Chlorine Presence in PbCl2-Derived Hybrid-Perovskite Solar Absorbers

Vanessa Pool^a, Christopher Tassone^a, Kevin Stone^a, Michael Toney^a, Aryeh Gold-Parker^a^,^b, Andrea Bowring^b, Michael McGehee ^b

^aSLAC National Accelerator Laboratory, 2575 Sand Hill Road MS 69, Menlo Park, 94025, United States

^bStanford University, Stanford, CA 94305, United States

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Vanessa Pool, 046
Publication date: 5th February 2015

One of the outstanding questions in the field of hybrid organic-inorganic perovskites (CH₃NH₃PbI_3-xCl_x) is the role that Cl plays in improving device performance. It has been shown that the presence of Cl affects the morphology of the film; however, the presence of Cl in the final film and its impact has been the subject of some debate. We have investigated the presence and chemical state of chlorine in PbCl₂-derivedmethylammonium lead trihalide perovskite thin films (CH₃NH₃PbI_3-xCl_x) using X-ray absorption near edge spectroscopy (XANES) of the Cl K-edge. Films were spin-cast from a 1:3 molar mixture of PbCl₂ and CH₃NH₃I in DMF, spin-coated on a Si₃N₄ windows to form a ~200nm thick film annealed in dry air at 95°C. Films were annealed at 0 min, 15min, 60min, 90min and 120min (fully converted). Using XANES at the chlorine K-edge and Pb M₃-edge, the amount of Cl remaining in the sample was quantified. This was done by comparing the Cl:Pb edge jump ratio, which is proportional to the quantity of Cl:Pb, and using PbCl₂ as a reference for this proportionality. We find that significant chlorine is present in all films, with a concentration of x=0.05±0.02 chlorine atoms per formula unit after complete anneal. Linear combination fitting of the XANES rules out the possibility that the remaining Cl is merely leftover precursor material (PbCl₂ and MAI). From this we conclude that the Cl is either substituting on the I site or interstitially, or collecting on boundaries such as grain boundaries or interfaces. We discuss the role of Cl in improving CH₃NH₃PbI_3-xCl_x device behavior.
In red the number of Cl atoms per formula unit in perovskite films annealed at 95°C for different lengths of time.
Colella, S., Mosconi, E., Fedeli, P., Listorti, A., Gazza, F., Orlandi, F., ... & Mosca, R. MAPbI3-xCl x Mixed Halide Perovskite for Hybrid Solar Cells: The Role of Chloride as Dopant on the Transport and Structural Properties. Chemistry of Materials 2013, 25, 22 4613-4618. Tidhar, Y., Edri, E., Weissman, H., Zohar, D., Hodes, G., Cahen, D., ... & Kirmayer, S. Crystallization of methyl ammonium lead halide perovskites: Implications for photovoltaic applications. Journal of the American Chemical Society, 2014, 136, 38, 13249-13256. Yu, H., Wang, F., Xie, F., Li, W., Chen, J., & Zhao, N. (2014). The Role of Chlorine in the Formation Process of “CH3NH3PbI3‐xClx” Perovskite. Advanced Functional Materials, 2014, 24, 45, 7102-7108.

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