Metal halide perovskites (ABX3), with their BX6 octahedron framework, exhibit excellent photoelectric properties, leading to power conversion efficiencies (PCE) exceeding 26% in perovskite solar cells (PSCs). However, the inherent fragility of this octahedron structure is a major cause of instability. Iodide ion migration along the PbI6 octahedron edges has the lowest activation energy, contributing significantly to PSC instability under operating conditions. Mobile iodides are susceptible to oxidation, accelerating perovskite degradation, leading to metal lead defects, irreversible perovskite collapse, and rapid degradation of the charge transport layer. Degradation typically initiates at the surface and grain boundaries, where defect density and ion diffusion are highest. Therefore, surface passivation is crucial for improving PSC performance and long-term stability. Current strategies focus on stabilizing the PbI6 octahedron by optimizing bonding characteristics, surface bond strength, and dimension management, but these methods often leave the surface susceptible to degradation. This paper introduces a novel approach to address this challenge.

Various passivation agents have been explored to suppress defect formation and ion migration in perovskite solar cells. These include ammonium-based salts, small organic molecules, polymers, and other passivation agents. Low-dimensional (LD) perovskites with bulky organic cations have also shown promise in stabilizing the perovskite surface by fixing the PbI6 framework and hindering ion migration through steric hindrance. Existing strategies primarily aim to improve the inherent stability of the perovskite structure itself, rather than creating a robust protective layer on its surface. The present work aims to improve upon these methods by forming a robust protective layer on the surface of the perovskite material.

This study introduces a lead iodide chelate, PbI2(DMEDA), where DMEDA (N, N'-Dimethyl-1,2-ethanediamine) acts as a bidentate ligand. PbI2(DMEDA) was synthesized by reacting DMEDA with FAPbI3 or PbI2 precursor solutions, resulting in white precipitates that were then recrystallized. Single-crystal X-ray diffraction (XRD) confirmed the crystal structure, revealing a robust chelated Pb octahedron framework. The exceptional stability of PbI2(DMEDA) against thermal, atmospheric, and light stressors was demonstrated by XRD after prolonged illumination and heating. A thin PbI2(DMEDA) layer was formed in situ on the FAPbI3 perovskite film by spin-coating a DMEDA solution in isopropanol (IPA). The reaction process is described by the following chemical equations: FAPbI3 + DMEDA → PbI2(DMEDA) + FAI (dissolved in IPA) PbI2 + DMEDA → PbI2(DMEDA) Scanning electron microscopy (SEM), atomic force microscopy (AFM), and XRD confirmed the formation of a thin, compact PbI2(DMEDA) layer (approximately 5 nm thick) on the perovskite surface. X-ray photoelectron spectroscopy (XPS) analysis further confirmed the presence of DMEDA on the treated surface. High-resolution transmission electron microscopy (HRTEM), high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM), and energy-dispersive X-ray spectroscopy (EDS) were employed to characterize the structure and composition of the modified perovskite. Ultraviolet-visible (UV-vis) spectroscopy and density functional theory (DFT) calculations determined the bandgap energy of PbI2(DMEDA) to be approximately 3 eV. Photoluminescence (PL) mapping, steady-state PL, and time-resolved PL (TRPL) measurements were used to assess the impact of the PbI2(DMEDA) layer on the optoelectronic properties of the perovskite. Kelvin probe force microscopy (KPFM) and ultraviolet photoelectron spectroscopy (UPS) determined the work function (WF) of the perovskite before and after treatment, showing improvement in surface potential. The performance of both hole transporting layer (HTL)-free devices and conventional n-i-p devices (FTO/SnO2/perovskite/spiro-OMeTAD/Au) was measured, demonstrating improved efficiency. The stability of unencapsulated devices was tested under AM1.5G illumination at maximum power point tracking, in both ambient air and nitrogen atmospheres, at various temperatures, including high temperatures. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) analysis was performed to examine the diffusion of iodine ions into the hole transport layer after prolonged operation. DFT calculations were conducted to determine the formation energies of iodine vacancies and activation energies for ion migration within the various perovskite structures.

