Recombination reduction by ammonium additives in ambient air process for organometal halide perovskite solar cells

Organometal halide perovskite solar cells (PSCs) have achieved power conversion efficiencies (PCE) greater than 26 %, making them attractive photovoltaic devices. However, achieving high-performance PSCs typically requires inert gas conditions and antisolvent processes, which are hurdles to the commercialization of PSCs. To overcome these problems, we adopted a vacuum flash-assisted process (VASP) under ambient air conditions. In addition, to minimize the effect of moisture and enhance crystal growth, we adopted an additive engineering strategy using ammonium salts (NH4X, X = I, Cl, and SCN). Among the various ammonium salts, ammonium iodide (NH4I) exhibited the best device performance, with a PCE of 19.8 %. The role of the ammonium salts was studied using photoluminescence (PL), scanning electron microscopy (SEM), grazing incidence wide-angle X-ray scattering (GIWAXS), and the device characteristics of the devices. In addition, we measured the wet film state using X-rays to study the effects of ammonium salts on the crystallization process. From these measurements and analyses, we found that the ammonium additives induced rapid crystallization of the perovskite layer, and NH4I induced a uniform crystalline state of the film in the vertical direction. The uniformity of the crystalline state was related to the reduction in charge recombination and the enhancement of the PCE in the NH4I added devices.