Interfacial Electromechanics Predict Phase Behavior of 2D Hybrid Halide Perovskites

Quasi - two dimensional mixed-cation hybrid halide perovskites (q-2DPK) have improved structural stability and device lifetime over their 3D perovskite counterparts. The addition of a large organic A’ cation to the bulk AMX₃ structure gives the q-2DPK chemical formula A’₂A_N-1M_NX_3N+1 and introduces new synthetic degrees of freedom through the composition index N. Ordered lamellar structures form via coordinated M-X bond breaking and peculiar critical phase behavior emerges as a function of N. We propose a thermodynamic model parametrized by first-principles calculations to generate a phase diagram for the q-2DPK. We find that the difficulty in synthesizing phase-pure samples in the high-N composition range arises from the energetic competition between electrostatic interactions of opposing interfacial dipole layers and mechanical relaxation of interfacial stress. Our model shows quantitative agreement with experimental observations and explains the non-monotonic evolution of the lattice parameters with composition index N. This model is generalizable to the entire family of q-2DPK and can guide the design of optoelectronic and photovoltaic materials with enhanced environmental stability and reduced excitonic degradations to carrier transport.