(390a) Controlling Crystallization for Large Ligand Incorporation into Quasi-2D Perovskite Solar Cells

Organic-Inorganic Hybrid Perovskite materials have swept the field of photovoltaics, offering ideal absorbing layers that feature long carrier lifetime and diffusion lengths, strong photoluminescence, and promising tunability. Furthermore, the solution-processing methods used to make these perovskites ensure that the solar cells will remain low-cost. One of the main issues holding back perovskites from becoming ubiquitous is their stability in how they degrade in the presence of water, thus leading to decreased device performance. One promising method found to improve the stability is to reduce the dimensionality of the perovskites into 2D perovskites. This is achieved by incorporating large organic ammonium ligands that are too large for the Goldschmidt Tolerance factor, thus reducing dimensionality of the perovskite. While these 2D variants have improved moisture stability over MAPbI₃, the incorporation of organic ligands introduces electronically-insulating layers that inhibit charge transport between the conducting inorganic slabs. Ordinarily this would not be very impactful, except that by general methods used to produce solar perovskite films prefer an in-plane alignment that causes there to be no path for photo-generated carriers to be extracted out of the device. To this end, vertical orientation or “out-of-plane,” orientation is critical for the highest efficiency devices. Several efforts have been made to determine the mechanism of growth to promote vertical-orientation in these quasi-2D perovskites involving methods such as hot casting, additive engineering, and other fabrication engineering methods. While devices with substantial power conversion efficiencies (PCEs) have been made over the past few years, the vast majority of the devices with PCEs over 10% feature small organic ligands such as butylammonium (BA) and phenylethylammonium (PEA), which severely limit the potential of 2D perovskites as absorbing layers. The goal now is to extend high efficiency 2D perovskite devices to longer ligands with multiple conjugated groups. It is thought that many of the current techniques emphasize two points in forming the perovskite layer: 1) Isolate nucleation on surface of film (liquid/vapor interface) and 2) have slow growth for high quality junctions. When extending these techniques to larger, multi-functional conjugated ligands, the differences in solubilities for precursors is more extreme, requiring tighter control of the nucleation and crystal growth process. Using solvent engineering methods involving more poorly coordinating solvents (with respect to PbI₆^2- octahedra) and creating a solubility-gradient at the vapor-liquid interface in thin films with high-vapor pressure cosolvents, we have determined a method to help vertically orient quasi-2D perovskite with large multi-conjugated group ligands, namely IH₃N(CH₂)₂(SC₄H₂)₂ aka 2T. The vertical orientation has been characterized via many diffraction methods (XRD, GIWAXS, etc) and shown an improvement of devices over 13% PCE, doubling the efficiency of similarly reported devices in literature. Furthermore, the devices have tremendous moisture stability mostly due to their bulky conjugated ligands, being able to retain their performance better than their BA counterparts, and far better than their 3D perovskite devices. In addition, through this co-solvent engineering method, the trap density of the perovskites has greatly improved in addition to creating pinhole-free films. While creating high-performing devices is important, this study contributes to the field even more by focusing on the understanding the control requirements on quasi-2D perovskite photovoltaics with larger conjugated ligands. This step is critical to the development of perovskites as solar cells because it can be extended to even larger and more complex semi-conduction ligands making the perovskite system more moisture stable, bringing us all one step closer to the realization of affordable, and ubiquitous solar energy.