Studies On The Absorber Composition And Interface For Kesterite Cztsse Solar Cells

Kesterite materials such as Cu₂ZnSnS₄,Cu₂ZnSnSe₄ and Cu₂ZnSn?S,Se?₄ are absorber materials for new generation thin-film solar cells.These materials have similar crystal structure and properties with chalcopyrite Cu?In,Ga?Se₂ material which performs well in thin-film solar cells.Kesterite materials have high absorption coefficient(greater than 10⁴ cm^-1),adjustable band gap?1.0-1.5 eV?and good stability under light.Moreover,kesterite materials contain elements which are abundant in earth and environment-friendly,which makes them suitable to develop high-efficiency,low-cost and stable solar cells.There are various of methods to fabricate kesterite materials and these methods can be divided into vacuum methods and non-vacuum methods.Among these methods,non-vacuum solution methods develop quickly due to its simplicity,high material utilization and facile control of the ratios.The highest efficiency of 12.6%is achieved by hydrazine solution method.However,hydrazine is explosive and toxic,which limit its industry application.More environment-friendly and safer solution system such as DMSO and amine-thiol solution system have been developed and applied.This thesis adopted ethanolamine-thioglycollic acid solution system to fabricate Cu₂ZnSn?S,Se?₄?CZTSSe?absorber layer and thin film solar cells.Ethanolamine-thioglycollic acid and other environment-friendly solution systems have more complex chemical component than hydrazine solution system,the mechanism of crystal growth is still not clear.The volatility of stannum and zinc makes it even complicated to control the crystal growth.The chemical potential has strong influence on the phase and defect of semiconductor material,so it is important to control the composition of CZTSSe absorb layer.This thesis regulated the Zn/Sn ratio in CZTSSe absorb layer to control the phase,defect,optical and electrical properties in CZTSSe.The UV-Vis spectra and PL spectra show that the E_g of CZTSSe decreases as Zn/Sn ratio increases,as a result the light current can be promoted.The Raman spectra shows that the secondary phase has relationship with the composition.C-V and DLCP results show that Zn/Sn ratio can influence the carrier density and defect density.The lifetime of carrier recombination was measured by our lab-made transient photovoltage system.Under the ratio of Zn/Sn=1.39,we obtained the CZTSSe thin film with least secondary phase and least defect density.As high as 10.08%photoelectric conversion efficiency has been achieved by this composition regulation.The interface is also very important for solar cells.There is a high temperature selenization process in the fabrication of CZTSSe absorb layer,which would cause a thick layer of Mo?S,Se?₂ between Mo and CZTSSe.Mo?S,Se?₂ is not favorable for the transportation of carriers.Moreover,CZTSSe could react with Mo substrate,which would cause the phase decomposition of CZTSSe.There needs a method to promote the back contact of CZTSSe solar cells.This thesis develops a high-temperature pre-annealing process for Mo substrate in air to grow a layer of MoO₃ on the substrate,which can resist the growth of Mo?S,Se?₂ and the reaction between CZTSSe and Mo.The annealing temperature has been optimized and the Mo substrate treated with 350?has the appropriate MoO₃ which can resist Mo?S,Se?₂ and will not influence the transportation of carriers.The solar cell fabricated on the 350?annealed substrate achieved 10.58%photoelectric conversion efficiency.Through the regulation of composition in CZTSSe absorber and the optimization of back contact in solar cell,the efficiency is improved from 6.78%to 10.58%.The regulation of composition diminishes the secondary phase and defects in the absorber material effectively.The lifetime of carrier recombination and light current get promoted.Annealed substrates resolve the problem of Mo?S,Se?₂ and decrease the series resistance of solar cell,as a result,the fill factor of solar cell is promoted.In the future,there need more studies on the mechanism of crystal growth and defect formation to guide the improvement of the performance of kesterite solar cells.With the development of research and technology,kesterite solar cells are promising for industrial application.