Origin of poor photovoltaic performance of bis(tetracyanoantrathiophene) non-fullerene acceptor

Light-induced EPR and out-of-phase electron spin echo techniques were applied to identify the origin of low photovoltaic performance of novel non-fullerene antrathiophene-based acceptor 2,2′-[2,2′-(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(anthra[2,3-b]thiophene-5,10-diylidene)]tetrapropanedinitrile (AT1), compared to conventional fullerene acceptors. These techniques revealed that average trapping energy of electrons in PCDTBT/AT1 blend is higher than that in the blends of PCDTBT with fullerene acceptors, while the distance of the initial photo-induced electron transfer is decreased. These two effects are suggested to enhance the efficiency of geminate recombination in PCDTBT/AT1 blend and reduce the yield of free charges, which is supposed to be the main reason for poor OPV performance of AT1-containing blends. Low electron mobility μ_e = 3.5 *10^-6 cm²V^-1s⁻¹ is estimated for AT1 from photo-CELIV experiment. Strong localization of the electrons on terminal acceptor units of AT1 consistently explains both efficient geminate recombination in blends of antrathiophene-based acceptor with polymer donors and low electron mobility in AT1.