Modulation of intermolecular interactions in the active layer enables highly efficient organic solar cells via introducing solid additives

Solid additives play a crucial role in developing highly efficient organic solar cells (OSCs) by improving intermolecular interactions between polymer donors and acceptors, forming optimal bulk heterojunction (BHJ) morphology. In this study, we report three volatile solid additives: 1,4-dibromobenzene (DBB), 1-bromo-4-iodobenzene (BIB), and 1,4-diiodobenzene (DIB). Their electrostatic potential on the molecular van der Waals surface decreases as iodine atoms replace bromine atoms. Fourier transform infrared spectroscopy indicates that these solid additives are completely removed during thermal annealing of the active layer. Theoretical calculations show that DIB exhibits the strongest intermolecular interactions with PM6 and BTP-eC9, respectively. The DBB shows the weakest intermolecular interactions with PM6 and BTP-eC9, respectively. Interestingly, the BIB-treated blend film exhibits the longest crystal coherence length (23.2 & Aring;) and the fastest hole transfer (0.17 ps), compared to DBB- (19.0 & Aring;, 0.25 ps) and DIB- (19.7 & Aring;, 0.23 ps) treated blend films. Consequently, the DBB- and DIB-treated active layers exhibit power conversion efficiencies (PCEs) of 17.14% and 18.25%, respectively, while the BIB-treated active layer achieves an excellent PCE of 18.91%. This work demonstrates that rational modulation of active layer intermolecular interactions through solid additives is essential for developing high-performance OSCs.