We　characterized　exciton　diffusion　and　dissociation　behaviour　in　two　ITIC　derivatives　non-fullerene　acceptors　(NFA)　by　time-resolved　spectroscopic　methods.　The　exciton　diffusion　length　was　determined　to　be　similar　to　26　and　similar　to　34　nm　in　ITIC　and　IT4F.　We　further　examined　the　dissociation　of　excitons　in　those　NFA　at　the　acceptor/donor　planar　heterojunction　interfaces　by　transient　absorption　measurements,　in　which　efficient　charge　generation　was　observed.　Finally,　we　fabricated　planar　heterojunction　solar　cells　using　PM6/NFA　bilayer　planar　heterojunctions;　a　power　conversion　efficiency　(PCE)　of　over　7　%　for　PM6/IT4F　bilayers　was　determined.　More　importantly,　the　spin-coating　of　top　layer　NFA　has　negligible　influence　on　the　morphology　of　the　PM6　layer,　suggesting　a　clear　bilayer　interface,　rather　than　a　quasi-bilayer　structure　with　a　portion　of　bulk　heterojunction.　The　results　suggest　that　enhanced　exciton　diffusion　length　and　efficient　exciton　dissociation　and　charge　generation　are　elemental　characters　to　realize　high　PCE　planar　heterojunction　organic　solar　cells.　We　established　direct　linking　between　the　exciton　diffusion　length　and　the　photocurrent　generation　in　NFA　layer　by　transfer　matrix　simulation.　The　large　exciton　diffusion　length　in　NFAs　makes　the　realization　of　high　efficiency　and　stable　bilayer　organic　solar　cells　feasible.