Switching From Acceptor to FRET Donor: How the Organic Solar Cell Architecture Can Change the Role of a Chromophore

Abstract The third component in a ternary organic solar cell (OSC) is generally selected to maximize absorption of the solar spectrum. The fused ring non‐fullerene acceptor 2,2′‐[({4,4,9,9‐tetra‐ n ‐octyl‐4,9‐dihydro‐ s ‐indaceno[1,2‐ b :5,6‐ b ']dithiophene‐2,7‐diyl}bis{benzo[ c ][1,2,5]thiadiazole‐7,4‐diyl})bis(methaneylylidene)]dimalononitrile ( o‐IDT‐BT‐DCV ) was investigated for use in binary and ternary OSCs. The optimized binary device with o‐IDT‐BT‐DCV as the acceptor and PM6 as the donor had a maximum power conversion efficiency (PCE) of 10.8%. Incorporation of o‐IDT‐BT‐DCV into a donor:acceptor PM6:Y6 blend delivered a ternary OSC with a maximum PCE of 16.2%. Femtosecond transient absorption spectroscopy (fs‐TAS), transient photovoltage (TPV), and transient photocurrent (TPC) measurements in combination showed that o‐IDT‐BT‐DCV in the ternary blend did not behave as an acceptor. Instead, it contributed to charge carrier generation through a sub‐picosecond energy transfer process to Y6, followed by a photoinduced hole transfer mechanism with PM6 and/or spontaneous exciton dissociation within the Y6 phase. Encapsulated ternary blend devices were found to be more stable than the binary blend solar cells. Under 1‐sun illumination and maximum power point (MPP) tracking, excluding the initial burn‐in loss, the ternary device retained ≈80% of its MPP over 1200 h compared to the 40% retained by the PM6:Y6 devices.