Researchers from Nanjing Tech University have developed a vacuum-deposition strategy that improves the performance of fully evaporated perovskite solar cells (PSCs) by precisely controlling the solid-state reaction pathway during film formation.
Vacuum deposition is widely viewed as a scalable and solvent-free alternative to solution processing, with potential advantages for industrial integration. However, devices fabricated using this approach have historically lagged behind their solution-processed counterparts due to defects formed during crystallization. The Nanjing Tech team addressed this limitation by introducing formamidinium acetate as a reactive precursor that fundamentally alters how the perovskite phase forms.
The key mechanism lies in the direct reaction between formamidinium acetate and lead iodide (PbI2). “We find that formamidinium acetate reacts directly with PbI2 to form FAPbI3 seed layers, substantially lowering the barrier to the solid-state conversion of precursors into the crystalline δ-phase and their subsequent transformation into the α-phase perovskite.” These seed layers act as nucleation centers, effectively guiding crystal growth and enabling a more controlled transition from precursor materials to the final perovskite structure.
By lowering the energetic barrier for phase conversion, the process promotes the formation of highly ordered crystalline films with larger grains. At the same time, excess acetate plays a secondary but critical role in defect management. “Moreover, the excess acetate passivates grain boundary defects, thereby suppressing non-radiative recombination.” This dual function - facilitating crystallization while reducing defect density - directly translates into improved optoelectronic quality.
Devices fabricated using this approach achieved a power conversion efficiency (PCE) of 25.53%, placing them among the highest-performing vacuum-deposited PSCs reported to date. In addition, the cells exhibited an electroluminescence external quantum efficiency (EQE_EL) of 18.38%, indicating low non-radiative losses and efficient charge recombination dynamics.
The solvent-free nature of the process also contributed to strong operational stability. The devices retained over 95% of their initial PCE after 1,000 hours under the ISOS-L-1 protocol, highlighting the robustness of the resulting perovskite films.
This work demonstrates how engineering the reaction pathway at the precursor level can overcome long-standing limitations in vacuum-deposited perovskites. By combining seed-assisted crystallization with in situ defect passivation, the approach provides a viable route toward high-efficiency, scalable PSC manufacturing.
