Superb solar-to-electricity conversion property of organic-inorganic halide perovskite has recently received considerable attention. Although the power conversion efficiency (PCE) reaches over 22%, severe current-voltage hysteresis in perovskite solar cells has been issues. To solve the hysteresis, we report here on interfacial engineering via post-treatment of perovsklte films with phenylalkylammonium iodide. The 145 degrees C-annealed FA(.9)Cs(.1)PbI(2.9)Br(.1) perovskite films are post-treated with series of phenylalkylammonium iodides with chemical formula of (C6H5)CnH2n(NH2I), where effects of concentration and solvent on photovoltaic performance and hysteresis are systematically investigated. For n = 2 phenylethylammonium iodide (PEAT), hysteresis is reduced by PEAT treatment but there is optimal concentration to minimize the hysteresis. Average PCE, estimated from reverse and forward scanned data, of 15.94% before PEAT treatment is improved to 17.62% after 4 mM PEAT treatment mainly due to increase in average FF from 0.696 to 0.751. At higher concentration of 10 mM, average PCE is declined again to 15.60% due to the large difference in FF between reverse and forward scan. Post-treatment with PEAT increases carrier life time from 500 ns (pristine) to 795 ns, which is responsible for the reduced hysteresis. Longer alkyl chain such as PEAT (n = 2) is found to be better to reduce the hysteresis than shorter one like PAT (n = 0) because of longer carrier life time, which indicates that dipole moment of phenylalkylammonium iodide is involved in time-dependent charge extraction.