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

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.