Modulation of Molecular Stacking via Tuning 2-Ethylhexyl Alkyl Chain Enables Improved Efficiency for All-Small-Molecule Organic Solar Cells

There is always a dilemma between strong N-N 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 SMEH-REH. It is found that EH-substitution on end groups (SM-REH) enables improved N-N 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 N bridge (SM-EH-R) significantly suppresses N-N stacking and increases the solubility, resulting in the lower efficiency of 11.9%. The further EH substitution on end-groups of SM-EH-R, namely, SM-EH-REH, recovers the N-N stacking strength and obtains a moderate efficiency of 14.4%. Despite the higher crystallinity and increased N-N 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 N-N 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.

Automated summary

There is a dilemma between strong N-N stacking/crystallinity and suitable domain size in all-small-molecule organic solar cells (ASM-OSCs), which requires careful design of molecular donors. A series of novel molecular donors with different 2-ethylhexy (EH) attachments were designed and synthesized. EH-substitution on end groups (SM-REH) improved N-N interaction and crystallinity but reduced solubility and phase size, resulting in higher efficiency. In contrast, EH-substitution on the N bridge (SM-EH-R) suppressed N-N stacking and increased solubility, leading to lower efficiency. Further EH substitution on end-groups of SM-EH-R, namely SM-EH-REH, recovered N-N stacking strength and achieved moderate efficiency.