Perovskite solar cells (PSCs), a rapidly advancing thin-film photovoltaic technology, have seen remarkable progress in their photoelectric conversion efficiency, climbing from an initial 3.8% to a current peak of 25.7% over just a decade. This impressive leap highlights their vast potential for commercial applications. Despite these advancements, challenges remain, particularly concerning the batch-to-batch reproducibility and long-term stability of high-efficiency NIP structured devices. These issues have become critical roadblocks to their widespread industrial adoption, yet the underlying causes of these limitations are still not fully understood.
Ma Changqi's team at the Suzhou Institute of Nanotechnology and Nanobionics of the Chinese Academy of Sciences conducted a comprehensive study on the ion migration behavior of NIP structure PSCs under air oxidation conditions. Their findings revealed that the Spiro-OMeTAD film undergoes oxidative treatment via a non-contact electrochemical approach. Here, oxygen and water molecules in the ambient air act as oxidizing agents for Spiro-OMeTAD, enhancing its conductivity. Importantly, this oxidation process triggers the migration of Li+ ions from the Spiro-OMeTAD layer into the cell, concentrating them at the SnO2/Perovskite interface. This migration and concentration of Li+ ions promote further oxidation of Spiro-OMeTAD and reduce the LUMO energy level of SnO2. Consequently, the internal electric field within the device is strengthened, boosting hole and electron extraction efficiencies at both the perovskite/Spiro-OMeTAD and perovskite/SnO2 interfaces. This improvement leads to enhanced overall device performance. The study offers a thorough mechanism-based explanation for the oxidative processes in NIP-type perovskite solar cells. The research findings were published in the 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."
Further investigations into the operational stability of NIP perovskite solar cells uncovered a phenomenon known as catastrophic failure during device operation. Photoluminescence (PL) imaging analysis pinpointed the short-circuit location to the edge of the metal Ag electrode. Subsequent SEM and TOF-SIMS analyses confirmed that Ag+ ions migrate and diffuse along the device edges, with no significant changes observed in the internal electrodes or perovskite layers. SEM characterization of Ag films deposited on Spiro-OMeTAD revealed that due to the non-wetting nature of Ag on Spiro-OMeTAD, Ag particles at the edges were smaller and more loosely packed compared to the central area. Based on these observations, the team proposed that the sudden short-circuit failure occurs when the perovskite film degrades under light exposure, forming polyiodine compounds that react with loose Ag clusters at the electrode edges. This reaction corrodes the Ag electrode, generating Ag+ ions that migrate through Spiro-OMeTAD into the perovskite, ultimately creating a conductive filament that shorts the circuit. To address this issue, a MoO3 film was deposited onto Spiro-OMeTAD to enhance Ag electrode deposition and achieve denser edge structures. Additionally, the MoO3 layer improved hole extraction efficiency at the Spiro-OMeTAD/Ag electrode interface, preventing hole accumulation and thus enhancing stability. With these modifications, the device operated stably for over 600 hours without experiencing the sudden failure mode. 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."
Although the operational stability of this configuration had improved, another challenge persisted: the rapid decline in efficiency, termed burn-in attenuation, typically occurring within the first dozens of hours of operation. This issue significantly diminished the stable output efficiency of the device. To tackle this, the research team examined the internal ion distribution and interface modifications during device design and stability tests. They discovered that the burn-in attenuation in this structure was 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 immobilized Li+ ions at the Perovskite/SnO2 interface, increasing the device’s built-in electric field and improving electron extraction efficiency. In the Cs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3 system, the modified device achieved an efficiency of 22.06%, maintaining 95% of its initial efficiency after 1000 hours of continuous light exposure, compared to 75% for the reference battery. Similarly, in the FAPbI3 system, the device attained a photoelectric conversion efficiency of 24.14%, eliminating the burn-in attenuation entirely. These results demonstrate the universality of using CL-PCBM interfacial modification to eliminate burn-in attenuation. Overall, reducing Li+ migration during device operation significantly curtails early-stage burn-in attenuation in perovskite solar cells, enhancing their stable power output. The study was published in Advanced Materials under the title "Boosting Perovskite Solar Cells Efficiency and Stability: Interfacial Passivation of Crosslinked Fullerene Eliminates the 'Burn-in' Decay."
[Image description: Figure 1 illustrates the mechanism of Li+ ion migration during the electrochemical oxidation of Spiro-OMeTAD in NIP structure perovskite solar cells. Figure 2 details the mechanism of "mutated failure" caused by Ag+ ion migration during operation and the role of MoO3 in improving operational stability. Figure 3 demonstrates how CL-PCBM interface modification suppresses Li+ ion migration, improving device efficiency and eliminating burn-in attenuation.]
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