Electronic Structure of Trions in Layered Hybrid Lead Halide Perovskites

A theoretical description of trions in layered hybrid organic-inorganic lead halide perovskites is presented. The description is based on a variational effective mass model, including polaronic effects by means of a Haken potential, spatial and dielectric confinements. We show that in thin layers, trions are formed by an exciton plus a more distant, orbiting carrier. Because lattice polarization in these materials is weak at short distances but strong at long ones, trion attractions are less screened than repulsions. The result are binding energies (shift between trion and exciton spectroscopic bands) as large as 70 meV in monolayered structures, greater than those of biexcitons. The ionic character of the bond (uneven sharing of the attractive carrier by the two repelling ones) reaches 50%, stimulated not only by the position-dependent dielectric constant, but also by dielectric confinement, the two effects being nonadditive. A definition of binding energy in confined systems, properly describing the correlated dynamics of particles, is introduced.