The Influence Of Co Doping On The Performance Of Cu₂ZnSn（S,Se）₄ Solar Cells

Recently,Cu₂ZnSn（S,Se）₄（CZTSSe）solar cells become the research hotspot of photovoltaic materials.CZTSSe is a quaternary selenide semiconductor with a direct bandgap,which has some advantages of abundant element reserves,low cost,environment friendly.The theoretical efficiency of CZTSSe solar cells was predicted to be 32.8%,however,the highest efficiency in the laboratory is only 12.6%.In the past two years,many research groups have achieved more than 10%efficiency.One of the important reasons to limit efficiency enhancement of CZTSSe solar cell is the low open circuit voltage and the low hole concentration.It is known that doping is an effective route to improve the hole concentration and weaken the Fermi level pinning effect.The transition metal（TM）element is considered as a good dopant candidate because it has variable chemistry valence.Especially,the partial filled d orbit can induce desired ferromagnetism in TM-doped CZTSSe.Many research groups attempted to dope magnetic TM elements into CZTSSe for altering the optical and electrical properties via partially or completely replacing Zn,but less work was done with Co doping.In this thesis,we proposed the idea of improving the optical and electrical properties of Cu₂ZnSn（S,Se）₄ absorption layer by adding Co into the layer.The main research content includes:1.Preparation of Co-doped CZTSSe（denoted as CZTSSe:Co）films with different Co concentrations using sol-gel method.It is found that the band gap and resistivity of CZTSSe:Co films decrease with the increase of the Co concentration.2.Structural,electrical,optical and magnetic properties of CZTSSe:Co films with high Co concentration.We prepared CZTSSe:Co films with high Co concentration and investigated the influence of Co doping on the properties.The results suggest that the Co doping leads to the significant changes in the electrical properties of CZTSSe:Co film.We found that the conduction mechanism of CZTSSe:Co film changes from thermal excitation conduction to nearest neighbor conduction and Mott variable-range conduction with the decrease of temperature.Meanwhile,the CZTSSe:Co thin films also exhibit room temperature ferromagnetism.3.Fabrication of CZTSSe-based solar cells with CZTSSe:Co absorption layer.We selected Co-doped CZTSSe with low Co concentration as absorption layer to fabricate solar cells.Our results indicate that the efficiency of the solar cell is improved from 3.4%（undoped）to 3.95%（Co-doped）.The improvement of efficiency is mainly attributed to the increase of short-circuit current derived from the hole concentration in Co-doped CZTSSe absorption layer.