Alignment of PbI2: Implications for Solar Cell Applications

• Lead iodide (PbI2) is a notable material in the realm of solar cell applications, particularly in the field of perovskite solar cells. The alignment and crystal structure of PbI2 play a significant role in determining the efficiency and performance of the resulting solar cells. Here are some implications of the alignment of PbI2 in solar cell applications:

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• Two-dimensional (2D) layered lead iodide (PbI2) is an important precursor and common residual species during the synthesis of lead–halide perovskites. There are currently debates and uncertainties about the effect of excess PbI2 on the efficiency and stability of the solar cell with respect to its energy alignment and energetics of defects. Herein, by applying first-principles calculations, we investigate the energetics, changes of work function, and defective levels associated with the iodine vacancy (VI) and interstitial iodine (II) defects of monolayer PbI2 (ML-PbI2). We find that PbI2 has very low formation energies of VI of 0.77 and 0.19 eV for dilute and high concentrations, respectively, reflecting the coalescence tendency of isolated VI. Similar to VI, a low formation energy of II of 0.65 eV is found, implying a high population of such defects. Both defects generate in-gap defective levels which are mainly due to the unsaturated chemical bonds of the p orbitals of exposed Pb or inserted I. Such rich defective levels allow the VI and II to be the reservoirs or sinks of electron/hole carriers in PbI2. Our results suggest that the remnant PbI2 in perovskite MAPbI3 (or FAPbI3) play dual opposite roles in affecting the efficiency of the perovskite: (1) Forming a Schottky-type interface with MAPbI3 (or FAPbI3) in which the built-in potential would facilitate the electron–hole separation and prolong the carrier lifetime; (2) acting as the recombination centers due to the deep defective levels. To promote the efficiency by the Schottky effect, our work reveals that the II defect is favored, and to reduce the recombination centers, the VI defect should be suppressed. Our results provide a deep understanding of the effects of defect engineering in ML-PbI2, which shall be beneficial for the related optoelectronics applications.

  
  

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• Crystal Structure: PbI2 has a layered crystal structure, with each layer consisting of lead ions surrounded by iodide ions. The alignment of these layers significantly affects the optical and electronic properties of the material, which in turn impacts the efficiency of the solar cell.

  
  

  Preferred Orientation: The alignment of PbI2 crystals influences the preferred orientation of the perovskite film. Controlling the alignment can help enhance the charge transport properties and improve the overall performance of the solar cell.

  
  

  Charge Transport: The alignment of PbI2 influences the charge transport pathways within the solar cell. Proper alignment can facilitate efficient charge transfer and reduce recombination losses, thereby improving the overall power conversion efficiency of the device.

  
  

  Stability and Durability: The alignment of PbI2 can impact the stability and durability of the perovskite solar cell. Proper alignment can enhance the material's resistance to degradation, ensuring long-term performance and reliability.

  
  

  Optical Properties: The alignment of PbI2 layers affects the light absorption and emission properties of the perovskite material. Optimizing the alignment can improve the light harvesting capability, thereby increasing the photocurrent and enhancing the overall efficiency of the solar cell.

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• Figure S1. Top view of relaxed structure of VI-2. The dark gray balls represent Pb atoms, the purple balls represent I atoms.

• Researchers and engineers in the field of solar energy continue to investigate methods to control and manipulate the alignment of PbI2 to optimize the performance of perovskite solar cells. By understanding and precisely controlling the crystal structure and alignment of PbI2, it is possible to design and develop more efficient, stable, and cost-effective solar cell technologies.
  

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• Figure S2. Top view of relaxed structure of VI-3. The dark gray balls represent Pb atoms, the purple balls represent I atoms.

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• Figure S3. The energy profiles corresponding to the formation of V-I pairs. The initial state (IS), final state (FS) and transition state (TS) of V-II-Pb is given in the inset.

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• Figure S4. The energy profiles corresponding to the recombination of V-I pairs. The initial state (IS), final state (FS) and transition state (TS) of V-II-Pb is given in the inset.

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• Figure S5. PDOS of pristine ML-PbI2 by PBE. The Pb-6 Pb-6p, I-5s and I-5p states are 𝑠, represented by black, red, green and blue lines, respectively.

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• Figure S6. The alignment of defective levels of VI-1 (left panel) and II-Pb (right panel) in different charged states (-1, 0, +1) relative to the band edges of perfect ML-PbI2.