Band alignment in Zn(1−x)MgxO:Al/SiOx/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 ( Δ E C ) between ZnO and (100) Si with a SiO x 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 Δ E C 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 SiO x interlayer. Furthermore, in the case of the undoped Zn 0.96 Mg 0.04 O layers, a ∼ 0.27 eV reduced Δ E C was found, with the difference with respect to Zn 0.94 Mg 0.06 O:Al attributed to the presence of a downward band bending toward the interface with SiO x . Full 1 × 1 cm test solar cells based on Zn 0.8 Mg 0.2 O:Al layers exhibited short circuit currents, open circuit voltages, fill factors, and efficiencies that varied in the ( 28 ± 1 ) mA / cm 2 , ( 430 ± 20 ) mV, ( 61 ± 2 ) %, and ( 7.2 ± 0.3 ) % ranges with the residual Δ E C ∼ 0.6 eV being among the main causes of the reduced device performances.