We studied ion transport in hybrid organic inorganic perovskite p-in devices as a function of applied bias under device operating conditions. Using electrochemical impedance spectroscopy (EIS) and equivalent circuit modeling, we elucidated various resistive and capacitive elements in the device. We show that ion migration is predictably influenced by a low applied forward bias, characterized by an increased capacitance at the hole transporting (HTM) and electron transporting material (ETM) interfaces, as well as through the bulk. However, unlike observations in n-i-p devices, we found that there is a capacitive discharge leading to possible ion redistribution in the bulk at high forward biases. Furthermore, we show that a chemical double layer capacitance buildup as a result of ion accumulation impacts the electronic properties of the device, likely by either inducing charge pinning or charge screening, depending on the direction of the ion induced field. Lastly, we extrapolate ion diffusion coefficients (~10-7 cm 2 s-1) and ionic conductivities (~10-7 S cm-1) from the Warburg mass (ion) diffusion response, and show that, as the device degrades, there is an overall depletion of capacitive effects coupled with an increased ion mobility. devices at different biases, X-ray diffraction of MAPbI3 under heat and illumination, X-ray diffraction of two-step method, evidence of Warburg diffusion, EIS fit values for one-step and two-step preparation methods, ionic conductivity (σion) values for one-step method, diffusion coefficients for both one and two-step method, comparison of geometric and double layer capacitance for two-step method, comparison of one-step and two-step methods, J-V characteristics of one-step and two-step method, double layer capacitance for devices with different HTMs, equivalent fit trends for the one-step method (PDF).

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 the substituted derivatives are autocatalytic, and we calculated rate coefficients for the degradation. All of the substituted hybrid organic-inorganic perovskites showed slower light-induced degradation compared to 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.

We demonstrate that using a hole transport bilayer composed of poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) and CuI leads to highly efficient and stable inverted, that is, p−i−n, perovskite solar cells (PSCs). The bilayerbased devices had an average power conversion efficiency (PCE) of 19.4% and a maximum PCE of 20.34%. In comparison, devices with a single PTAA interlayer showed an average PCE of 17.7%. Perovskite films fabricated on PTAA/CuI bilayers showed improved crystallinity and larger grain sizes. Data from ultraviolet photoelectron spectroscopy and Mott−Schottky analysis of impedance suggest an increased built-in potential within the device with enhanced upward band bending at the CuI interface. The bilayered HTL devices have minimum current−voltage hysteresis and stable current output at the maximum power point. These devices were stable for more than 4500 h under ambient light in an inert atmosphere.

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Using ion-selective impedance spectroscopy to study cation transport in hybrid organic–inorganic perovskites.