Perovskite materials have garnered significant attention due to their exceptional optoelectronic properties, including high carrier mobility, tunable bandgaps, and strong light absorption. A critical aspect influencing their performance is the surface and interface science, which governs charge transport, recombination, and stability. This article delves into the fundamental aspects of perovskite surfaces, including termination effects, surface reconstruction, and charge trapping mechanisms. Additionally, characterization techniques and passivation strategies are discussed to provide a comprehensive understanding of surface engineering in perovskites.

Termination effects play a pivotal role in determining the electronic and chemical properties of perovskite surfaces. Perovskites typically exhibit mixed termination layers, with common terminations being lead iodide (PbI2) and methylammonium iodide (MAI) for MAPbI3. The PbI2-terminated surface is often electron-rich due to undercoordinated Pb atoms, while the MAI-terminated surface tends to be more neutral. These terminations influence surface reactivity, defect formation, and charge carrier dynamics. For instance, PbI2-terminated surfaces are prone to forming deep-level traps, which act as non-radiative recombination centers. In contrast, MAI-terminated surfaces exhibit fewer defects but may suffer from instability due to volatile organic cations. The balance between these terminations is crucial for optimizing surface properties.

Surface reconstruction is another key phenomenon observed in perovskites, driven by the dynamic nature of their ionic lattices. Under ambient conditions, perovskite surfaces can undergo structural rearrangements to minimize surface energy. For example, PbI2-rich surfaces may reconstruct to form PbI2 clusters or lead to the formation of iodine vacancies. Such reconstructions can introduce mid-gap states that trap charge carriers, degrading optoelectronic performance. Temperature and humidity further exacerbate these effects, accelerating surface degradation. Understanding reconstruction pathways is essential for designing stable perovskite materials.

Charge trapping at perovskite surfaces is a major challenge, primarily due to undercoordinated ions and dangling bonds. Shallow traps, often associated with iodide vacancies, can reversibly capture and release charge carriers, while deep traps, such as those from Pb clusters, lead to permanent losses. The density and energetics of these traps are influenced by surface terminations and reconstructions. For instance, PbI2-terminated surfaces exhibit a higher density of deep traps compared to MAI-terminated ones. Charge trapping not only reduces carrier lifetimes but also contributes to hysteresis in electronic measurements.

Characterization techniques are indispensable for probing perovskite surfaces and interfaces. X-ray photoelectron spectroscopy (XPS) is widely used to analyze surface composition and chemical states. It can identify the presence of Pb0 species, indicative of PbI2 decomposition, or detect organic cation loss through shifts in binding energies. Atomic force microscopy (AFM) provides topographic and mechanical insights, revealing surface reconstructions and phase segregation at nanoscale resolution. Kelvin probe force microscopy (KPFM), an extension of AFM, maps surface potentials, highlighting charge trapping sites and work function variations. Photoluminescence (PL) spectroscopy offers a non-destructive method to assess trap densities by analyzing emission quenching and lifetime decay. Secondary ion mass spectrometry (SIMS) complements these techniques by profiling elemental distribution and identifying impurity segregation at surfaces.

Passivation strategies are essential for mitigating surface defects and enhancing perovskite stability. Ligand capping is a prominent approach, where molecules such as alkylammonium iodides or thiols bind to undercoordinated Pb atoms, neutralizing traps. For example, phenethylammonium iodide (PEAI) forms a thin layer on perovskite surfaces, reducing non-radiative recombination and improving PL quantum yield. Lewis base passivation, using molecules like pyridine or thiocyanate, coordinates with Pb2+ ions, filling iodine vacancies and suppressing deep traps. Inorganic passivation, involving materials like Al2O3 or graphene oxide, provides a protective barrier against environmental degradation while maintaining electronic properties. Each strategy must balance defect passivation with minimal disruption to charge transport.

Surface energy engineering is another effective method to control termination and reconstruction. By tuning precursor stoichiometry or post-deposition treatments, the dominance of PbI2 or MAI terminations can be adjusted. For instance, excess MAI in precursor solutions promotes MAI-terminated surfaces, reducing trap densities. Annealing in controlled environments can also drive surface reconstruction toward more stable configurations. However, excessive organic cation content may lead to phase impurities, necessitating precise optimization.

Environmental factors such as humidity, oxygen, and light exposure significantly impact perovskite surfaces. Moisture accelerates the decomposition of organic cations, while oxygen can oxidize Pb2+ to Pb0, creating metallic clusters that act as recombination centers. Light-induced ion migration further complicates surface stability, leading to phase segregation and defect formation. Encapsulation techniques, combined with passivation, are critical for mitigating these effects in practical applications.

In summary, perovskite surface and interface science is a multifaceted field that dictates material performance and stability. Termination effects and surface reconstructions directly influence electronic properties, while charge trapping remains a central challenge. Advanced characterization techniques enable detailed surface analysis, guiding the development of effective passivation strategies. By addressing these fundamental aspects, researchers can unlock the full potential of perovskite materials for optoelectronic applications. Future work should focus on in-situ studies to capture dynamic surface processes and the development of universal passivation protocols for diverse perovskite compositions.