Bipolar Carrier Transport of GaO _x via Doping and Defect Engineering for High‐Efficiency Silicon Heterojunction Solar Cells

Abstract Passivating contact serves as the critical enabler for fully unlocking the potential of high‐efficiency crystalline silicon (c‐Si) solar cells. Transition metal oxides (TMOs) have attracted significant attention as passivating contact layers due to their wide bandgap, tunable work function (WF), and effective surface passivation. Gallium oxide (GaO _x ) exhibits ultrawide‐bandgap (≈4.8 eV), high electron mobility, and excellent field‐effect passivation due to abundant fixed charges, yet its application in passivating contacts remains unexplored. This work demonstrates tunable carrier selectivity of GaO _x through ion doping for c‐Si solar cells. The Cu ²⁺ doping in GaO _x reduces oxygen vacancies (V _O ), leading to an increased WF of 5.64 eV and a decreased valence band offset with p‐Si, thereby achieving a low contact resistivity (ρ _c ) of 35.09 mΩ cm ² . In contrast, the Si ⁴⁺ doping in GaO _x increases the V _O , resulting in a reduced WF of 3.52 eV and a decreased conduction band offset with n‐Si, enabling a low ρ _c of 24.19 mΩ cm ² . As a result, the p‐Si/Al ₂ O ₃ /GaO _x :Cu ²⁺ /Au and n‐Si/SiO ₂ /GaO _x :Si ⁴⁺ /Mg heterojunctions have achieved high conversion efficiencies of 21.82% and 21.49%, respectively. This work demonstrates that GaO _x exhibits tunable and bipolar transport properties through doping and defect engineering, paving for the development of passivating contacts for highly efficient c‐Si solar cells.