Perovskite solar cells (PSCs) have shown remarkable promise in renewable energy due to their high power conversion efficiency (PCE) and ease of fabrication. However, their full potential is currently limited by charge-carrier recombination stemming from imperfect passivation at interfaces. While chemical passivation methods aim to reduce defect density at the perovskite/electron transport layer (ETL) or hole transport layer (HTL) interfaces, the underlying mechanisms and interplay between chemical and field-effect passivation remain inadequately understood. This study investigates the impact of interfacial energy offset and defects on PSC performance using both experimental and computational approaches. The research hypothesis is that a carefully engineered energy offset at the perovskite interface can significantly improve defect tolerance and boost efficiency, potentially surpassing the performance gains achieved solely through chemical passivation. The importance of this work lies in providing a deeper understanding of the fundamental loss mechanisms in PSCs and offering a novel design strategy for high-efficiency, stable devices. The efficient and stable operation of PSCs is crucial for their widespread adoption as a sustainable energy source; thus, addressing the limitations caused by charge recombination is paramount.

Recent advancements in perovskite solar cell technology have led to impressive efficiency gains, with certified record power conversion efficiencies exceeding 25%. Studies have focused on various strategies to enhance device performance, including improved perovskite synthesis, interface engineering, and the introduction of 2D perovskite layers. However, the detailed interplay between interfacial energy offsets and defect densities, and their influence on charge-carrier recombination, remains an active area of investigation. Previous research has explored the benefits of chemical passivation techniques to reduce defect density at interfaces, but the potential of field-effect passivation through optimized energy level alignment has not been fully explored. The literature also highlights the challenge of achieving both high efficiency and long-term stability in PSCs, with ion migration frequently cited as a key factor contributing to device degradation. This paper builds upon existing knowledge by quantitatively assessing the individual and combined effects of chemical and field-effect passivation on PSC performance and stability, offering a more comprehensive understanding of these critical factors.

This study employed a multi-faceted approach combining device physics modeling, material characterization, and device fabrication. **Device Physics Modeling:** Electrical simulations were performed on a regular PSC structure (SnO2/perovskite/spiro-OMeTAD) to investigate the influence of interfacial energy offset (bi) and defect density on device performance. The model allowed for systematic variation of these parameters to evaluate their individual and combined effects on power conversion efficiency (PCE), open-circuit voltage (VOC), and fill factor (FF). Simulations provided insights into charge-carrier concentration profiles and electric field distributions within the device. **2D/3D Perovskite Heterojunction Fabrication:** A 2D/3D perovskite heterojunction was fabricated by incorporating a 2D perovskite layer (using BABr and BAI precursors) onto a 3D perovskite layer. The thickness of the 2D layer was controlled to be around 30 nm. The resulting devices were characterized using a variety of techniques, including X-ray diffraction (XRD), grazing-incidence wide-angle X-ray scattering (GI-WAXS), X-ray photoelectron spectroscopy (XPS), Kelvin probe force microscopy (KPFM), ultraviolet photoelectron spectroscopy (UPS), transient photoluminescence (TrPL), transient absorption spectroscopy (TAS), and electrochemical impedance spectroscopy (EIS). These techniques provided detailed information on material structure, energy level alignment, charge-carrier dynamics, and defect densities. **Density Functional Theory (DFT) Calculations and Finite-Element Simulations:** DFT calculations were conducted to investigate the electronic structure of 2D and 3D perovskites, focusing on the impact of halide composition (Br- and I-) on the valence band offset (VBO). Finite-element simulations were coupled with the experimental results to evaluate the influence of VBO and interfacial defect densities on PSC performance. The simulations helped quantify the interplay between chemical and field-effect passivation and provided further insights into the impact of 2D perovskite mobility on the overall efficiency. **Device Characterization and Stability Testing:** The fabricated PSCs (both small-area devices and modules) were characterized using current-voltage (J-V) measurements under standard illumination conditions to determine their PCE, VOC, Jsc, and FF. External quantum efficiency (EQE) measurements were performed to correlate the J-V results with the spectral response of the devices. Long-term stability tests under continuous illumination were conducted to assess the operational lifetime of the devices.

