Regulation Of Small Molecular Selenium Diffusion For Cu₂ZnSn（S,Se）₄ Thin Film Photovoltaic Performance

Copper zinc tin sulfur selenium（Cu₂Zn Sn（S,Se）₄,CZTSSe）with kesterite structure is regarded as one of the ideal photovoltaic materials in the new generation,due to its non-toxicity,tunable band gap,stable structure,earth-abundant composition,and high theoretical efficiency.Currently,the highest certified power conversion efficiency（PCE）of CZTSSe solar cells based on the solution method has reached 14.9%,which is still far from the PCE of Cu（In,Ga）Se₂（CIGS）solar cells（23.35%）due to the much higher open-circuit voltage deficit(V_oc,def)of CZTSSe solar cells than that of CIGS solar cells.There are many reasons that contribute to high V_oc,def,including band gap,electrostatic potential fluctuations,tail states,deep level defects,secondary phases,and recombination at interfaces.These reasons are closely related to the chemical environment,phase evolution,and crystal growth in the selenization process.Therefore,optimizing the selenization strategy is one of the most important procedures to improve the photovoltaic performance of CZTSSe.The main research contents of this thesis are as follows:（1）To improve insufficient selenium mass transfer in the traditional process,the selenium pre-evaporation stage is introduced to provide sufficient source,thereby promoting the phase evolution from CZTS to CZTSSe and grain growth,namely two-step selenization.In the heat preservation period at 260-360°C（selenium pre-evaporation stage）,it is possible to regulate the content of selenium clusters entering the precursor before phase evolution and grain growth,thereby improving the lack of selenium mass transfer in the film.Compared with one-step selenization,the crystallinity of the absorber is improved,and the bulk defects density and interface defect density are decreased significantly in two-step selenization.The photovoltaic performance of the device prepared by two-step selenization is significantly improved in particular the open-circuit voltage(V_oc)is increased from 0.427 V to 0.501 V,and the V_oc,def is decreased from 0.419 V to 0.347 V.Under the two-step selenization strategy,the PCE of the CAZTSSe solar cell reaches 11.07%.（2）Small-molecule selenium is introduced to improve the activity of the selenium source.Selenium clusters have low diffusion activity and chemical activity due to their large molecular chains,which will hinder their participation in mass transfer and phase evolution.Based on two-step selenization,a simple and novel pre-selenization strategy is proposed by spin-coating and annealing（150°C）in the air at the bottom of the precursor with Na₂（Se₂S）solution.This strategy can directly introduce small-molecular selenium（Se₂,Se₃,Se₄）into the precursor.The results show that the small-molecular selenium with high activity selenium in the precursor could promote the formation of liquid Cu_2-xSe,which is conducive to promoting the diffusion of Zn from the Zn-rich microregions to Zn-poor microregions,ensuring the optimal range of Cu/（Zn+Sn）and Sn/Zn and the small fluctuations in different microregions,thereby effectively inhibiting Cu _Zn and Sn _Zn defects and related[2Cu _Zn+Sn _Zn]defect clusters.Finally,the strategy effectively suppresses non-radiative recombination and tail states,and significantly reduces the V_oc,def.This work increased the V_oc of DMF-based CAZTSSe solar cells to 0.520 V and the PCE is increased to 12.86%,attribute to the small-molecular selenium.