Inorganic alkali lead iodide semiconducting APbI₃ (A = Li, Na, K, Cs) and NH₄PbI₃ films prepared from solution: Structure, morphology, and electronic structure

Abstract

APbI3 alkali lead iodides were prepared from aqueous (A= Na, Cs, ammonium NH4+, and methyl­ammonium CH3NH3+) and acetone (A= Li, K) solutions by a self-organization low temperature process. Diffraction analysis revealed that the methylammonium-containing system (MAPbI3) crystallizes into a tetragonal perovskite structure, whereas the alkali and NH4+ systems adopt orthorhombic structures. Morphological inspection confirmed the influence of the cation on the growth mechanism: for A = Cs and NH4+, needle-like crystallites with lengths up to 3–4 mm; for A = K, thin stripes with lengths up to 5–6 mm; and for A = MA+, dodecahedral crystallites were observed. For A = Li and Na, the APbI3 systems typically resulted in polycrystalline aggregates. Optical absorption measurements demonstrated large energy band gaps for the alkali and ammonium systems with values between 2.19 and 2.40 eV. For electronic and chemical characterization by photoelectron spectroscopy, the as-prepared powders were dissolved in di-methylformamide and re-crystallized as thin films on F:SnO2 substrates by spin-coating. The binding energy differences between Pb4f and I3d core levels are highly similar in the investigated systems and close to the value measured for PbI2, indicating similar relative partial charges and formal oxidation states. The binding energies of the alkali ions are in accordance with oxidation state +1. The X-ray excited valence band spectra of the investigated APbI3 systems exhibited similar line shapes in the region between the valence band maximum and 4.5 eV higher binding energy due to common PbI6 octahedra which dominate the electronic structure. While the ionization energy values are quite similar (6.15 ± 0.25 eV), the Fermi-level positions of the unintentionally doped materials vary for different cations and different batches of the same material, which indicates that the position of the Fermi level can be influenced by changing the process parameters.

Keywords

• solution process
• alkali lead iodide
• ammonium lead iodide
• X-ray photoelectron spectroscopy
• ultra-violet photoelectron spectroscopy