Seite - 98 - in Photovoltaic Materials and Electronic Devices

better transport properties [22]. Little difference of electronic properties was represent among orthorhombic, tetragonal and cubic phases of MAPbI3 [23], however, the valance-band-maximum and conduction-band-minimum states can be mainly derived from iodine ions at some unique positions, Cl´ substitution can strengthen the unique position of the ions and result in more localized charge density [24]. Thus, lower carrier recombination rate and enhanced carrier transport ensued. For the interface, the (001) and (110) surfaces tend to favor hole injection to 2,21,7,71-tetrakis(N,N-di-p-methoxyphenylamine)-9,91-spirobifluorene (Spiro-MeOTAD), while the (100) surface facilitates electron transfer to [6,6]-phenyl C61-butyric acid methyl ester (PCBM) [25]. A better structural matching between adjacent rows of perovskite surface halides and TiO2 under coordinated titanium maybethereasonfor the (110)-orientedgrowthofMAPbI3-xClx andMAPbI3 [26]. InterfacialCl´maythus furtherstabilize the (110)surfaceandmodify the interface electronicstructurebetweenMAPbI3 andTiO2 [26]. Despite the absence of Cl´ in the perovskite, it still played an important role in the crystallization process. For instance, the morphology of MAPbI3-xClx was compared with MAPbI3 [27] and a model in which the Cl´ rich phase modifies the morphologies of perovskite was proposed and fit well with the results from scanning electronmicroscopy(SEM)[27]. Inaddition, thetransmissionelectronmicroscopy(TEM) offreeze-driedperovskiteMAPbI3-xClx precursorsolutionshowedthepresenceofPbCl2 nanoparticles [28] and this is in agreement with the dynamic light scattering (DLS) investigations of MAPbI3-xClx precursor solution [29]. Thus, references [28,29] further proved the model of the heterogeneous nucleation by PbCl2 nanoparticles proposed in reference [27]. However, the formation mechanism of the crystal structure remains underminedandthiswillbediscussedinthefollowingpartsofthisarticle. 2. MethodsforFabricatingMAPbI3-xClx In Section 3, we discuss the crystal structure of MAPbI3-xClx according to the depositionmethod. Asthefabricationmethodswerediscussedindetail inreference[30], here we add a brief introduction about the preparation methods of MAPbI3-xClx. For the one-step deposition method, MAI:PbI2/PbCl2 (molar ratio 1:1 or 3:1) [31,32] were dissolved in γ-butyrolactone (GBL) or DMF, spin-coated on the substrates and annealed to form perovskite. Different annealing conditions result in different morphology of the MAPbI3-xClx layer. While a rapid thermal annealing at 130 ˝C resulted in micron-sized perovskite grains [33], two-step annealing, such as 90 ˝C for 30 min then at 100 ˝C for 2 min [34] or 60 ˝C then ramping to 90 ˝C [35], resultedinoptimalPCEonpoly(3,4-ethylenedioxythiophene)poly(styrene-sulfonate) (PEDOT:PSS) substrates. A full coverage of perovskite can be achieved by rapid cooling after annealing [36]. To increase the solubility of Cl´, 1,8-diiodooctane [37] orotheralkylhalideadditives[38]ordimethylsulfoxide[9]canbeemployed. Adding 98