Suppressing The Undesirable Defects Of Cu₂ZnSn?S,Se?₄ Absorber Layer Via Ga³⁺-Doping

Kesterite Cu₂ZnSn?S,Se?₄?CZTSSe?has attracted much attention as an ideal absorber for the thin film solar cells due to its earth-abundant,low-cost elemental constituents and high absorption coefficient(>104 cm^-1).But nowadays,the record power conversion efficiency of CZTSSe is only 12.62%,which is mainly caused by the high VOC deficit.The Cu Zn deep-level defect has lower formation energy due to the very similar ionic radii and chemical properties between Cu+ and Zn2+,which act as recombination centers and induce electron-trapping states.And even worse,the defect clusters [Cu Zn+Zn Cu] and [2Cu Zn+Sn Zn] will form through donor-acceptor compensation,which can cause bandgap fluctuations or electrostatic potential fluctuations in the CZTSSe absorber layer and lower the open-circuit voltage.Therefore,how to suppress the Cu Zn antisite defect and the defect clusters [Cu Zn+Zn Cu] and [2CuZn+SnZn] in the CTZSSe absorber layer is a big challenge.In order to suppress the Cu Zn antisite defect and defect clusters [Cu Zn + Zn Cu] and [2CuZn+SnZn],we introduced Ga³⁺ into the CZTSSe compound to form Cu₂ZnSn?Ga??S,Se?4?CZTGSSe?absorber layer.In Ga³⁺-doping CZTGSSe absorber,two Ga³⁺ substituted for one Zn2+ and one Sn4+ respectively,aiming to simultaneously inhibit the Cu Zn antisite defect and relevant defect clusters in the bulk phase of CZTSSe absorbing layer.The work of this paper mainly including the following:In the first part,we tried to introduce Ga³⁺ into CZTSSe to inhibit the formation of Cu Zn antisite defect,the Ga-doping was preliminarily achieved by inserting a layer of Ga-?HSCH₂CH₂HS?x solution in the middle of the absorption layer.Characteristics by XRD and Raman found that Ga³⁺ was successfully incorporated into the CZTSSe host lattice to form a homogeneous CZTGSSe alloy material without producing impurity phases;The XPS results showed that Ga³⁺ doping did not affect the valence states of other elements in CZTSSe compound,and the valence state of Ga element in CZTSSe showed oxidation state of +3;EDS scanning together with SIMS characterization indicated that the gallium element was mainly distributed in the fine-grain layer near the molybdenum back electrode.We accomplished the CZTSSe film with different Ga³⁺ doping concentrations,the results showed that Ga³⁺ doping could improve the performance of photovoltaic devices.These preliminary experimental results laid a solid foundation for us to further use Ga³⁺ doping to suppress the undesirable defects of the CZTSSe absorption layer.In the second part,we investigated the effect of uniform Ga³⁺-doping on the performance of CZTSSe photovoltaic devices.Different from the sandwich absorption layer structure in which Ga³⁺ was mainly distributed in the fine-grain layer near the molybdenum back electrode in the previous part of work,we obtained a homogeneous and stable CZT?G?SSe precursor solution by mixing CZTSSe and Ga?Se?solution,aiming to achieve the uniform Ga³⁺-doping.Through EQE characterization,we found that the bandgap of CZTSSe would be slightly reduced after the introduction of gallium,which was in good agreement with our theoretical calculation.According to the capacitance-voltage curve,the Wd of the Ga-doped device?0.305 ?m?is much higher than that of the pristine device?0.227 ?m?.It is well known that the Wd value of CZTSSe materials reflects the charge separation ability.Hence,the larger Wd of the Ga-doped device demonstrated that the Ga³⁺ doping can improve the effective collection of charge carriers.Through analyzing Arrhenius plots derived from the DLTS spectra,with the introduction of Ga³⁺,the NT of Cu Zn,Cu Sn,and Sn Zn defect respectively decreased from 3.19×10¹³,1.31×10¹² and 6.11×10¹² cm^-3 to 1.15×10¹²,2.55×10¹¹ and 5.06×1012 cm-3,indicating that the doping of Ga³⁺ into CZTSSe films can suppress the formation of the Cu Zn antisite defect and defect clusters [CuZn + ZnCu] and [2CuZn+SnZn].As expected,the devices with homogeneous Ga³⁺ doping exhibited much higher VOC and PCE.The PCE reached a maximum of 12.96% when the value of Ga/?Ga+Zn+Sn?was 10%,compared with the pristine photovoltaic device,the VOC was increased from 435 m V to 508 m V,likely as a result of significantly lower carrier recombination losses due to decreased defect density.