Fabrication And Optimization Of CZTSSe Thin Film Solar Cells In Low Selenization Temperature

Thin film solar cells based on Cu?In,Ga?Se₂?CIGS?have reached up to the best photoelectricconversion efficiency?PCE?of 23.35%.Nevertheless,its large-scale application and development are limited due to its expensive,toxic and rare earth elements,In and Ga.Direct-gap semiconductor Cu₂ZnSn?S,Se?₄?CZTSSe?which has similar structure has emerged as one of the most promising absorber materials of thin film solar cells for next-generation photovoltaic technology because of its high theoretical PCE of 32.2%,high absorbtion coefficient,tunable band gap 1-1.5 eV,and especially its cheap,nontoxic and abuandant elements.There must undergo a high temperature selenization process in the fabrication of CZTSSe thinfilm solar cells.And this temperatue is always 550°C or higher in previous researches.On the one hand,this high temperature annealing process has high energy consumption.On the other hand,the process has many detrimental side effects on CZTSSe thin films.Firstly,SnSe becomes volatile at this temperature.This will cause the loss of Sn,obtain the inaccurate stoichiometric ratio and promote the decomposition of CZTSSe to binary secondary phases.Secondly,there will form an unwanted thick MoSe₂ between Mo and CZTSSe absorber layer which certainly increase series resistance and degrade the device performance.Thirdly,the range of substrates upon which CZTSSe devices can be integrated will become smaller.Flexible substrates and roll-to-roll manufacturing production will be limited.However,low-temperature selenization will increase the number of electronic disorders,decrease grain sizes,increase the number of grain boundaries and finally degrade the device performance.So,it is extremely necessary to study how to lower the selenization temperature and keep good performance simultaneously.Throughout the literature of CIGS and CZTSSe related to low-temperature selenization,we foundthat there are two commen methods.One is that Sb-incorporation in CIGS which could significantly lower the selenization temperature and improve its crystallization.But,the researches of Sb-incorporation in CZTSSe is very few and its mechanism is not clear enough.Meanwhile,Ag⁺substitution of Cu⁺is the only method which could obviously lower the selenization temperature to 480°C and obtain good device efficiency at the same time.However,it can also reduce carrier concentration and make the electrical performance of CZTSSe worse,thus failto further improve the PCE of CZTSSe thin film solar cell at low temperature.This article mainly includes two parts corresponding tosolve the above two issuses.The first part of my work is directly dissolving Sb₂O₃ into CZTSSe precursor solution toobviously lower selenization temperature and improve PCE of CZTSSe at low temperature.By adjusting the concentration of Sb-incorporation,the grain size,fine grain layer,defect and secondary phase,carrier density and band bending are simultaneously improved.Consequently,we achieved 9.02%at 40°C lower than normal selenizationtemperature and achieved comparable efficiency at 80°C lower than normal selenizationtemperature.On the one hand,the highest efficiency of 8.20%is still relatively low compared with the level at home and abroad.On the other hand,due to 4 mol%Sb has reached the maximum at470°C,more Sb will significantly reduce the adhesion between CZTSSe thin film and Mo back contact which result in the failure of CZTSSefabrication and performance improvement at low temperature.Then,we choose ACZTSSe which could be fabricated at 480°C and possess good device performance at that temperature as basis to carry out my following second part.The second part of my work is introducing LiF-PDT into ACZTSSe to improve its electricalproperties which is weaken by Ag-incorporation.It can be found from EQE,C-V,DLTS,EIS and EBIC measurements that LiF-PDTcan significantly increase the light absorption at long wavelength,improve the carrier concentration,carrier life and carrier collection efficiency,and thereby reduce the recombination loss and improve the carrier transport characteristics in the bulk of ACZTSSe.In addition,AFM measurement shows that LiF-PDT can inverse band bending at grain boundary thereby repell electrons away from GBs and reduce carrier recombination,and thus achieve the effect of passivation of grain boundary.Through the dual effect of LiF-PDT on ACZTSSe thin film solar cells,the device parameters of ACZTSSe thin film solar cells prepared at low temperature are all improved.Finally,the highest efficiency of 10.29%for ACZTSSe thin film solar cell with LiF-PDT is achieved.