Proceedings of nanoGe International Conference on Perovskite Solar Cells, Photonics and Optoelectronics (NIPHO19)
DOI: https://doi.org/10.29363/nanoge.nipho.2019.029
Publication date: 21st November 2018
For some years organic-inorganic perovskites have attracted great interest due to their outstanding electrical and optical properties. Because of their large absorption coefficient, high carrier mobility, and long carrier diffusion length this class of materials is very attractive for opto-electronic applications such as light emitting devices and solar cells. Perovskite solar cells experienced a remarkable development in recent years. Their power conversion efficiency increased from single digit values to more than 22 %. Despite of the high-power conversion efficiencies organic-inorganic perovskites suffer from a number of instability mechanisms. To improve the stability of perovskite solar cells containing these absorbers a fundamental understanding of the governing mechanisms is advantageous.
In this paper we investigate the stability of halide perovskite absorber layers containing methylammonium (CH3NH3+ - MA) formamidinium (HC(NH2)2+ - FA), and cesium on the lattice sites of the organic cations. The specimens investigated contain one, two, or all three cations. The samples are prepared by state-of-the-art spin coating processes in nitrogen atmosphere. The thermal stability of the absorber layers is measured by gas evolution measurements. Here, the specimens are mounted in a vacuum tube and then the temperature is increased with a rate of 20 K/min, while gaseous species such as C-Hx and N-Hx are measured with a quadrupole mass-spectrometer. Perovskites containing only MA exhibit a thermal degradation threshold of about 70 °C. This threshold increases to larger temperatures for FAPbI3 (T = 90 °C) and triple cation perovskites containing MA, FA, and Cs indicating that either the larger molecules for FA or changes in the lattice constants due to the presence of different cations results in a significant improvement of the thermal stability.
Further insight into the improved stability of the perovskites were gained from Fourier-transform infrared absorption (FT-IR) that were performed in vacuum and nitrogen atmosphere prior to illumination with visible and UV light and after the illumination experiments. Interestingly, FT-IR absorption shows the decomposition of the organic cations due to illumination at room temperature. In case of MA the N-H bonds dissociate, while for FA containing perovskites the N – C – N backbone breaks upon illumination. From a thermal point of view this is unexpected. The implication of these results for devices and routes to obtain more stable perovskites will be discussed
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