Key Takeaways & Executive Findings
- •• • The use of MBP as a self-assembled monolayer on NiOx yields a contact angle reduction from 70° (bare NiOx) to 30° with Sn-PVSK precursor, enhancing film uniformity and reducing interfacial defects, leading to improved fill factor and open-circuit voltage in tin perovskite solar cells. • • Replacing acidic PEDOT:PSS with NiOx/MBP hole transport layer eliminates acid-induced degradation of the perovskite layer, extending device operational lifetime by over 500 hours under continuous illumination (ISOS-L-1 protocol) compared to PEDOT:PSS-based devices. • • The optimized NiOx/MBP interface achieves a power conversion efficiency (PCE) of 14.2% for tin-based perovskite solar cells, a 20% relative improvement over devices with 2PACz or MBC SAMs, attributed to better energy level alignment and reduced non-radiative recombination. • • The molecular design of MBP, featuring a phosphonic acid anchoring group and a methyl-substituted biphenyl core, provides a dense, well-ordered monolayer that passivates under-coordinated tin atoms at the buried interface, reducing trap density by an order of magnitude (from 10^16 cm^-3 to 10^15 cm^-3) as measured by thermal admittance spectroscopy.
Abstract
The advancement of lead-free perovskite photovoltaics, particularly tin-based devices, has been hindered by interfacial instability and energetic mismatches at the buried interface. This study, building on the foundational work of Qi and co-workers, establishes a clear paradigm: the path to stable, lead-free perovskite photovoltaics depends not only on material composition but also on interfacial engineering at the molecular scale. By transforming a historically problematic buried interface into a structurally coherent and energetically optimized contact, the research sets a new benchmark for tin-based devices. The work demonstrates that molecular design of self-assembled monolayers (SAMs) on nickel oxide (NiOx) hole transport layers can significantly enhance device performance and stability. Specifically, the use of phosphonic acid-based SAMs, such as MBP, results in improved surface wettability, reduced contact angle with the perovskite precursor, and superior current density-voltage characteristics. The findings underscore the critical role of interfacial chemistry in achieving high-efficiency, durable tin perovskite solar cells. This research brings perovskite solar cells closer to the long-sought balance of sustainability, efficiency, and durability, essential for real-world adoption. The study also highlights the importance of replacing acidic PEDOT:PSS with non-acidic alternatives to prevent device degradation. Overall, this work provides a comprehensive strategy for interfacial engineering that can be universally applied to other perovskite systems, paving the way for commercial viability of lead-free perovskite photovoltaics.
1. Introduction
The commercialization of perovskite solar cells has been dominated by lead-based compositions, which pose significant environmental and health concerns. Lead-free alternatives, particularly tin-based perovskites, offer a promising route to sustainable photovoltaics. However, their performance and stability lag far behind their lead counterparts, primarily due to the facile oxidation of Sn2+ to Sn4+ and the resulting high defect densities. A critical bottleneck lies at the buried interface between the hole transport layer (HTL) and the perovskite absorber. Conventional HTLs such as PEDOT:PSS are acidic and hygroscopic, leading to interfacial degradation and poor device stability. Moreover, the energy level mismatch and poor wettability of the HTL surface cause non-uniform perovskite film growth, exacerbating non-radiative recombination losses. These interfacial issues have stymied the efficiency and longevity of tin-based devices, preventing their transition from laboratory curiosities to commercial products.
This research addresses these challenges through molecular-scale interfacial engineering. By designing a novel phosphonic acid-based self-assembled monolayer (SAM), MBP, on a NiOx inorganic HTL, the study achieves a structurally coherent and energetically optimized buried interface. The MBP molecule not only improves the wettability of the HTL surface, facilitating the formation of high-quality perovskite films, but also passivates interfacial defects and enhances charge extraction. The result is a significant boost in power conversion efficiency and operational stability, setting a new benchmark for tin-based perovskite solar cells. This work underscores the importance of interfacial chemistry in unlocking the full potential of lead-free perovskites, offering a universal strategy that could be extended to other emerging photovoltaic materials.
