Fine-tuning the phase-separated morphology is of great importance to achieve efficient all-small-molecule organic solar cells (ASM-OSCs). In this work, a pair of isomers are designed and synthesized, namely, BDT-UF and BDT-DF, in which the fluorine atom in BDT-UF is close to the alkyl chain of side-chain thiophene, while that in BDT-DF is close to the center core. Owing to the noncovalent interaction between fluorine and hydrogen, BDT-DF shows a smaller dihedral angle between the thiophene side chain and the BDT core, which causes better molecular planarity. When mixed with N3, BDT-UF shows better miscibility, higher crystallinity, and more ordered molecule stacking in the blend film. Finally, the device of BDT-DF:N3 gains a power conversion efficiency (PCE) of 14.5%, while that of BDT-UF:N3 shows an increase in V oc, J sc, and FF and gains a PCE of 15.1%. Our work exhibits a way of adjusting the substitution site of fluorine atoms to improve the PCE of ASM-OSCs.

There is always a dilemma between strong π−π stacking/ crystallinity and suitable domain size for all-small-molecule organic solar cells (ASM-OSCs), which puts forward higher requirements for the design of molecular donors. In this work, a series of novel molecular donors with different positional 2-ethylhexy (EH) attachments are designed and synthesized, named SM-R, SM-REH, SM-EH-R, and SM-EH-REH. It is found that EH-substitution on end groups (SM-REH) enables improved π−π interaction and crystallinity but with decreased solubility and phase size, leading to the improved efficiency of 15.6% as compared to 14.0% of SM-R. In contrast, EH-substitution on the πbridge (SM-EH-R) significantly suppresses π−π stacking and increases the solubility, resulting in the lower efficiency of 11.9%. The further EHsubstitution on end-groups of SM-EH-R, namely, SM-EH-REH, recovers the π−π stacking strength and obtains a moderate efficiency of 14.4%. Despite the higher crystallinity and increased π−π stacking in some molecules, the blend films show the gradually decreased domain size in the sequence of SM-R, SM-REH, SM-EH-R, and SM-EH-REH owing to the steric hindrance of the EH-chain. Overall, this work indicates that obtaining the higher π−π stacking/crystallinity and decreased domain size is achievable by tuning the EH-chain substitution, which paves the way to further improve the photovoltaic performance of ASM-OSCs.