Recently,the two-dimensional (2D) ruddlesden-popper (RPP) perovskite has been successfully attracting great attention owing to their excellent electronic property and superior ambient stability.But 2D perovskite solar cells (PVSCs) with insulating large cations show a worse performance than three-dimensional (3D) PVSCs in general because of the worse charge transportation.In this work,dimethyl sulfoxide (DMSO) and KI were incorporated simultaneously to produce a synergistic effect on both film quality and orientation of 2D perovskite.With this strategy,a cavity-free 2D perovskite film was formed with vertically oriented crystal,and high quality film was obtained with decreased defects and increased crystallinity.Besides,profitable multiple phases were obtained for better spontaneous carrier separation and transportation.The 2D PVSCs based on (PEA)2(MA)n-1PbnI3n+1(n=5) delivered a higher power conversion efficiency (PCE) of 13.4％.In addition,the perovskite with KI and DMSO contained more stable low-dimension phase at the bottom ofperovskite film,which could act as a barrier to prevent moisture from further eroding internal perovskites.The optimized 2D PVSCs remained 90％ of the PCE after being exposed in air (50％-60％humidity,room temperature) with a continuous illumination for 300 h.