Abstract
The interest of the scientific community on methylammonium lead halide perovskites (MAPbX3, X = Cl, Br, I) for hybrid organic-inorganic solar cells has grown exponentially since the first report in 2009. This fact is clearly justified by the very high efficiencies attainable (reaching 20% in lab scale devices) at a fraction of the cost of conventional photovoltaics. However, many problems must be solved before a market introduction of these devices can be envisaged. Perhaps the most important to be addressed is the lack of information regarding the thermal and thermodynamic stability of the materials towards decomposition, which are intrinsic properties of them and which can seriously limit or even exclude their use in real devices. In this work we present and discuss the results we obtained using non-ambient X-ray diffraction, Knudsen effusion-mass spectrometry (KEMS) and Knudsen effusion mass loss (KEML) techniques on MAPbCl3, MAPbBr3 and MAPbI3. The measurements demonstrate that all the materials decompose to the corresponding solid lead (II) halide and gaseous methylamine and hydrogen halide, and the decomposition is well detectable even at moderate temperatures (~60 °C). Our results suggest that these materials may be problematic for long term operation of solar devices.
Highlights
We present the results of two parallel investigations on MAPbCl3, MAPbBr3 and MAPbI3
Thermodynamic data necessary to assess the stability of the compounds in diverse conditions, such those encountered during the realization of the devices and during their operation
The diffractograms of MAPbCl3, MAPbBr3 and MAPbI3 are presented in Fig. 1 and all of them show only the reflections of the desired compounds[12,13], with no lead (II) halides detected
Summary
The basic experimental data are the ion intensities, Ii+, recorded as a function of the temperature of the molecular source These can be converted into partial pressures of the corresponding neutral species in the Knudsen cell through the relation[25]: Pi = k f i Ii+T (16). The effusion source, whose mass is monitored, was modified in our laboratory[29] in order to allow an optimal temperature measurement and to maximize the uniformity of the sample temperature Both the Knudsen cell and a Pt100 platinum resistance thermometer are inserted into a capped copper cylinder; so that the temperature of the molecular source is directly measured instead of the usual “dummy” cell placed in the isothermal section of the furnace.
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