Strain is ubiquitous in perovskite based photovoltaic cells, but can be hardly well released in most perovskite films, which usually has an adverse effect on devices' performance and lifetime. Here, a molecular buffer interface that enabled strain-free hybrid perovskite films is demonstrated via a sequential deposition method. A quaternary ammonium iodide buffer layer can effectively release the in-plane tensile stress of PbI2via its flexibility and weak interaction, which enlarges the PbI2 layer spacing and favors the subsequent perovskite phase conversion. Drift-diffusion simulations further suggest optimized interfacial band edges after the removal of residual PbI2, which enhances the charge extraction of solar cell devices. The optimized perovskite solar cell device attains an efficiency of 22.14% with a high fill factor of 0.846 and good long-term stability.