Solvent coordination governs phase purity and crystallographic order in quasi-2D Ruddlesden–Popper perovskite thin films

Quasi-2D Ruddlesden–Popper (RP) perovskites have emerged as high-stability alternatives to three-dimensional metal-halide systems, with the bulky organic spacer 2-naphthylammonium (2-NA) offering enhanced resistance to moisture and thermal degradation. [1] However, the steric demand of aromatic spacers introduces significant kinetic complexity during film formation: the competition between spacer intercalation, inorganic framework condensation, and solvent evaporation collectively determines the final phase distribution. Controlling this competition through solvent engineering is therefore the central challenge in producing phase-pure, well-ordered quasi-2D films.

We investigate how solvent coordination strength and volatility modulate the structural organization of films targeting a nominal n = 2 stoichiometry, formulated as (2-NA)₂(MA,FA)Pb₂I₇ using 2-naphthylamine hydrochloride (2-NACl) as the spacer precursor. Four 3:1 (v/v) solvent mixtures were evaluated: A (DMSO:DMF), B (DMF:DMSO), C (NMP:DMF), and D (DMF:NMP). The series was designed to independently vary coordination strength and boiling point: DMF (b.p. 153°C) promotes rapid crystallization, while DMSO (b.p. 189°C) and NMP (b.p. 202°C) stabilize PbI₂ coordination complexes and slow nucleation. NMP was selected specifically for its superior solvation of aromatic species, which may facilitate the self-assembly of the 2-NA spacer layer more effectively than DMSO. [2][3]

X-ray diffraction (XRD) characterization reveals that solvent identity is the decisive factor in both phase purity and long-range crystallographic order. The NMP-dominant formulation (sample C) produced the highest structural quality, with sharp diffraction reflections (FWHM ≈ 0.16°) and well-resolved n = 1 and n = 2 phases — consistent with a slow, thermodynamically guided growth mechanism that allows the 2-NA spacers to adopt a uniform, tilted configuration (~55–65° relative to the inorganic plane).[4] In contrast, the DMF-dominant NMP formulation (sample D) exhibited a kinetically trapped phase landscape comprising n = 1, n = 2, and intermediate species, alongside measurable lattice expansion, suggesting that faster solvent evaporation traps metastable configurations before equilibrium packing can be achieved. The DMSO-based systems (A and B) performed poorly in comparison to their NMP counterparts at equivalent DMF ratios, indicating that NMP's combination of higher boiling point and aromatic solvation capacity provides a qualitatively different growth environment rather than a simple boiling-point effect.

Analysis of the d-spacing data confirms that the 2-NA spacer adopts a consistently tilted geometry in the best-performing films, with tilt-angle uniformity degrading markedly in samples where competitive crystallization kinetics were not adequately suppressed.

These findings establish that while the precursor stoichiometry targets n = 2, the realized phase distribution is primarily determined by the solvent coordination environment during film formation.[5] Ongoing work — encompassing UV–Vis absorption, steady-state and time-resolved photoluminescence (PL, TRPL), photoluminescence excitation (PLE) spectroscopy, and scanning electron microscopy (SEM) — will correlate these structural outcomes with the energy landscape of the resulting phase gradients and charge-transfer dynamics across phase boundaries. Device integration in photovoltaic and light-emitting architectures will follow. The results to date provide a clear design principle: for bulky aromatic spacers in quasi-2D perovskites, maximizing solvent coordination lifetime — through high-boiling, strong Lewis-basic solvents with affinity for aromatic species — is the primary lever for achieving phase-pure, orientationally ordered films.