Research On Improving The Performance Of Cu₂ZnSn（S,Se）₄ Solar Cells By Defect Control

Recent years,Cu₂Zn Sn（S,Se）₄（CZTSSe）solar cells have attracted the attention of researchers due to their low material consumption,abundant and non-toxic precious metal content,high coefficient of absorption(>10⁴ cm^-1)and adjustable band gap（1.0-1.5 e V）,it is considered as a promising absorber material to replace chalcopyrite Cu（In,Ga）Se₂（CIGS）for the preparation of thin-film solar cells.However,so far,the photovoltaic conversion efficiency（PCE）of CZTSSe solar cells is far below their theoretical efficiency based on the Shockley-Queisser limit（～33%）and the recorded efficiency of 23.35%for CIGS solar cells,with a maximum of only13.6%.It is shown that the main factors limiting the PCE of CZTSSe solar cells are short carrier lifetime and large interfacial recombination.The poor crystal quality of the CZTSSe absorber and the presence of deep defects in the energy level(e.g.,Sn _Znand[Cu _Zn+Sn _Zn]defect clusters)are the main reasons for the short carrier lifetime.Therefore,it has become a key scientific issue in the research of CZTSSe cells to develop techniques for preparing high-quality CZTSSe thin films and to explore methods to suppress or passivate deep energy-level defects and surface interfacial defects to reduce carrier recombination.To solve the problem of absorber quality,this paper adopts the method of selenization of Cu₂Zn Sn S₄（CZTS）precursor thin film after annealing in air to prepare CZTSSe absorber,and investigates the law and mechanism of the effect of air annealing on the absorber quality and photovoltaic performance of CZTSSe solar cells.In order to reduce deep energy level defects and interfacial defects,this paper adopts germanium uniform doping of CZTSSe absorber to study the effect of germanium doping on the deep energy level defects associated with Sn and the photovoltaic performance of CZTSSe solar cells.The specific research carried out and results obtained are as follows.（1）Different from the traditional method of preparing CZTSSe thin films,in this thesis,CZTS precursor films were annealed in air at 100～500℃for 10 minutes and then selenized,then the CZTSSe solar cells with traditional structure were prepared.The results showed that as the annealing temperature increased,the PCE of CZTSSe solar cells showed a trend of first increasing and then decreasing,PCE increased from7.50%from unannealed to 8.94%of annealed at 100℃.And when the annealing temperature reached 500℃,PCE decreased to 5.09%.The contribution of V_oc,J_sc and FF to PCE and the contribution of J₀,R_s and R_sh to V_oc,J_sc and FF were quantitatively analyzed,and it was concluded that the influence of air-annealing on PCE mainly came from its influence on J₀,followed by the influence on R_s.XRD and Raman test results show that J₀ is mainly affected by interfacial recombination induced by harmful defect clusters[2Cu _Zn+Sn _Zn],which is related to the surface O content of air-annealed CZTS.R_s is mainly determined by the crystal quality and surface phase composition of the absorber.（2）CZTS precursor solutions with different Ge doping concentrations were prepared by the metal salt thiourea method using Ge Cl₄ as the Ge source and DMSO as a dissolving agent with nominal doping concentrations of Ge/（Ge+Sn）=0,5,10,20%,respectively.The Ge doped CZTS precursor films were selenized to prepare Ge doped CZTSSe（CZTGSSe）.CZTGSSe was prepared as an absorber and then prepared into conventional structure of CZTGSSe solar cells.The results have shown that with the increasing of Ge doping concentration,the PCE of CZTGSSe solar cells showed a trend of first increasing and then decreasing,PCE increased from 7.98%without doping to 9.80%of doping concentration of 5%.And when the doping concentration reached 20%,PCE decreased to 7.58%.The quantitative analysis of the contribution of J₀,R_s,and R_sh to V_oc,J_sc,and FF and the contribution of V_oc,J_sc,and FF to PCE concluded that the effect of Ge doping on the enhancement of PCE is mostly related to the decrease of J₀;the effect on the reduction of PCE is mostly related to the increase of J₀,then followed by the decrease of J_L.The results of Raman,SEM and variable temperature J-V show that the decrease of J₀is mostly caused by the decrease of the density of[2Cu _Zn+Sn _Zn]defect clusters on the surface of the absorber,and the increase in J₀ is essentially attributed to the increase in the number of small particles and holes at the bottom of the absorber.As the Ge content increases,J_L decreases.On the one hand,Ge doping leads to the increase of CZTSSe band gap,and on the other hand,depletion layer width decreases.