Probing defect states in 2D perovskites by two-photon absorption

Two-dimensional (2D) perovskites have better ambient stability against moisture degradation than three-dimensional (3D) perovskites but retain almost the same photophysical properties of 3D perovskites. These appealing properties make 2D perovskites very promising in optoelectronic applications. However, the aqueous synthesis procedure usually brings in defects in 2D perovskites unintentionally, which may hinder their performance in optoelectronic devices. Hence, investigating the types and characteristics of defects in the synthesized crystals are essential steps for the optimization and application of high quality 2D perovskites. Here, we investigate the intrinsic defects in a prototypical 2D perovskite, (C6H5C2H4NH3)2PbI4, by two-photon absorption spectroscopy and density functional theory (DFT) calculations. The defect was revealed to be lying 380 meV below the conduction band minimum. DFT calculations demonstrate that this defect was a deep level in-plane I vacancy (Ivac_in). The findings in this work potentially provide the physical basis for understanding the properties of defects in 2D perovskites.