甲胺基-甲脒基混合钙钛矿的第一性原理研究

First-Principles Study of Mixed Cation Methylammonium-Formamidinium Hybrid Perovskite

有机铅卤钙钛矿APbX_3的稳定性是制约其应用的重要因素,对APbX_3钙钛矿中A和X采用不同种类离子混合的化学组分调控是改进其稳定性最有效的方式之一。其中,A位点采用不同比例的甲脒离子(FA)和甲胺离子(MA)是当前研究的热点方向。本文通过第一性原理计算,系统研究了FA_(1-x)MA_xPbI_3体系的结构和光电性质。研究发现FA与MA的混合增加体系的稳定性,其中FA_(0.5)MA_(0.5)PbI_3最稳定。通过分析不同混合比例的结构,揭示了晶格常数随x的增加线性减小;以及带隙随x减小而线性增加。此外,计算结果发现MA所占比例增加时吸收光谱蓝移。研究表明通过FA和MA离子的混合能有效调控钙钛矿的光电性质,从而获得更有效的钙钛矿太阳能电池。

Organic-inorganic hybrid perovskites have attracted substantial attention due to their outstanding performance in solar energy conversion and optoelectronic applications.However,the poor stability of organic lead halide perovskites(APbX3,A:organic cations;X:halide anions)hinders their practical application.Compositional engineering,optimizing the concentration of different A and/or X,is one of the most effective ways to enhance the stability of APbX3.In particular,mixing formamidinium(FA)and methylammonium(MA)cations has been widely investigated.Here,we performed first-principles calculations to systematically investigate the structures and optical properties of FA1-xMAxPbI3.We obtained several stable crystal structures of FA1-xMAxPbI3 in the cubic phase for different ratios of FA and MA cations.By analyzing the structures of these mixed cation perovskites,we revealed that the lattice parameters decreased linearly with increasing concentration of MA cations,which was consistent with previous experimental results.For APbX3 perovskites,the Goldschmidt tolerance factor has been shown to be an effective index for predicting the stability.The mixing of organic cations is an effective method for adjusting this factor and thus the stability of these perovskite materials.In this work,the formation energy difference(ΔE)was calculated and our results showed that mixing of FA and MA cations could increase the thermodynamic stability compared to pure FAPbI3 and MAPbI3.FA0.5MA0.5PbI3 was found to be the most stable of all composites investigated.Furthermore,the band gap decreased with increasing proportion of FA cations,indicating an effective strategy for extending the absorption range of organic-inorganic hybrid perovskites into the infra-red region of the solar spectrum.By analyzing the Pb―I bond lengths for different FA:MA ratios,we showed that the decrease of band gap mainly comes from the increase of the maximum valence band energy.In addition,the effective masses were similar,irrespective of the cation mixture,indicating that the mixing of organic cations does not affect the carrier transport mechanism in this material.Density of states(DOS)analysis revealed that the DOS of valence band edge increased with increasing proportion of FA cations,enhancing transitions between the valence and conduction bands.Finally,detailed analysis of the optical absorption coefficients of all composition was performed;the absorption spectra were blue shifted with increasing MA cation concentration.These results demonstrated the possibility of controlling the optoelectronic properties of organometallic lead halide perovskites by mixing FA and MA cations and hence,further improving the efficiency of perovskite solar cells.