Influence Of Trace Metallic Ion(Ni²⁺,Mg²⁺) Doping On The Performance Of Cu₂ZnSn（S,Se）₄ Thin Film Solar Cells

As one representative of inorganic thin film solar cells,kesterite Cu₂Zn Sn（S,Se）₄（CZTSSe）thin film solar cells have a great deal of advantages:low cost,high absorption coefficient,earth-abundant component elements,high theoretical conversion efficiency,adjustable band gap etc.These advantages make CZTSSe thin film solar cells have great potential for future commercial applications.At present,the optimal photoelectric conversion efficiency of CZTSSe thin film solar cells reaches13.2%,with too much harmful defects and band tailing,serious open circuit voltage loss,and low filling factor.In order to overcome the above problem,scholars find the grain quality of films by tuning the selenization process,reducing the harmful defects and secondary phase;and the band tail state can be reduced by doping the additional cations,decreasing open circuit voltage loss.The research content of this dissertation is as follows:1.Exploring the optimal selenization process for thin films.Though optimizing the low-temperature and high-temperature conditions of the two-step selenization process,the optimal selenization conditions are determined to be LT400-10&HT550-10.At this time,the small particle layer of the film completely disappears,forming homogeneous large-grain structure of absorber layer.The surface morphology of the film is compact,and the average surface roughness is low.Under a bias voltage of 1.5 V,the average surface current of the film is high,reaching over 2 n A,which is beneficial for carrier transport in thin films;and the short circuit current density,open circuit voltage,fill factor,photoelectric conversion efficiency,and other photovoltaic parameters of thin film devices can be improved.2.On the basis of the optimal two-step selenization process,Ni doped CZTSSe thin films are prepared by introducing metal nickel ions(Ni²⁺)into the precursor solution.The addition of trace amounts of Ni²⁺can suppress the generation of reverse defects and defect clusters,improve the morphology of the film,increase the average grain size of the film,improve the concentration of photogenerated carriers,and increase the short-circuit current density of the device.When a trace amount of 0.5%Ni²⁺is added,the average surface current of CZTSSe thin film reaches 7 n A,the open circuit voltage of the device increases to 416.20 m V,and the short circuit current density increases to 33.88 m A·cm^-2.3.On the basis of the optimal two-step selenization process,CZTSSe thin films with Mg doping are prepared by introducing metal magnesium ions(Mg²⁺)into the precursor solution.Adding extra Mg²⁺can significantly inhibit the formation of secondary phases and harmful defects in the film,optimize the surface morphology of the film,promote the growth of crystal grains,increase the concentration of photo generated carriers,and improve the electrical properties of the film.When the doping amount of Mg²⁺is 2%,the carrier concentration of CZTSSe thin film increases from 2.57×10¹⁶ to 3.24×10¹⁶ cm^-3,the open circuit voltage of the device increases from 404.22 to 411.74 m V,the parallel resistance of the devices increases from 240.60 to 1750.27Ω·cm²,and the photoelectric conversion efficiency of the battery increased to 8.18%.In this thesis,the grain growth mechanism are systematically studied,and the optimal selenization process is developed;Moreover,the electrical properties of films and photovoltaic properties of devices are improved by Ni or Mg doping.This conclusion can provide a demonstration basis for the commercialization of CZTSSe thin film solar cells as soon as possible.In addition,the results expands the road for further optimization of copper-based solar cells.