Influence of Lewis base HMPA on the properties of efficient planar MAPbI3 solar cells fabricated by one-step process assisted by Lewis acid-base adduct approach

To fabricate efficient MAPbI(3) perovskites using the Lewis acid-base adduct approach, we used a Lewis base hexamethylphosphoramide (HMPA) having a high donor number (D-N), and investigated the effect of high-D-N HMPA treatment on the properties of planar MAPbI(3)-based solar cells. The main functions of the HMPA treatment were to passivate the perovskites by forming well-distributed PbI2 phases and to densify the perovskites by controlling the rate constants. The treatment with Lewis base HMPA by one-step spin coating and subsequent solvent washing promoted the formation of pinhole-free perovskite films by increasing the coordination ability of HMPA with Pb2+ adducts and controlling the nucleation and growth rates of the perovskites. In addition, treatment with various concentrations of HMPA led to the formation of a residual PbI2 phase that acted as a passivation layer, even though stoichiometric quantities of precursors were used. Through a series of advanced electron microscopy studies, the exact three-dimensional locations of the PbI2 passivation layers were determined. Furthermore, by calculating the decreased active trap concentrations and carrier densities, we verified the passivation effect of remnant PbI2. Importantly, we achieved the best power conversion efficiency of 17.09% by the controlled addition of Lewis base HMPA, thus demonstrating a much improved performance than that obtained without HMPA treatment (15.73%).