Unlocking charge transport in Dion-Jacobson perovskite solar cells through rational spacer design

Dion-Jacobson (DJ) phase perovskite solar cells have emerged as a promising platform for achieving exceptional stability without sacrificing charge transport efficiency. However, their performance has long been hindered by inefficient out-of-plane carrier extraction, primarily due to the insulating nature of conventional organic spacer cations. This review systematically examines and critically assesses recent advances in the nanoscale engineering of spacer molecules to transform them from passive structural units into active electronic and functional components. We critically analyze recent spacer engineering strategies, including π-conjugated and semiconducting spacers, molecular rigidity and size modulation, functional group incorporation, and mixed-spacer compositional gradients. We show how these approaches can collectively suppress dielectric and quantum confinement, promote vertical crystal orientation, reduce exciton binding energies, and enable direct orbital coupling across organic–inorganic interfaces. Beyond performance enhancement, we explicitly discuss the inherent trade-offs between electronic activation, environmental stability, and manufacturability, highlighting the emerging challenges associated with synthetic complexity, phase separation, and scalability. By integrating mechanistic insights with practical limitations, this review formulates realistic design principles for multifunctional spacers and outlines critical pathways toward DJ perovskite solar cells that simultaneously achieve high efficiency, operational stability, and technological relevance. • Spacer design governs crystallization, orientation, charge transport, and stability in DJ perovskites. • π-Conjugated spacers reduce dielectric confinement and promote efficient out-of-plane charge transport. • Functional groups regulate defect passivation, interlayer coupling, and buried-interface crystallization kinetics. • Mixed-spacer strategies enable phase regulation and built-in energy-funneling in DJ perovskites. • Future DJ spacer design balance efficiency, stability, synthetic simplicity, and scalability.