Quasi two-dimensional (2D) perovskites have garnered significant research attention due to their promising optoelectronic properties such as high emission quantum yields and absorption coefficients. Their structural diversity makes them suitable for an array of optoelectronic applications such as light emitting diodes, field effect transistors, and solar cells. The structure of quasi-2D perovskites feature layers of perovskite octahedra intercalated along the {001} direction by bulky organic spacer cations such as n-butylammonium. These cations increase the hydrophobicity and lattice rigidity of the material, resulting in improved operational stability over their three-dimensional perovskite counterparts - a key consideration for their commercial viability in solar cells. 

Despite these benefits, the layered structure of quasi-2D perovskites leads to reduced out of plane charge mobility, hindering their efficiency in solar cells. The use of additives to improve charge transport has been shown to improve the power conversion efficiency of these solar cells. Typically, thiocyanate (SCN-) additives are thought to induce the vertical orientation of 2D perovskite layers, thereby improving performance by enabling charges to be transported between electrodes along the in-plane perovskite layer, where the mobility is much greater. However, the understanding of this process is limited, and as such we are restricted in the identification of potential additives and their use across different perovskites. 

In this presentation I will discuss the structural effect of ammonium thiocyanate (NH4SCN) in quasi-2D perovskites, focussing on the formation and distribution of 2D and 3D perovskite domains in thin films. I will show our studies correlating spectroscopic and morphological measurements with structural properties, and demonstrating resultant improvements in photovoltaic performance. Here, we demonstrate that rather than inducing vertical orientation, the addition of NH4SCN enables the 2D perovskite to act as a scaffold, templating the growth of highly crystalline MAPbI3. These results expand our understanding of the role of additives in the fabrication of high-performance quasi-2D perovskite solar cells. Finally, I will show that the effect of NH4SCN differs with perovskite composition, in particular highlighting the pivotal role of the spacer cation on the choice of additive. We believe that this work provides important insights for improving charge transport in quasi-2D perovskites, enabling highly stable and efficient perovskite photovoltaics.