Alkyl chain modulation of asymmetric hexacyclic fused acceptor synergistically with wide bandgap third component for high efficiency ternary organic solar cells

Herein, two asymmetric hexacyclic fused small molecule acceptors (SMAs), namely BP4F-HU and BP4F-UU, were synthesized. The elongated outside chains in the BP4F-UU molecule played a crucial role in optimizing the morphology of blend film, thereby improving charge mobility and reducing energy loss within the corresponding film. Notably, the PM6:BP4F-UU device exhibited a higher open-circuit voltage (V_oc) of 0.878 V compared to the PM6:BP4F-HU device with a V_oc of 0.863 V. Further, a new wide bandgap SMA named BTP-TA was designed and synthesized as the third component to the PM6:BP4F-UU host binary devices, which showed an ideal complementary absorption spectrum in PM6:BP4F-UU system. In addition, BTP-TA can achieve efficient intermolecular energy transfer to BP4F-UU by fluorescence resonance energy transfer (FRET) pathway, due to the good overlap between the photoluminescence (PL) spectrum of BTP-TA and the absorption region of BP4F-UU. Consequently, ternary devices with 15wt% BTP-TA exhibits broader photon utilization, optimal blend morphology, and reduced charge recombination compared to the corresponding binary devices. Consequently, PM6:BP4F-UU:BTP-TA ternary device achieved an optimal power conversion efficiency (PCE) of 17.83% with simultaneously increased V_oc of 0.905 V, short-circuit current density (J_sc) of 26.14 mA/cm², and fill factor (FF) of 75.38%.