Effect Of Pre-deposited Molybdenum Disulfide Layers On The Performance Of CZTSSe Solar Cells

Kesterite structured Cu₂Zn Sn（S,Se）₄（CZTSSe）has now become a hot material for the preparation of absorber layers for solar cells in the field of photovoltaics.It has high reserves of its constituent elements in the earth’s crust and has the advantages of low toxicity,high absorption coefficient and tunable band gap.At present,the maximum power conversion efficiency（PCE）of CZTSSe absorption layer based solar cells is predicted to be 32%,but the efficiency of this cell only reaches 13%In the world,which is much lower than the former,and much lower than 22.9%achieved by Cu（In,Ga）Se₂ thin film solar cells with the same crystal structure.One of the key problems leading to the low efficiency of CZTSSe solar cells is that the formation of MoS₂（or MoSe₂）intermediate layer is inevitably between absorption layer the Mo back electrode in the process of the sulfur（or selenization）process,and the thickness of intermediate layer is not easy to control.This uncontrolled intermediate layer is unfavorable to the improvement of the performance of CZTSSe solar cell.In this thesis,a certain thickness of MoS₂ intermediate layer was pre-deposited on the Mo back electrode using magnetron sputtering method.By optimizing the thickness of MoS₂ intermediate layer,the performance of CZTSSe solar cells was improved.The main research content of this thesis is as follows:（i）The structure,optical and electrical properties of MoS₂ thin films were first investigated.MoS₂ films were prepared using RF magnetron sputtering at room temperature and sulfureted by tube furnace at different temperatures.The effects of sulfuration temperature on the crystal structure and optical properties of MoS₂ films were studied.It was found that the MoS₂ films prepared at room temperature are amorphous,and the crystal MoS₂ thin films with the hexagonal structure are obtained after annealing at 500℃and 540℃.The optical band gap of MoS₂ thin films are determined and the optical band gap shows a narrowing with the increase of annealing temperature.（ii）The electrical properties of amorphous MoS₂ films were studied via measuring temperature-dependent resistivity.The results suggest that the amorphous MoS₂ at low temperatures follows the conduction mechanism of Mott variable range hopping and Efros-Shklovskii variable range hopping mechanism.（iii）Amorphous MoS₂ films with various thickness were designedly introduced between the CZTSSe absorption layer and the Mo back electrode.The influence of different thickness of MoS₂ intermediate layer on the performance of CZTSSe solar cells were studied,focusing on the effect of 1min MoS₂ films on the cell performance.It was observed that the crystalline quality of the absorber layer was significantly improved and the generation of secondary phases was suppressed when a 1 min MoS₂layer was added.The highest photoelectric conversion efficiency（PCE）of 9.64%was obtained.Further analysis indicates that the rise of PCE is mainly attributed to the raise of J_SC and V_OC.The increase in J_SC is contributed from the increase in J_L,which is attributed to the increase of depletion layer width as inserting the MoS₂ layer with an appropriate thickness.The increase of V_OCoriginates mainly from the decrease of J₀.Our results suggest that the introduction of an appropriate thickness of MoS₂intermediate layer is profitable to reduce the back interface recombination in CZTSSe solar cells and enhances the photovoltaic performance of CZTSSe solar cells.