The　open-circuit　voltage　(V-OC)　of　cesium-based　perovskite　solar　cells　(Cs-PSCs)　is　severely　limited　by　carrier　recombination　both　in　bulk　and　at　the　interface　of　the　perovskite　layer,　which　mainly　roots　in　its　elevated　fabrication　temperature.　Enhancing　the　quality　of　perovskite　film　and　optimizing　the　device　structure　are　effective　strategies　to　improve　the　performance　of　Cs-PSCs.　Here,　the　V-OC　is　markedly　promoted　by　constructing　CsPbI3/Cs(1-x)DMA(x)PbI(3)　bulk　heterojunction　(BHJ)　structure,　which　was　spontaneously　formed　by　a　precisely　controlled　spatially　selective　phase　transition　method.　The　perovskite　BHJ　structure　not　only　facilitates　the　charge　separation　and　collection　process　by　enhancing　the　built-in　potential　but　also　obviously　reduces　the　carrier　recombination　loss.　Importantly,　a　maximum　V-OC　of　1.23　V　was　obtained　as　the　V-OC　deficit　was　0.45　V,　and　the　champion　power　conversion　efficiency　(PCE)　reached　20.32%　(certified　PCE　of　19.66%).　Our　finding　indicates　that　junction　engineering　will　be　a　new　strategy　for　efficient　perovskite　devices.