Impact of iodine antisite (IPb) defects on the electronic properties of the (110) CH3NH3PbI3 surface

Interaction of C60 with Methylammonium Lead Iodide Perovskite Surfaces: Unveiling the Role of C60 in Surface Engineering

Understanding the interaction between fullerene (C60) and perovskite surfaces is pivotal for advancing the efficiency and stability of perovskite solar cells. In this study, we investigate the adsorption behavior of C60 on methylammonium lead iodide (MAPbI3) surfaces using periodic density functional theory calculations. We explore various surface terminations and defect configurations to elucidate the influence of surface morphology on the C60-perovskite interaction, computing the adsorption energy and transfer of charge. Our results reveal distinct adsorption energies and charge transfer mechanisms for different surface terminations, shedding light on the role of surface defects in modifying the electronic structure and stability of perovskite materials. Furthermore, we provide insights into the potential of C60 to passivate surface defects, playing a relevant role in the surface reconstruction after the formation of defects. This comprehensive understanding of C60-perovskite interactions offers valuable guidelines about the role of fullerenes on surface structure and reconstruction.

Impacts of the Electron Transport Layer Surface Reconstruction on the Buried Interface in Perovskite Optoelectronic Devices

Using density functional theory combined with ab initio molecular dynamics, we comprehensively investigated the performance enhancement mechanism of the device after surface reconstruction by passivating different halogen groups (i.e., F or Cl) at the ETL/perovskite interface. We demonstrated that the halogen group at the ETL layer could stabilize the geometric structure of the perovskite surface by balancing the interfacial interaction, ionic migration, and lead iodide framework. Even though halogen passivation decreased and increased the interface charge transfer at the O- and SnO-terminated MAPbI₃/SnO₂ interfaces, respectively, halogen passivation optimized surface reconstruction and could theoretically relieve the interface carrier recombination according to the changes in conduction band offsets generated by halogen passivation. Furthermore, the interfacial carrier recombination of the MAPbI₃/SnO₂ interface was also connected to the interfacial gap states, which were smaller for O-terminated MAPbI₃/SnO₂ interfaces with halogen passivation-induced surface reconstruction but larger for the SnO-terminated cases. Hence, our findings have implications for the design of buried interface optimization in perovskite optoelectronic devices.

Origin, Influence, and Countermeasures of Defects in Perovskite Solar Cells

Defects are considered to be one of the most significant factors that compromise the power conversion efficiencies and long-term stability of perovskite solar cells. Therefore, it is urgent to have a profound understanding of their formation and influence mechanism, so as to take corresponding measures to suppress or even completely eliminate their adverse effects on device performance. Herein, the possible origins of the defects in metal halide perovskite films and their impacts on the device performance are analyzed, and then various methods to reduce defect density are introduced in detail. Starting from the internal and interfacial aspects of the metal halide perovskite films, several ways to improve device performance and long-term stability including additive engineering, surface passivation, and other physical treatments (annealing engineering), etc., are further elaborated. Finally, the further understanding of defects and the development trend of passivation strategies are prospected.

Role of Chemistry and Crystal Structure on the Electronic Defect States in Cs-Based Halide Perovskites

The electronic structure of a series perovskites ABX3 (A = Cs; B = Ca, Sr, and Ba; X = F, Cl, Br, and I) in the presence and absence of antisite defect XB were systematically investigated based on density-functional-theory calculations. Both cubic and orthorhombic perovskites were considered. It was observed that for certain perovskite compositions and crystal structure, presence of antisite point defect leads to the formation of electronic defect state(s) within the band gap. We showed that both the type of electronic defect states and their individual energy level location within the bandgap can be predicted based on easily available intrinsic properties of the constituent elements, such as the bond-dissociation energy of the B-X and X-X bond, the X-X covalent bond length, and the atomic size of halide (X) as well as structural characteristic such as B-X-B bond angle. Overall, this work provides a science-based generic principle to design the electronic states within the band structure in Cs-based perovskites in presence of point defects such as antisite defect.

Unraveling the Photogenerated Electron Localization on the Defect-Free CH3NH3PbI3(001) Surfaces: Understanding and Implications from a First-Principles Study

The localization of photogenerated electrons in photovoltaic and photocatalytic materials is crucial for reducing the electron-hole recombination rate. Here, the photogenerated electron localization is systematically investigated on the CH₃NH₃PbI₃ (MAPbI₃) perovskite using first-principles calculations. It is found that under vacuum conditions, the photogenerated electron is delocalized in the MAPbI₃ bulk as well as on the stochiometric MAPbI₃(001) surface with the CH₃NH₃I (MAI) termination, while it is trapped on the defect-free PbI₂-terminated surface. Our ab initio molecular dynamics simulations reveal that the introduction of solutions will prompt the formation of localized electronic states. The photogenerated electron is discovered to be localized on both the MAI- and PbI₂-terminated surfaces in the presence of solutions with different concentrations of HI, from pure water to the saturated solution. We demonstrate that the Pb-I bond weakening or breaking resulting in an unsaturated coordination of a Pb site is the prerequisite to trap the photogenerated electron.

Realizing Reduced Imperfections via Quantum Dots Interdiffusion in High Efficiency Perovskite Solar Cells

Realization of reduced ionic (cationic and anionic) defects at the surface and grain boundaries (GBs) of perovskite films is vital to boost the power conversion efficiency of organic-inorganic halide perovskite (OIHP) solar cells. Although numerous strategies have been developed, effective passivation still remains a great challenge due to the complexity and diversity of these defects. Herein, a solid-state interdiffusion process using multi-cation hybrid halide perovskite quantum dots (QDs) is introduced as a strategy to heal the ionic defects at the surface and GBs. It is found that the solid-state interdiffusion process leads to a reduction in OIHP shallow defects. In addition, Cs⁺ distribution in QDs greatly influences the effectiveness of ionic defect passivation with significant enhancement to all photovoltaic performance characteristics observed on treating the solar cells with Cs_0.05 (MA_0.17 FA_0.83 )_0.95 PbBr₃ (abbreviated as QDs-Cs5). This enables power conversion efficiency (PCE) exceeding 21% to be achieved with more than 90% of its initial PCE retained on exposure to continuous illumination of more than 550 h.