Preparation And Performance Regulation Of Cu₂ZnSn?S,Se?₄ Thin-film Solar Cell Absorbers

Cu₂ZnSn?S,Se?₄?CZTSSe?with kesterite structure is considered as the most promising light-absorbing materials in the next-generation thin-film solar cells due to its high elemental abundance,low toxicity,large optical absorption coefficient and adjustable band gap.In recent years,CZTSSe has become a research hotspot in the field of renewable energy.The absorbers of high quality are usually prepared by magnetron sputtering and chemical solution deposition,but the chemical solution deposition is more popular for its easy operation and low cost as well as no need for the vaccum.So far,the CZTSSe thin-film solar cell with the world-record photoelectric conversion efficiency?12.6%?is still the one that prepared by the hydrazine-based solution method.However,the toxic hydrazine-based solution is against the green and eco-friendly goal in developing the solar cells.Therefore,it is very important and meaningful to develop a non-toxic solution method for preparing CZTSSe solar cells.In this thesis,the CZTSSe absorption layers were respectively prepared by magnetron co-sputtering and non-toxic water-based solution method.The influence of Ge or Cd doping on the properties of CZTSSe absorber as well as their impacts on the performance of CZTSSe-based solar cell were studied,and the results are listed as follows:1.CZTSSe films were obtained by selenizing Cu₂ZnSnS₄?CZTS?precursors prepared by co-sputtering of Cu₂S,SnS and ZnS targets.The results reveal that the quality of CZTSSe films could be effectively promoted by the selenization temperature.The obtained CZTSSe films have formed a kesterite structure at the selenization temperatures of 500-560?and their compositions are all Cu-poor.With increasing the selenization temperature,more Se element is incorporated into the CZTSSe films.Especially after being selenized at 540?for 30 min,the CZTSSe film possesses a Cu-poor/Zn-rich and suitable ratio of metal atoms to sulfur selenium atoms?Cu/?Zn+Sn?=0.67,Zn/Sn=1.20,?Cu+Zn+Sn?/?S+Se?=1.03?,which is very close to the composition of high quality CZTSSe absorber layer reported before.Moreover,it shows strong optical absorption,and its optical band gap?E_g?is found to be 1.41 eV.2.The CZTSSe films were prepared from the CZTS precursors as deposited by water-based solution method followed by a selenization process under different conditions.The structure,morphology,chemical composition,valence state of the elements and electrical properties were analyzed.When the amount of selenium powder,selenide time and selenide temperature were 100 mg,10 min and 550?respectively,a pure CZTSSe thin film with a Cu-poor/Zn-rich compostition?Cu/?Zn+Sn?=0.85 and Zn/Sn=1.16?,high crystallinity,large grain size and good electrical performance was obtained.Subsequently,the CZTSSe thin film solar cell with the efficiency of 4.63%was fabricated.In this work,Cu₂ZnSn_1-xGe_x?S,Se?₄?CZTGSSe?thin films with varied Ge composition?x=0,0.2,0.4,0.6,0.8 and 1?were prepared by using a water-based solution method.The fabricated kesterite CZTGSSe thin films were investigated systematically in terms of phase structure,morphology,valence states of elements,optical properties as well as their electrical properties.It is found that Ge element was successfully incorporated into the films and the CZTGSSe films were formed.As the increase of Ge content,the crystal grains of the films which show uniform morphology on sample surfaces grow larger.By adjusting the content of Ge element,the band gaps of CZTGS and CZTGSSe can be controlled in the range of 1.49-2.00 eV and 1.21-1.51 eV,respectively.Furthermore,the doping of the Ge element can effectively improve the electrical properties of the absorbing layer film.Finally,the CZTGSSe?x=0.2?solar cell with an efficiency of 5.39%was prepared.This results indicate that the non-toxic water-solution based CZTGSSe films with tunable optical band gap show promising applications in solar cell.3.Cu₂Zn_1-xCd_xSn?S,Se?₄?CZCTSSe?films with different Cd components?x=0,0.2,0.4,0.6,0.8 and 1?were prepared by water-based solution method at the above process conditions.The phase structure,morphology,chemical composition,element valence state,optical properties and electrical properties of the films were studied systematically.It was found that Cd element entered successfully the CZTSSe lattice and synthesized CZCTSSe.As the Cd content increases,the grain size increases.When x is equal to 0.6,the crystal grains are largest and evenly distributed.The thin films are all Cu-poor and Zn-rich,especially when x is equal to 0.4 or 0.6,the ratio of Cu/?Zn+Cd+Sn?and?Zn+Cd?/Sn is the closest to those of high-efficiency CZTSSe solar cells.By adjusting the content of Cd elements,the band gaps of CZCTS and CZCTSSe can be precisely controlled in the range of 1.39-1.51 eV and 1.10-1.21 eV,respectively.The conductivity and photoelectric properties of the absorbing layer film can be improved by doping Cd element.The film has the highest conductivity when the Cd content is x=0.6.In addition,CZCTSSe/CdS has good contact,which has formed the p-n junction.The results show that CZCTSSe thin films prepared by water-based solution method have high research value in solar cells.