Enhancing Crystallinity Of CZTSSe Absorber By Adjusting Amine-thiol Precursor

The copper zinc tin sulfur selenium（CZTSSe）solar cell device,which derived from copperindium gallium selenium（CIGS）solar cell,has become the potential future photovoltaic star,owing to its elemental abundance of the absorption layer,facile preparation process and high light absorption coefficient.Compared with the physical method,the precursor solution method is not only easy to operation but also the highest certification efficiency（13%）has been achieved,which makes the precursor solution method become the main method to assemble CZTSSe devices.Among them,the amine-mercaptan solvent system is an earlier method developed by our group to fabricate CZTSSe precursor film.With the continuous exploration and optimization for the past several years,12.54%of the efficiency for CZTSSe device has been achieved,which is comparable to that from other solvent systems.The main problem for amine-mercaptan system is the insufficient selenidation of the absorption layer.It could only obtain a double-layer structured absorption layer with top large grain/thick bottom small grain layer,which is obviously unlike the good grain layers from other systems,such as DMF,DMSO,ethylene glycol methyl ether and so on.The small grains in the absorption layer would cause much more grain boundaries,thus increase the density of defect states and lead to the recombination of photogenerated carriers.In addition,the small grains in the absorption layer could create new shunt paths and cause adverse impacts on R_sh and FF of the device,and ultimately decrease the efficiency of the device.Therefore,the reduction of the grain boundaries,the enlargement of the grain size and the improvement of the compactness of the absorption layer would be helpful for further improving the performance of CZTSSe solar cell devices prepared by amine-mercaptan solvent system.In view of the current status of CZTSSe solar cell devices prepared by amine-thiol solvent system,and our full understanding of its solvent system,we will adjust the ratio of amine and thiol,and optimize the ratio of sulfur and selenium in the solvent system to improve the crystallinity of the absorption layer,in order to obtain a penetrating CZTSSe absorber layer with good crystallinity,and on this basis,further optimize the device assembly process such as buffer layer and window layer to improve the efficiency of CZTSSe devices.Therefore,three works were carried out as following.1.The optimization of the crystallinity of CZTSe with the adjustment of the amine/mercaptanratio:Aiming at the multi-layer problem in the preparation of CZTSSe thin film by ethylenediamine（EN）-ethyldithiol（EDT）method,the factors that affect the photoelectric conversion performance,such as the dosage of ethylenediamine,the dosage of ethyldithiol,the ratio of EN/EDT,the temperature of the hot bench for precursor film annealing,were investigated.The results show that the effect of the dosage of ethyldithiol is more significant than that of ethylenediamine,while the ratio of EN/EDT plays a major role.This is mainly attributed to that the ratio of EN/EDT could significantly affect the complexation ability of metal cations,and subsequently the formation of film and the properties of absorption layer.The optional device achieved a photoelectric conversion efficiency of 10.02%,with the values of V_OC,J_SC and FF are421.0 m V,37.3 m A/cm² and 63.9%respectively.2.To improve the crystallinity of CZTSSe by adjusting the sulfur/selenium ratio:In the first part,although the crystallinity of the surface and the cross section of CZTSSe layer are improved,the VOC of the device is only 421.0m V,which is still far from the high-efficiency CZTSSe device.The theoretical and experimental investigations indicated that the ratio of S/（S+Se）would affect the band gap of CZTSSe layer and the performance of the device.In this part,we aimed to improve the performance of the CZTSSe device by adjusting the ratio of sulfur/selenium by varying the dosage of Se powder in the precursor solution,the addition mode of sulfur source and the content of various stabilizers.The results indicated that the appropriate amount of sulfur powder could make the surface of CZTSSe layer much denser.In addition,the crystallinity of the CZTSSe surface would become much better,as well as the ratio of the thickness of the large grain and the fine grain increased,as the decrease of S content in S/（S+Se）.Finally,it is regulated by stabilizers,the dense large grain is formed on CZTSSe film surface.Moreover,the large grain runs through the cross section of CZTSSe layer.The photoelectric conversion efficiency of the final device reached 10.89%with the values of V_OC,J_SC and FF are 494.6 m V,34.1 m A/cm² and 64.5%respectively.The open circuit voltage is 73.6m V higher than that of the optimization of the amine/mercaptan ratio.3.The optimization of buffer layer and window layer:In the previous work,although themorphology of the CZTSSe layer was improved,the photoelectric conversion efficiency of the device is still far from that the high-efficiency CZTSSe solar cell.In this part,based on the previous optimized CZTSSe film,the other two key layers in the device,that is,the buffer layer and the window layer,was further optimized.By regulating the crystallization character of Cd S buffer layer and the thickness of window layer,the device performance reaches the maximum when the Cd S deposition time is 440 s,the zinc oxide sputtering time is 2100 s and the ITO sputtering time is 3100 s.Finally,the optimized processes are superimposed to obtain a high-efficiency device with the highest photoelectric conversion efficiency of11.71%.