Page - 99 - in Photovoltaic Materials and Electronic Devices

poly-(vinylpyrrolidone) (PVP)canalso improvethesurfacecoverageofperovskite [39]. It is interestingtonotethat, forMAPbI3-xClx, asimpleannealingstepisenoughtoform agoodcoverage[6,40], but forMAPbI3, a special step, suchasmulti-deposition[41], adding N-cyclohexyl-2-pyrrolidone (CHP) [42], fast deposition [43–45], or air flow duringspincoating[46,47], isneeded. The sequential deposition method was mainly applied for MAPbI3 perovskite. Inatypicalsynthesis, thesolutionofPbI2 inDMFwasspunonasubstrateas thefirst step then the substrate was dipped in a solution of MAI in 2-propanol (IPA) to form MAPbI3 crystalsas thesecondstep[48]. For the inclusionofchloride, in thefirst step the PbCl2 can be mixed with PbI2 in DMF or dimethyl sulfoxide (DMSO) [49–52], and/or the second step MACl can be added [53–55]. For vapor based deposition methods, the MAPbI3-xClx can be formed by co-evaporating MAI and PbCl2 onto thesubstrates [56,57]orbyreactingPbCl2 onsubstrateswithMAIvapor [58,59]. 3. TheCrystalStructureFormandFormation 3.1. CrystalStructureofMAPbI3Layer The parameters and transitions of phases of bulk MAPbI3 were included in references [60,61]. Here, we focus on the tetragonal and cubic phases [62]. In fact, therearenocriticaldifferencesbetweenthetwophases,exceptaslight rotationof PbI6 octahedra alongthe c-axis. The atomicstructures of MAPbI3 of the two phases areshowninFigure1A,B.Thus, the tetragonalphasecanbetreatedasapseudocubic phase with a* = a/ ? 2, c* = c/2 [63]. Below 54 ˝C, the cubic phase of MAPbI3 can be transformed into the tetragonal phase [60], and the opposite transition occurs by annealing at 100 ˝C for 15 min [41]. In Figure 1C, the X-ray diffraction (XRD) patternsof the twophasesareshown. After transformationto the tetragonalphase, the (100) and (200) peaks of cubic MAPbI3 split, also new (211) and (213) peaks show up. Here, we use the peak splitting as indictor for phase transformation. Analysis of theMAPbI3-xClx usuallyshowsthecubicphaseofMAPbI3,however,withamuch more preference along (100) and (200). This will be discussed in the Sections 3.2 and3.3. Anotherphasewhichshouldbenotedis theamorphousphase. Pairdistribution functionanalysisofX-rayscatteringshowedthatafterannealingat100˝Cfor30min, theMAPbI3 inmeso-porousTiO2 hasabout30atom%inmediumrangecrystalline order and the other 70 atom% in a disordered state with a coherence length of 1.4 nm [66]. The poor crystallization of the MAPbI3 in meso-porous TiO2 was studied by high-resolution TEM [67]. Quartz crystal microbalance measurements suggest that during the sequential method only half of PbI2 is converted to MAPbI3 instantly, while the other half is involved in reversible transformation with MAPbI3. Additionally, the amorphous character with a very small average crystallite size may 99