Hybrid organic-inorganic perovskites (HOIPs) are a highly promising class of materials for advanced applications in nonlinear optics, especially for second-harmonic generation (SHG), a process that occurs only in materials with a noncentrosymmetric crystal structure. The breaking of inversion symmetry is not only crucial for SHG but also enables other important functionalities, including ferroelectricity and the bulk photovoltaic effect (BPVE). The BPVE can produce ultrafast, dissipation-less photocurrents without the need for heterostructures or interfaces, making these materials highly attractive for next-generation photovoltaics and self-powered photodetectors

A sure way to achieve noncentrosymmetry is through the incorporation of homochiral organic ligands, which inherently forces the material to crystallize in one of the chiral Sohncke space groups. While this is a guaranteed route to an SHG-active material, it limits the structural possibilities to chiral space groups, among which only a part is polar. Thus, non-chiral acentric structures can crystallize in a broader range of noncentrosymmetric space groups, potentially offering greater chance for e.g. ferroelectric properties.

This presentation will provide an overview of rational design strategies to engineer SHG-active hybrid perovskites and key techniques and challenges for comprehensive characterization of their temperature-dependent SHG responses. We will focus on crystal engineering techniques that induce noncentrosymmetry in achiral systems. Ligand halogenation, also known as halogen engineering, has proven to be a potent tool for inducing polar distortions and symmetry breaking, often through the formation of halogen bonds.[1] The introduction of specific organic cations, such as methylhydrazinium (MHy⁺), while being a nonchiral molecule, represents another powerful approach known to promote the formation of noncentrosymmetric strcture in lead halide perovskites.[2-4] These organic cations can induce structural distortions and break inversion symmetry through their unique geometric and electronic properties.

Through a series of case studies, this talk will demonstrate how these synthetic approaches can be leveraged to design and obtain novel hybrid perovskites with tailored SHG activity, feature multinoncentrosymmetry (the presence of multiple distinct temperature-dependent noncentrosymmetric crystal phases within a single material system) and other noncentrosymmetry-induced functionalities. The combination of rational crystal engineering strategies and comprehensive variable-temperature characterization provides a pathway toward developing next-generation nonlinear optical materials with enhanced performance characteristics.