Study On Surface Passivation Of Absorption Layer Of Sb₂Se₃ Thin Film Solar Cells

Since entering the 21st century,people are increasingly aware of the hidden dangers brought by the energy crisis and environmental problems.Green development has become the theme of the development of the world today.As one of the clean energy sources,solar cell have received widespread attention from scholars at home and abroad.At present,the commercialized solar cells,such as single crystal silicon,cadmium telluride,gallium arsenide,etc.,their photoelectric conversion efficiency has reached more than20%,which has great development potential.Sb₂Se₃ is one of the very suitable light absorption materials.Because of its appropriate band gap,high light absorption coefficient,rich reserves,low prices,etc.,it has attracted the attention and favor of scholars.Sb₂Se₃ has a unique quasi-dimensional crystal structure which is arranged in parallel（Sb₄Se₆）_n ribbons.When the（Sb₄Se₆）_n ribbons grow along along[hk1],carriers are easy to transport at the interface and the recombination loss is small.On the other hand,if the（Sb₄Se₆）_n ribbons grow in the direction of[hk0],they need to overcome van der Waals forces between the（Sb₄Se₆）_n ribbons,and the transmission efficiency will be reduced.Therefore,as long as the grains orientation of the Sb₂Se₃ are controlled along the appropriate direction,thin film solar cells with excellent performance can be obtained.However,the maximum efficiency of the Sb₂Se₃ thin film solar cell is only 10.57%,which is still a large gap between the theoretical Shockley-Queisser（S-Q）limit（about 32%）,which is mainly caused by two reasons:First,the free carrier concentration of the undoped Sb₂Se₃ is low,which will limit the improvement of the built-in voltage(V_bi)and affect the splitting of the Fermi level.Second,due to the low symmetry of Sb₂Se₃,its internal defect properties are very complex.Defects formed at different sites have significantly different defect energy levels and defect formation energies,and defects at non-equivalent sites cannot be regarded as the same defects.Due to the above two disadvantages,the open-circuit voltage loss of the Sb₂Se₃ thin film solar cell is very serious,which also reduces the photoelectric conversion efficiency of the cell.Therefore,in view of the above problems,this paper will perform corresponding optimization treatment on the basis of the Sb₂Se₃ thin film prepared by rapid thermal evaporation to prepare high-quality and high-performance Sb₂Se₃ thin film.First of all,exploring the temperature of thin film preparation,and selecting the appropriate temperature to prepare smooth and dense Sb₂Se₃ thin film with good surface morphology.Then spin coating it with different concentrations of Na₂S solution to increase the doping concentration of Sb₂Se₃,exploring the influence of Na₂S solution concentration on Sb₂Se₃ film.Subsequently,the reasons for the performance improvement were explored through optical characterization and electrical characterization.Finally,the deep-level defects in Sb₂Se₃ films were passivated by introducing trace oxygen in the air annealing process to reduce the recombination of carriers,thus reducing the open-circuit voltage loss and improving the photoelectric conversion efficiency of battery devices.The specific research content and research results of this paper are as follows:1.Influence of Na₂S treatment on Sb₂Se₃ thin film solar cellsSb₂Se₃ thin films were prepared by rapid thermal evaporation method.Firstly,the preparation process of Sb₂Se₃ thin films was explored to facilitate the subsequent optimization treatment.Sb₂Se₃ films with different morphologies were prepared at different temperatures.Characterization analysis showed that the film at 480℃had larger grain size,smooth and compact film,suitable thickness,and higher photoelectric conversion efficiency.Then,the surface of Sb₂Se₃ film at 480℃was coated with different concentrations of Na₂S solution.When the concentration of Na₂S solution was 0.1 mol/L,the solar cell efficiency was the best of 7.00%.Energy dispersive X-ray spectroscopy（EDS）and ultraviolet photoelectron spectroscopy（XPS）showed that Na₂S were uniformly distributed on the surface of Sb₂Se₃ thin film.According to the fitted diode parameters and the Urbach energy（E_u）fitted by external quantum efficiency（EQE）,it can be seen that after spin coating Na₂S,the band trailing effect of the device is decreased,the recombination of heterojunction interface and space charge region is inhibited,the loss of photogenerated carriers is significantly reduced,and the quality of the diode is improved.Then,the electrochemical impedance spectroscopy（EIS）and capacitance-voltage（C-V）tests were performed.The test results showed that the spin coating of Na₂S could reduce the charge recombination of the device,passivate the surface defects of Sb₂Se₃ absorbing layer,and thus increase the open circuit voltage(V_OC),and the photoelectric conversion efficiency of the device was increased from 6.08%to 7.00%.2.Influence of air annealing treatment on Sb₂Se₃ thin film solar cellsIn order to further reduce V_OC loss and improve device performance,the prepared Sb₂Se₃ thin films were subjected to air annealing treatment,and trace oxygen was introduced to passivate the deep-level defects of Sb₂Se₃ thin films.The effects of different annealing temperatures on Sb₂Se₃ thin films were investigated.The results show that the performance of Sb₂Se₃ thin films annealed at 350℃is the best,which is 7.96%.The results of energy dispersive X-ray spectroscopy（EDS）and ultraviolet photoelectron spectroscopy（XPS）show that a small amount of Sb₂O₃ is generated on the surface of the device annealed at 350℃.Moreover,according to the capacitance-voltage（C-V）and drive-level capacitance profiling（DLCP）test results,the air annealing treatment reduces the defect concentration of Sb₂Se₃/Cd S heterojunction interface,and increases the depletion width（W_d）and the built-in voltage(V_bi),which is favorable for the separation and extraction of photogenerated carriers.Finally,deep-level defects of Sb₂Se₃ thin film solar cells are detected by deep-level transient spectroscopy（DLTS）.The results show that the concentration of V_Se and Se _Sb defects can be effectively reduced after 350℃air annealing,and the deep-level defects can be passivated and the carrier lifetime can be improved.