A study on the nature of the thermal decomposition of methylammonium lead iodide perovskite, CH3NH3PbI3: an attempt to rationalise contradictory experimental results

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Abstract

Thermal decomposition of methylammonium lead triiodide was determined to proceed through two different pathways driven by thermodynamics vs. kinetics competition.

Abstract

The nature of the gas phase product released during the thermal decomposition of CH ₃NH ₃PbI ₃ (methylammonium lead iodide) to PbI ₂ (lead diiodide) under vacuum is discussed on the basis of thermodynamic predictions, recently published experimental results, and new experiments presented here. From the limited data currently available, the nature of the main decomposition path is not clear because, both, the process releasing HI(g) + CH ₃NH ₂(g) (1) and that leading to NH ₃(g) + CH ₃I(g) (2) were observed under different conditions. Our thermodynamic analysis showed that process (2) is largely favoured for all the CH ₃NH ₃PbX ₃ (X = Cl, Br, I) compounds. However, Knudsen effusion mass spectrometry experiments (temperature range 140–240 °C) showed that HI(g) and CH ₃NH ₂(g) were the predominant species in the vapor, with process (2) occurring to a much smaller extent than suggested by the thermodynamic driving force, thus being of minor importance under effusion conditions. We also found that this process was comparatively enhanced by high temperatures and low effusion rates (high impedance orifice). Our experimental evidence suggested that the thermodynamically favoured process (2) was affected by a significant kinetic hindrance. Overall, the prevailing decomposition path is likely to markedly depend on the actual operative conditions.

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Thermal degradation of CH3NH3PbI3perovskite into NH3and CH3I gases observed by coupled thermogravimetry–mass spectrometry analysis

Emilio Juarez-Perez, Zafer Hawash, Sonia Raga … (2016)

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Making and Breaking of Lead Halide Perovskites.

Joseph Manser, Makhsud Saidaminov, Jeffrey A Christians … (2016)

A new front-runner has emerged in the field of next-generation photovoltaics. A unique class of materials, known as organic metal halide perovskites, bridges the gap between low-cost fabrication and exceptional device performance. These compounds can be processed at low temperature (typically in the range 80-150 °C) and readily self-assemble from the solution phase into high-quality semiconductor thin films. The low energetic barrier for crystal formation has mixed consequences. On one hand, it enables inexpensive processing and both optical and electronic tunability. The caveat, however, is that many as-formed lead halide perovskite thin films lack chemical and structural stability, undergoing rapid degradation in the presence of moisture or heat. To date, improvements in perovskite solar cell efficiency have resulted primarily from better control over thin film morphology, manipulation of the stoichiometry and chemistry of lead halide and alkylammonium halide precursors, and the choice of solvent treatment. Proper characterization and tuning of processing parameters can aid in rational optimization of perovskite devices. Likewise, gaining a comprehensive understanding of the degradation mechanism and identifying components of the perovskite structure that may be particularly susceptible to attack by moisture are vital to mitigate device degradation under operating conditions. This Account provides insight into the lifecycle of organic-inorganic lead halide perovskites, including (i) the nature of the precursor solution, (ii) formation of solid-state perovskite thin films and single crystals, and (iii) transformation of perovskites into hydrated phases upon exposure to moisture. In particular, spectroscopic and structural characterization techniques shed light on the thermally driven evolution of the perovskite structure. By tuning precursor stoichiometry and chemistry, and thus the lead halide charge-transfer complexes present in solution, crystallization kinetics can be tailored to yield improved thin film homogeneity. Because degradation of the as-formed perovskite film is in many ways analogous to its initial formation, the same suite of monitoring techniques reveals the moisture-induced transformation of low band gap methylammonium lead iodide (CH3NH3PbI3) to wide band gap hydrate compounds. The rate of degradation is increased upon exposure to light. Interestingly, the hydration process is reversible under certain conditions. This facile formation and subsequent chemical lability raises the question of whether CH3NH3PbI3 and its analogues are thermodynamically stable phases, thus posing a significant challenge to the development of transformative perovskite photovoltaics. Adequately addressing issues of structural and chemical stability under real-world operating conditions is paramount if perovskite solar cells are to make an impact beyond the benchtop. Expanding our fundamental knowledge of lead halide perovskite formation and degradation pathways can facilitate fabrication of stable, high-quality perovskite thin films for the next generation of photovoltaic and light emitting devices.