Benign and detrimental chemical decomposition routes depleting methylammonium cation in hybrid perovskites
ARAID, Government of Aragon, Zaragoza, Spain Institute of Nanoscience of Aragon (INA), University of Zaragoza, Spain, Campus Río Ebro, Edif. I+D, C/ Mariano Esquillor Gómez, s/n, Zaragoza, 50018, ES
Lead halide based perovskites have emerged as promising active materials for photovoltaic cells. Enormous efforts have been devoted to device fabrication and optimization leading to power conversion efficiencies exceeding 22%, which gives perovskite solar cells the competitive advantage over many other well-known solar technologies. Despite superb efficiencies achieved in laboratory-scale devices, it was soon recognized that long-term stability was rapidly compromised under ambient conditions and such instability could jeopardize the future of perovskite solar cells.
In the present communication, a clear demarcation line between photo- and thermal degradation processes in methylammonium (MA) based perovskite is traced[1-3] including characterization techniques employed identifying decomposition gases released. Based on experimental evidence, a clear distinction between true decomposition reactions and evaporation-like processes suffered by MA is done. The halide effect on stability is discussed in terms of energy barriers during degradation reactions showing a better stability of Br based perovskite ascribed to two aspects: (i) lower Brönsted-Lowry acidity of HBr compared to HI and (ii) higher nucleophilic character of CH3NH2 compared to NH3. Finally, current strategies to overcome the long-term chemical instability in hybrid perovskite are analyzed.
 Juarez-Perez, E. J., Hawash, Z., R. Raga, S., Ono, L. K., Qi, Y., "Thermal degradation of CH3NH3PbI3 perovskite into NH3 and CH3I gases observed by coupled thermogravimetry - mass spectrometry analysis" Energy Environ. Sci. 2016, 9, 3406-3410.
 García-Fernández, A., Juarez-Perez, E. J., Castro-García, S., Sánchez-Andújar, M., Ono, L. K., Jiang, Y., Qi, Y., "Benchmarking chemical stability of arbitrarily mixed 3D hybrid halide perovskites for solar cell applications" Small Methods 2018, , 1800242.
 Juarez-Perez, E. J., Ono, L. K., Maeda, M., Jiang, Y., Hawash, Z., Qi, Y., "Photodecomposition and thermal decomposition in methylammonium halide lead perovskites and inferred design principles to increase photovoltaic device stability" J. Mater. Chem. A 2018, 6, 9604-9612.
 Juarez-Perez, E. J., Ono, L. K., Uriarte, I., Cocinero, E. J., Qi, Y., "Degradation Mechanism and Relative Stability of Methylammonium Halide Based Perovskites Analyzed on the Basis of Acid-Base Theory" ACS Appl. Mater. Interfaces 2019, 11, 12586-12593.