third component on charge generation and recombination mechanisms remains unclear. Here, we investigate the photoexcited dynamics of a ternary heterojunction blend (L2:FNIC4:FBTIC) incorporating a star-shaped FBTIC molecule as the third component using broadband transient absorption spectroscopy. Singular value decomposition and global-target analysis reveal that FBTIC plays three critical roles. First, it serves as a complementary absorber, extending light-harvesting capabilities and facilitating energy transfer to the electron acceptor, FNIC4. Second, it accelerates hole transfer (0.79 ps vs 4.9 ps) and charge separation (95 ps vs 115 ps) via a hole-transfer pathway with a low driving force of 0.08 eV, thereby improving the internal quantum efficiency of the solar cell. This enhancement is attributed to morphological modulations induced by FBTIC, consistent with improved electron mobility in the ternary blend. Third, FBTIC suppresses charge recombination by diluting the L2-FNIC4 heterojunction interface, resulting in an increased open-circuit voltage. These synergistic effects collectively enhance the PCE of ternary OSCs, providing valuable insights for the design of next-generation photovoltaic materials and devices.