title: Unreacted PbI2 as a Double-Edged Sword for Enhancing the Performance of Perovskite Solar Cells.
abstract: Lead halide perovskites have over the past few years attracted considerable interest as photo absorbers in PV applications with record efficiencies now reaching 22%. It has recently been found that not only the composition but also the precise stoichiometry is important for the device performance. Recent reports have, for example, demonstrated small amount of PbI2 in the perovskite films to be beneficial for the overall performance of both the standard perovskite, CH3NH3PbI3, as well as for the mixed perovskites (CH3NH3)x(CH(NH2)2)(1-x)PbBryI(3-y). In this work a broad range of characterization techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), photo electron spectroscopy (PES), transient absorption spectroscopy (TAS), UV-vis, electroluminescence (EL), photoluminescence (PL), and confocal PL mapping have been used to further understand the importance of remnant PbI2 in perovskite solar cells. Our best devices were over 18% efficient, and had in line with previous results a small amount of excess PbI2. For the PbI2-deficient samples, the photocurrent dropped, which could be attributed to accumulation of organic species at the grain boundaries, low charge carrier mobility, and decreased electron injection into the TiO2. The PbI2-deficient compositions did, however, also have advantages. The record Voc was as high as 1.20 V and was found in PbI2-deficient samples. This was correlated with high crystal quality, longer charge carrier lifetimes, and high PL yields and was rationalized as a consequence of the dynamics of the perovskite formation. We further found the ion migration to be obstructed in the PbI2-deficient samples, which decreased the JV hysteresis and increased the photostability. PbI2-deficient synthesis conditions can thus be used to deposit perovskites with excellent crystal quality but with the downside of grain boundaries enriched in organic species, which act as a barrier toward current transport. Exploring ways to tune the synthesis conditions to give the high crystal quality obtained under PbI2-poor condition while maintaining the favorable grain boundary characteristics obtained under PbI2-rich conditions would thus be a strategy toward more efficiency devices.
authors.affiliation: University of Cambridge , Department of Chemistry, Lensfield Road, Cambridge CB2 1EW, U.K.|Laboratory for Photomolecular Science, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne , CH-1015-Lausanne, Switzerland.
authors.forename: T Jesper
authors.initials: TJ
authors.lastname: Jacobsson
authors.affiliation: Laboratory for Photomolecular Science, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne , CH-1015-Lausanne, Switzerland.
authors.forename: Juan-Pablo
authors.initials: JP
authors.lastname: Correa-Baena
authors.affiliation: Laboratory for Photomolecular Science, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne , CH-1015-Lausanne, Switzerland.|Department of Materials Engineering, Isfahan university of Technology , Isfahan, 84156-83111, Iran.
authors.forename: Elham
authors.initials: E
authors.lastname: Halvani Anaraki
authors.affiliation: Department of Physics and Astronomy, Uppsala University , Box 516, 75120 Uppsala, Sweden.
authors.forename: Bertrand
authors.initials: B
authors.lastname: Philippe
authors.affiliation: Research Laboratory of Electronics, Massachusetts Institute of Technology , 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States.|Cavendish Laboratory , JJ Thomson Avenue, Cambridge CB3 0HE, U.K.
authors.forename: Samuel D
authors.initials: SD
authors.lastname: Stranks
authors.affiliation: Photochemical Dynamics Group, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne , CH-1015-Lausanne, Switzerland.
authors.forename: Marine E F
authors.initials: ME
authors.lastname: Bouduban
authors.affiliation: Laboratory of Photonics and Interfaces, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne , CH-1015-Lausanne, Switzerland.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Tress
authors.affiliation: École Polytechnique Fédérale de Lausanne , CH-1015-Lausanne, Switzerland.
authors.forename: Kurt
authors.initials: K
authors.lastname: Schenk
authors.affiliation: Photochemical Dynamics Group, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne , CH-1015-Lausanne, Switzerland.
authors.forename: Joël
authors.initials: J
authors.lastname: Teuscher
authors.affiliation: Photochemical Dynamics Group, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne , CH-1015-Lausanne, Switzerland.
authors.forename: Jacques-E
authors.initials: JE
authors.lastname: Moser
authors.affiliation: Department of Physics and Astronomy, Uppsala University , Box 516, 75120 Uppsala, Sweden.
authors.forename: Håkan
authors.initials: H
authors.lastname: Rensmo
authors.affiliation: Laboratory for Photomolecular Science, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne , CH-1015-Lausanne, Switzerland.|Department of Chemistry - Ångström Laboratory, Uppsala University , Box 538, 75121 Uppsala, Sweden.
authors.forename: Anders
authors.initials: A
authors.lastname: Hagfeldt