Effect Of Zn_1-xMg_xO Window Layers On The Performance Of Cu₂ZnSn?S,Se?₄ Solar Cells

With the rapid development of the social progress era,the development of green environmental protection and renewable energy has become our top priority.Solar energy has attracted widespread research and attention as the most promising green energy among many renewable energy sources.Thin film solar cells photovoltaic devices can convert solar energy into electricity at low cost.At present,Cu₂ZnSn?S,Se?₄?CZTSSe?with kesterite structure have become the research focus in the field of solar cells due to their adjustable band gaps,earth-abundant elemental composition,high visible light absorption coefficient and non-toxicity and harmlessness.Based on the results from theoretical calculations,the theoretical efficiency of solar cells with CZTSSe as the absorption layer is 32.8%,but the highest efficiency achieved in experiments is only 12.6%,which is far from the expected value,which limits the improvement of its photoelectric conversion efficiency.There are many reasons to result in the low efficiency,such as the interface recombination,the crystalline quality of the absorption layer,the collection of carriers and loss of light,and so on.Among these reasons,the quality of window layers is an important factor to affect the efficiency of solar cells.Some studies indicated that the Mg doping in ZnO thin films can lead to the widening of the band gap and improve the optical quality.Thus,the absorption layer can absorb more photons.Meanwhile,the ionic radii of Mg²⁺and Zn²⁺are close and the incorporation of Mg does not cause significant lattice distortion.It is expected that Mg-doped ZnO can replace the pure ZnO as a window layer in CZTSSe solar cells for improving the efficiency.However,there are seldom experimental studies on Mg-doped ZnO as the window layer of solar cells.Therefore,in this thesis,for improving the optoelectronic performance of the CZTSSe solar cells,we try to reduce the light loss caused by the window layer via inducing Mg-doped ZnO as the window layers.The results are as follows:1.Zn_1-xMg_xO alloy targets with different Mg concentrations were prepared by the solid-phase reaction method,and Zn_1-x-x Mg_xO alloy films with different Mg concentration were successfully prepared on a sapphire substrate using a rf magnetron sputtering method.The experimental results indicate that Mg doping at a certain content will not affect the apparent lattice distortion of ZnO.As the Mg content gradually increases,the optical band gap of the Zn_1-xMg_x O films gradually increases and the transmittance of the films also gradually increases.2.Based on the performance characteristics of Zn_1-xMg_xO thin films with various Mg content,Zn_1-xMg_xO alloy film was used as a window layer instead of pure ZnO to prepare CZTSSe solar cells using the traditional techniques.The performance of CZTSSe solar cells was also characterized.The results suggest that the efficiency of the solar cell with the Zn_0.94Mg_0.06O window layer reached 6.60%,enhancing 55%with respect to the efficiency of 4.27%for the solar cell with a traditional ZnO window layer.The main reason is that the incorporation of Mg improves the optical performance of the window layer,reduces the light loss of the solar cells,and leads to the improvement of the short-circuit current density.3.In order to further analyze the influence of the Mg doping concentration on the band alignment of CdS/Zn_0.94Mg_0.06O heterojunction,we carried out a detailed investigation based on experiments and first-principles calculations.In experiments,the XPS measurements were used to determine the core levels of the elements to align the valence-band maximum?VBM?.The determined the energy band alignment for the CdS/Zn_0.94Mg_0.06O heterojunction is type-II.In the parts of theoretical calculations,the average electrostatic potential was used to align the VBM,and calculated the band alignment is also type-II,which is consistent with the results from XPS measurement.