We report the development of a solution-processed In2O3/ZnO heterojunction electron transport layer (ETL) and its application in high efficiency organic bulk-heterojunction (BHJ) and inorganic colloidal quantum dot (CQD) solar cells. Study of the electrical properties of this low-dimensional oxide heterostructure via field-effect measurements reveals that electron transport along the heterointerface is enhanced by more than a tenfold when compared to the individual single-layer oxides. Use of the heterojunction as the ETL in organic BHJ photovoltaics is found to consistently improve the cell's performance due to the smoothening of the ZnO surface, increased electron mobility and a noticeable reduction in the cathode's work function, leading to a decrease in the cells’ series resistance and a higher fill factor (FF). Specifically, non-fullerene based organic BHJ solar cells based on In2O3/ZnO ETLs exhibit very high power conversion efficiencies (PCE) of up to 12.8%, and high FFs of over 70%. The bilayer ETL concept is further extended to inorganic lead-sulphide CQD solar cells. Resulting devices exhibit excellent performance with a maximum PCE of 8.2% and a FF of 56.8%. The present results highlight the potential of multilayer oxides as novel ETL systems and lay the foundation for future developments.