This PDF file includes:

• fig. S1. Comparative analysis of crystal structures of PbI₂·xHI and HPbI₃.
  
• fig. S2. Morphology of EDAPbI₄ films.
  
• fig. S3. Schematic structure of (110) layered 2D films.
  
• fig. S4. The organic compositions of CsPbI₃·xEDAPbI₄ films.
  
• fig. S5. Characterization of CsPbI₃ + 0.05PbI₂ with or without EDAI₂.
  
• fig. S6. Effect of EDAPbI₄ on the optical properties.
  
• fig. S7. Effect of EDAPbI₄ on the transient photovoltage behavior.
  
• fig. S8. Hysteresis behavior of CsPbI₃·0.025EDAPbI₄-based device.
  
• fig. S9. Effect of EDAPbI₄ on the phase stability of CsPbI₃·xEDAPbI₄ perovskite films.
  
• fig. S10. Phase stability of CsPbI₃·0.025EDAPbI₄ perovskite film under room temperature.
  
• fig. S11. Phase stability of CsPbI₃·0.025EA2PbI₄-based films.
  
• fig. S12. Device performance of CsPbI₃·0.025EA2PbI₄-based solar cell.
  
• fig. S13. Phase stability of CsPbI₃·0.025BA₂PbI₄-based films.
  
• fig. S14. Effect of CsPbI₃·0.025BDAPbI₄ and CsPbI₃·0.025EDBEPbI₄ 2D perovskite component on the evolution of morphology.
  
• fig. S15. Phase stability of CsPbI₃·0.025BDAPbI₄ and CsPbI₃·0.025EDBEPbI₄ films.

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