Polarons in Lead-Halide Perovskites: What the Structural Evidence Does and Does Not Establish

Abstract

Photogenerated carriers in lead-halide perovskites are widely believed to move as large polarons, whose accompanying lattice distortion is thought to screen the carrier from charged defects. Three recent findings call this picture into question. First, time-resolved structural probes now measure the photoinduced lattice distortion directly; yet calculations show that thermal motion of the lattice alone can raise the carrier effective mass-an increase previously taken as evidence for polarons-so that it remains unclear whether the lattice distortion around a carrier is really a polaron. Second, the carrier-lattice coupling strengthens as the material is thinned from 3D to 2D. In the two main families of 2D perovskites, Ruddlesden-Popper and Dion-Jacobson, the carrier couples to the lattice through different channels, and their contrasting behavior is set by which coupling channel dominates rather than by lattice stiffness alone. Third, the first direct test of defect tolerance does not point to polaron screening; its authors attribute defect tolerance to the soft, disordered lattice. New transport data show that polaron screening reduces scattering between carriers, but not that it protects carriers from defects. Polaron formation, even if established, therefore does not establish polarons as the cause of defect tolerance. Separating the two requires measuring the lattice distortion as a function of excitation density, over a range spanning the low densities of a solar cell under sunlight and the much higher densities used in most ultrafast experiments. On this basis, we propose experiments that can clarify how photogenerated carriers behave in lead-halide perovskites.