Impedance Spectroscopic Indication for Solid State Electrochemical Reaction in (CH₃NH₃)PbI₃ Films

Halide perovskite-based solar cells still have limited reproducibility, stability, and incomplete understanding of how they work. We track electronic processes in [CH₃NH₃]­PbI₃(Cl) (“perovskite”) films <i>in vacuo</i>, and in N₂, air, and O₂, using impedance spectroscopy (IS), contact potential difference, and surface photovoltage measurements, providing direct evidence for perovskite sensitivity to the ambient environment. Two major characteristics of the perovskite IS response change with ambient environment, viz. -1- appearance of negative capacitance <i>in vacuo</i> or post<i>-vacuo</i> N₂ exposure, indicating for the first time an electrochemical process in the perovskite, and -2- orders of magnitude decrease in the film resistance upon transferring the film from O₂-rich ambient atmosphere to vacuum. The same change in ambient conditions also results in a 0.5 V decrease in the material work function. We suggest that facile adsorption of oxygen onto the film dedopes it from n-type toward intrinsic. These effects influence any material characterization, i.e., results may be ambient-dependent due to changes in the material’s electrical properties and electrochemical reactivity, which can also affect material stability

Deleterious Effect of Negative Capacitance on the Performance of Halide Perovskite Solar Cells

Negative capacitance in photovoltaic devices has been observed and reported in several cases, but its origin, at low or intermediate frequencies, is under debate. Here we unambiguously demonstrate a direct correlation between the observation of this capacitance and a corresponding decrease in performance of a halide perovskite (HaP; CsPbBr3)-based device, expressed as reduction of open-circuit voltage and fill factor. We have prepared highly stable CsPbBr3 HaPs that do not exhibit any degradation over the duration of the impedance spectroscopy measurements, ruling out degradation as the origin of the observed phenomena. Reconstruction of current-voltage curves from the impedance spectroscopy provided further evidence of the deleterious role of negative capacitance on photoconversion performance

What Is the Mechanism of MAPbI₃ p‑Doping by I₂? Insights from Optoelectronic Properties

Obtaining insight into, and ultimately control over, electronic doping of halide perovskites may improve tuning of their remarkable optoelectronic properties, reflected in what appear to be low defect densities and as expressed in various charge transport and optical parameters. Doping is important for charge transport because it determines the electrical field within the semiconducting photoabsorber, which strongly affects collection efficiency of photogenerated charges. Here we report on intrinsic doping of methylammonium lead tri-iodide, MAPbI₃, as thin films of the types used for solar cells and LEDs, by I₂ vapor at a level that does not affect the optical absorption and leads to a small (<20 meV, ∼9 nm) red shift in the photoluminescence peak. This I₂ vapor treatment makes the films 10× more electronically conductive in the dark. We show that this change is due to p-type doping because we find their work function to increase by 150 mV with respect to the ionization energy (valence band maximum), which does not change upon I₂ exposure. The majority carrier (hole) diffusion length increases upon doping, making the material less ambipolar. Our results are well-explained by I₂ exposure decreasing the density of donor defects, likely iodide vacancies (V_I) or defect complexes, containing V_I. Invoking iodide interstitials, which are acceptor defects, seems less likely based on calculations of the formation energies of such defects and is in agreement with a recent report on pressed pellets

Screening Aluminum-Based Compounds as Low‑κ Dielectrics for High-Frequency Applications

Advances in telecommunications require electronics that operate at ever-increasing frequencies, exemplified by 5G or fifth-generation technologies that operate in the GHz regime. At high frequencies, electrical circuits are plagued by so-called RC delays, arising from the time constant τ = RC for electrical signals which is the product of the resistance R and the capacitance C, respectively, of conductors and their insulating substrates. Besides using high quality, low-R electrical conductors such as high-purity Cu with low surface roughness, small RC delays are achieved by lowering the dielectric constant κ of the materials used in printed circuit board substrates. These largely comprise particles of an inorganic material, notably functionalized SiO2, embedded in a polymer-based matrix. The value of κ of the composite is primarily dictated by κ of the inorganic material. The properties of the inorganic component also impact other relevant parameters such as the quality factor, mechanical strength, and thermal expansion of the substrate. Here, we ask whether there are inorganic compounds with dielectric constants (measured at 10 GHz) that are lower than that of SiO2 and potentially replace it in electronics. We describe the key characteristics for low-κ materials and develop a framework for screening such compounds by employing some guiding principles, followed by using a combination of empirical estimates and density functional perturbation theory-based calculations. We then report experimental results on two promising aluminum-based low-κ compounds for high-frequency applications. The first is the cristobalite form of AlPO4. The second is the simplest 3D metal–organic framework, aluminum formate Al(HCOO)3. The measured values of κ at 10 GHz, which are 4.0 for AlPO4 and 3.8 for Al(HCOO)3, compare well with what is measured on SiO2 particles

Hybrid Iodide Perovskites of Divalent Alkaline Earth and Lanthanide Elements

Hybrid halide perovskites AMIIX3 (A = ammonium cation, MII = divalent cation, X = Cl, Br, I) have been extensively studied but have only previously been reported for the divalent carbon group elements Ge, Sn, and Pb. While they have displayed an impressive range of optoelectronic properties, the instability of GeII and SnII and the toxicity of Pb have stimulated significant interest in finding alternatives to these carbon group-based perovskites. Here, we describe the low-temperature solid-state synthesis of five new hybrid iodide perovskites centered around divalent alkaline earth and lanthanide elements, with the general formula AMIII3 (A = methylammonium, MA; MII = Sr, Sm, Eu, and A = formamidinium, FA; MII = Sr, Eu). Structural, calorimetric, optical, photoluminescence, and magnetic properties of these materials are reported

Soft-Chemical Synthesis, Structure Evolution, and Insulator-to-Metal Transition in Pyrochlore-like λ‑RhO₂

λ-RhO2, a prototype 4d transition metal oxide, has been prepared by the oxidative delithiation of spinel LiRh2O4 using ceric ammonium nitrate. Average-structure studies of this RhO2 polytype, including synchrotron powder X-ray diffraction and electron diffraction, indicate the room-temperature structure to be tetragonal, in space group I41/amd, with a first-order structural transition to cubic Fd3̅m at T = 345 K on warming. Synchrotron X-ray pair distribution function analysis and 7Li solid-state nuclear magnetic resonance measurements suggest that the room-temperature structure displays local Rh–Rh bonding. The formation of these local dimers appears to be associated with a metal-to-insulator transition with a nonmagnetic ground state, as also supported by density functional theory-based electronic structure calculations. This contribution demonstrates the power of soft chemistry to kinetically stabilize a simple binary oxide compound