Insights into the Charge-Transfer Mechanism of Organic Photovoltaics: Effect of Domain Size

A great effort has been devoted into understanding the mechanisms of charge generation and charge separation processes in bulk heterojunction solar cells, with the aim of improving their performance. Theoretical methods, such as density functional theory (DFT), have been used to shed light into these complex processes, but the computational cost associated with the simulations limits the model size and thus its accuracy with respect to real heterojunctions. To overcome this limitation, a linear-scaling reformulation of time-dependent DFT is employed, allowing to move beyond the simple polymer–fullerene models and to consider larger complexes composed of more than a single oligomer chain and numerous fullerene molecules. In this work, the interaction between an analogue of PBTZT-stat-BDTT-8, a high-performance D–A statistical copolymer developed by Merck, and phenyl-C₆₁-butyric acid methyl ester is explored, with a focus on (i) the effect of the size of the polymer’s acceptor (A) blocks and (ii) the effect of the domain size. Results suggest that large acceptor blocks enhance the probability of a charge transfer (CT) to occur and that CT states are more significantly affected by the size of the polymer rather than the fullerene phase. Evidence of long-range CT states in the low-energy part of the excited-state manifold is also observed.