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Spatially Heterogeneous Chlorine Incorporation in Organic-Inorganic Perovskite Solar Cells Yanqi Luo, Shany Gamliel, Sally Nijem, Sigalit Aharon, Martin Holt, Benjamin Stripe, Volker Rose, Mariana I. Bertoni, Lioz Etgar, and David P Fenning Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b02065 • Publication Date (Web): 30 Aug 2016 Downloaded from http://pubs.acs.org on September 2, 2016
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Chemistry of Materials
Spatially Heterogeneous Chlorine Incorporation in Organic-Inorganic Perovskite Solar Cells Yanqi Luo,a Shany Gamliel,b Sally Nijem,b Sigalit Aharon,b Martin Holt,c Benjamin Stripe,d Volker Rose,c,d Mariana I Bertoni,e Lioz Etgar,b and David P. Fenning a,* a, *
Department of Nanoengineering, University of California San Diego, La Jolla, CA 92093 Tel: (858) 246-0864; E-mail:
[email protected] b The Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, 91904, Israel c Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Il 60439 d Advanced Photon Source, Argonne National Laboratory, Argonne, Il 60439 e School of Electrical, Computer and Energy Eng., Arizona State University, Tempe, AZ 85287 ABSTRACT: Spatial heterogeneities in the chemical makeup of thin film photovoltaic devices are pivotal in determining device efficiency. We report the in-plane spatial distribution and degree of chlorine incorporation in organic-inorganic lead halide perovskite absorbers by means of non-destructive synchrotron-based nanoprobe X-ray fluorescence. The presence of chlorine is positively identified in CH3NH3PbI3 films synthesized with Cl-containing precursors and as an impurity in some films synthesized with nominally Cl-free precursors. The impurity may be introduced from precursors or as contaminants during film synthesis. The films formed from Cl-containing precursors contain roughly an order of magnitude higher amount of chlorine, with Cl:I values greater than 0.02 found whether Cl is present in either the organic or the inorganic precursor for both one- and two-step fabrication processes. A spatial variation in the Cl incorporation is observed within single particles and as well as between particles within a given film, and the standard deviation of the Cl:I ratio across the films is up to 30% of the average value. Understanding and controlling the heterogeneous distribution of chlorine in hybrid perovskite layers may offer a path to improve their photovoltaic performance.
The rapid development of solid-state organometal halide perovskite solar cells has garnered significant attention to the relationship between their hybrid chemistry and their exceptional optoelectronic properties1–3 that could enable wide-ranging potential application.4–7 The power conversion efficiency of perovskite solar cells has risen to above 22% (independently certified) with extensive optimization of film properties and device architectures.8–12 The performance of this organic-inorganic halide cell is related to the optoelectronic properties of the organic lead halide absorber layer, most commonly CH3NH3PbX3 (X = I and Br), and can be influenced by chemical composition, film morphology and device architecture.7,13–17 Recent reports have shown that using chloride containing salts or precursors (methylammonium chloride, MACl, and/or PbCl2) can assist perovskite crystal growth because chloride ions or the formation of PbCl2 nanocrystals (in a one-step deposition) can act as a nucleation center11,18 due to size of Cl and PbCl2 solubility. Additionally, the presence of MACl can also enhance the adsorption of lead iodide perovskite absorber on the TiO2 substrate and produce large oriented crystallite domains by allowing slow crystal growth.18,19 In perovskite solar cell synthesis, during the final annealing step the majority of the chlorine sublimes, resulting in a low degree of chlorine incorporation.20 A number of reports with varying film preparations have failed to identify Cl incorporation with X-ray photoelectron spectroscopy (XPS) or energy
dispersive X-ray spectroscopy (EDS),21–23 although bulk incorporation of chlorine as CH3NH3PbI3-xClx with x
