Interpenetrating interfaces for efficient perovskite solar cells with high operational stability and mechanical robustness

Perovskite solar cells (PSCs) have rapidly gained prominence in photovoltaics due to their low cost, high efficiency, and flexibility. Organic-inorganic halide perovskites (OIHPs), such as formamidinium lead iodide (FAPbI3) and methylammonium lead iodide (MAPbI3), are commonly used and have achieved power conversion efficiencies (PCEs) rivaling silicon cells. This high performance stems from their favorable optoelectronic properties: high absorption coefficients, long carrier diffusion lengths, and high defect tolerance. Solution processing at low temperatures allows for lightweight and flexible PSCs, opening possibilities for building-integrated photovoltaics. However, a major limitation hindering widespread adoption is their operational stability. Interface engineering is a crucial approach to improve both efficiency and stability in PSCs. Traditional methods involve inserting additional layers or surface modifications to optimize energy-level alignment, improve contacts, suppress defects, and mitigate hysteresis. These techniques can, however, introduce additional processing steps and potentially compromise the mechanical integrity of the interfaces, crucial for long-term stability under continuous illumination and mechanical stress (particularly in flexible devices). This study introduces a novel approach to address these limitations by designing a holistic, interpenetrating interface between the perovskite and ETL layers. This is achieved through a reaction between a pre-deposited FAI-incorporated SnO2 (FI-SnO2) ETL and a PbI2-excess OIHP layer. The SnO2-OIHP interface was chosen for initial demonstration. This method offers simplicity and scalability compared to previous approaches. Advanced characterization techniques, including TOF-SIMS and TEM, are used to confirm the interpenetrating structure and its impact on device performance and stability. The research aims to demonstrate that this novel interface design leads to highly efficient, stable, and mechanically robust PSCs.

The literature extensively documents the pursuit of high-efficiency and stable PSCs through interface engineering. Studies have explored various strategies, including the insertion of inorganic nanoparticles, polymers, or molecules between the perovskite and charge transport layers [14–16]. Surface modifications using functional organic groups (thiophene, pyridine) or inorganic dopants (chlorine, alkali) have also been investigated [17–19]. These methods aim to optimize energy-level alignment, enhance interfacial contacts, reduce defect density, minimize hysteresis, and improve surface hydrophobicity [14,17,20]. However, many of these methods add complexity to the fabrication process and may compromise mechanical robustness. Previous research on SnO2-OIHP interfaces [21–24] provides a foundation for this work, but the interpenetrating interface design offers a significant advancement, aiming for a synergistic enhancement of both functional and mechanical properties. The existing literature lacks a comprehensive approach focusing on the creation of robust and efficient perovskite interfaces through an interpenetrating structure produced via a controlled interfacial reaction.

The fabrication of the interpenetrating interface begins with the synthesis of the FI-SnO2 ETL. FAI powder is dissolved in a SnO2 nanocrystal colloidal solution in isopropanol (IPA), resulting in a color change. This triggers SnO2 nanocrystal regrowth, possibly due to a hydrolysis reaction. The solution is spin-coated onto an FTO-coated glass substrate, annealed at 80 °C, and subjected to UV-ozone treatment. The FAI concentration is optimized for optimal photovoltaic performance. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) confirm the uniformity of the FI-SnO2 ETL. Conductive AFM (C-AFM) mapping reveals a more uniform electrical conductivity compared to pristine SnO2, indicating improved distribution of SnO2 nanocrystals. Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses confirm the presence of FA+ ions within the ETL. The OIHP layer is deposited using a solution containing PbI2, PbBr2, CsI, FAI, and MABr, resulting in a Cs0.04(FA0.84MA0.16)0.96Pb(I0.84Br0.16)3 composition with excess PbI2. During annealing at 100 °C, a reaction occurs between the excess PbI2 in the OIHP and the FA halide in the FI-SnO2 ETL, creating the interpenetrating structure. Cross-sectional SEM, TOF-SIMS, TEM, and energy-dispersive X-ray spectroscopy (EDX) are employed to confirm the interpenetration of OIHP and FI-SnO2 phases. Ultraviolet photoemission spectroscopy (UPS) is used to determine the energy levels of the layers, revealing a favorable cascade electronic structure for photocarrier transfer. In-operando Kelvin probe force microscopy (KPFM) is used to examine the potential profile across the interface, demonstrating a wider depletion region and larger potential difference compared to the pristine SnO2 interface, enhancing carrier separation and collection. Time-resolved photoluminescence (TRPL) and steady-state PL spectroscopy further confirm the enhanced charge dynamics. Rigid PSCs are fabricated by depositing spiro-OMeTAD hole-transporting layers (HTLs) and Au contacts on the OIHP/ETL structures. Current density-voltage (J-V) curves are measured to assess performance. Flexible PSCs are fabricated similarly but on PEN/ITO substrates. Long-term operational stability is evaluated under continuous one-sun illumination, and mechanical endurance is assessed via cyclic bending tests. Cross-sectional SEM imaging is used to analyze the morphological changes of the devices during bending.

