Suppressing trap density and energy loss via skeleton asymmetry strategy enables highly efficient all-small-molecule organic solar cells

Reducing energy loss while ensuring the required charge collection is of vital importance to high-performance all-small-molecule organic solar cells (SM-OSCs) limited by high trap density (1016 ∼ 1018 cm−3) in bulk heterojunction films. Herein, we show that the trap density in SM-OSCs can be dramatically reduced by designing a small-molecule donor (SMD) using a skeleton asymmetry strategy. Compared with its symmetric counterpart, TBD-BCl with an asymmetric thienobenzodithiophene (TBD) central core has different rotational energy barriers and a transformed C-shaped configuration, endowing blend films with strong intermolecular interactions and higher crystallinity. Thus, blending TBD-BCl with the L8-BO acceptor leads to a low trap density of 3.21 × 1015 cm−3 and density of state of 46 meV relative to BDT-BCl:L8-BO films. In addition, joint experimental and theoretical studies revealed that the TBD-BCl:L8-BO complex with a much lower driving force can still facilitate exciton dissociation, suppress charge-carrier recombination, and significantly reduce energy loss in devices. As a result, SM-OSCs based on TBD-BCl:L8-BO deliver a higher efficiency of 16.2% and an improved Voc of 0.91 V. Overall, the experiments and theory calculations in this study provide a new perspective for designing SMDs that suppress trap states and reduce the energy loss in SM-OSCs.