First-principles Study On Photovoltaic Properties Of Novel Lead-free Double Perovskite Materials

Photovoltaic materials achieve zero-carbon emissions by converting light energy into electricity,which can effectively solve the problems of environmental pollution and energy shortages faced by the current society.In recent years,a new generation of perovskite photovoltaic materials,represented by MAPbI₃(MA=CH₃NH₃),has been developed due to its suitable band gap value(?1.56 e V),strong light absorption performance(?10~5 cm^-1)and relatively high carrier mobility and has attracted much attention.However,it has always been a major pain point for such materials,because the lead element it contains is harmful to the human body and pollutes the environment.The researchers used alkaline earth metal ions,transition metal ions and other divalent cations to directly replace the lead element at the B position to achieve the purpose of removing toxicity.However,the properties of materials synthesized by direct substitution are far inferior to MAPb I₃,because the lone pair electrons contained in Pb²⁺have a significant impact on the photoelectric properties of the material.In this paper,we will mainly focus on the lead-free modification of traditional perovskite materials using cationic transmutation strategy.Combined with the first-principles calculation method,a series of double perovskites Cs₂MM'Br₆(M=Cu,Ag,Au;M'=Ga,In,Bi,Sb)have been systematically studied on their electronic structure and optical properties.Meanwhile,we find that the material properties also can be regulated by halogen site replacement,strain application,and vacancy doping.The main research contents are summarized as follows:The double perovskite Cs₂MM'Br₆(M=Cu,Ag,Au;M'=Ga,In,Bi,Sb)all have strong stability and non-toxic properties.Physical quantities such as band gap,effective mass and absorption capacity are used as indicators to screen out semiconductor materials with excellent optoelectronic properties.The research results show that Cs₂AgInBr₆(1.47e V)and Cs₂AgGaBr₆(1.37 e V)have the characteristics of direct band gap,and compared with the traditional perovskites MAPb I₃(?1.56 e V),their band gap values are closer to the optimal value of 1.34 e V required by the Shockley-Queisser limit.At the same time,the energy conversion efficiencies of the two candidate materials can reach 31.9%and 32.45%,respectively,and the light absorption coefficients are both above 10~5 cm^-1in the visible light range.At room temperature,the electron mobility of Cs₂AgGaBr₆ reaches 160.8 cm~2 V^-1s^-1,which is very close to the 165 cm~2V^-1s^-1 of MAPb I₃.The above results show that the inorganic double perovskite materials designed by the cation transmutation strategy not only effectively remove toxic ions,but also have excellent photovoltaic properties,which is a class of photovoltaic materials with broad development prospects in the future.The experimentally synthesized Cs₂AgBiBr₆ double perovskites have the characteristics of indirect band gap and large forbidden band width,which hinder the application in photovoltaic field.In this paper,it is proposed to control the optoelectronic properties by replacing the Br atoms at the halogen sites with I element.The research results show that Cs₂AgBiI_xBr_6-x(x=0,1,2,3,4,5,6)has good stability,and the band gap value gradually decreases from 2.21 e V with the increase of the doping concentration of I element to 1.51 e V.Bandgap tuning of the material is effectively achieved,which is largely attributed to the formation of antibonding states between the 5p orbital of I and the 4d orbital of Ag,thereby reducing the bandgap value.At the same time,in the visible light range,the light absorption coefficient is significantly improved compared with Cs₂AgBiBr₆.The energy conversion efficiency of Cs₂AgBiI₂Br₄material with space group I4/mmm is about 2%higher than that of Cs₂AgBiBr₆.Finally,the study of mechanical properties shows that the incorporation of different degrees of I atoms can effectively tune the elastic modulus of the material.By applying strain and vacancy doping to Cs₂AgBiBr₆,we hope to realize the regulation of double perovskite properties from other perspective.The results show that the tensile and compressive strains can increase and decrease the material band gap value on the basis of Cs₂AgBiBr₆,respectively.With the increase of compressive stress,the effective mass of holes decreases significantly,which greatly improves the carrier mobility.It is shown that stress is an effective means to tune the electronic properties of the double perovskite Cs₂AgBiBr₆.At the same time,the vacancy doping causes the phase transition of Cs₂AgBiBr₆ from semiconductor phase to metal phase.