First Principles Study On The Electronic Properties And Passivation Mechanism Of Cu₂ZnSnS₄ Thin Film Solar Cell

In recent years,Cu2ZnSnS4(refered to as CZTS)thin film solar cells has attracted much more researchers' attention for its possesses excellent properties and contains earth abundant elements Zn and Sn.CZTS thin film solar cells have a complex layered structure: Soda lime glass|Mo back electrode|CZTS light absorbing layer|Cd S buffer layer|ZnO window layer| front electrode,this is the physical basis of the photovoltaic effect.The photoelectric properties of CZTS cells depend not only on the composition and properties of the materials each layer,but also on the composition,structure and properties of the interface between the layers,this is because the abrupt change of structure at the interface between adjacent layers will lead to the appearance of the interface states and change the band str ucture of the semiconductor,and will have a serious negative impact on battery performance.Therefore,it is important to study the structure of the interface and eliminate the interface states.In this thesis,we have systematically studied the local lattice structure and electronic properties of CdS(110)/WZ-ZnO(112)interface and CZTS(102)/WZ-ZnO(110)interface of CZTS thin film solar cells by using first principles calculations method from the atomic level.Based on our calculation,the passivation mechanism of the interface states near the Fermi level of the CZTS/WZ-ZnO interface is studied.Our main research contents and results are as follows:(1)We calculated the lattice structure and electronic properties(including band structure,density of states,optical properties and electronic charge)of CdS bulk,WZ-ZnO bulk,CdS(110)surface,WZ-ZnO(112)surface and WZ-ZnO(112)/CdS(110)interface from atomic level.The main finding as bellow: 1)The mismatch of WZ-ZnO(112)/CdS(110)interface is 4.3%,which indicates that the WZ-ZnO(112)/CdS(110)interface formed by the buffer layer CdS and the win dow layer WZ-ZnO can match well.2)The interface binding is-0.61J/m2,and the atoms near the interface moved along different directions,and the length of the atomic bonds slightly elongated and shortened after relaxtion.3)There is no interface states near the Fermi level of the WZ-ZnO/CdS interface,which will be beneficial to the electrons and holes passing through this interface.4)The s,p,d atomic electron orbitals of Cd,S,Zn,O atoms are hybridization in different degree.There is a amount of charge transfer on the interface.these electronic orbital hybridizati on and charge transfer will promote WZ-ZnO(112)/CdS(110)interface combination.(2)The Cu2ZnSnS4(102)/WZ-ZnO(110)interface lattice structure and electronic properties is investigated.The main finding as bellow:1)the mismatch of Cu 2ZnSnS4(102)/WZ-ZnO(110)interface is less than 3.2%,and the interface b inding is-0.21 J/m2.2)the results of band offsets indicated that the bonding of Cu2ZnSnS4(102)/WZ-ZnO(110)is belong to type I hererojunction,and the conduction band offsets is 0.4 e V,which will lead to the decrease of photogener ated carriers at the interface.3)the CZTS/WZ-Zn O interface has a large number of interface states near Feimi level.Our anlysis show that the interface states mainly derived from the Cu-3d orbital,Sn-5sorbital S-3s and 3s orbital on the CZTS first layer.The 4s-orbital and the 3d-orbital of Zn atoms,the 2s-orbital and the 2p-orbital of the O atoms on the first WZ–Zn O layer.4)The atoms at the interface have larger orbital hybridizations and charge transfers.The irregular bonding and large interface sates have a negative impact on interface bind and electrons tansport,which is one of the reasons for the low conversion efficiency of the CZTS thin film solar cells.(3)There is a high interface states near the Fermi level of the CZTS/WZ-ZnO interface,and the interface states are mainly contributed by the Sn atom 3 d electron orbital.Therefore,we discussed the passivation method s and effect of F,Cl and H atom to eliminate the Cu2ZnSnS4(102)/WZ-ZnO interface states.Our main finding is: 1)The F,Cl and H atoms can more or less passivation the interface states near the CZTS/WZ-ZnO interface Fermi level.2)The H and F atoms have a more efficient passivation effect than that of Cl atom.3)The charge density difference and Bader atomic charge analysis suggests that F,Cl and H can get part of the electrons leading to interface states and that the interface states can be passivated by F,Cl or H atoms.