Reaction Pathways and Kinetics during the Formation of Mixed-halide Perovskites by In-situ Optical Spectroscopy combined with In-situ X-ray Spectroscopy and Diffraction
Thomas Unold a, Charles J. Hages a, Pascal Becker a, Justus Just a, Eva Unger a b, Klara Suchan a b
a Lund University, Sweden, Kämnärsvägen 10H, Lund, 22645, Sweden
Proceedings of Perovskite Thin Film Photovoltaics (ABXPV17)
València, Spain, 2017 March 1st - 2nd
Organizers: Henk Bolink and David Cahen
Oral, Justus Just, presentation 027
Publication date: 18th December 2016

Solution based processes show great potential for synthesis of mixed-halide perovskite thin-films, as they do not need cost intensive vacuum technology and no high-temperature annealing steps. However little is known about the exact mechanisms during film formation. We present a detailed investigation of the chlorine derived synthesis of MAPbI3 perovskite. By simultaneously applying synchrotron based in-situ quick scanning X-ray absorption spectroscopy (QEXAFS), X-ray diffraction (XRD) as well as X-ray fluorescence (XRF) in combination with in-situ optical reflection and photoluminescence spectroscopy a conclusive picture of the film formation can be drawn. Product and educt phases as well as their reaction kinetics were identified by combining these measurements with ex-situ high energy resolution fluorescence detected (HERFD)-XAS. The reaction is conducted in a custom designed in-situ reactor with control of temperature and atmosphere. Using this set of complementary real-time time -resolved characterization techniques simultaneously, we can correlate the evolution of the chemical reaction from XAS, the formation of crystalline phases from XRD and the chemical composition from XRF with the evolution of the optoelectronic properties of the film on one single timeline.Combining complementary results we are able to draw a comprehensive picture describing the film formation and reaction route. Furthermore, the combination of compositional and structural properties with the evolution of optoelectronic properties allows a meaningful interpretation of the optoelectronic data. Hence, we are able to correlate the delayed formation of crystalline MAPbI3 with the decreasing chlorine content of the sample and will present a detailed view on the kinetics of all involved formation and decomposition reactions. Correlating changes of the optoelectronic properties with simultaneously measured structural and compositional properties allows an interpretation of lab-based in-situ optical spectroscopy. Our results give insight into the detailed formation process and can help to provide a mechanistic understanding of the reactions and intermediates involved in the formation of MAPbI3 from Cl-containing precursors. This knowledge could be utilized to optimize synthesis strategies and tunability of the optoelectronic properties of organic-metal-halide perovskite semiconductors.

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