Identifying and Overcoming the Polaron-Induced Mobility Limit in a 2D Germanium Halide Perovskite.

Germanium-based halide perovskites (GHPs) are promising nontoxic alternatives to their lead-based counterparts, yet their charge transport properties remain poorly understood. Probing the intrinsic mobility of these materials has been challenging due to the lack of single-crystal devices. Here, we report the fabrication of single-crystal field-effect transistors from a 2D Ruddlesden-Popper GHP, (PEA)₂GeI₄. Temperature-dependent measurements reveal that its intrinsic charge transport is thermally activated (∂μ/∂T > 0), a hallmark of small polaron hopping, which stands in stark contrast to the band-like transport (∂μ/∂T < 0) of its tin-based analogue ((PEA)₂SnI₄). We provide direct spectroscopic evidence that this behavior is driven by exceptionally strong electron-phonon coupling in the GHP lattice. Critically, we validate this by demonstrating that rational cation engineering to suppress this coupling switches the transport mechanism back to the more efficient band-like regime, enhancing mobility by over an order of magnitude. This work not only identifies small polaron formation as the primary performance limit in GHPs but also demonstrates a clear strategy to overcome it, contributing to the rational design of high-performance, nontoxic perovskite optoelectronics.