The formation of a robust chelated lead octahedron surface (CPOS) using the bidentate ligand DMEDA significantly improved the efficiency and stability of perovskite solar cells. Key findings include: * **Enhanced efficiency:** The champion perovskite solar cell achieved a power conversion efficiency (PCE) of 25.7% (certified 25.04%), a substantial improvement over the control device (23.18%). This increase is attributed to improvements in open-circuit voltage (Voc), short-circuit current density (Jsc), and fill factor (FF). * **Improved optoelectronic properties:** The CPOS treatment resulted in a more uniform and significantly higher photoluminescence (PL) intensity, indicating reduced nonradiative recombination. Time-resolved PL (TRPL) measurements revealed a substantial increase in carrier lifetime from 1.6 µs to 2.6 µs. Kelvin probe force microscopy (KPFM) showed an increase in work function from 4.4 eV to 4.7 eV, further suggesting improved surface potential and reduced trap states. * **Increased stability:** Unencapsulated devices with the CPOS treatment exhibited remarkably enhanced operational stability, maintaining over 88% of their initial efficiency after 150 hours under AM1.5G illumination in ambient air and exceeding 90% after nearly 1000 hours under continuous illumination in a nitrogen atmosphere at 50°C. This stability is attributed to the inhibition of iodide ion migration and the formation of a robust, water-resistant surface layer, as supported by ToF-SIMS results. In high temperature tests with PTAA instead of Spiro-OMeTAD, CPOS devices still showed improved stability over control devices. * **Suppressed ion migration:** ToF-SIMS depth profiles confirmed significantly suppressed iodide ion migration in the CPOS devices compared to control devices, explaining the observed enhanced stability. Temperature-dependent conductivity measurements showed increased activation energy for iodide ion migration in the CPOS film (0.67 eV) compared to the control (0.35 eV), indicating reduced ion mobility. * **DFT calculations:** DFT calculations corroborated the experimental observations by showing that the formation energy of iodine vacancies in PbI2(DMEDA) (1.48 eV) was higher than in FAPbI3 (0.84 eV) and PbI2 (1.18 eV), and that the energy barrier of ion migration is substantially higher in PbI2(DMEDA). * **Scalability:** High-performance was also observed in larger area (1 cm²) devices, exhibiting a PCE of 23.8% in reverse scan and 23.2% in forward scan. * **Compatibility:** The CPOS passivation strategy is compatible with both n-i-p and p-i-n device architectures, showcasing its versatility.

The results demonstrate that the in-situ formation of a robust PbI2(DMEDA) layer on the perovskite surface effectively passivates defects, inhibits iodide ion migration, and improves the optoelectronic properties and stability of PSCs. The significant increase in PCE and long-term stability highlight the effectiveness of this approach. The improved work function and suppressed nonradiative recombination, as indicated by PL and TRPL measurements, are crucial factors contributing to the enhanced device performance. The suppressed iodide ion migration directly relates to the improved stability. These findings offer a promising pathway towards the development of highly efficient and durable perovskite solar cells for commercial applications.

This work presents a highly effective surface passivation strategy using a robust chelated lead octahedron layer (PbI2(DMEDA)) for perovskite solar cells. The resulting devices exhibit a champion PCE of 25.7% (certified 25.04%) and exceptional long-term stability. This approach successfully addresses the critical issue of perovskite instability by creating a protective surface layer while simultaneously improving optoelectronic properties. Future work could explore other bidentate ligands to further optimize the CPOS layer and investigate the scalability of this method for large-area solar cell modules.

While the study demonstrates remarkable improvements in efficiency and stability, there are limitations to consider. The study focused primarily on FAPbI3 perovskite; the generalizability of this approach to other perovskite compositions needs further investigation. The long-term stability tests were conducted under specific conditions; further studies are needed to assess the stability under a wider range of environmental conditions. The cost and scalability of producing PbI2(DMEDA) on an industrial scale also requires assessment.