Ammonium thiocyanate enables in-situ structural templating of MAPbI3 via quasi-2D perovskite scaffold

Ammonium thiocyanate (NH4SCN) has been used extensively to improve the current extraction and device performance of quasi-two-dimensional perovskite solar cells. Whilst this additive has been shown to influence quasi-2D perovskite crystallisation, a detailed understanding of its mechanism has not yet been established. Through spectroscopic and structural studies, we demonstrate that NH4SCN enables quasi-2D perovskite phases to structurally template the dominant 3D phase. Contrary to previous reports, we suggest that the benefit of this additive comes from an improvement to the 3D perovskite structure, not from charges transferring through vertically oriented 2D perovskite layers. We show that this additive provides kinetic control over the quasi-2D phase formation, suppressing the unfavourable n = 1 perovskite phase and promoting the growth of high-n 2D scaffold phases by extending the nucleation window. This promotes the in-situ templated growth of MAPbI3. We provide robust structural analysis demonstrating highly (110)-ordered 3D perovskite, with the mechanism confirmed by close lattice matching between this phase and quasi-2D phases. Furthermore, we show that this structural control is cation dependent, requiring both close lattice matching between quasi-2D and 3D perovskite phases, and successful kinetic control of the relevant quasi-2D phases.