Electrodeposition Preparation And Performance Optimization Of CZTSe-Based Thin Film Solar Cells

With the increasing problems of energy shortage and environmental degradation,the effective development of green and renewable energy is extremely urgent.The inexhaustible solar energy is an important representative of renewable energy,and the key to solving the human energy crisis and environmental problems.Therefore,solar cells that directly convert solar energy into electrical energy have become one of the research priorities of researchers in recent years.Currently,the commercialized copper indium gallium selenide?CIGS?and CdTe solar cells have been limited due to their lack of indium resources and poisoning of Cd.Thus,new alternatives are urgently needed.Among many compound semiconductor materials,CZTSSe due to its advantages of abundant and environmentally friendly elements,continuously adjustable energy gap,high light absorption coefficient,good stability,no photoinduced recession and high theoretical conversion efficiency?32.2%?,has become a popular new energy semiconductor material.Compared with the vacuum-based methods,the solution-processed CZTSSe solar cell has obvious advantages in preparation cost,and has the highest photoelectric conversion efficiency of 12.6%.However,the use of toxic and volatile solvents in this method inevitably leads to a reduction in the safety factor and the improvement of equipment requirements.Therefore,the exploration of economic,environmentally friendly and efficient CZTSSe preparation needs to be further extended.From the two aspects of equipment requirements and risk index,electrochemical deposition?electrodeposition?is the most potential method for preparating CZTSSe absorption layer due to its simple equipment,safe and convenient operation,no use of toxic reagents,and lowest risk factor.According to different synthetic way,electrodeposition of CZTSSe films can be divided into two categories:stacked elemental layer?SEL?and multi-element co-electrodeposition?MCE?.The biggest advantage of the MCE method is to simplify the deposition process.However,due to the difference in diffusion coefficient and deposition potential of multimetals,the stability of the deposition solution,the uniformity of film composition and thickness are largely restricted to a great extent.The SEL method is a separate sequential deposition of a prepared layer,which can accurately control the composition and thickness by controlling the deposition potential and deposition time of each metal layer,thereby constructing a dense and uniform prefabricated layer.However,this method also has problems,such as the fact that the reduction potentials of Zn and H₂O are very similar?-0.762 V and-0.829 V,respectively?,which not only increases the difficulty of Zn deposition,but also reduces the density and quality of the film.Due to the differences of diffusion coefficient and expansion coefficient of multimetals,the internal absorption layer prepared by stacked electrodeposition is inhomogeneous,leading to more defects/impurities and low efficiency.Therefore,it is necessary to explore novel treatment or construction methods to comprehensively improve the deposition of Zn and suppress the internal heterogeneity,and improve the quality of the film to achieve high efficiency construction of the electrodeposited CZTSSe solar cells.The electrodeposition preparation and performance optimization of CZTSSe film are mainly studied in this paper.Firstly,the catalytic effect of CuZn alloy and the induction effect in the processes of electrodeposition are used to effectively improve the deposition of Zn layer and construct a dense prefabricated layer with a reverse element distribution by using a modified Cu/Sn/Zn stack element layer method,Secondly,selenization is optimized to reduce internal defects and improve the quality of CZTSe film through the pre-alloying treatment.The solar cells have realized a breakthrough of the highest efficiency of 9.1%.Thirdly,the CZTSSe thin films of“uniform distribution”and“pre-gradient distribution”were constructed by“co-sulfur-selenization”and“selenization followed by sulfuration”,respectively,and the efficiency of the CZTSSe photovoltaic device was further improved.The specific research contents are as follows:1.Preparation of CZTSe thin film solar cells by SEL methodThe Cu/Sn/Zn pre-formed film was prepared by a green,simple and controllable potentiostatic SEL method.Through the in-depth systematic study of the prefabricated film,the intrinsic mechanism of electrodeposition to prepare the prefabricated film was investigated.That is,the catalytic effect of CuZn alloy and the induction effect during electrodeposition,effectively improve the deposition of Zn layer and enhance the diffusion among the metals,and successfully prepare a dense pre-formed Cu/Zn/Sn layer with a reverse element distribution;The effects of selenization temperature on the morphology,composition,crystal phase and photovoltaic performance of CZTSe absorber film were investigated in detail,and the relationship between them was revealed,which laid a foundation for further optimization of photovoltaic performance.2.Optimizing selenization to prepare high quality CZTSe thin film solar cellsIn order to promote the diffusion and suppress the internal unevenness caused by different diffusion coefficients and expansion coefficients among metals,the optimized selenization method of pre-alloy was adopted to improve the quality of the film.The effects of different pre-alloy temperature and selenization atmosphere on the morphology and crystallinity of the film were investigated.A high-quality CZTSe absorber layer with a dense,large-grained and fine-particle layer-free morphology was prepared.And the photoelectric conversion efficiency of CZTSe solar cells has been increased from 5.4%to 9.1%,being the highest efficiency of current electrodeposited CZTSe photovoltaic devices.It provides a potential development direction for preparing high performance Cu-kesterite solar cells using a low cost,green electrodeposition process.3.Effect of different sulfur doping methods on CZTSSe filmThe CZTSSe films of"uniform distribution"and"pre-gradient distribution"were successfully constructed by exploring two ways of"co-sulfur selenization"and"selenization before sulfuration",respectively,in an attempt to increase the open circuit voltage without seriously affecting the photocurrent,so as to further improve the efficiency of CZTSSe photovoltaic devices.Compared with"co-sulfur selenization","selenization before sulfuration"avoids the disadvantage of the hard control of sulfur-selenium vapor pressure,and the content and depth introduced by S are precisely controlled to prepare high quality CZTSSe film by adjusting S content or time.The efficiency of CZTSSe photovoltaic devices has increased to 7.72%,and laying a solid theoretical and practical foundation for the development and efficiency of CZTSSe-based solar cell devices.