Organometallic　lead　halide　perovskites　are　excellent　light　harvesters　for　high-efficiency　photovoltaic　devices.　However,　as　the　key　component　in　these　devices,　a　perovskite　thin　film　with　good　morphology　and　minimal　trap　states　is　still　difficult　to　obtain.　Herein　we　show　that　by　incorporating　a　low　boiling　point　alkyl　halide　such　as　iodomethane　(CH3I)　into　the　precursor　solution,　a　perovskite　(CH3NH3PbI3-xClx)　film　with　improved　grain　size　and　orientation　can　be　easily　achieved.　More　importantly,　these　films　exhibit　a　significantly　reduced　amount　of　trap　states.　Record　photoluminescence　lifetimes　of　more　than　4　mu　s　are　achieved;　these　lifetimes　are　significantly　longer　than　that　of　pristine　CH3NH3PbI3-xClx　films.　Planar　heterojunction　solar　cells　incorporating　these　CH3I-mediated　perovskites　have　demonstrated　a　dramatically　increased　power　conversion　efficiency　compared　to　the　ones　using　pristine　CH3NH3PbI3-xClx.　Photoluminescence,　transient　absorption,　and　microwave　detected　photoconductivity　measurements　all　provide　consistent　evidence　that　CH3I　addition　increases　the　number　of　excitons　generated　and　their　diffusion　length,　both　of　which　assist　efficient　carrier　transport　in　the　photovoltaic　device.　The　simple　incorporation　of　alkyl　halide　to　enhance　perovskite　surface　passivation　introduces　an　important　direction　for　future　progress　on　high　efficiency　perovskite　optoelectronic　devices.