A metal-oxide broken-gap heterojunction between p-Cu2O and n-In2O3 is proposed and studied for use as a tunnel junction in polycrystalline CuIn1-xGaxSe2 (CIGS) based tandem solar cells using numerical device simulation. Specifically, a tandem solar cell with a CuGaSe2 (CGS) absorber top cell and a CuInSe2 (CIS) absorber bottom cell was considered. The ballistic transport model explains well the carrier transport in the broken-gap heterojunction. Broken-gap heterojunctions provide linear current-voltage characteristics even if one side is lightly doped and the junction resistance is much lower than typical values of series resistance in solar cells. However, electron affinities of metal oxides for broken-gap band alignment may induce an energy barrier between the tunnel junction and the top and bottom solar cells, degrading performance. These barriers can be reduced using buffer layers. For example, NiO and graded In2O3-ZnO buffer layers are proposed beneath CGS top solar cell and above the CIS bottom solar cell, respectively. With these buffer layers, the efficiencies of the top and bottom cells are 17.5% and 6.5% respectively. The modeled efficiency of the CGS/CIS tandem solar cell is 24.1% and there is virtually no significant efficiency loss due to the presence of the broken-gap junction. (C) 2014 Elsevier B.V. All rights reserved.