Over 22% Efficient Silicon Heterojunction Solar Cells via Enhanced Oxygen Migration in Mg-Doped ZnO Electron Transport Layers

In silicon heterojunction (SHJ) solar cells, wide-band gap metal-oxide electron transport layers (ETLs) are being actively explored as dopant-free alternatives to conventional doped silicon contacts. Yet, for ZnO-based ETLs, aliovalent donor doping (e.g., Al or Ga) can raise electron concentration but often introduces lattice mismatch and defect-related recombination. The structural perturbation compromises carrier selectivity and limits the open-circuit voltage (VOC) in the device. Here, we demonstrate a Mg-doped ZnO (ZnMgO) ETL that overcomes these limitations by exploiting oxygen migration as a controllable tuning knob for electron-selective contacts. Owing to the similar ionic radii of Mg2+ and Zn2+, Mg incorporation mitigates lattice distortion while enabling oxygen redistribution and oxygen-vacancy formation near the metal interface. Using radio frequency magnetron sputtering as an industry-compatible deposition route, followed by rapid thermal processing to activate donors, we reconstruct the interfacial band alignment and enhance electron extraction at the ZnMgO/c-Si contact. Density functional theory calculations show that Mg lowers the oxygen-vacancy formation energy and reduces the migration barrier for lattice oxygen, while complementary structural, spectroscopic, and electrical measurements experimentally verify Mg-enabled donor activation and oxygen migration in ZnMgO. As a result, a low specific contact resistivity of 63.89 mΩ·cm2 is achieved, enabling SHJ solar cells with a power conversion efficiency of 22.08% and an open-circuit voltage of 717 mV─representing a high open-circuit voltage while maintaining competitive efficiency among sputtered ZnO-based ETLs. This work establishes oxygen-migration engineering in ZnMgO as a scalable strategy for efficient, stable dopant-free electron-selective contacts in high-performance crystalline silicon photovoltaics.