Researchers from Korea's Jeonbuk National University have developed a rational bilayer surface passivation strategy to address one of the key bottlenecks in vacuum-deposited perovskite solar cells (PSCs): defect-rich, non-uniform crystallization arising from solid-state film formation.
Schematic diagram of device structure. Image from: Advanced Functional Materials
Vacuum thermal evaporation is widely regarded as a scalable route for PSC fabrication, offering precise control over thickness, stoichiometry, and large-area uniformity. However, the absence of a solvent limits mass transport and surface diffusion during film growth, often leading to incomplete crystallization and a high density of surface and grain-boundary defects. In particular, halide vacancies generate uncoordinated Pb2+ sites, which act as nonradiative recombination centers, suppress quasi-Fermi level splitting and reduce the achievable open-circuit voltage (VOC). These defect sites also accelerate ion migration and environmental degradation, undermining device stability. To overcome these limitations, the researchers introduced an “Anchor-and-Seal” bilayer passivation approach based on sequential deposition of ethylenediammonium diiodide (EDAI2) and 4-methoxy-phenethylammonium iodide (4MeO-PEAI).
The design leverages complementary molecular properties to achieve more complete defect passivation than conventional single-layer treatments. In this system, the divalent EDA2+ cations act as a rigid anchoring layer, binding strongly to surface defects through hydrogen bonding and Lewis base coordination with undercoordinated Pb2+. However, due to steric limitations, EDAI2 alone cannot fully access nanoscale voids present on the irregular polycrystalline surface. The second layer, composed of flexible 4MeO-PEAI molecules, infiltrates these remaining nanometer-scale gaps, effectively “sealing” residual electronic voids. This hierarchical coverage results in a more uniform and defect-suppressed interface.
Beyond defect passivation, the bilayer also modifies the interfacial energetics. The treatment induces an upward shift of the Fermi level, improving energy-level alignment at the perovskite/electron transport layer interface and facilitating more efficient charge extraction. As a result, non-excitonic recombination losses are reduced, leading to enhanced device performance.
Devices incorporating this bilayer strategy achieved a champion power conversion efficiency (PCE) of 20.59% in a p–i–n architecture, with a notably improved VOC and fill factor. In addition to efficiency gains, the hydrophobic nature of the 4MeO-PEAI capping layer contributes to enhanced environmental robustness. The devices retained 94% of their initial efficiency after 1000 hours in an inert atmosphere, and 90% after 200 hours of continuous maximum power point tracking under ambient conditions.
Importantly, the approach also demonstrates scalability. A lab-scale mini-module with an active area of 2.4 cm2 achieved a PCE of 18.46%, highlighting the compatibility of this strategy with large-area vacuum processing.
Despite these advances, the study notes that interfacial passivation alone cannot fully eliminate residual bulk defects formed during vacuum deposition. As a result, vacuum-processed PSCs still lag behind the best solution-processed devices. The authors suggest that further improvements will require complementary strategies such as improved stoichiometric control during co-evaporation, bulk passivation via vacuum-compatible additives, and vapor-assisted post-annealing to reduce grain boundary density.
Overall, the work demonstrates how spatially and electronically complementary molecular design can address the unique defect landscape of vacuum-deposited perovskites, providing a practical pathway toward high-efficiency, scalable photovoltaic technologies.
