Thiocyanate Ligands Induce a Trade-Off between Carrier Transport and Nonradiative Relaxation through Halide Tuning in Two-Dimensional Lead Halide Perovskites.

Pseudohalides such as thiocyanate (SCN-) can stabilize and functionalize two-dimensional (2D) metal halide perovskites, yet their influence on excited-state lifetimes remains unclear. Here, first-principles calculations and nonadiabatic molecular dynamics are used to examine all-inorganic Ruddlesden-Popper (RP) Cs2Pb(SCN)2X2 perovskites (X = Cl, Br, I). Ordered SCN- ligands favor nonpolar Pmmn frameworks and reconstruct the valence band through Pb-N/S hybridization, producing composition-dependent band-edge delocalization, dielectric screening, exciton binding, and carrier mobility. Cs2Pb(SCN)2Br2 and Cs2Pb(SCN)2I2 exhibit lighter carriers, stronger screening, lower exciton binding energies, and higher room-temperature mobilities, although polar optical phonon scattering remains dominant. Nonadiabatic molecular dynamics further show that these transport advantages do not extend excited-state lifetimes. Instead, SCN--containing lattices show larger band-edge gap fluctuations, faster decoherence, and accelerated phonon-assisted nonradiative relaxation driven by low-frequency ligand, cation, and framework motions, revealing a trade-off between static transport properties and excited-state stability.