Page - 101 - in Photovoltaic Materials and Electronic Devices

3.2. ConvertingLeadHalides toPerovskite In the sequential deposition method, PbI2 or/and PbCl2 were first dissolved in a solvent. As PbI2 crystal has a layered structure, DMF can intercalate into the PbI2 interlayerspaceandscreenPbI2 viaPb-Obonding[71–73]. WhenDMFis intercalated, theXRDpeakof thePbI2 (001)planeredshifts from14.8˝ to7.94˝ [72,73]. Thered-shift of thisXRDpeakto9.17˝also indicates the intercalationofDMSO[43]. WhilePbCl2 doesn’tpossessasimilar layeredstructureasPbI2, its solubility ispoorwherePbCl2 nanoparticles may only suspend in the solvent [28]. However, depositing a mixture of PbI2 and PbCl2 on the substrates result in a new PbICl phase [74], whose crystal structure issimilar toPbCl2 [75]. At the beginning of the reaction of PbI2 and MAI, a predominant peak at (220)appeared(asshowninFigure2B). Inotherwords, theMAPbI3 preferentially grows along (220) plane at first. The annealing process increases the long range crystalline order and results in the predominant (110) peak instead. Noting the (220) is only a short range of (110), thus, another possible reason for the (110)-oriented growthofMAPbI3-xClx andMAPbI3 maybebecause layeredcrystalstructureofPbI2 (growthalong(001)planesofPbI2 like the liquidcatalystclustermodelmentioned inreference [76]). The latticeplanesof tetragonalMAPbI3 areshowedinFigure3. Materials  2016,  9,  123  5  of  13    Figure  2.  XRD  patterns  of  glass  substrates  with  vapor  deposition  of  (A)  a  layer  of  PbI2;  and  (B)  a  layer  of  PbI2  followed  by  a  layer  of  MAI,  repeating  this  step  7  times;  XRD  pattern  of  the  (C)  annealed  7‐time  deposited  PbI2/MAI  layer;  (D)  1‐time  deposited  PbI2  (50  nm  thickness)/MAI  (50  nm  thickness)  layer;  and  (E)  1‐time  deposited  PbI2  (150  nm  thickness)/MAI  (150  nm  thickness)  layer.  Reprinted  from  reference  [77],  Copyright  ©  2015,  Royal  Society  of  Chemistry.    Figure2. XRDpatternsofglasssubstrateswithvapordepositionof (A)a layerof PbI2; and (B) a l yer of PbI2 followed by a layer of MAI, repeating this step 7 times; XRD pattern of the (C) annealed 7-time deposited PbI2/MAI layer; (D) 1-time deposited PbI2 (50 nm thickness)/MAI (50 nm thickness) layer; and (E) 1-time depositedPbI2 (150nmthickness)/MAI(150nmthickness) layer. Reprintedfrom reference [77],Copyright©2015,RoyalSocietyofChemistry. 101