Alkylammonium Spacer-Directed Charge-Transfer in 2D|3D Perovskite Solar Cells

Long-term stability is a key challenge for Perovskite Solar Cells (PSCs). A promising strategy is to construct two-dimensional|three-dimensional (2D|3D) lead-halide perovskite heterostructures, which integrate the high efficiency of 3D phases with the high durability of 2D layers. These layers are constructed on 3D perovskites by spreading spacer solutions onto their surfaces, forming 2D|3D structures. Here, we systematically investigate the role of two alkylammonium spacers by using butylammonium iodide (BAI, a Ruddlesden-Popper spacer) and butyl-1,4-diammonium diiodide (BDAI₂, a Dion-Jacobson spacer) in two different concentration regimes. At low spacer concentrations (5 mmol L^-1 BAI and 0.5 mmol L^-1 BDAI₂), both spacers primarily acted as surface passivators, yielding efficiencies comparable to those of pristine methylammonium lead iodide perovskite. High spacer concentrations (50 mmol L^-1 BAI and 5 mmol L^-1 BDAI₂) induced layered 2D|3D phases with contrasting effects. BAI promoted structural flexibility (by varying n from 1 to 2) and improved moisture resistance, thereby enhancing device stability. BDAI₂ unexpectedly leads to rapid degradation under ambient processing conditions. Photoluminescence, conductive atomic force microscopy, and electrochemical impedance spectroscopy confirmed that excessive spacer incorporation introduces insulating barriers that hinder charge transport. Optimized concentrations suppress nonradiative recombination without compromising conductivity. Durability tests demonstrated that BAI consistently prolonged device lifetime, while BDAI₂ devices degraded rapidly. These results reveal that spacer chemistry and concentration critically determine the trade-off between charge transport and environmental resilience in 2D|3D PSCs. By clarifying these mechanisms, this work establishes insights into the rational design of spacer-assisted perovskites, offering a pathway toward more durable and commercially viable perovskite photovoltaics.