Device Research And Process Optimization Of Lead-Free Tin-Based Perovskite Solar Cells

As the third-generation solar cell,perovskite solar cells have attracted widespread attention from researchers all over the world because of their excellent optoelectronic characteristics,easy fabrication,lower cost and application for flexible device.However,the element Pb in perovskite is toxic,which violates the original intention of energy conservation and environmental protection,hindering the commercialization of perovskite solar cells.This paper aims to improve the device performance of lead-free Sn-based perovskite solar cells.The main work is carried out in the following three aspects:（1）The key parameters,such as annealing temperature for fabricating perovskite films,the kind of anti-solvent and the thickness of PCBM,were adjusted to optimize the performance of the device.A fabricating process of perovskite solar cell has been explored in which the perovskite absorption layer is based on FASnI_3.The experimental results demonstrate that device of the best performance was obtained when choosing diethyl ether as an anti-solvent,perovskite films annealing at 70℃for10 min and spin coating PCBM with the thickness of 70 nm as electron transport layer.As a result,this device has its PCE reach up to 1.24%,while its V_OC,J_SC and FF is 0.25 V,11.02 mA·cm^-2 and44.05%,respectively.（2）Poly-α-methylstyrene（PAMS）is doped into the perovskite film by dissolving PAMS in an anti-solvent and dropping it during the perovskite spin coating process.The results show that the lower concentration of PAMS doping effectively improves the surface morphology of the film,reduces the roughness of the film surface,inhibits the non-radiative recombination,and improves the carrier mobility.In this study,PAMS was dissolved in anti-solvent diethyl ether at concentrations of0 mg/mL,0.3 mg/mL,0.5 mg/mL,1 mg/mL,2 mg/mL,and 3 mg/mL,respectively.At the same time,the device performance,surface morphology,etc.were compared in these conditions.The experiment result shows that with the concentration of PAMS in anti-solution increases,the efficiency of the device first increased to the best and then decreased.When the concentration is 0.5 mg/mL,the perovskite solar cell achieves the highest photovoltaic conversion efficiency,resulting in the short-circuit current density increases from 12.68 mA·cm^-2 to 17.64 mA·cm^-2 and the device efficiency increases from 1.295%to 2.278%.However,higher concentration leads to the formation PAMS agglomerate on the surface of the perovskite films,which blocks carriers transport into the electrode of the device,resulting in lower PCE.In addition,the doping of PAMS in the absorption layer significantly improves the hydrophobicity of the perovskite films,providing a new method for the fabricating stable Sn-based perovskite solar cells.（3）The band gap is adjusted by replacing the elements in the ABX₃ structure of Sn-based perovskite to increase the open circuit voltage(V_OC)and PCE of the device.Two types of Sn-based perovskite solar cells with mixed-halide(FASnBr_x I_3-x)and mixed-cation(（FAI）_1-x（MABr）_xSnI₂)were studied systematically in terms of their photovoltaic conversion efficiency,surface morphology and photoelectric properties,where x equals 0,0.2,0.4 and 0.6,respectively.Since the ionic radii of Br and MA are less than that of I and FA,respectively,for FASnBr_xI_3-x perovskites,as x increases,lattice shrinkage,band gap broadening,and UV absorption intensity increase occur.The photovoltaic conversion efficiency of the same batch of devices increased from 1.576%to 1.919%,and the open circuit voltage increased from 0.26 V to 0.396 V.For the（FAI）_1-x（MABr）_xSnI₂ perovskite,as the x increases,the lattice shrinkage of the perovskite becomes more pronounced,and the unit cell size on the surface of the film becomes larger.When x=0.4,the photovoltaic conversion efficiency of the same batch of devices increased from 0.452%to 1.904%,and the open circuit voltage increased from0.252 V to 0.385 V.The results of ultraviolet photoelectron spectroscopy（UPS）indicate that the（FAI）_0.6（MABr）_0.4SnI₂ perovskite has a band structure,which energy level is better matching to that of the transport layers.