Page - 96 - in Photovoltaic Materials and Electronic Devices

CrystalStructureFormationof CH3NH3PbI3-xClx Perovskite ShiqiangLuoandWalidA.Daoud Abstract: Inorganic-organic hydride perovskites bring the hope for fabricating low-cost and large-scale solar cells. At the beginning of the research, two open questionswereraised: thehysteresiseffectandtheroleofchloride. Thepresenceof chloridesignificantly improvesthecrystallizationandchargetransferpropertyof the perovskite. However, thoughthe longhelddebateoverof theexistenceofchloride in theperovskiteseemstohavenowcometoaconclusion,nopriorworkhasbeen carried out focusing on the role of chloride on the electronic performance and the crystallizationof theperovskite. Furthermore,currentreportsonthecrystalstructure of the perovskite are rather confusing. This article analyzes the role of chloride in CH3NH3PbI3-xClx onthecrystalorientationandprovidesanewexplanationabout the (110)-orientedgrowthofCH3NH3PbI3 andCH3NH3PbI3-xClx. ReprintedfromMaterials. Cite as: Luo,S.; Daoud,W.A.CrystalStructureFormation ofCH3NH3PbI3-xClx Perovskite.Materials2016,9, 123. 1. Introduction Since the first organic-inorganic halide perovskite solar cell was reported [1], perovskites have attracted growing interest and the power conversion efficiency (PCE) has reached 20.1% [2]. It is not very common that a photovoltaic device canexperiencesucharapiddevelopment. While thestructureof thecellsevolved from sensitized meso-structure to planar structure [3], both inorganic and organic materials can be applied as electron and hole transfer materials [4]. Furthermore, by tuning the composition of the perovskite, the band gap can be easily modified [5]. Giventhenumerousadvantagesofperovskite,aclearunderstandingof thecrystal structure is crucial and the role of chloride in the formation of CH3NH3PbI3-xClx (hereafter,weuseMAshort forCH3NH3) isoneof themostpressingtopics. Ithasbeenreportedthat thepresenceofchlorideintheperovskite improvesthe uniformity of its layer [6] and results in an increase of the carriers’ diffusion length fromca.100nmtoover1µm[7]. However, thelonghelddebateoverof theexistence of chloride in the perovskite seems to have now come to a conclusion. First, when synthesizingtheperovskitebytheonestepmethodwithprecursorsolutionofMACl andPbI2 (1:1molarratio) inanhydrousN,N-dimethylformamide(DMF), theresulting crystal isnotMAPbI2ClbutamixtureofMAPbI3 andMAPbCl3 [8]. Thisprovidesdirect evidencethatchloride(Cl´) cannotsubstitute iodine(I´) in theperovskite to forma 96