Synthesis And Photovoltaic Application Of Novel Thiocyanate-based Organometal Perovskite Materials

Recently,the solar cells based on organometal halide perovskites have attracted considerable research attention for the power conversion efficiency?PCE?of these cells has skyrocketed to a certified 21.1% from 3.8% back in 2009.As solar-absorbing materials,organic-inorganic hybrid perovskites are self-assembled via organic and inorganic molecular layer accumulated alternately to form the quantum well structure.Since perovskite materials have successfully combined organics' good functionality and workability with inorganics' high carrier mobility,mechanical stability and thermostability,plus with appropriate optical band gap as well as broad spectral response range,it has been emerging as the forerunner in the next generation photovoltaic material.Currently,the most used light absorber materials in perovskite solar cells are MAPbX₃?MA = CH₃NH₃⁺;X = I,Cl,Br?,which all have three-dimensional?3D?frameworks and can only be dissolved in solutions with high-boiling points.Therefore,in the process of fabricating solar cell devices by solution-processed method,thermal annealing shall be carried out to remove the solvents in order to crystallize perovskite thin films,which is detrimental for achieving excellent surface morphology of the films.Owing to inappropriate thermal annealing,such as excessive annealing time and over high temperature,undesirable pinholes,intraholes,or cracks through the films will be formed due to fast evaporation of solvents or uncontrollable decomposition of perovskite.Consequently,poor perovskite morphology may increase the probability of charge recombination,which may make a big difference to achieve high performance of solar cell devices.Hence to find a novel perovskite material that can be dissolved in low boiling point solvent?LBPS?is of great significance to overcome the shortages resulted from thermal annealing.Based on this idea,we considered modifying molecular structures as an essential way to change the solubility of materials.We believe that an implantation of larger anions into the Y-site of organic-inorganic hybrid perovskite with formula ABY₃ to design 2D molecular frameworks will be a good idea.Upon this evaluation,a novel layered material?MA?₂Pb?SCN?₂X₂ has been successfully prepared and characterized in this paper,in which thiocyanide?SCN^-?possessing rigid linear structures and extended conjugated ? electron systems was adopted to enhance photoelectron transfer and nonlinear optical reactivity.A novel layered organometal halide perovskite material?CH₃NH₃?₂Pb?SCN?₂I₂ that shows great solubility and crystallinity in low boiling-point solvent tetrahydrofuran was acquired.The molecular structure of this material was characterized by X-ray single crystal diffraction,through which the thiocyanides were proved to be coordinated as terminal ligands on the lead center.A variety of spectral tests reveal material's energy gap?Eg?and valence band?VB?as 2.0 eV and-5.76 e V,respectively.The concentration and the mobility of holes have also been located as up to 1.07×10¹⁴ cm^-3 and 4.34 cm²/v·s via Hall effect measurement.On the basis of the synthesis and characterization of materials,this article also presents a detailed research of how the several influence factors,such as solution dosage,spin rate,annealing temperature and standing time,exert impact to the crystallinity and surface topography.And through this elaborate work,an easy one-step technique to prepare perovskite thin films in microstructure without any thermal annealing was developed,which provides a significant report focusing on the interaction between device performance and the morphology of thin film.