Formamidinium (FA) lead halide (alpha-FAPbI(3)) perovskites are promising materials for photovoltaic applications because of their excellent light harvesting capability (absorption edge 840 nm) and long carrier diffusion length. However, it is extremely difficult to prepare a pure alpha-FAPbI(3) phase because of its easy transformation into a nondesirable delta-FAPbI(3) phase. In the present study, a "perovskite" template (MAPbI(3)-FAI-PbI2-DMSO) structure is used to avoid and suppress the formation of delta-FAPbI(3) phases. The perovskite structure is formed via postdeposition involving the treatment of colloidal MAI-PbI2-DMSO film with FAI before annealing. In situ X-ray diffraction in vacuum shows no detectable delta-FAPbI(3) phase during the whole synthesis process when the sample is annealed from 100 to 180 degrees C. This method is found to reduce defects at grain boundaries and enhance the film quality as determined by means of photoluminescence mapping and Kelvin probe force microscopy. The perovskite solar cells (PSCs) fabricated by this method demonstrate a much-enhanced short-circuit current density ( J(sc)) of 24.99 mA cm(-2) and a power conversion efficiency (PCE) of 21.24%, which is the highest efficiency reported for pure FAPbI(3), with great stability under 800 h of thermal ageing and 500 h of light soaking in nitrogen.