Influence Of Trace Rare Earth Ions(Ce³⁺,La³⁺)Doping On The Performance Of Cu₂ZnSn（S,Se）₄ Thin Film Solar Cells

Due to its advantages of earth-abundant,low-toxic,and environmental friendly constituents,kesterite Cu₂ZnSn（S,Se）₄（CZTSSe）semiconductor material has been concerned by researchers.However,in terms of the current development of CZTSSe solar cells,the highest efficiency（12.7%）achieved by the undoped devices is still lower than the efficiency（23.35%）of parent compound Cu（In,Ga）（S,Se）₂（CIGSSe）solar cells.And it is far below its Shockley-Queisser limit（～33%）,which is attributed to the severe open circuit voltage deficit(V_oc,_def).There are two reasons for the V_oc,_def:（i）the shallow defects(such as Cu _Zn,Zn _Cuand[V_Cu+Zn _Cu])lead to Cu/Zn atoms disorder and the serious band tail state;（ii）the deep defects(such as Sn _Zn,V_S,and[2Cu _Zn+Sn _Zn])enhance the electron capture capability of the non-radiative recombination center.It has been indicated that cationic doping is effective methods for passivation defects and V_oc,def.Based on these experimental experiences,in order to improve the performance of devices,this thesis focuses on studying the influence of annealing conditions and rare earth ions(Ce³⁺,La³⁺)doping on the performance of CZTSSe thin film solar cells.Some meaningful conclusions are as follow:1.Cu₂ZnSnS₄（CZTS）precursor solution was prepared by acid-amine ionic solution system,and CZTSSe thin film was deposited by spin-coating-sintering process and two-step annealing method.With the help of the RTP furnace,different seleniztion conditions（temperature and time）have the effect on the properties of CZTSSe thin films were studied.Combing with the phase structure,microstructure,and elemental valence state of film,the optimal annealing condition is regarded at 400-10＆560-10,and the corresponding device can achieve the efficiency of 6.77%.2.Influence of trace rare earth Ce³⁺doping on the CZTSSe thin film solar cells was studied.Though analysising the phase structure and element distribution of films,the incorporated Ce is thought to be distributed at grain boundary and small-grain layer as a Na-Ce-Se phase.The CZTSSe grain growth is improved by the interaction between Ce and Na.Then,combing with surface current distribution of films and urbach energy of devices,the doped Ce can inhibit Cu/Zn disorder and Sn-related defects,and improve the electrical properties of absorbers.Finally,the efficiency is enhanced from6.77%（Ce0）to 9.14%（Ce4）with the reduced V_oc,def.3.Influence of trace rare earth La³⁺doping on the CZTSSe thin film solar cells was studied.According to the characterization of phase structure and element distribution in the film samples,the incorporated La is also thought to be located at grain boundary and small-grain layer,which can inhibit the harmful defects of the CZTSSe materials and improve its electrical properties.Further combining with the results of C-V and J-V measurements,the efficiency of 7.70%is found to be achieved by the increased hole concentration and V_oc.In addition,excessive La³⁺ions will boost the impurity phase（La₂Se₃）and the thickness of small-grain layer,leading to the decrease of efficiency.Our conclusions not only demonstrate rare earth ions(Ce³⁺,La³⁺)doping can reduce V_oc,defof devices,improving the devices performance,but also provide the new insights for further increasing CZTSSe-based solar cell efficiency by defect engineering.