Enhancement of ternary organic solar cell performance using sterically shielded non-fullerene acceptor additives

Ternary organic solar cells (TOSC) have recently gained attention due to their superior performance over two-component organic solar cell (OSC) counterparts. In this study, we investigate three new structural derivatives of the conventional nonfullerene acceptor (NFA) material ITIC, in which the acceptor-donor-acceptor (A-D-A) molecular structure is switched to a D-A-D or A-D-A-D-A configuration. The corresponding materials R-ITIC, R-INIC, and C-ITIC were synthesized using the Janus-like electron-accepting building block 1,5-bis(dicyanomethylene)-3,7-dioxo-1,2,3,5,6,7-hexahydro-s-indacene (CIDC). Computational simulations, photophysical measurements, thin film energy level investigations, and OSC tests indicate that the newly synthesized compounds exhibit poorer solid-state packing ability than the reference compound ITIC, primarily due to increased sterical shielding at the molecular surface. When used as additives in TOSCs with the composition PM6:Y7:additive (10:10:1), the synthesized NFAs increase the power conversion efficiency (PCE) of the devices. The observed enhancement in PCE reaches 1.5 %, from 10.5 % in additive-free devices to 12 % in the case of the best-performing compound, C-ITIC. The improvement in device performance stems from the optimized light absorption profile of the devices, as the additives fill the absorbance gap between PM6 and Y7 components. The absence of ordered aggregates and the resulting inability to participate in charge separation facilitate cascade Förster resonance energy transfer (FRET) from the additive to Y7, since the photogenerated charges are not trapped in small inclusions of the additive material. These findings provide a novel conceptual approach to developing TOSC-designated NFA materials.