Gap States in Methylammonium Lead Halides: The Link to Dimethylsulfoxide?

Abstract

Understanding the origin and distribution of electronic gap states in metal halide perovskite (MHP) thin films is crucial to the further improvement of the efficiency and long‐term stability of MHP‐based optoelectronic devices. In this work, the impact of Lewis‐basic additives introduced in the precursor solution on the density of states in the perovskite bandgap is investigated. Ultraviolet photoemission spectroscopy and contact potential difference measurements are conducted on MHP thin films processed from dimethylformamide (DMF)‐based solutions to which either no additive, dimethylsulfoxide (DMSO), or N‐methylpyrrolidine‐2‐thione (NMPT) is added. The results show the presence of a density of states in the gap of methylammonium lead halide films processed from DMSO‐containing solution. The density of gap states is either suppressed when the methylammonium concentration in mixed cation films is reduced or when NMPT is used as an additive, and eliminated when methylammonium (MA) is replaced with cesium or formamidinium (FA). These results are consistent with the notion that reaction products that result from DMSO reacting with MA+ in the precursor solution are responsible for the formation of gap states.

The pursuit of stability in halide perovskites: the monovalent cation and the key for surface and bulk self-healing

“…3b-Cs, because Cs oxides are known low work-function (B1 eV) materials. 39,54 We note that neither lead oxide nor lead bromate are stable in acid environment. MAPbBr 3 and FAPbBr 3 are reactive towards oxides due to the acidity of the A + cation and would transform to PbBr 2 .…”

“…Ref. 39 shows tail states in high-resolution photoemission spectroscopy, if DMSO is used in HaP film preparation. The assumption in this explanation is that solvent molecules are readily removed from the surface by evaporation, but remain trapped in the bulk.…”

Electronic properties of metal halide perovskites and their interfaces: the basics

“…The transition between the two regimes is not abrupt because of a finite density of states of the corresponding bands at which pinning occurs. Additionally, the presence of tail states near the band edges 32 leads to a more gradual transition. Another observation is that the apparent F sub at which the transition occurs seemingly depends on the MHP film thickness.…”

Mechanism and Timescales of Reversible p‐Doping of Methylammonium Lead Triiodide by Oxygen

“…[ 13 ] One important issue, which is at the center of this work, is the strong variation of the Fermi level ( E F ) position in the energy gap of MHPs, with nominally undoped MHPs reported to exhibit behavior ranging from n‐type to p‐type. [ 14–20 ] For example, Schulz et al. showed that E F in methylammonium lead triiodide (MAPbI 3 ) can be shifted by as much as 0.7 eV solely by varying the substrate from TiO 2 to NiO x .…”