Organic-inorganic hybrid perovskite solar cells (PSCs) have seen significant progress, achieving a power conversion efficiency (PCE) of 26.7% for single-junction cells. Monolithic all-perovskite tandem solar cells (TSCs), integrating a wide-bandgap (WBG) top subcell (~1.7–1.9 eV) with a narrow-bandgap (NBG) bottom subcell (~1.2–1.3 eV), offer the potential to surpass the Shockley–Queisser limit. Certified PCEs of all-perovskite TSCs have reached 30.1%, highlighting their efficiency advantages. However, challenges remain, including high open-circuit voltage (Voc) deficit and low fill factor (FF) in all-perovskite TSCs. These limitations stem from significant non-radiative carrier recombination at the interface between the Sn-Pb mixed perovskite and the fullerene (C60)-based electron transport layer (ETL) in the bottom cell. Defects on the surface of Sn-Pb mixed perovskite films result from the uncontrolled crystallization process, leading to Sn²⁺ ion aggregation, oxidation to Sn⁴⁺, and the formation of organic ammonium cation and iodine vacancies (Va and Vi). These defects contribute to non-stoichiometric ratios, particularly on the film surface, necessitating strategies to minimize non-radiative recombination losses. Single surface passivation proves insufficient to address both Sn⁴⁺ defects and Va/Vi related defects. This research introduces a novel surface reconstruction strategy to overcome these limitations.

Previous studies have explored various approaches to improve the efficiency of all-perovskite tandem solar cells. Efforts have focused on fabricating high-quality subcells and interconnecting layers to minimize optical and electrical losses. However, the challenges of high Voc deficit and low FF due to non-radiative recombination at the Sn-Pb perovskite/C60 interface persist. Researchers have investigated different surface passivation techniques to mitigate these defects; however, a single passivation strategy has proven insufficient. Studies focusing on the crystallization dynamics of Sn-based perovskites, and surface reconstruction techniques, have highlighted the importance of addressing both Sn⁴⁺-related defects and organic cation/halide vacancies for optimal performance. The use of additives and various surface modification strategies have been explored to address these issues. The current study builds upon this existing knowledge by introducing a combined chemical polishing and passivation approach.

This research employed a two-pronged surface modification strategy using 1,4-butanediamine (BDA) for chemical polishing and ethylenediammonium diiodide (EDAI₂) for surface passivation. The optimal concentration of BDA in isopropyl alcohol (IPA) was determined to be 0.1 mg mL⁻¹. Various characterization techniques were used to analyze the impact of this treatment on the perovskite films. X-ray photoelectron spectroscopy (XPS) was used to assess surface composition and stoichiometry, revealing a reduction in Sn⁴⁺ and a closer-to-ideal I/(Pb+Sn) ratio after BDA treatment. Kelvin probe force microscopy (KPFM) showed a more uniform surface potential distribution after BDA polishing, indicating fewer defects. Fourier transform infrared (FTIR) and ¹H nuclear magnetic resonance (¹H NMR) spectroscopy investigated the interaction between BDA and the perovskite components, suggesting Lewis acid-base coordination and hydrogen bonding mechanisms. Grazing-incidence wide-angle X-ray scattering (GIWAXS) confirmed the 3D perovskite structure remained intact after BDA treatment, with no evidence of low-dimensional perovskite formation at low BDA concentrations. Density functional theory (DFT) calculations supported the effectiveness of BDA in binding to V₁ defects and inhibiting Sn²⁺ oxidation. Photoluminescence (PL) and time-resolved photoluminescence (TRPL) spectroscopy demonstrated enhanced carrier lifetime and PL intensity after BDA-EDAI₂ modification, indicating suppressed non-radiative recombination. Ultraviolet photoelectron spectroscopy (UPS) revealed improved energy level alignment at the perovskite/C₆₀ interface after BDA-EDAI₂ treatment, facilitating charge extraction. Grazing-incidence X-ray diffraction (GIXRD) indicated a reduction in residual tensile strain after BDA-EDAI₂ modification. Finally, photovoltaic devices were fabricated using inverted architecture, and their performance was evaluated under standard AM1.5G simulated sunlight. Electrochemical impedance spectroscopy (EIS), thermal admittance spectroscopy (TAS), Mott-Schottky analysis, transient photovoltage decay (TPV), and transient photocurrent decay (TPC) were used to characterize charge transport and recombination properties. Electroluminescence (EL) measurements were performed to quantify non-radiative voltage losses. The universality of the treatment was investigated using different perovskite compositions, including FASnI₃ and FA₀.₆MA₀.₃Cs₀.₁Pb₀.₅Sn₀.₅I₃. All-perovskite TSCs were fabricated and tested, followed by module-level device fabrication and stability analysis.

