Page - 104 - in Photovoltaic Materials and Electronic Devices

In addition, the MAI:PbI2 (molar ratio3:1) precursor solutionon compact TiO2 canalsoformMAPbI3 withapredominant(110)plane,buttheannealingtemperature need to be above 150 ˝C [84,88,90]. The different sublimation temperature of MAI andMAClandtheevidenceofresidueMAIorMACl intheresultingperovskitemay explain thehigherannealingtemperatureneededforMAPbI3 [84,91]. The XRD patterns of the resulting MAPbI3-xClx prepared from different chloride-containing precursors are summarized in Figure 5. All the patterns showed predominantcrystallizationalongthe (110)and(220)planes. Interestingly, the (220) peak split at a high MACl (x = 2) concentration in Figure 5B [27]. This split was also observedinreference [29]. In thesequentialdepositionmethod, the (110)and(220) crystallizationpreferencemaybeduetoan insitu transformationprocess [92]ofPbI2 to MAPbI3, as discussed in Section 3.2. However, the PbI6 octahedra aremore likely tobefullydissociated in theonestepprecursorsolution. [29,93–95]AsMACldoes notfit intheMAPbI3 structure, it couldbepossible thatMAClmaybeexpelledalong the (110) planes of the MAPbI3 and that is why the MAPbI3-xClx always showed the(110)and(220)orientationpreference. Thisassumptioncanbeprovedby(220) peak split in Figure 5B, as excess of MACl breaks down the crystal range along (110)planesresulting inpeaksplit. However,MAIcanfit in theMAPbI3 structure, (110)-oriented growth is just theresultof cubic phase in high temperature (150 ˝C inrefereneces [84,88,90]). Surprisingly,amainXRDpeakof (310)wasobservedfor the one step deposition prepared MAPbI3 [96]. The main peak of (310), which is distinct fromthe(110)peak,mayhaveresultedfromthefact that theMAIwasadded into theprecursorsolutionafter thePbI2 wascompletelydissolvedinsteadofboth MAIandPbI2 beingpresentat thesametime[96],or the fact that the (310)planeof MAPbI3 may match the crystal structure of the substrate better. Then the magnitude of the (110) peak of MAPbI3-xClx and the (310) peak of MAPbI3 further increases after5weeks [96]. Thus,webelieve that theannealingprocessmayonlyreinforce the crystallization preference as it is initially formed and the effects of substrate also contribute to the crystal structure formation of the perovskite in some cases. Returning to Figure 5, if excess of MACl breaks down the growth along the (110) plane,webelieveMAClcanalsobreakdownthecrystallineorderrange. Sincealarge amountofMAPbI3 existed intheamorphousphase form, thecubicphaseofMAPbI3 maybemore favorable inshortcrystallineorderrangethanthe tetragonalphase. 104