Electronic structure of Li+@C60 adsorbed on methyl-ammonium lead iodide perovskite CH3NH3PbI3 surfaces

Abstract

It was recently shown by Jeon et al. [Angew. Chem., 2018, 57, 4607] that air stability for more than 1000 hours under light illumination can be achieved in methyl-ammonium (MA) lead iodide perovskite solar cells when Li⁺ is replaced by Li⁺@C₆₀ in a dopant material used in the p-type donor layer. In order to demonstrate the role of Li⁺@C₆₀ in the hole transporting material, we carried out first-principles electronic structure calculations of Li⁺@C₆₀ adsorbed on MAPbI₃ using the van der Waals corrected density functional theory. We used finite slab models of tetragonal MAPbI₃ (001) surfaces with MAI terminations. Using the most stable MAPbI₃ surfaces, we introduced one Li⁺@C₆₀ molecule on various positions of the MAI surface and performed geometrical optimizations. Then, we found that the Li⁺@C₆₀ was adsorbed at the aboveC position with almost 100% ionization. There is not much difference between the top and bottom positions of Li⁺ inside C₆₀, but the bottom position is slightly more stable than the top position. In the MAPbI₃/Li⁺@C₆₀ system, the Kohn–Sham (KS) wave functions of the HOMO and HOMO-1 levels are spread out in MAPbI₃ and those of the LUMO, LUMO+1, LUMO+2 levels are localized in Li⁺@C₆₀; the energy gap between these levels is 0.6 eV. Thus, in the neutral, spin-polarized MAPbI₃/Li@C₆₀ system, the KS wave functions of the HOMO (SOMO, – spin) and LUMO (+spin) levels are both localized in Li@C₆₀ and their energy gap of 0.125 eV is very small. This HOMO (SOMO) level behaves as a hole level when it loses an electron to become Li⁺@C₆₀. Since there are three almost degenerate LUMO+1 and LUMO+2 levels, it is expected to behave as a good hole conductor. As such, we conclude that the use of Li⁺@C₆₀ molecules in place of the bare Li-ions as dopant materials in the MAPbI₃ solar cells can prevent degradation. In any case, regardless of the adsorption geometry, our results clearly show the existence of the energetically isolated hole level localized in the Li@C₆₀, which may successfully contribute to the suppression of the oxidation of the MAPbI₃ surface.