Density Functional Theory Study On Band-gap Engineering And Optical Properties Of Organic-inorganic Hybrid Perovskites

The harmonious development between man and nature,as the main trend of the progress of modern human society,and its future development are bound to face two major issues,one is energy,the other is environment.Renewable energies have now been a hot research field that governments highly focus on,due to the advantages of green non-pollution and sustainability.So far,renewable energies mainly consist of solar energy,wind power,water power,biofuel,geothermal energy and so on.Among those,solar energy attracts considerable attention,since that its development is without any geographic limits.The main path to utilize solar energy is solar cells.At present,solar cells are comprised of compound solar cells,silicon-based thin film cells,dye-sensitized solar cells?DSSCs?,organic solar cells and so on.In the present work,perovskite solar cells?PSCs?is focused on,which are from the evolution of DSSCs.In 2009,the research group,leaded by Professor Miyasaka in Toin University of Yokohama,first used methylammonium lead iodide?MAPbI₃?as visible-light sensitizer?directly contacting with electrolyte?in DSSCs.Although the obtained photoelectric conversion efficiency?PCE?was only 3.8%at that time,the significance of the finding is that PSCs gradually became a hot research from then on.In the following ten years,the PCE has been rapidly increased to the recent 23%at an unprecedented rate.The predominant properties of organic-inorganic hybrid perovskites?OIHPs?mainly include long carrier diffusion length,high cost-effectiveness,facile solution processing.On account of these unique advantages,OIHPs have been drawn wide attention and stood out from numerous photovoltaic materials.In the thesis,how to improve optical absorption capability and structural stability of hybrid perovskite materials is studied,and its inherent mechanism is also explored.To improve the PCE of PSCs,in the present work,the effect of mono-doping or co-doping in the A-site,the B-site and the X-site on prototype perovskite with the formula of ABX₃,is explored,in terms of component engineering.The results indicate that organic cations in the A-site almost have no contributions to the electronic orbits at band-gap edge,however,band gap can be effectively tuned by lattice deformation induced by ion doping in the A-site.As for ion-doping in the B-site,it is clearly unveiled that the electronic orbit at the conduction band minimum?CBM?is occupied by divalent metal ion.In other words,band gap of primitive perovskite can tuned by ion doping in the B-site.Moreover,valence band maximum?VBM?is mainly composed of I-5p electronic states,hence,band gap is also tuned by ion doping in the X-site.Meanwhile,it has been demonstrated that long carrier diffusion length can be achieved in the chloride-or bromide-ion doped perovskite compounds.Therefore,with regard to ion co-doping in A-site/B-site and X-site,theoretically,band gap can be tuned.Meanwhile,long carrier diffusion length can also be realized.Furthermore,it has been experimentally demonstrated that metal cesium ion can enhance the stability of perovskite materials,and the inherent mechanism for that is also probed.Specifically,the main research results are listed below during four-year doctoral learning:?1?band gap engineering and visible-light absorption capability of archetype perovskite by germanium/tin ion dopingGermanium/tin in the same main group is chosen as substitution doping of lead.Due to different doping sites,Ge-doping?i?,Ge-doping?ii?,Ge-doping?iii?and Ge-doping?iv?are herein studied.With respect to tin doping,the same four cases are also investigated.The relevant results are listed as follows,The results suggest that band gap of doped compounds MAPb_1-?X?I₃?X=Ge,Sn?of can be effectively tuned,and the effect of the germanium-doped is superior than the tin-doped.The electronic density of state?DOS?imply that without spin orbit coupling?SOC?the main occupied electronic states in the band-gap edge are I-5p and Pb-6p,with SOC those become Pb-6s and I-5s.In other words,SOC significantly changes not only band gap but also occupied electronic orbit in the band-gap edge.Charge density distribution shows that the interaction between methylammonium cation and Pb-I inorganic framework mainly embodies between CH₃/NH₃ functional group and iodide ion.In addition to that,charge density distribution around methylammonium ion and iodide ion almost has no change by comparing with SOC and without SOC,which implies that SOC has no remarkable impact on these two ions.The defect formation energy shows that four inequivalent structures are possible to be synthesized simultaneously under the same growth condition.Furthermore,red shift of absorption spectrum in the germanium-doped is more significant than in the tin-doped.?2?The inherent mechanism exploration on improving optical absorption by organic cation mixingFormamidinium cation is here chosen as substitution doping of methylammonium cation.Taking four MAPbI₃ units in prototype perovskite into account,the different doping ratios are respectively studied,including x=0.0,x=0.25,x=0.5,x=0.75 and x=1.0.Among the doped compounds MA_1-xFA_xPbI₃,x=0.0 is pure MAPbI₃,and x=1.0 is pure FAPbI₃.As for