Band alignment in Zn(1−<i>x</i>)Mg<i>x</i>O:Al/SiO<i>x</i>/Si heterostructures for photovoltaic applications realized by atomic layer deposition: Effects of Al doping and Mg alloying

In this work, the impact of Al doping and Mg alloying on the conduction band misalignment (ΔEC) between ZnO and (100) Si with a SiOx interlayer was studied by combining capacitance vs voltage, Hall and x-ray diffraction measurements, energy-dispersive x-ray spectroscopy, secondary mass spectrometry, and high-resolution scanning transmission electron microscopy. To decouple the effect of the high carrier density in the ZnO-based layers due to the Al introduction, the measured ΔEC was corrected for the conduction band lowering effect taking into account the conduction band non-parabolicity of ZnO. Then, from the Mg content dependence, using the interface-induced gap states approach, branch point energies referred to the valence band maximum equal to (2.7±0.2) and (3.6±0.4) eV were extracted for ZnO and MgO, respectively. These branch point energies were obtained under the assumption of a linear variation between the respective values of the corresponding two binary compound semiconductors, ZnO and MgO, and taking into account the presence of the SiOx interlayer. Furthermore, in the case of the undoped Zn0.96Mg0.04O layers, a ∼0.27 eV reduced ΔEC was found, with the difference with respect to Zn0.94Mg0.06O:Al attributed to the presence of a downward band bending toward the interface with SiOx. Full 1 × 1 cm test solar cells based on Zn0.8Mg0.2O:Al layers exhibited short circuit currents, open circuit voltages, fill factors, and efficiencies that varied in the (28±1)mA/cm2, (430±20) mV, (61±2)%, and (7.2±0.3)% ranges with the residual ΔEC∼0.6 eV being among the main causes of the reduced device performances.