Study On The Interface Contact Characteristics And Interface Engineering Between Cu₂ZnSnS₄ Thin Film And Flexible Metal Substrates

Due to its suitable band gap,high absorption coefficient and p-type conductivity,kesterite Cu2ZnSnS4(CZTS)has attracted increasing research attentation.Compared with the constituent elements of Cu(In,Ga)Se2 thin films,all of the constituent elements of CZTS thin films are nontoxic,earth-abundant,and low cost.Hence,the Cu(In,Ga)Se2 absorber layer is replaced by CZTS absorber.The numerous studies have concentrated on flexible CZTS thin film solar cells due to their ease of installation on non-planar platforms,low cost and high power/mass ration.A metal substrate has low resistivity,and it can simplify the fabrication of CZTS thin films solar cells.Therefore,the metal substrate is a suitable selection of the back electrode of flexible CZTS solar cells.The transports of photogenerated carriers and properties of CZTS thin films are influenced by the interface contact characteristic between CZTS and back electrode.The interface properties between CZTS and metal substrates have an impact on the photovoltaic performances of solar cells.Therefore,it is worth to study the interface properties and interface engineering between CZTS and flexible metal substrates.In this dissertation,the CZTS thin films on the Mo,W,Ti and Al metal substrates were fabricated by magnetron sputtering and post-sulfrization.The influences of the various metal foils on the crystal phases,morphologies,element distributions,and electrical contact properties of CZTS/metal foils interface were studied.And then,the influences of annealed Mo foil,solid sulfur powder mass,Si buffer layer and Mo buffer layer on the interface contact characteristics of CZTS/Mo were investigated.The effects of conductivity type of MoS2,carrier concentration of MoS2,thickness of MoS2,band gap of MoS2,and recombination velocity of MoS2/Mo interface on the photovoltaic performances of solar cells were studied by AFORS-HET v2.5 software.Finally,the effects of sulfurization temperature and oxidized Ti foil on the diffusion of Ti were studied.The results were listed as follows:(1)The formation of MoS2 and WS2 phases are observed at the interfaces of CZTS/Mo and CZTS/W,respectively.However,Ti-S and Al-S phases are not formed at the CZTS/Ti and CZTS/Al interfaces.Compared with the particles size of back of CZTS on the other substrates,the particles size of back of CZTS on Ti substrate is larger.Too-thick WS2 interface layer can be seen at the interface between CZTS and W because the preferred orientation of W substrate is(211)plane.The diffusion of Ti atoms is due to the large average hop distance defects in the CZTS thin film and the relatively low activation energy of Ti atoms.The ohmic contacts of the CZTS/Mo and CZTS/Ti interfaces are better than ohmic contact of CZTS/W interface.The Schottky contact presents at the CZTS/Al interface.(2)The effects of annealed Mo foil and solid sulfur powder mass on the characteristic of CZTS thin film and the interface contact properties of CZTS/Mo interface were investigated.Compared with the sample on unannealed Mo substrate,the surface morphology of CZTS on annealed Mo substrates becomes compact.The thickness of MoS2 interface layer decreases from 1.9μm to 1.5μm.The slope of I-V curves of CZTS/Mo structure increases from 0.06S to 0.27S.The resistance of CZTS/Mo structure decreases from 16.7Ω to 3.7Ω.As the mass of sulfur powder decreases from 11g to 0.5g,the FWHM of CZTS diffraction peak in the XRD patterns increases with decreasing the sulfur powder mass.The formation of no MoS2 interface layer at the interface between CZTS and Mo is due to the reduction of sulfur powder mass.The slope of I-V curves increases from 3.2×10-5S to 0.24S.The resistance of CZTS/Mo structures decreases from 3.2×104 Ω to 4.2Ω.(3)Compared with the sample without Si buffer layer,the grain size of CZTS thin film is uniformly distributed when a 30nm Si buffer layer is inserted between CZTS and Mo.No MoS2 interface layer is observed at the interface between CZTS and Mo.The slope of I-Vcurves of CZTS/Mo structure increases from 0.20S to 0.40S.The resistance of CZTS/Mo structure decreases from 5Ω to 2.5Ω.Compared with the CZTS thin film without Mo buffer layer,when the thickness of Mo buffer layer increases to 373nm,no MoS2 interface layer presents at the interface between CZTS thin film and Mo.Firstly,the slope of I-Vcurve of CZTS/Mo structure increases to 0.21S,then decreases to 0.16S.The resistance of CZTS/Mo structure decreases from 11.1Ω to 4.8Ω,then increases to 6.3Ω.(4)The effects of MoS2 interface layer on the photovoltaic performance were studied by the AFORS-HET v2.5 software.The change of carrier concentration of MoS2 interfac layer proves that p-type MoS2 layer promotes the photovoltaic performance of CZTS solar cells.The n-type MoS2 layer with suitable carrier concentration improves the photovoltaic performances of solar cells slightly.The change of thickness of MoS2 interfac layer indicates that the MoS2 layer with suitable thickness improves the photovoltaic properties of CZTS solar cells.In addition,the barrier is always observed at the interface of MoS2/Mo.The transport of photogernated carrier is limited by the barrier of MoS2/Mo interface.The reduction of the barrier of MoS2/Mo interface improves the photovoltaic performances of solar cell.The suitable recombination velocity of MoS2/Mo interface enhances the photovoltaic performances of solar cells.(5)The effect of sulfurization temperature and oxidized Ti subsrrate on the diffusion of Ti into the CZTS thin films was studied.As the sulfurization temperature increases from 400℃ to 450℃,the percentage content of Ti element in CZTS thin film increases from 0.21%to 30.01%.The percentage content of Ti element in CZTS thin film decreases from 30.01%to 5.03%when the sulfurization temperature increases from 450℃ to 550℃.As the sulfurization temperature increases from 550℃to 600℃,the percentage content of Ti element in CZTS increases from 5.03%to 30.02%.When the sulfurization temperature is 550℃,the grains at the bottom of CZTS thin film grow densly.It can limit the diffusion of Ti atoms.The percentage content of Ti element in CZTS thin film decreases from 2.65%to 0.43%when the Ti substrate is oxidized.In addition,the oxidized Ti substrate improves the growth of grains at the bottom of CZTS thin film.