Perovskite solar cells have emerged as a promising photovoltaic technology due to their high power conversion efficiencies and low fabrication costs. However, the presence of defects in perovskite films, including Pb2+ vacancies, halide vacancies, and grain boundaries, significantly impacts device performance and long-term stability. Nanomaterial additives such as fullerene derivatives and MXenes have shown potential in passivating these defects, improving both efficiency and durability.
Bulk defects in perovskite films primarily consist of undercoordinated Pb2+ ions, halide vacancies, and disordered grain boundaries. These defects act as non-radiative recombination centers, reducing charge carrier lifetimes and open-circuit voltage. Pb2+ vacancies, for instance, create deep-level trap states that hinder electron transport. Halide vacancies facilitate ion migration, accelerating degradation under operational conditions. Grain boundaries serve as pathways for moisture and oxygen ingress, further compromising stability.
Fullerene derivatives, particularly [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), have been widely investigated for defect passivation. The electron-rich carbonyl groups in PCBM interact with undercoordinated Pb2+ ions, forming coordination bonds that neutralize trap states. Studies indicate that optimal doping concentrations of PCBM range between 0.5 to 2 wt%, beyond which aggregation occurs, disrupting perovskite crystallinity. Uniform distribution is critical, achieved through anti-solvent engineering or precursor solution blending. Devices incorporating PCBM exhibit reduced trap density and enhanced charge extraction, with reported improvements in power conversion efficiency from 18% to over 21%.
MXenes, a class of two-dimensional transition metal carbides and nitrides, offer complementary passivation mechanisms. Their abundant surface terminations (-O, -F, -OH) interact with both Pb2+ and halide vacancies. Ti3C2Tx MXenes, for example, bind with undercoordinated Pb2+ ions via oxygen functional groups, while fluorine terminations passivate halide vacancies. MXenes also serve as nucleation templates, promoting larger grain sizes and fewer grain boundaries. Optimal concentrations typically fall below 1 wt%, as higher loadings impede charge transport due to excessive sheet stacking.
Synergistic effects arise when combining fullerene derivatives and MXenes. Fullerene derivatives primarily passivate Pb2+ vacancies, while MXenes address halide vacancies and grain boundary defects. This dual approach reduces trap-assisted recombination and suppresses ion migration. Devices employing both additives demonstrate improved operational stability, retaining over 90% of initial efficiency after 1000 hours under continuous illumination.
Distribution control of nanomaterial additives is crucial for uniform defect passivation. Solution processing techniques, such as blade coating or slot-die coating, require careful optimization of solvent systems to prevent phase segregation. In-situ incorporation during perovskite crystallization ensures homogeneous dispersion, whereas post-deposition treatments may lead to uneven coverage. Advanced characterization techniques, including grazing-incidence X-ray scattering and energy-dispersive X-ray spectroscopy, verify additive distribution at the nanoscale.
Long-term stability is enhanced through defect passivation, but interfacial compatibility must also be considered. Fullerene derivatives can migrate toward charge transport layers over time, necessitating crosslinking strategies or alternative molecular designs. MXenes exhibit superior environmental stability but may oxidize at elevated temperatures. Encapsulation techniques mitigate these issues, with atomic layer deposition of Al2O3 proving effective in preventing additive degradation.
Industrial compatibility of nanomaterial additives depends on scalability and cost. Fullerene derivatives are commercially available but require purification to eliminate batch-to-batch variations. MXene synthesis involves selective etching, which can be scaled using continuous flow reactors. Both materials integrate well with roll-to-roll manufacturing, though ink formulation and drying kinetics require optimization for high-throughput production.
Alternative passivation strategies, such as polymer encapsulation or ionic liquids, can be combined with nanomaterial additives for further improvements. Polymers provide mechanical reinforcement, while ionic liquids passivate interfacial defects. The multi-functional nature of nanomaterials enables hybrid approaches, where defect passivation, moisture resistance, and ion migration suppression are simultaneously addressed.
In conclusion, nanomaterial additives like fullerene derivatives and MXenes offer a versatile platform for bulk defect passivation in perovskite films. Their interactions with Pb2+ vacancies, halide vacancies, and grain boundaries enhance device performance and longevity. Optimal doping concentrations, distribution control, and synergistic integration with other strategies are key to maximizing their potential. As fabrication techniques advance, these nanomaterials are poised to play a pivotal role in the commercialization of perovskite photovoltaics.