| Organic-inorganic hybrid perovskite materials have exhibited unparalleled potential application in photoelectric fields such as solar cells,photodetectors and LED,which has attracted the world’s attention.However,as the deepening of research,more and more problems are emerging,such as the toxicity of lead,the stability of perovskite materials and devices,the band-gap engineering,and the unclear intrinsic physical properties of single crystals.All the above hinder the further commercial application of such materials.Therefore,it is imperative to tune the optical band gap of the hybrid perovskites by designing a simple and effective method,reduce the toxicity of lead,improve the stability of its materials and devices,and explore new lead-free,high stability,narrow band gap perovskite materials,and further investigate their intrinsic physical properties.Based on the above reasons,series of new hybrid perovskite single crystals were designed and grown.Furthermore,in order to broaden their photoelectric applications,the optical bandgap of the perovskite materials was tuned,the toxicity of Pb was reduced,and the stability of the materials was also improved by several processes such as gas induction and doping.The new phenomena found in the research process are systematically explained.Moreover,the single crystal structures,the thermology and photoelectric properties and other intrinsic physical properties are studied in detail.The main results are as follows:1.A convenient,effective and universal conversion process from single crystal to single crystal induced by CH3NH2(MA)gas for a variety of hybrid perovskite materials was designed and the transformation mechanism was also revealed.Most significantly,the hybrid perovskite single crystals keep their original morphology,and improve their photoelectric properties dramatically after induced conversion by MA gas,especially for DMAPbI3.The optical band gap of DMAPbI3 was reduced from 2.58 eV to 1.53 eV with their carrier mobility enhanced more than 2 times.Its photo-response performance was also enhanced greatly(>40 times).This transition is actually a process in which the organic group in perovskite materials was replaced by MA molecules.With the prolongation of induction time,MAPbI3 materials will be obtained eventually and their original morphology will be retained.Moreover,this process were also used in other perovskites such as BA2PbI4,HA2PbI4,OA2PbI4 and GAPbI3,which exhibited a similar phenomenon as DMAPbI3,and their optical band gap was significantly narrowed,indicating the universality of this process.2.Due to the toxicity of Pb element and the phase separation phenomenon caused by halide ion migration when mixed halide anions(Br-,I-)compounds to tune bandgaps,we designed and grown the mixed metal Pb/Sn halide perovskite single crystals.The aim is to reduce the toxicity of Pb and improve the phase stability of perovskite materials on the premise of tuning the band gap.By designing,exploring and optimizing the crystal growth conditions,series of large size and high quality MAPbxSn1-xBr3 perovskite crystals with different Pb/Sn ratios was obtained.All the MAPbxSn1-xBr3 crystals belong to Cubic system and Pm-3m space group.In addition,MAPbxSn1-xBr3 crystals exhibit excellent semiconductor properties and good thermodynamic stability.Especially,these perovskite single crystals not only exhibited much narrower optical band gap(1.77 eV)than those of MAPbBr3 and MASnBr3,but also possessed higher stability even exposed to air over one month than that of MASnBr3.3.An intriguing and reversible band gap narrowing behavior of the lead-free single crystal DMASnI3 with excellent phase stability in water was found for the first time and the mechanism of this phenomenon was also revealed in detail.Combined with the analysis of XRD,XPS,Raman spectroscopy and first-principles calculation,we revealed the mechanism of band gap narrowing behavior which was iodine ion migration induced defect.In addition,the photoelectric properties of the materials were systematically investigated by UV-diffuse reflectance spectra,PL spectra,TRPL spectra,SCLC measurements and Hall Effect measurements and indicated that the DMASnI3 shown good semiconductor properties.It should be noted that DMASnI3 crystals exhibit excellent stability to water,and their XRD spectra remain unchanged after immersion in water for 16 h.Based on its good water phase stability,we first investigated the photocatalytic hydrogen evolution activity in deionized water,and got the hydrogen evolution successfully,which greatly broaden the application of this perovskite material.4.In order to expand the optoelectronic applications of Sb-based and Bi-based perovskite-like materials,a general Sn2+ doping strategy was designed to narrow the wide band gap of Bi-based and Sb-based perovskite-like materials.Obviously,Sn-doped MA3Sb2I9 and Sn-doped MA3Bi2I9 exhibit narrower band gap than that of MA3Sb2I9 and MA3Bi2I9.Combined with the first-principles calculation,the changes of electronic structures of Sn-doped MA3Sb2I9 were shown clearly,and the reasons for the band gap reduction are revealed.The electrical properties of MA3Sb2I9 before and after doping were systematically studied by SCLC and Hall Effect tests.The results show that after the doping of Sn2+ ions,the carrier mobility of MA3Sb2I9 materials was enhanced about two times and both of Sn-doped MA3Sb2I9 and Sn-doped MA3Bi2I9 remained stable even exposed to air over one month.5.A series of lead-free,high stability,narrowing bandgap Te-based perovskite single crystals were designed and grown.All the crystal structures were determined by the X-ray single crystal diffraction.By changing the organic cations(MA+,FA+,BA+)and halogens(Br-,I-),these Te-based perovskite crystals exhibit a tunable band gap(1.42-2.02 eV).The UV-vis-NIR diffuse reflectance spectra show that the optical absorption cut-off edges of MA2TeI6,FA2TeI6 and BA2TeI6 are 843,871 and 812 nm,respectively which is similar to that of MAPbI3.Taking MA2TeI6 and MA2TeBr6 as examples,their photoelectric properties are studied systematically.Encouragingly,MA2TeI6 exhibits low trap density(～1010cm-3),and high mobility(～65 cm2 V-s-1),while the MA2TeBr6 possesses a long carrier lifetime of 5958 ns and corresponding carrier diffusion lengths of～38 μm,which are ideal characteristics for a material of tandem solar cells.Moreover,A2TeX6 perovskites are relatively robust in ambient conditions,being stable for at least two months without showing any signs of phase change.In conclusion,the Te-based perovskite materials have narrowing band gap and good stability,and exhibit the potential photoelectric applications. |
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