Over 19% Efficient Inverted Organic Photovoltaics Featuring a Molecularly Doped Metal Oxide Electron-Transporting Layer

Molecular doping is commonly utilized to tune the charge transport properties of organic semiconductors. However, applying this technique to electrically dope inorganic materials like metal oxide semiconductors is challenging due to the limited availability of molecules with suitable energy levels and processing characteristics. Herein, n-type doping of zinc oxide (ZnO) films is demonstrated using 1,3-dimethylimidazolium-2-carboxylate (CO₂-DMI), a thermally activated organic n-type dopant. Adding CO₂-DMI into the ZnO precursor solution and processing it atop a predeposited indium oxide (InO_x) layer yield InO_x/n-ZnO heterojunctions with increased electron field-effect mobility of 32.6 cm² V⁻¹ s⁻¹ compared to 18.5 cm² V⁻¹ s⁻¹ for the pristine InO_x/ZnO bilayer. The improved electron transport originates from the ZnO's enhanced crystallinity, reduced hydroxyl concentrations, and fewer oxygen vacancy groups upon doping. Applying the optimally doped InO_x/n-ZnO heterojunctions as the electron-transporting layers (ETLs) in organic photovoltaics (OPVs) yields cells with improved power conversion efficiency of 19.06%, up from 18.3% for devices with pristine ZnO, and 18.2% for devices featuring the undoped InO_x/ZnO ETL. It is shown that the all-around improved OPV performance originates from synergistic effects associated with CO₂-DMI doping of the thermally grown ZnO, highlighting its potential as an electronic dopant for ZnO and potentially other metal oxides.