Non-competitive additives regulate molecular assembly for compact monolayers in perovskite/silicon tandems.

Achieving densely packed self-assembled monolayers (SAM) is essential for constructing well-defined functional interfaces, yet monolayer formation is often hindered by molecular aggregation and additive-induced competition for surface anchoring sites. Here we report a non-competitive additive strategy that regulates molecular assembly during SAM deposition. Using 1,3,5-tris(trifluoromethyl)benzene (TTMB) as a non-anchoring additive, aggregation of the SAM molecule is suppressed while all substrate binding sites remain available for molecular anchoring. Because TTMB interacts only weakly with the surface, it does not participate in surface binding but increases the population of anchoring-capable molecules and strengthens SAM-substrate interactions, enabling the formation of densely packed monolayers. The resulting interfaces exhibit improved energetic alignment, reduced non-radiative recombination, and suppressed shunt pathways. When implemented in monolithic perovskite/silicon tandem solar cells, the compact SAM enables a certified stabilized power conversion efficiency of 33.66%. This non-competitive additive concept provides a general strategy for directing molecular assembly in monolayer materials.