Understanding the Degradation of Methylenediammonium and Its Role in Phase-Stabilizing Formamidinium Lead Triiodide

Abstract

Formamidinium lead triiodide (FAPbI3) is the leading candidate for single-junction metal–halide perovskite photovoltaics, despite the metastability of this phase. To enhance its ambient-phase stability and produce world-record photovoltaic efficiencies, methylenediammonium dichloride (MDACl2) has been used as an additive in FAPbI3. MDA2+ has been reported as incorporated into the perovskite lattice alongside Cl–. However, the precise function and role of MDA2+ remain uncertain. Here, we grow FAPbI3 single crystals from a solution containing MDACl2 (FAPbI3-M). We demonstrate that FAPbI3-M crystals are stable against transformation to the photoinactive δ-phase for more than one year under ambient conditions. Critically, we reveal that MDA2+ is not the direct cause of the enhanced material stability. Instead, MDA2+ degrades rapidly to produce ammonium and methaniminium, which subsequently oligomerizes to yield hexamethylenetetramine (HMTA). FAPbI3 crystals grown from a solution containing HMTA (FAPbI3-H) replicate the enhanced α-phase stability of FAPbI3-M. However, we further determine that HMTA is unstable in the perovskite precursor solution, where reaction with FA+ is possible, leading instead to the formation of tetrahydrotriazinium (THTZ-H+). By a combination of liquid- and solid-state NMR techniques, we show that THTZ-H+ is selectively incorporated into the bulk of both FAPbI3-M and FAPbI3-H at ∼0.5 mol % and infer that this addition is responsible for the improved α-phase stability.

Direct observation of phase transitions between delta- and alpha-phase FAPbI3via defocused Raman spectroscopy

“…The defocused Raman setup, therefore, allows us to resolve much weaker Raman modes than was previously possible. To demonstrate this, we reported the molecular modes of the organic cation FA + in a recent study 37 (see also Fig. S4†) which have previously only been detected in FAPbI 3 with laser energies below the band gap.…”

Methoxy‐Based Passivator 4‐Methoxyphenethylammonium Iodide as Multifunctional Passivator for High‐Efficiency 3D/2D Flexible Perovskite Solar Cells over 23%

“…We grow the single crystals via the inverse temperature crystallization method. [7,18] Unfortunately, a significant number of defects were inevitably introduced. Figure 1a illustrates the optimization mechanism of the perovskite thin film and shows the chemical structure of p-MeOPEA.…”

“…The increase in bandgap caused by the variation in the ABX 3 structure, due to the presence of smaller cations, is not desirable for single‐junction solar cells. Additionally, the compositional inhomogeneity introduced during the growth process of mixed‐ion perovskites exacerbates ion separation [ 18,21 ] and non‐radiative recombination losses in the material during operation. [ 22 ] Developing a strategy that can optimize all aspects of performance while enhancing the outstanding photovoltaic conversion performance of perovskite materials and improving their stability would be highly valuable.…”

“…[14][15][16][17] However, under ambient conditions (light, heat, humidity), the cubic α-phase of FAPbI 3 is thermodynamically unstable compared to the transformation into the light-inactive hexagonal δ-phase polymorph. [18] It is crucial to address the stability issue of the PSC systems in FAPbI 3 . The key to solving this problem is to suppress the α to δ phase transition, to stabilize the perovskite structure, a commonly used approach is to alloy with FA þ by incorporating smaller A-site cations.…”

Enhancing the inherent stability of perovskite solar cells through chalcogenide-halide combinations

“…The advancement in PSCs increased their efficiency to around 26% 5,24–33 through structural optimization. These improvements involve mesoporous titanium dioxide, the design of organic and inorganic carrier transport layers, adjustment of doping and composition with various additives, and enhancement in device structures, including the development of laminated solar cells.…”