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QI et al. (2026). Interfacial Molecular Engineering for Stable Lead-Free Tin Perovskite Solar Cells: A Paradigm Shift in Buried Interface Optimization. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3832-2
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Frequently Asked Questions
What are the specific failure mechanisms of tin-based perovskite solar cells under operational stress, and how does the NiOx/MBP interface mitigate them?
Tin-based perovskites suffer from Sn2+ oxidation to Sn4+, creating deep-level defects that act as non-radiative recombination centers. Additionally, the acidic nature of PEDOT:PSS can etch the perovskite and cause interfacial degradation. The NiOx/MBP interface replaces the acidic HTL with a chemically stable inorganic oxide, while the MBP SAM passivates under-coordinated tin atoms at the interface, reducing trap density by an order of magnitude. This suppresses non-radiative recombination and prevents oxidation, leading to enhanced operational stability. Under continuous illumination (ISOS-L-1), devices retain 80% of initial efficiency after 500 hours, whereas PEDOT:PSS-based devices degrade to 50% within 200 hours.
How does the molecular structure of MBP influence the energy level alignment and charge extraction efficiency at the buried interface?
MBP features a phosphonic acid anchoring group that binds strongly to NiOx, forming a dipole that shifts the work function of the HTL to better match the valence band of the tin perovskite. The methyl-substituted biphenyl core provides a hydrophobic surface that improves precursor wetting, leading to a more uniform perovskite film. This optimized energy alignment reduces the interfacial energy barrier, enhancing hole extraction efficiency. Time-resolved photoluminescence measurements show a faster hole transfer rate (decay time of 1.2 ns) compared to devices with 2PACz (1.8 ns), indicating more efficient charge extraction.
What are the scalability challenges of using self-assembled monolayers like MBP in industrial-scale production, and how can they be overcome?
Scalability of SAMs is challenged by the need for uniform monolayer coverage over large areas and the cost of molecular synthesis. However, MBP can be deposited via simple spin-coating or slot-die coating from a dilute solution, compatible with roll-to-roll processing. The synthesis of MBP involves a two-step reaction with high yield (>85%), making it cost-effective. Furthermore, the NiOx layer can be deposited via sputtering or atomic layer deposition, both scalable techniques. The key is to optimize the deposition parameters to ensure complete monolayer formation without aggregation, which can be achieved by controlling solution concentration and annealing temperature.
How does the performance of tin-based perovskite solar cells with NiOx/MBP compare to state-of-the-art lead-based devices in terms of efficiency and stability?
While lead-based perovskite solar cells have achieved efficiencies exceeding 25%, tin-based devices with NiOx/MBP reach 14.2% PCE, which is among the highest for tin-based cells. The stability, however, is comparable: under ISOS-L-1, tin-based devices retain 80% of initial efficiency after 500 hours, whereas lead-based devices typically retain 90% after 1000 hours. The lower efficiency is due to the inherently higher defect density and lower open-circuit voltage in tin perovskites. Nevertheless, the elimination of lead toxicity and the improved stability make tin-based devices a viable alternative for applications where environmental impact is a priority.
What is the industrial relevance of replacing PEDOT:PSS with NiOx/MBP in terms of cost and manufacturing compatibility?
PEDOT:PSS is acidic and hygroscopic, causing device degradation, and its removal simplifies the manufacturing process by eliminating the need for acidic additives. NiOx is an inexpensive, abundant material that can be deposited via low-cost methods such as sputtering or chemical bath deposition. MBP is a custom-designed molecule, but its synthesis is straightforward and scalable. The overall cost of the HTL stack is estimated to be comparable to PEDOT:PSS, while offering superior stability. Moreover, the NiOx/MBP stack is compatible with both rigid and flexible substrates, and can be integrated into existing perovskite solar cell production lines with minimal modification.
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