Hybrid Perovskites with Larger Organic Cations Reveal Autocatalytic Degradation Kinetics and Increased Stability under Light.

Hybrid organic-inorganic perovskites have shown incredible promise as active materials for photovoltaic devices, but their instability to light remains a significant roadblock in realizing these applications. Changing the organic cation has been shown to affect light-induced degradation. As a strategy for increasing the stability of these materials, we replaced varying percentages of methylammonium ion in the archetypical methylammonium lead iodide (MAPbI3) hybrid organic-inorganic perovskite with three significantly larger organic ammonium cations: imidazolium, dimethylammonium, and guanidinium. We were able to synthesize hybrid organic-inorganic perovskites with the same 3D perovskite structure as MAPbI3 with substitution of the larger ions as high as 20-30%. These substituted hybrid organic-inorganic perovskites retained similar optoelectronic properties. We discovered that the light-induced degradation in MAPbI3 and its substituted derivatives is autocatalytic, and we calculated rate coefficients for the degradation. All of the substituted hybrid organic-inorganic perovskites showed light-induced degradation slower than that of MAPbI3, up to a 62% decrease in degradation rate coefficient, at all substitution percentages up to 20%. This work provides evidence that a high percentage of a variety of large ammonium cations can be substituted into the hybrid organic-inorganic perovskite lattice without compromising its desirable optoelectronic properties. Insight into the autocatalytic mechanism of light-induced degradation will be valuable for designing additional strategies to improve the stability of hybrid organic-inorganic perovskites. We also offer insights into how factors other than size, such as hydrogen bonding, influence the stability of the materials. Overall, we have shown that substitution of methylammonium ion for the much larger imidazolium, dimethylammonium, and guanidinium cations in MAPbI3 is a valid strategy for creating stable hybrid organic-inorganic perovskite derivatives by slowing the rate of light-induced degradation.

Correction: The use of ion-selective membranes to study cation transport in hybrid organic-inorganic perovskites.

Correction for 'The use of ion-selective membranes to study cation transport in hybrid organic-inorganic perovskites' by Emily C. Smith et al., Phys. Chem. Chem. Phys., 2019, 21, 20720-20726.

The use of ion-selective membranes to study cation transport in hybrid organic-inorganic perovskites.

Using a methylammonium selective membrane in conjunction with electrochemical impedance spectroscopy, we measured ion migration in methylammonium lead triiodide (MAPbI3) with a millisecond (ms) time constant under illumination. These values were consistent with the reported values of ionic conduction in thin-film perovskite solar cells. We monitored an electrochemical impedance response arising from ionic conductivity through MAPbI3 and a methylammonium selective layer. We could fit this complex impedance response to an intuitive circuit model, which revealed an ionic species moving on a ms time scale. Electrospray ionization mass spectrometry (ESI-MS) revealed direct chemical evidence of methylammonium diffusion into the ion-selective layer. We found no experimental evidence indicating the mobility of lead ions or protons, suggesting that the mobile species observed under illumination is likely methylammonium.