Disentangling the Effects of Organic Spacer Fluorination on Lattice Stability Using an Identical 2D Perovskite...

Metal halide perovskite single crystals have emerged as promising optoelectronic materials, yet their long-term stability remains limited by severe ion migration arising from insufficient lattice stabilization. Although fluorination is widely employed to enhance stability, whether fluorination necessarily provides cumulative benefits remains unclear, primarily because fluorination can alter crystal symmetry, making its intrinsic effects difficult to isolate. Herein, by incorporating mono- and trifluorinated spacer cations into the {EA2Pb3Br10} template framework, two novel 2D perovskite single crystals, (SFEA)2(EA)2Pb3Br10 (SF) and (TFEA)2(EA)2Pb3Br10 (TF), were synthesized, both crystallizing in the polar space group Cmc21. This platform enables a direct comparison of the effects of fluorination content while minimizing symmetry-related variations. Interestingly, the more fluorinated TF exhibits enhanced photoresponse owing to the increased dielectric constant, whereas the less fluorinated SF likely benefits from stronger organic-inorganic interactions, contributing to reduced dark-current drift (2.51 × 10-7 nA cm-1 s-1 V-1), indicating suppressed ion migration and enhanced lattice rigidity. Consequently, photodetectors based on SF single crystals retain 63.7% ± 10.7% of their initial photocurrent after 96 days of ambient aging, significantly outperforming TF counterparts (25.5% ± 3.1%). This work provides a template-based strategy for disentangling fluorination effects, offering insights into the rational design of highly stable optoelectronic materials.