Breaking the symmetry of interfacial molecules with push-pull substituents enables 19.67% efficiency organic solar cells featuring enhanced charge extraction

The symmetry of a molecule governs its electronic structure, dipole moment, electrostatic potential, and molecular interactions. Symmetry breaking is frequently adopted in donor and acceptor materials for efficient charge separation in organic solar cells (OSCs). In this work, we extend this strategy to interfacial materials and enhance the OSC charge extraction. In particular, we rationally designed an unsymmetrical interfacial phosphonic acid, BrDECz, by introducing an electron-donating and an electron-withdrawing group through straightforward coupling reactions for a push-pull self-assembled monolayer/multilayer (SAM). We systematically show that the unsymmetrical structures induce a larger dipole moment, optimized energy levels, higher adsorption energy, and enhanced conductivity as confirmed by KPFM and C-AFM measurements. These factors collectively contribute to enhanced charge extraction and collection as demonstrated by transient technologies. Consequently, we achieved a 19.67% PCE in binary single junction OSCs, one of the highest reported efficiencies for this type of device. Importantly, the designed unsymmetrical BrDECz interfacial layer is universally applicable to other systems, and offers improved thermal stability compared to PEDOT:PSS. The unsymmetrical interfacial molecule strategy offers valuable insights into the design and application of interfacial materials, presenting a promising approach for further enhancing the photovoltaic performance of OSCs. © 2025 The Royal Society of Chemistry.