Proceedings of International Conference on Perovskite Thin Film Photovoltaics, Photonics and Optoelectronics (ABXPV18PEROPTO)
Publication date: 11th December 2017
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 methylammonium (CH3NH3+ - MA) and formamidinium (HC(NH2)2+ - FM) lead iodide perovskite films using visible and ultra violet light in oxygen atmosphere and in vacuum. Insight into the degradation mechanisms was obtained from in-situ Fourier-transform infrared absorption (FT-IR), photoluminescence, and gas effusion measurements. We revisited the light-induced degradation of MAPbI3 in the presence of oxygen. Illumination in O2 atmosphere results in a swift degradation. Isotope experiments clearly show that O2 acts as a catalyst decomposing MA ions into CH3NH2 and hydrogen. In case of FMPbI3 perovskites illumination in the presence of O2 results in a more complex reaction; decomposition of the FM ions occurs at the N – C – N bonds and as a result CO2 and C = O molecules are formed that rapidly diffuse out of the crystalline lattice.
In addition, we present experimental evidence of a hitherto unknown but fundamental degradation mechanism of MAPbI3 and FMPbI3 perovskite layers due to exposure to visible and ultra violet light. This degradation mechanism does not require the presence of oxygen or other constituents. Prolonged illumination causes the dissociation of MA ions into molecular hydrogen and CH3NH2. Interestingly, FM ions also decompose into CH3NH2. The resulting molecules are highly mobile at room temperature and diffuse out of the perovskite layer. As a result, the concentration of localized defects increases and quenches the photoluminescence. Our data indicate that the molecular orbitals of the organic ions are not in resonance with the energy bands of the perovskite.
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