Realizing the Lowest Bandgap and Exciton Binding Energy in a Two-Dimensional Lead Halide System

Finding stable analogues of three-dimensional (3D) leadhalideperovskites has motivated the exploration of an ever-expanding repertoireof two-dimensional (2D) counterparts. However, the bandgap and excitonbinding energy in these 2D systems are generally considerably higherthan those in 3D analogues due to size and dielectric confinement.Such quantum confinements are most prominently manifested in the extreme2D realization in (A)( m )PbI4 (m = 1 or 2) series of compoundswith a single inorganic layer repeat unit. Here, we explore a newA-site cation, 4,4 & PRIME;-azopyridine (APD), whosesize and hydrogen bonding properties endow the corresponding (APD)PbI4 2D compound with the lowest bandgap and exciton binding energyof all such compounds, 2.19 eV and 48 meV, respectively. (APD)PbI4 presents the first example of the ideal Pb-I-Pbbond angle of 180 & DEG;, maximizing the valence and conduction bandwidthsand minimizing the electron and hole effective masses. These effectscoupled with a significant increase in the dielectric constant providean explanation for the unique bandgap and exciton binding energiesin this system. Our theoretical results further reveal that the requirementof optimizing the hydrogen bonding interactions between the organicand the inorganic units provides the driving force for achieving thestructural uniqueness and the associated optoelectronic propertiesin this system. Our preliminary investigations in characterizing photovoltaicsolar cells in the presence of APD show encouraging improvements inperformances and stability.