With their high efficiencies and potentially low manufacturing costs, metal halide perovskites are poised to revolutionize the solar cell industry. However, the same low energy of formation that enables low temperature and low-cost fabrication, also makes them highly vulnerable to degradation. To improve their long-term stability, the two-dimensional analogs of these materials have been investigated. These materials consist of alternating layers of organic ligands and metal-halide octahedra forming an array of quantum wells. The organic layer serves to increase the hydrophobicity and protects the metal-halide layer from environmental degradation. On top of the highly tunable perovskite composition, the layered perovskite structure provides two additional degrees of freedom: the choice of the organic ligand, and the number of metal-halide octahedral layers between each organic layer. Until now, a detailed understanding of how each of these changes affect the material energetics has not yet been achieved.
In this work, the energetic properties of a host of 2D layered perovskites are examined using a combination of experimental and theoretical techniques resulting in accurate ionization energies and electron affinities. It is shown that modeling these materials like quantum wells sheds light on the origin of their band gap, exciton binding energy and other optoelectronic properties. As the number of consecutive metal-halide octahedral layers increases, the confinement decreases thereby increasing the electron affinity and decreasing the ionization energy, consistent with the quantum well picture. Four different mechanisms by which the choice of organic ligand affects the electronic energy levels are identified and explained. These include the width of the organic layer, the energy levels of the organic ligand in the perovskite environment, the polarizability, and the bonding motif.
For 2D layered perovskites to be incorporated effectively into devices, understanding the nature of their surfaces and the interfaces they make is of critical importance. For this reason, the energetics of an interface joining a 2D and a 3D perovskite is probed directly. The effects of surface photovoltage and doping are also discussed.