x=0.5,three cases of x=0.5a,x=0.5b and x=0.5c are discussed,respectively,in terms of two formamidinium-cation sites.The related results are listed as follows,The electronic DOS shows that VBM is mainly composed of I-5p electronic state while CBM consists of Pb-6p electronic state.Although the main occupied electronic orbits at band-gap edge is almost changed by methylammonium cation doping,charge density distributions at VBM and CBM are significantly modified.In addition that,band gap of perovskite materials can be tuned by means of methylammonium cation doping.Both Bader charge analysis and charge density distribution suggest that I-5p electronic state at VBM is chiefly from the iodide ion in the equatorial direction within octahedron[PbI₆]⁴⁺.The defect formation energy for the doped series MA_1-xFA_xPbI₃ are negative,which means that the doped compounds can be experimentally synthesized.Among those,the defect formation energy for compound FA_0.75MA_0.25PbI₃ is minimum,which suggest that the compound is easier synthesized under the same condition.Furthermore,absorption spectrum simulation shows that red shift of absorption spectrum is achieved by virtue of the mixing methylammonium cation and formamidinium cation.Especially,within the visible-light spectrum range of 416-565 nm,the absorption coefficient of doped compound FA_0.75MA_0.25PbI₃ is significantly enhanced as compare with that in archetype perovskite.?3?The effect of mono-doping/co-doping of formamidinium cation and halide ion on optical capability of archetype perovskiteLong carrier diffusion length can be achieved over 1 micrometer in the chloride-doped perovskite compound,which has been experimently demonstrated.And formamidinium cation doping can also tune band gap,as discussed in our previous study.Hence we focus on the impact of mono-dopin/co-doping of formamidinium cation and chloride ion?or bromide ion?on optical absorption.Note that co-doped compounds consist of four cases,including FACl-a,FACl-e,FABr-a and FABr-e,due to different doping site of halide ion.The DOS is in the level of HSE06+SOC,while optical properties and band structure are in DFT+D2 level.The relevant results on the investigated doped compounds MA_1-?FA?PbI_3-?X?are shown below,Electronic transition mainly happen between I-5p electronic state and Pb-6p electronic state.The electronic orbit at VBM and CBM almost keep the same with that in prototype perovskite,no matter what for mono-doped or co-doped.Interestingly,the doped compounds are direct bandgap semiconductor,which is favorable generation of electron-hole pairs.The doping site of halide ion is closely related to charge density distribution at VBM,and it has further effect on carrier diffusion length.Besides,absorption spectrum indicates that compound FACl-a is a better choice among the doped series.?4?The effect of mono-doping/co-doping of divalent metal ion and halide ion on band gap and optical absorption of prototype perovskiteExperiment observation find that halide ion doping is favorable to achieve long carrier diffusion length.And band gap of perovskite can also tuned effectively by means of germanium/tin doping,as confirmed in our previous study.On account of these progresses,electronic properties and optical properties of the mono-doped of FA-momo,Cl-mono,Br-mono,and the co-doped of Ge-Cl,Ge-Br,Sn-Cl,Sn-Br are here explored.The results on the doped series MAPb_1-?X?I_3-?Y??X=Ge,Sn;Y=Cl,Br?are listed below,The equatorial Pb-I bond interaction is significantly stronger than that in the apical Pb-I bond within[PbI₆]⁴⁺,which is suggested by both charge density distribution and bond length change.Moreover,I-5p electric state at VBM is mainly from the equatorial iodide ion within octahedron[PbI₆]⁴⁺.Although halide ion doping can increase carrier diffusion length,meanwhile it reduces optical absorption coefficient.Consequently,doping proportion of halide ion should be strictly controlled to synthesize perovskite compounds.As for absorption spectrum,the compound of MAPb?16?-?Ge?I?18?-?Br?presents superior visible-light harvesting capability as compared the others.?5?The effect of ion doping of metal cesium ion and bromide ion on structural stability and optical propertiesMetal cesium ion doping has been experimentally confirmed to improve the stability of perovskite materials.Meanwhile thin-film compound FA_1-xCs_xPbI_3-yBr_y is also confirmed to achieve large-size solar cells.On account of these findings,compounds of MAPbI_3-yBr_y and MA_1-x-x Cs_xPbI_3-yBr_y are here investigated to probe inherent mechanism of cesium-ion doping enhancing structural stability.The relevant results are shown as follows,Metal cesium ion doping can tune band gap of prototype perovskite,but almost have no impact on electronic orbit at VBM and CBM.Charge density distribution implies that organic cation in the A site is mainly used for maintaining charge neutrality.Meanwhile,within octahedron[PbI₆]⁴⁺charge density distribution around apical iodide ion,which means that stability is enhanced.It is further confirmed that the importance of cesium ion doping is the improvement of stability not the enhancement of optical absorption.Additionally,both visible-light harvesting capability and structural stability of compound MA_0.75Cs_0.25PbI₂Br are superior than those of the others.