Molecular origin of efficient hole transfer from non-fullerene acceptors: insights from first-principles calculations†
Due to the strong exciton binding energy (Eb) of organic materials, the energy offset between donor (D) and acceptor (A) materials is essential to promote charge generation in organic solar cells (OSCs). Yet an efficient exciton dissociation from non-fullerene acceptors (NFAs) began to be observed in D/A blends even at very low driving force for hole transfer (ΔHh). The mechanism behind this efficient photoinduced hole transfer (PHT) remains unclear since current estimates from calculations of isolated molecules indicate that Eb > ΔHh. Here we rationalize these discrepancies using density functional theory (DFT), the total Gibbs free energy method and the extended Hückel theory (EHT). First, we employed DFT to calculate Eb for NFAs of three representative groups (perylene diimide derivatives, indacenodithiophene and subphthalocyanines) as well as for fullerene acceptors (FAs). Considering isolated molecules in the calculations, we verified that Eb for NFAs is lower than for FAs but still higher than the experimental ΔHh in which efficient PHT has been observed. Finding the molecular geometry of the excited state, we also obtain that the structural relaxation after photoexcitation tends to further decrease (increase) Eb for NFAs (FAs). This effect helps explain the delayed charge generation measured in some NFA systems. However, this effect is still not large enough for a significant decrease in Eb. We then applied EHT to quantify the decrease of Eb induced by energy levels coupling between stacked molecules in a model aggregate. We then estimated the number of stacked molecules so that Eb approaches ΔHh's. We found that small NFA aggregates, involving around 5 molecules, are already large enough to explain the experiments. Our results are justified by the low energy barrier to the generation of delocalized states in these systems (especially for the hole delocalization). Therefore, they indicate that molecular systems with certain characteristics can achieve efficient molecular orbital delocalization, which is a key factor to allow an efficient exciton dissociation in low-driving-force systems. These theoretical findings provide a sound explanation to very recent observations in OSCs.