On the Demystification of “HPbI₃” and the Peculiarities of the Non‐innocent Solvents H₂O and DMF

Abstract

Detailed investigations by XRD reveal that the precursor “HPbI3” that was obtained by reaction of aq. conc. hydroiodic acid HI and PbI2 in DMF is (CH3)2NH2PbI3. (CH3)2NH2+ is formed by solvent reaction as already described in the literature but not properly assigned. Attempts to synthesize HPbI3 by gas phase reaction of PbI2 with aq. conc. HI yielded light‐yellow crystals of the oxonium salt H18O8Pb3I8 (Pbam, Z = 2, a = 10.075, b = 30.162, c = 4.5664 Å). The crystal structure of H18O8Pb3I8 consists of trimeric ribbons of edge‐sharing PbI6 octahedra. These ribbons [Pb3I8]2– are separated by protonated fragments of crystalline ice [H18O8]2+ or (H2O)6(H3O+)2. H18O8Pb3I8 can also be precipitated from PbI2 and aq. conc. HI. H18O8Pb3I8 is not stable at room temperature but transforms within a few days to light‐yellow (H3O)2x(H2O)2–2xPb1–xI2 with x ≈ 0.23 (R3m, Z = 3, a = 4.5554, c = 29.524 Å). The crystal structure represents a CdCl2‐type layer structure with H2O/H3O+ in between. Charge compensation is achieved by Pb2+ vacancies. Via topotactic reaction (H3O)2x(H2O)2–2xPb1–xI2 releases H2O/HI and forms crystals of the pristine PbI2. All steps were characterized by P‐XRD, IR/Raman spectra, and UV/Vis spectra. H18O8Pb3I8 acts as a precursor for the synthesis of MAPbI3 because the reaction with gaseous CH3NH2 yields MAPbI3, so it can mimic a composition “HPbI3”.

Unveiling Property of Hydrolysis-Derived DMAPbI3 for Perovskite Devices: Composition Engineering, Defect Mitigation, and Stability Optimization

“…We synthesized the DMAPbI 3 powder following the commonly used method by reacting PbI 2 and HI in DMF (details in Methods section), which is used as the starting material for making the Cs-based perovskite films (Figure 1A). This method was once widely believed to produce the “mythical” HPbI 3 (Wang et al., 2015, Pang et al., 2016, Long et al., 2016), whereas a recent study challenged such claim and proposed that the product would be DMAPbI 3 (Ke et al., 2018, Daub and Hillebrecht, 2018), which is now confirmed, finally, by our detailed analysis as described below.

Figure 1Synthesis and Characterization of the DMAPbI 3 Powder(A) Schematic diagram for synthesizing DMAPbI 3 powder.(B) XRD pattern of the DMAPbI 3 powder.(C and D) SEM images of the DMAPbI 3 powder with different resolutions, See also Figure S1.(E–H) Energy-dispersive spectrometric (EDS) mapping for N, Pb, and I elements in the DMAPbI 3 powder: EDS mapping of (E) N, Pb, and I elements overlay, (F) N element, (G) Pb element, and (H) I element distribution of the DMAPbI 3 powder.

…”

“…For example, it has recently been suggested that the “mythical” hydrogen lead trihalide (HPbI 3 , also known as PbI 2 ∙ x HI), the often-assumed reaction product of HI and PbI 2 , does not actually exist (Ke et al., 2018). Instead, adding acid to DMF is known to generate a weak base dimethylamine (DMA) through hydrolysis (Noel et al., 2017, Sutherland, 2017, Daub and Hillebrecht, 2018), and with the presence of PbI 2 the actual final product is believed to be a compound of DMAPbI 3 (DMA + = dimethylammonium, (CH 3 ) 2 NH 2 + ). Despite the broad adoption of such reaction route in fabricating Cs-based perovskite materials, systematic investigation of DMAPbI 3 as the starting material and its effect on the performance of the resultant PSCs has been largely missing, breeding continued debates and confusion.…”

Unveiling the Effects of Hydrolysis‐Derived DMAI/DMAPbIx Intermediate Compound on the Performance of CsPbI3 Solar Cells

“…In fact, inorganic perovskites are still hybrid organic‐inorganic perovskites . Here, HI acts with PbI 2 containing DMF to remove H 2 O, PbO, and PbO 2 in PbI 2 and also eventually form dimethylammonium (DMA + ) group . Similarly, Liu and coworkers reported hydrolysis‐derived materials (i.e., DMAPbI 3 ) and analyzed its role in producing high‐quality PSCs in detail by changing the ratio of CsI/DMAPbI 3 in the precursor .…”

New Acidic Precursor and Acetone-Based Solvent for Fast Perovskite Processing via Proton-Exchange Reaction with Methylamine

“…In the presence of dissolved PbI 2 and excessive iodide anions, the main product of this reaction, dimethylammonium (DMA + ), forms a solid phase DMAPbI 3 , which was previously mistaken for the mythical "HPbI 3 " [12,13]. Forming non-perovskite phase, the DMA affects the properties of the resulting perovskite film [11,13,14] (for more detailed discussion see SI). However, even a more significant problem of using DMF-based acid solutions is almost uncontrollable changing of solution composition upon preparation, storage, and processing.…”