Perovskite solar cells (PSCs), as a rapidly advancing technology in thin-film photovoltaics, have seen their power conversion efficiencies skyrocket from 3.8% to 25.7% over the past decade, signaling immense potential for commercial applications. Despite these impressive gains, the challenge of achieving consistent performance across batches and ensuring long-term stability remains a critical barrier to their widespread adoption. Currently, the scientific community lacks a comprehensive understanding of the factors contributing to the variability and degradation issues in high-efficiency PSCs.
A team led by Ma Changqi at the Suzhou Institute of Nanotechnology and Nanobionics of the Chinese Academy of Sciences has conducted an in-depth investigation into the ion migration behavior in NIP-structured PSCs under air oxidation conditions. Their findings reveal that the oxidation of the Spiro-OMeTAD layer occurs via a non-contact electrochemical process, where oxygen and moisture in the air act as oxidizing agents, enhancing the conductivity of the Spiro-OMeTAD film. Importantly, this oxidation triggers the migration of Li+ ions from the Spiro-OMeTAD layer into the cell, concentrating them at the SnO2/Perovskite interface. This redistribution of Li+ ions accelerates the oxidation of Spiro-OMeTAD, lowers the LUMO energy level of SnO2, strengthens the internal electric field, and improves charge extraction at both the perovskite/Spiro-OMeTAD and perovskite/SnO2 interfaces, ultimately boosting the overall device efficiency. This work offers a detailed mechanistic explanation of the Spiro-OMeTAD oxidation process in NIP-type PSCs, and the results were published in *Journal of Materials Chemistry A* under the title "Synergetic Effects of Electrochemical Oxidation of Spiro-OMeTAD and Li+ Ions Migration in Improving the Performance of NIP-Type Perovskite Solar Cells."
In subsequent studies focused on the operational stability of NIP PSCs, the research team uncovered a phenomenon known as catastrophic failure, characterized by abrupt device short-circuits during operation. Photoluminescence imaging identified the short-circuit locations near the edges of the Ag metal electrode. Further examination using SEM and TOF-SIMS confirmed that Ag+ ions migrate and accumulate at the electrode edges. Interestingly, no significant changes were observed in the electrodes or perovskite layers within the cell. SEM analysis of Ag films deposited on Spiro-OMeTAD revealed that due to the non-wetting nature of Ag on Spiro-OMeTAD, the Ag particles at the edges were smaller and less dense compared to those in the central region. Based on these observations, the team proposed that the sudden short-circuit failures occur when the perovskite film decomposes under illumination to form polyiodide compounds. These compounds diffuse and react with loose Ag clusters at the electrode edges, leading to Ag electrode corrosion and the subsequent migration of Ag+ ions into the perovskite via Spiro-OMeTAD. This process ultimately creates a conductive filament, causing a short circuit. To address this issue, the team introduced a MoO3 layer on Spiro-OMeTAD, which not only improved the uniformity of Ag electrode deposition but also enhanced hole extraction efficiency, preventing hole accumulation at the interface. As a result, devices equipped with this modification exhibited stable operation for over 600 hours without experiencing the aforementioned catastrophic failures. These findings were published in *Advanced Functional Materials* under the title "Revealing the Mechanism behind the Catastrophic Failure of N-i-p Type Perovskite Solar Cells under Operating Conditions and How to Suppress It."
While the operational stability of this architecture has shown improvement, many devices still encounter a rapid decline in efficiency—known as burn-in attenuation—within the first few tens of hours of operation. This phenomenon significantly impacts the long-term performance of the cells. To tackle this issue, the research team explored the internal ion distribution and interface dynamics during device design and stability testing. They determined that the burn-in attenuation in this architecture is linked to the migration of Li+ ions from SnO2 to the perovskite/hole transport layer interface. By introducing a thin layer of crosslinked PC61BM (CL-PCBM) at the SnO2/perovskite interface, they successfully mitigated this effect. TOF-SIMS analysis confirmed that the CL-PCBM layer immobilizes Li+ ions at the perovskite/SnO2 interface, while simultaneously increasing the built-in electric field and improving electron extraction efficiency. In Cs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3-based perovskite cells, this approach yielded an efficiency of 22.06%, with 95% of the initial efficiency maintained after 1000 hours of continuous light exposure. By contrast, the reference device retained only 75%. Similarly, in FAPbI3-based systems, the efficiency reached 24.14%, with the burn-in attenuation process entirely eliminated. These results underscore the universality of using CL-PCBM interfacial modifications to enhance both efficiency and stability. Overall, by curbing Li+ migration during operation, the burn-in attenuation in early-stage stability tests can be drastically reduced, paving the way for more reliable and stable perovskite solar cells. These findings were published in *Advanced Materials* under the title "Boosting Perovskite Solar Cells Efficiency and Stability: Interfacial Passivation of Crosslinked Fullerene Eliminates the 'Burn-In' Decay."
Through these advancements, researchers continue to push the boundaries of perovskite solar cell technology, addressing critical challenges to realize its full commercial potential.
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