Over 20% Efficient Water-Based Layer-by-Layer Organic Solar Cells with High Thickness Tolerance Enabled by Surfactant Promoted Electrostatic Interaction.

Aqueous processing represents a promising eco-friendly fabrication route for organic solar cells (OSCs), aligning with growing industrial sustainability requirements. While water-dispersed semiconducting nanoparticles (NPs) offer an attractive solution, the essential surfactants required for NP stabilization typically compromise device performance. In this study, surfactant-engineered donor NPs are systematically evaluated for constructing optimized active layers through a sequential layer-by-layer (LBL) deposition approach. The surfactant named sodium dodecyl phosphate (SDP), featuring dual anionic charges, generates exceptional electrostatic potential (ESP) differences that promote strong donor-acceptor interactions. This electrostatic engineering enables the formation of a pseudo-planar heterojunction structure (PPHJ) with ideal vertically graded morphologies in thick active layers. Therefore, the PM6:L8-BO binary OSC processed by mesostructured NP (mn)-LBL (SDP) strategy shows excellent thickness tolerance and achieved a PCE of 18.9% (certified as 18.3%) with a 300 nm active layer. Furthermore, the mn-LBL OSCs with the ternary PM6:L8-BO:BTP-eC9 deliver a champion PCE of 20.3% (certified as 19.9%) processed by a non-halogenated water/toluene solvent system. This work establishes a general surfactant selection paradigm that simultaneously addresses the conflicting demands of nanoparticle stabilization, morphological control, and device performance, paving the way for sustainable manufacturing of high-efficiency OSCs.