Page - 97 - in Photovoltaic Materials and Electronic Devices

stablecrystal. Then, twocontradictoryresultswerethenreported. X-rayphotoelectron spectroscopy (XPS) showed that the molar ratio C:N:Pb:I:Cl of the perovskite is ca. 1:1:1:2:1, whenprepared from aprecursor of MAI:PbCl2 (molar ratio3:1) [9]. On the otherhand,energydispersiveX-ray(EDX)analysisshowedthatnoCl´waspresent in theperovskitepreparedfromPbI2 +MAI+MACl[10]. Notingthat theXPSwasunable todeterminetheexistenceofMAPbI2ClcrystalandthatEDXhasitsdetectinglimitation, more precise characterizations were needed. Later on, the simultaneous Fourier transforminfraredspectroscopyanalysisof theexpelledgasduringthedecomposition of MAPbI3-xClx showed the presence of Cl´, angle-resolved XPS [11] and X-ray fluorescencespectroscopy(XFS)[12]notonlyconfirmedtheexistenceofCl´,butalso showedthatCl´waslocatedat theinterfacebetweentheperovskiteandtheelectron transport TiO2 layer, and not in the perovskite structure [11,12]. Moreover, scanning transmission microscopy-energy dispersive spectroscopy (STEM-EDS) detected no trace of Cl´ in the perovskite. Even though there is a strong Cl´ signal, no N was observed indicting the presence of only PbCl2 [13]. Thus, Cl´ only appears at the interfacebetweenMAPbI3 andtheanode. Twomorereportshavefurtherconfirmed thisconclusion. XPSanalysisshowedonlyweakCl´ signalafteretchingthesurface of MAPbI3-xClx by a 50 nm thickness [14]. Hard X-ray photoelectron spectroscopy andfluorescenceyieldX-rayabsorptionspectroscopyshowednoCl´at thesurfaceof MAPbI3-xClx withhigheraverageconcentrationofClthroughouttheperovskite layerat thedeepbeneath[15]. Here,werefer toMAPbI3-xClx asMAPbI3 that ispreparedusing chloride-containingprecursors. However,as theconditionfordepositingMAPbI3-xClx differs, Cl´may still remain in the resulting perovskite layer. For instance, X-ray absorptionnearedgestructure(XANES)resultsshowedthatx=0.05˘0.03Clatoms per formula unit remain in the films after annealing at 95 ˝C for 120 min [16]. The results fromphotothermal inducedresonance(PTIR)showedthat theMAPbI3-xClx film consistsofamixtureofCl-rich(xlocal <0.3)andCl-poorphasesafteramildannealing (60˝C,60min)andhomogeneousCl-poorer(xlocal <0.06)phaseuponfurtherannealing (110˝C)[17]. Inaddition,first-principlescalculationresultsprovidesomegoodexplanation. For the crystal structure, Cl´ concentration was found below 3%–4% [8] and if the Cl´ ions enter the crystal structure, they preferentially occupy the apical positions in the PbI4X2 octahedra [18]. For the electronic property, while the molecular orientations of CH3NH3+ result in three times larger photocurrent responsethantheferroelectricphotovoltaicBiFeO3,Cl´ substitutionattheequatorial site inducesa largerresponse thandoessubstitutionat theapical site [19]. Results also showed that, using Cl´ precursor can avoid forming the PbI defects [20]. IntroducingCl´wouldreduce the latticeconstantwhichcan inhibit the formationof interstitial defects [21]. As excitons may be screened by collective orientational motion of the organic cations, Cl´ might hinder this motion and results in 97