A Comparison of Thermal Stabilities of Methylammonium Lead Iodide and Methylammonium Lead Bromide Perovskites Using X-Ray Photoelectron Spectroscopy.
Udit Tiwari a, Karen Syres a, Andrew Thomas b, Mark Jackman b, Alexander Generalov c, David Lewis b, Michael Wagstaffe d
a University of Central Lancashire, University of Central Lancashire, Preston PR1 7QR, UK, Preston, United Kingdom
b University of Manchester, MSS Tower, Manchester, United Kingdom
c Lund University, Sweden, Kämnärsvägen 10H, Lund, 22645, Sweden
d DESY - Deutsches Elektronen-Synchrotron, Hamburg, Notkestraße, 85, Hamburg, Germany
Proceedings of International Conference on Frontiers in Electrocatalytic Transformations (INTERECT)
València, Spain, 2021 November 22nd - 23rd
Organizers: Elena Mas Marzá and Ward van der Stam
Poster, Udit Tiwari, 007
Publication date: 10th November 2021

Metal halide perovskites are a new class of semiconductor material which show great promise for thin film solar cells. Although, they have efficiencies competitive with monocrystalline silicon, the advantages of flexibility, low cost, and easy processing, makes them superior candidates over conventional inorganic solar cells [1]. Perovskite based thin film solar cells have now reached an efficiency of 25.6% as of April 2021 [2]. Perovskite materials are distinguished by tuneable bandgap [3], ambipolar charge transport [4], long carrier lifetimes [5], and long charge diffusion lengths [6]. At present, the most studied organic-inorganic perovskites are represented by the formula CH3NH3PbX3 where X is a halide ion (Cl, Br, or I).

The major obstacle facing perovskite solar cells is their limited service life under operating conditions owing to the deterioration of the absorber material in contact with humidity, heat, and light [7, 8]. Understanding the degradation mechanism in perovskites is of crucial importance for device design and optimization. The degradation mechanism of perovskites has been a topic of extensive exploration [9-16]. Although various degradation pathways have been proposed, the exact route is still under debate. A few XPS studies have shown the presence of metallic lead in the perovskite films but the pathway of lead precipitation is not very well understood [17].

In this study, we aim to compare the thermal degradation mechanism of the two main lead halide perovskites: CH3NH3PbI3 (MAPbI3) and CH3NH3PbBr3 (MAPbBr3) using X-ray photoelectron spectroscopy (XPS). The samples were heated to around 200 °C and XPS data was recorded before and after heating. We observed that the perovskite peak in the MAPbI3 C1s spectra changes to MA+ after heating, whereas the perovskite peak still remains after heating in the case of the MAPbBr3 sample. This suggests that the perovskite phase in the MAPbI3 sample fully degraded upon heating but the perovskite phase in the MAPbBr­­3 sample only partially degraded. Similarly, in the N 1s spectra of the MAPbI3 sample the perovskite peak disappeared after heating and a new peak corresponding to the binding energy of MAI was observed post heating. In contrast, the perovskite peak was seen before and after heating in MAPbBr3 sample as observed in the C1s spectra. We also observed metallic lead peaks (as has been previously reported) in both the samples before and after heating. However, the intensity of the peaks increased post heating indicating that thermal stress accelerated the precipitation of metallic lead. We observed a decrease in the concentration of iodine and nitrogen after heating and an increase in the surface lead concentration in the MAPbI3 sample. In contrast, only slight changes in concentration were observed in the MAPbBr3 spectra. Based on these observations we propose a three-step degradation mechanism of perovskites. The degradation mechanism is used to explain the observed concentration changes and the role of halide ions in the structural stability of perovskites.

The experimental measurements were carried out at beamline D1011 at MAX-lab synchrotron facility in Sweden  The authors would like to thank DTA3 COFUND H2020/Marie Skłodowska Curie PhD Fellowship programme for partially funding this work. (Grant Agreement Number: 801604). 

© Fundació Scito
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