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

Abstract 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.