The combined BDA polishing and EDAI₂ passivation strategy resulted in significant improvements in the quality and performance of Sn-Pb mixed perovskite films and devices. XPS analysis showed that BDA effectively removed the Sn-rich, I-deficient surface layer, leading to a more stoichiometric surface composition. KPFM images indicated a more uniform surface potential, reducing potential barriers for charge transport. FTIR and ¹H NMR data revealed the chemical interactions between BDA and the perovskite components, confirming the polishing mechanism. DFT calculations provided theoretical support for the enhanced defect passivation. PL and TRPL measurements showed significantly improved carrier lifetimes and PL intensities after the BDA-EDAI₂ treatment, demonstrating the reduction of non-radiative recombination. UPS spectra revealed improved energy level alignment at the perovskite/C₆₀ interface, facilitating efficient charge transfer. GIXRD measurements showed a reduction of residual tensile strain after BDA-EDAI₂ modification, indicating improved crystal quality. The resulting PSCs exhibited notably enhanced PCEs, with values of 22.65% and 23.32% achieved for cells with 1.32 and 1.25 eV bandgaps, respectively. These efficiencies were significantly higher than the control devices and accompanied by improved Voc and FF. The superior performance was attributed to a reduction in non-radiative recombination losses at the perovskite/ETL interface. Two-junction all-perovskite TSCs fabricated using the modified NBG perovskite achieved a certified PCE of 28.49%, which is among the highest values reported for such devices. Moreover, the improved interfacial properties translated into enhanced device stability, with encapsulated devices retaining 79.7% of their initial PCE after 550 hours of continuous operation. The effectiveness of the BDA-EDAI₂ treatment was also demonstrated for different perovskite compositions, highlighting its universality. Module-level devices demonstrated the practical applicability of the approach, achieving a PCE of 23.39%.

This study successfully demonstrates a highly effective surface reconstruction strategy for enhancing the performance and stability of all-perovskite tandem solar cells. The synergistic combination of BDA chemical polishing and EDAI₂ passivation effectively addresses the critical issue of non-radiative recombination at the perovskite/ETL interface, a major limiting factor in achieving higher efficiencies. The results highlight the importance of comprehensive surface engineering for optimizing the properties of Sn-Pb perovskite films. The significant improvement in PCE, Voc, and FF of both single-junction and tandem solar cells underscores the efficacy of this approach. The long-term stability test results further validate the practical implications of this method for developing highly efficient and durable all-perovskite solar cells. The universality of the treatment across different perovskite compositions opens up new avenues for optimizing various types of perovskite-based devices. Future research could focus on further optimizing the surface treatment process, exploring other suitable polishing and passivation agents, and investigating the scalability of the approach for large-scale manufacturing.

This work presents a novel surface reconstruction strategy using BDA and EDAI₂ to enhance the performance of all-perovskite tandem solar cells. The synergistic effect of chemical polishing and passivation leads to high-quality Sn-Pb mixed perovskite films with significantly reduced non-radiative recombination. Remarkably high PCEs were achieved for single-junction and tandem devices, demonstrating the efficacy of the approach. The excellent stability also proves the practical potential of this method. Future research could explore different combinations of polishing and passivating agents, and further optimization of the processing parameters.

While this study demonstrates significant improvements, certain limitations exist. The study primarily focused on specific perovskite compositions. While the universality of the method was explored with a few compositions, further investigations on a broader range of perovskite materials are necessary. The long-term stability tests were conducted under specific conditions, and further investigations under varying environmental conditions (temperature, humidity, light intensity) are needed to fully assess the stability. The study did not delve into the detailed mechanisms of the BDA and EDAI2 interactions at a molecular level. More in-depth investigations could utilize advanced characterization techniques to further illuminate the interactions and reaction kinetics.