Introduction : Double doping of zinc oxide thin films to achieve good surface morphology and excellent band gap energy value remains a challenge in materials science in the field of thin film solar cells.

Methods : In this work, we prepared a series of zinc oxide thin films double-doped with aluminum (Al) and magnesium (Mg) atoms using the sol-gel technique via spin-coating equipment. Double doping is carried out in the following proportions : (1%Al,1%Mg), (3%Al,3%Mg), (5%Al,5%Mg) and (7%Al,7%Mg). The double-doped (Al,Mg) thin films were: (1) synthesized by spin-coating process, (2) characterized by complementary techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) and UV-Vis-NIR spectroscopy, and, (3) used for 2-T perovskite/CIGSSe tandem solar cells.

Results : The characterization results assessed that the double-doped (Al,Mg) samples are oriented in the c-axis direction in a wurtzite structure, and the grain sizes range from 40 to 92 nm. Uniform, dense thin films were obtained on the glass substrates, and the samples consisting of spherically shaped nanograins forming homogeneous layers. In addition, optical transmittance measurements show good values between 87 and 91%, and the band gap energy, E_g, for the double-doped ZnO:(Al,Mg) materials has been determined using Tauc plots (E_g: 3.22 to 3.26 eV). SCAPS-1D software simulation results under the AM 1.5G spectrum show that an optimum efficiency of 20.62% (V_OC = 0.786 V, J_SC = 42.28 mA/cm², and FF = 62.03%) was achieved with the (1%Al, 1%Mg) layer.

Conclusions : This research paper introduces double doping to modulate the band gap energy, E_g, of doped ZnO:(Al,Mg) materials for applications in 2-T perovskite/CIGSSe tandem solar cells.