The key findings demonstrate the successful fabrication of an interpenetrating OIHP/ETL interface with significantly improved performance and stability compared to PSCs with a traditional SnO2 ETL. * **Enhanced Efficiency:** PSCs with the interpenetrating interface exhibit high PCEs: 22.2% (rigid) and 20.1% (flexible), exceeding those of devices with the pristine SnO2 ETL (19.7% and lower, respectively). * **Improved Stability:** Long-term operational stability tests show that the rigid devices retain 82% of their initial PCE after 1000 hours of continuous operation, a substantial improvement over the control group which shows a significantly faster degradation. * **Mechanical Robustness:** The flexible PSCs demonstrate remarkable mechanical endurance, maintaining 85% of their initial PCE after 2500 bending cycles, compared to only 60% for the control devices, highlighting the enhanced structural integrity offered by the interpenetrating interface. * **Mechanistic Insights:** Advanced microscopic characterizations (TOF-SIMS, TEM, EDX, KPFM, TRPL, and steady-state PL) provide strong evidence for the interpenetration of the OIHP and FI-SnO2 phases, correlating this microstructure with the observed performance enhancement. KPFM reveals a wider depletion region and increased potential drop at the interpenetrating interface, facilitating efficient charge separation and collection. TRPL data shows faster carrier recombination times, indicating suppressed recombination losses. SEM imaging confirms that the interpenetrating interface significantly improves the resistance to delamination during mechanical stress. The interfacial reaction between excess PbI2 and FA halide produces a more robust interface resisting delamination under mechanical stress, explaining the improved mechanical durability.

The results clearly demonstrate the effectiveness of the interpenetrating interface design in addressing the challenges of stability and mechanical robustness in PSCs. The observed performance enhancements are directly attributed to the improved structural and electronic integrity of the OIHP/ETL interface. The interpenetration of the OIHP and FI-SnO2 phases creates a more damage-tolerant interface compared to traditional sharp interfaces, effectively reducing delamination during mechanical stress and improving long-term operational stability. The wider depletion region and larger potential difference at the interpenetrating interface, as revealed by KPFM, promote efficient charge separation and collection, leading to enhanced device performance. The faster PL decay observed in TRPL supports this conclusion, demonstrating the reduction of non-radiative recombination processes. The findings highlight the crucial role of interface engineering in achieving high-performance and durable PSCs.

This study successfully demonstrates the fabrication of highly efficient, operationally stable, and mechanically robust perovskite solar cells through the design of an interpenetrating OIHP/ETL interface. This novel interface design, created via a controlled interfacial reaction, shows significant advantages over traditional approaches. The enhanced performance is attributed to improved interface integrity, efficient charge separation, and reduced recombination losses. This work underscores the importance of interfacial engineering for developing next-generation high-performance PSCs suitable for various applications, including flexible electronics and building-integrated photovoltaics. Future research can explore the adaptability of this interpenetrating interface design to other perovskite compositions and ETL materials for further performance improvements and broader applicability.

While this study demonstrates significant advancements in PSC performance and stability, some limitations exist. The study primarily focuses on a specific perovskite composition and ETL material; further investigation is needed to assess the generalizability of this approach to other material systems. The long-term stability tests were conducted under controlled laboratory conditions. Real-world environmental factors (e.g., humidity, temperature fluctuations) could influence long-term stability. The mechanical bending tests used a specific bending radius; investigating the impact of different bending radii on device performance would provide a more complete understanding of mechanical robustness. A deeper exploration of the underlying reaction mechanisms that lead to interpenetration of the two materials would enhance the understanding and control of the process.