Phase-Pure Thiophene-Based Quasi-2D Perovskite Single Crystals via Cosolvent-Controlled Crystallization

Quasi-two-dimensional (Q2D) metal halide perovskites (PVSKs) have attracted great attention due to their improved environmental stability over three-dimensional ones. However, solution-based synthesis commonly yields mixed phases in the PVSKs that introduce an energetic disorder, limiting efficient carrier transport and thus device performance. In spite of its high demand, achieving phase-pure Q2D PVSKs remains challenging particularly for highly hydrophobic spacers due to supersaturation at the liquid-air interface and uncontrolled nucleation during crystallization. Here, we report a cosolvent-controlled crystallization method via an aqueous route, enabling the synthesis of phase-pure Q2D PVSKs single crystals based on 2-thiophenemethylammonium (TMA) with n = 1-3. Introducing sulfolane as the cosolvent increases the solubility of TMA and reduces the surface excess concentration of PVSK precursors, suppressing supersaturation and random nucleation. As a result, we obtain highly crystalline, phase-pure Q2D PVSK single crystals, confirmed by PL, XRD, and single-crystal XRD. Photodetectors fabricated from the phase-pure crystals exhibit low dark current, a high on/off ratio, high responsivity, high specific detectivity, and fast rise and fall time. This work establishes an effective strategy to overcome spacer-induced phase inhomogeneity and expands the library of phase-pure Q2D PVSKs for stable, high-performance optoelectronics.