Preparation Of CZTSSe Thin Film Based On DMSO Precursor Solution And Its Photovoltaic Properties

Cu₂ZnSn（S,Se）₄（CZTSSe）is considered to be a potential absorber material for photovoltaic applications due to its earth-abundant and non-toxic constituent elements,similar crystal structure and excellent optoelectronic properties to the chalcopyrite Cu In Ga Se₂（CIGS）.Due to the serious deficit of open circuit voltage(V_OC),however,the conversion efficiency of CZTSSe thin film solar cell has not been effectively promoted in recent years.The key factors limiting the device performance include:the crystal quality of the absorber,the interface matching between the absorber and the buffer layer,and the contact between the back electrode and the absorber.Among them,high-quality absorber is the prerequisite towards high device performance.Therefore,based on a low-cost direct solution coating method,we first systematically studied the effects of the volume ratio of N-dimethylformamide（DMF）and dimethyl sulfoxide（DMSO）in a low-toxic precursor solution system on the film morphology,crystallinity and device performance.Upon achieving the optimal solvent ratio,a simple and effective method was then proposed to reduce the organic residuals in the precursor film and improve the CZTSSe film quality.Finally,effects of the Sn precursors with different oxidation state(Sn²⁺and Sn⁴⁺)in the precursor solution on the formation of absorber layer and device performance were studied,and underlying physical mechanisms were revealed.The optimal processing conditions of obtaining high-quality absorber layer and high-efficiency CZTSSe solar cells were also established.The main results are as follows:1.A mixed solvent formed by introducing DMF into DMSO was utilized to improve the film forming of precursor solutions.Effects of different DMF adding contents on the morphology and optoelectronic properties of CZTSSe as well as device performance were investigated.The results show that,compared with pure DMSO solvent,mixed solvent with appropriate amount of DMF can effectively increase the wettability of precursor solution on the substrate,leading to the enhanced grain size and surface compactness of CZTSSe absorbers.An optimal DMF volume content occurs at about 30%,corresponding to an increase of carrier lifetime from 2.78 ns to 5.07 ns in prepared CZTSSe absorbers and efficiency promotion from 4.77%to 6.03%in solar cell devices.When DMF content exceeds40%,however,the film quality and device performance decrease.2.The preparation of CZTSSe absorbers via non-hydrazine solution process often suffers from poor crystallization caused by organic residues.To address this issue,ultraviolet（UV）-ozone treatment was introduced in preparing Cu₂ZnSnS₄（CZTS）precursor film based on DMSO/DMF solutions.It was found that exposing precursor films in ultraviolet ozone with proper duration before post selenization effectively reduces the organic residues stemming from solvent and/or component sources.Comparing with the untreated reference sample,the crystallization and morphology of CZTSSe layers with UV-ozone treatment are evidently improved,leading to enhanced carrier lifetime and solar cell device performance.The highest conversion efficiency of solar cell devices with the optimal UV-ozone treatment time（30 min）reaches 7.07%.This work provides a feasible way to solve the common problem involving organic residues during CZTSSe preparation via a non-hydrazine solution process.3.The effect of the valence state of Sn ions in the precursor solution on the quality of CZTSSe absorbers was systematically studied at different selenization temperatures.Upon morphological and structural analysis of both precursor layer and absorbers,it was confirmed that different component ratio of Sn⁴⁺/(Sn⁴⁺+Sn²⁺)in the precursor solution corresponds to different dominant reaction mechanism in the formation of CZTSSe.For precursor solutions with higher Sn²⁺content,a relatively low selenization temperature should be adopted based on the multi-phase reaction involving SnSe₂.For precursor solutions with higher Sn⁴⁺content,a relatively high selenization temperature should be adopted based onSe substitution growth from homogeneous CZTS phase.However,too high selenization temperature will lead to crystal degradations due to the thermal decomposition of the absorber layer.High-efficiency CZTSSe solar cells with an average conversion efficiency of about 8%can be obtained both at 530 ℃ using the precursor solution with Sn⁴⁺/(Sn⁴⁺+Sn²⁺)of 0.4 and at 550 ℃ using the precursor solution with pure Sn⁴⁺.