Improving The Performance Of CZTSSe Devices By Gradient Doping Near Surface Of Absorber

The photoelectric conversion efficiency of Cu₂ZnSn（S,Se）₄（CZTSSe）thin-film solar cells has achieved a new breakthrough recently,reaching 14.9%.But it is still far below Shockley-Queisser（SQ）theoretical limit efficiency.One of the main reasons obstructing the development of devices is the significant open-circuit voltage(V_oc)loss,resulting from severe carrier recombination at CZTSSe/Cd S heterojunction interface（the front interface）.The reasons causing severe carrier recombination at heterojunction interface are as the following two aspects:（1）There are high concentrations of harmful Cu _Zn,Cu _Sndefects and[2Cu _Zn+Sn _Zn]defect clusters near surface of absorber.（2）Band alignment between CZTSSe absorber and Cd S buffer layer need to be optimized.Up to now,several strategies had been adopted to solve the above problems,such as insertion of intermediate passivation layer,cation substitution and heterojunction annealing.However,inserting of intermediate passivation layer may introduce new interfaces,thus increasing the complexity and uncontrollability of carrier separation and collection.During heterojunction annealing,elements interdiffusion between CZTSSe and Cd S are complex and uncontrolled.For cation substitution,cations may migrate rapidly during selenization process and are difficult to concentrate on surface of absorber for defects passivation.Herein,a convenient selenization method was proposed to realize elements gradient doping near surface of absorber.In this way,it will realize the synchronous optimization of defect environment and band structure,so as to inhibit interface recombination.Based on the advantages of Cd in defects suppression and conduction band regulation,this dissertation firstly constructed Cd gradient near surface of absorber.Furthermore,Ag and Cd double gradient were constructed near surface of absorber for the regulation of VBM of CZTSSe absorber at surface.Ag and Cd will cooperate to further improve the quality of CZTSSe/Cd S heterojunction interface.The main works are as follows:In the first part,Cd gradient doping near surface of absorber inhibits carrier recombination:During selenization process,CZTSSe precursor film was placed in close contact with Cd S film prefabricated on soda-lime glass,so that Cd gradually diffused into the formed CZTSSe absorber at high selenization temperature.As a result,Cd gradient was formed at the surface of CZTSSe absorber,thus forming a conduction band gradient at the surface of CZTSSe absorber.The formed conduction band gradient would drive electrons transport to the front interface and reduce carrier recombination.Deep level transient spectrometer characterization indicated that the activation energy as well as concentration of Cu _Zndefects and[2Cu _Zn+Sn _Zn]defect clusters were significantly reduced.As a result,interface recombination was significantly suppressed,facilitating V_ocof device increased from 432 m V to 486 m V（increased by 54m V）while photoelectric conversion efficiency increased from 10.16%to 12.14%.However,the higher concentration of Cd reduced the band gap near surface of absorber.In addition,Cu _Sndeep level defects were not effectively suppressed.All these factors would limit the further improvement of V_oc.In the second part,Ag and Cd double gradient doping near surface of absorber inhibits carrier recombination:In order to solve the aforementioned problems in first part,Ag and Cd double gradient was introduced to the surface of CZTSSe absorber.The introduction of Ag will reduce the VBM of CZTSSe absorber,thus alleviating the reduction of band gap near CZTSSe absorber surface.The optimized energy band structure would facilitate transport and collection of carriers,thus further reducing carrier recombination at the CZTSSe/Cd S heterojunction interface.Detailed defect characterization indicated that Ag,Cd double gradient significantly reduced the concentration of Cu _Sndefects,Cu _Zndefects and[2Cu _Zn+Sn _Zn]defect clusters,implying less interface recombination.Compared with Cd gradient device,Ag,Cd double gradient device possess better defect environment and more beneficial band structure at CZTSSe/Cd S heterojunction for carrier transport.As a result,the V_ocof device was further increased to 496m V,and a champion device with 12.66%photoelectric conversion efficiency was achieved.