The study yielded several key findings: 1. **Impact of Interfacial Energy Offset:** Simulations demonstrated that a favorable interfacial energy offset (bi) significantly improves defect tolerance. A positive bi of 0.2 eV increased defect tolerance by three orders of magnitude compared to devices without an energy offset. This superior defect tolerance reduced the need for stringent chemical passivation to achieve high PCEs. The simulations showed that a positive bi value reduces minority carrier concentration (electrons in this case), leading to a decrease in nonradiative recombination and an increase in VOC. However, excessively large bi values (>0.4 eV) can hinder carrier transport, lowering the FF and overall PCE. 2. **2D/3D Heterojunction Performance:** The introduction of a 2D perovskite layer at the perovskite/HTL interface created a heterojunction that improved charge-carrier transport and extraction. The 2D perovskite acted as both a chemical and field-effect passivator, leading to both reduced defect density and favorable energy level alignment. BAI-based 2D/3D heterojunction PSCs achieved a record-breaking PCE of 25.32% (certified 25.04%) for small-area devices and 21.39% for a large-area module (29 cm²). The use of different halide types (Br- and I-) allowed for tuning of the energy band alignment and subsequent optimization of device performance. TrPL, TAS, and KPFM studies confirmed enhanced charge extraction and reduced recombination losses in 2D/3D heterojunction PSCs. 3. **Suppression of Ion Migration:** The 2D/3D heterojunction effectively suppressed ion migration, resulting in dramatically improved long-term stability. Unencapsulated small-area devices maintained 90% of their initial efficiency after 2000 hours of continuous operation at the maximum power point. Kinetic measurements showed that the activation energy for ion migration was significantly higher in 2D/3D PSCs than in control devices. 4. **DFT and Simulation Validation:** DFT calculations corroborated the experimental findings, showing that halide composition in the 2D perovskite significantly influenced the VBO and bandgap. Simulations using these parameters demonstrated the strong correlation between VBO, interface defect density, and device PCE. Optimal PCEs were observed for positive VBO values below 0.42 eV, even with high interface defect densities. The simulations also indicated that a proper VBO could compensate for reductions in PCE due to low 2D perovskite mobility.

The findings of this study significantly advance our understanding of charge recombination loss mechanisms in PSCs and demonstrate the critical role of both chemical and field-effect passivation. The results clearly show that a favorable interfacial energy offset plays a more significant role in enhancing PCE than reducing the defect density alone. The successful implementation of 2D/3D heterojunctions highlights the potential of this approach to simultaneously achieve high efficiency and long-term stability. The ability to tune the energy level alignment by varying the halide composition of the 2D perovskite provides a powerful tool for optimizing device performance. The improved stability of the 2D/3D PSCs, attributable to the suppression of ion migration, is a critical advancement toward the practical application of PSCs. These results have important implications for the design and development of next-generation, high-performance PSCs, including the exploration of other 2D materials and the optimization of interfacial engineering techniques.

This research demonstrates, for the first time, the quantitative contributions of interfacial energy offset and defect density in 2D/3D perovskite heterojunctions to PSC performance. The findings show that a properly tuned interfacial energy offset can dramatically enhance defect tolerance and effectively suppress non-radiative recombination. This led to the development of highly efficient and stable PSC modules. The work provides a novel design strategy for future high-efficiency, stable, large-scale PSCs. Future work could explore a wider range of 2D materials and investigate the impact of different interface engineering techniques on device performance and stability.

The study focuses on a specific type of 2D/3D perovskite heterojunction. The findings may not be directly generalizable to all 2D/3D combinations or different device architectures. Further investigation is needed to explore the long-term stability under different environmental conditions, such as high humidity and temperature. The simulations used idealized models and may not fully capture the complexities of real-world devices. The scale-up of the fabrication process for large-area modules needs further optimization to ensure consistent performance.