Correlation between molecular configuration and charge transfer dynamics in highly efficient organic solar cells

Molecular design and charge transport dynamics play an important role in achieving highly efficient organic solar cells (OSCs). In this work, non-fullerene acceptor Y6, its alkyl chain modified derivative N3 and halogen-replaced derivative BTP-BO-4Cl are selected to fabricate OSCs. The OSCs obtain a high power conversion efficiency (PCE) of 16.86%. The impact of different modification methods on molecular configuration and energy level matching is investigated systematically by time-resolved photoluminescence (TRPL) and transient absorption spectroscopy (TAS). The results demonstrate that side chain modification has a positive effect on molecular layer spacing, domain area size and charge carrier mobilities. More importantly, halogen atom replacing has the similar but more positive impact, bringing a higher PCE than side chain modification. The overall processes of charge generation, dissociation and recombination in OSCs are studied systematically for the first time. For the best-performance device, the lifetimes of singlet exciton recombination (τ1), charge transfer (CT) state recombination (τ2), singlet exciton dissociation (τ3) and CT state dissociation (τ4) are calculated as ∼500 ps, ∼2 ns, ∼200 fs and ∼1 ps, respectively. The correlation between molecular configuration and charge transfer dynamics is established, which can provide a guideline for designing materials of highly efficient OSCs.