Study On The Effect Of Periodical Precursors On The Properties Of CZTS Thin Films

As a quaternary compound,CZTS has been a promising solar cell absorbing layer material to substitute the traditional Cu(In,Ga)Se2 materials in recent years.The four elements of CZTS thin film are earth-abundant,low cost and non-toxic.CZTS thin film has a band gap of 1.5e V,which belongs to the ideal band gap value of single junction solar cells absorbers.And also has absorption coefficient as high as 10-4cm-1 in visible light.Therefore,CTZS is a promising,efficient and sustainable solar cell absorber film material.This dissertation used magnetron sputtering to prepare the multi-periodic layer of CZT precursor,and then sulfurization the CZTS thin films using two step processing method.The CZTS thin film obtained by periodic sputtering precursor show the characteristic of broader and faster during the sulfurization reaction.In this dissertation,the sulfurization mechanism has been mainly studied.The effect of the order of sputtering CZT,the pre-annealing conditions,the sulfurization process conditions and the influence of precursor doping on the properties of the thin film have also been studied.The results obtained are as follows:The CZTS thin film obtained by the periodic sputtering precursor was more favorable to the growth of CZTS than the conventional single periodic sputter precu rsor.It was found that during the process of sulfurization,periodic metal layers show multiple interface reaction at the same time between reaction of metal sulfide in the latter sulfurization process,makes the reaction wider and faster,single peridic layers show the normal contact between the two layers.The phase of CZTS is formed earlier by the four periodic sputtering than the single periodic in the process of sulfurization heating.The phase of CZTS was found at 400 ? in the four periodic thin films while the in the single periodic thin films the CZTS phase appeared at 500 ?.When the CZTS thin films were prepared by periodic sputtering precursor,it is better to use Zn/Sn/Cu order to depositmetal precursor than Sn/Zn/Cu and Cu/Sn/Zn sputtering sequences.CZTS thin films using this sequence have better adhesion to the Mo back contact layer,and also effectively reduce the pores on the surface of CZTS thin films.Cu sputtered on the most outlayer of the precursor metal layer reducing the loss of other elements in the film.Pre-annealing of CZT metal precursors before sulfurization processcan make metal layers pre-alloyed.Excessive annealing temperature can result in the Sn loss in the films,the film appears pore,and the the surface grains come to decomposition.The crystallization ability of CZTS thin films pre-annealing20 min at 150 ? is better than samples without pre-annealing,but with the increase of pre-annealing time,the crystallization ability of films decreases.In the process of two step sulfurization,the degree of crystallinity of CZTS films after pre-sulfurization is better than that of the samples without pre-sulfurization treatment.Pre-sulfurization treatment can inhibit the volatilization of Sn within a certain temperature range.When the pre-sulfurization temperature is 350 ?,the crystallinity of the thin films is best.When the pre-sulfurization temperature is 350 ?,the increase of the pre-sulfurization time is beneficial to complete sulfurization reaction.Excessive pre-sulfurization time is not conducive to the growth of single phase CZTS.It will also lead to the more easily appearance of secondary phase,which affects the quality of the film.When pre-sulfurization time is 10 min and the pre-sulfurization temperature is 350 ?,the CZTS thin film has the best crystallinity and no pore on the surface.It is found that the insufficient sulfurization time will lead to the secondary phase of metal sulfide during the secondary process of sulfurization.Properly increasing the sulfurizationtime can improve the quality of CZTS thin films.But excessive sulfurization time will make the crystallinity become worse.The samples with prolonged sulfurization time to 60 min are copper rich zinc poor,and the surface is uniform and dense,has a good crystallization performance.(AgxCu1-x)2ZnSnS4 films and Cu2ZnSiS4 films were prepared by periodic sputtering precursor..With the increase of Ag content in(Agx Cu1-x)2ZnSnS4,the crystallinity of the film becomes worse,the reaction degree between metal and sulfur is weaken,and the loss of Sn is accelerated.Proper improvement of sulfurization temperature can enhance the crystallinity of AZTS sample,but result in a rough surface morphology.When the metal ratio of target is Ag/(Ag+Cu)=0.1 in the sputtering target,the composition of the film is copper poor and zin crich,close to the best ratio value,and the crystallization is better than the CZTS sample without Ag doping.When sulfurization temperature was 500 ?,the single-phase ACZTS was obtained.With the increase of sulfurization temperature,the crystallinity of Cu2ZnSiS4 increased,and the composition ratio of the sample was closer to the ideal element composition ratio of CZTS film.When the sulfurization temperature is 800 ?,the crystallinity of Cu2ZnSiS4 is the best,and have the best component ratio and optical band gap of 1.48 e V.