Lead　halide　perovskites　(LHPs)　with　a　three-dimensional　(3D)　octahedral　structure　possess　outstanding　optoelectronic　properties　for　photovoltaic　application,　yet　the　toxicity　of　lead　has　consistently　impeded　their　commercialization.　Among　the　various　candidates,　copper　halide-based　perovskites　(CHPs)　have　drawn　considerable　interest　due　to　their　low　cost,　high　stability　and,　especially,　eco-friendliness.　Currently,　the　performance　of　CHP　solar　cells　is　suboptimal,　primarily　due　to　their　low　electronic　dimensionality.　Reports　on　3D　CHPs　with　an　octahedral　structure　are　yet　to　emerge.　In　this　study,　the　structure　of　the　alloyed　3D　CsPb1-xCuxBr3　constructed　by　corner-sharing　[PbBr6]　and　[CuBr6]　octahedra　are　predicted　using　ab　initio　density　functional　theory　(DFT)　calculations.　By　comparing　different　functionals,　we　found　that　the　HSE06　+　SOC　method　provides　a　more　accurate　understanding　of　the　electronic　structure　of　the　Cu-Pb　alloyed　system,　as　opposed　to　PBE　or　PBE　+　SOC.　Our　work　reveals　that　the　more　ionic　Cu(II)　cations　lead　to　a　competition　between　the　individual　stability　of　[PbBr6]/[CuBr6]　octahedron　and　the　overall　stability　of　the　3D　perovskite　network.　Smaller　Cu(II)　cations　also　cause　a　shrinkage　in　the　lattice　and　promote　the　hybridization　between　Cu　3d　and　Br　4p　orbitals.　This　introduces　acceptor　bands　near　the　valence　band　maximum,　resulting　in　very　rare　degenerate　p-type　semiconductors.　Among　them,　Cu-rich　CsPb0.875Cu0.125Br3　is　a　very　promising　candidate　to　replace　LHP　absorbing　layer　in　solar　cells　by　exhibiting　a　near-ideal　direct　bandgap　of　1.359　eV　and　strong　light　absorption.　In　contrast,　Cu-poor　CsPb0.25Cu0.75Br3,　identified　as　an　indirect　bandgap　semiconductor,　exhibits　superior　hole　mobility　compared　to　electron　mobility,　along　with　weak　light　absorption/reflection.　These　properties　position　it　as　the　hole　transport　layer　for　inverted　perovskite　solar　cells.　This　work　sheds　new　light　on　applying　3D　structured　CHPs　in　photovoltaics　and　other　optoelectronics.　©　2025　Elsevier　Ltd