A 2D lead halide hybrid system with the lowest bandgap and exciton binding energy

Despite the rapid developments of hybrid lead halide perovskites as light harvesters and hole-transport materials, the inadequate environmental stability of these materials imposes an obstruction on the way to its commercialization. Insertion of large, hydrophobic cations at the A-site enhances the environmental stability at the cost of breaking 3D structure to obtain its 2D derivatives, resembling quantum well type structures with high bandgap (E_g) as well as large exciton binding energy (E_b), detrimental to the photovoltaic performances. In this work, a new 2D hybrid system is synthesised with a new organic spacer molecule. Unlike other n=1 2D systems reported in the literature, the Pb-I-Pb angle of the synthesized material is 180º, which implies an absence of any inter-octahedral tilt distortions in the inorganic lattice, leading to the smallest (2.19 eV) amongst the series of (A)_nPbI₄ (where n = 1 or 2) with any organic spacer molecule (A). The estimated (~50 meV) from a series of temperature dependent absorption measurements is close to that of the 3D limit of the hybrid perovskites. Moreover, the material exhibits exceptional water stability, making it an excellent candidate for photovoltaic applications.