Detection and relevance of ion conduction in hybrid organic-inorganic halide perovskites for photovoltaic applications

In recent years, hybrid organic-inorganic halide perovskites have attracted much attention with respect to their potential use as sensitizers in solar cells.[1] These materials show many outstanding properties, such as high absorption coefficients, ideal bandgap for solar light absorption (1.5 eV), long electron-hole recombination lengths and high charge carrier mobilities[1-3], that leads to a photo-conversion efficiency of hybrid-perovskite-containing devices exceeding 20%.[4] However, anomalous behaviors have been reported for these materials, such as high apparent dielectric constants at low AC frequencies or photocurrent hysteresis of solar cell devices during operation.[5] In this study[6] we measure the electrical transport properties of CH3NH3PbI3, by means of DC galvanostatic polarization, AC impedance spectroscopy and open circuit voltage measurements in electrochemical cells. By using ion-blocking electrodes, we detect a clear stoichiometric polarization behavior, from which we can separate electronic and ionic contributions to the total conductivity. We show that, under certain conditions, ionic conductivity can substantially exceed electronic conductivity and we assess the nature of the migrating ions (iodine ions). It is noteworthy that such ionic conductivity can naturally explain the above mentioned anomalies (Figure 1). Moreover, from the experimental data, a strong trapping of the electronic carriers due to ionic defects is ascertained. As a natural follow-up to better understand the defect chemistry of such materials, conductivity response to different atmospheres (I2, O2) has been measured and acceptor doping has been successfully applied. Please click Additional Files below to see the full abstract

Non-stoichiometry and ion transport in halide perovskites: Equilibrium situation and light effects

In recent years, hybrid halide perovskites have been attracting great attention due to their exceptional photo-electrochemical properties.[1-2] When used as light-harvesters in solar cells, device efficiencies exceeding 22% can be realized. We showed that a deeper understanding of (i) functionality, (ii) stability, as well as (iii) the possibility to improve the performance require a thorough insight into non-stoichiometry and ion transport.[3-5] In this contribution, we study the nature of the ionic conductivity in methylammonium lead iodide (MAPbI3), the archetypal halide perovskite, by means of a great number of electrochemical and nuclear magnetic techniques.[4] To aid the experimental investigation, we include detailed defect chemical modelling describing the effects of iodine partial pressure (Fig. 1a), doping and interaction with oxygen.[5] We also discuss results that show the significance of ion redistribution phenomena for relevant interfaces. By extending this study to the situation under illumination, we observe a striking enhancement of ionic conductivity by more than 2 orders of magnitude in MAPbI3, alongside the expected increase in electronic conductivity.[6] We provide a mechanistic explanation of this astonishing phenomenon and discuss its relevance for future light-triggered ionic devices (“opto-ionics”, see Fig. 1b). Please click Additional Files below to see the full abstract

Hill climbing hysteresis of perovskite-based solar cells: a maximum power point tracking investigation

The surge of the power conversion efficiency of metal halide lead perovskite solar cells comes with concerns, such as the long-term ecotoxicity of lead compounds, their sensitivity toward moisture and oxygen, or the scarcity of some of their components. Most perovskite solar cells still suffer from serious stability problems when measured under real working conditions (maximum power point tracking at 60 degrees C). In the long run, stability will certainly decide on the fate of CH3NH3PbI3 and related lead perovskites for their use in photovoltaic modules. Herein, we show an effective and inexpensive strategy to perform ageing of perovskite solar cells under maximum power point tracking. For the first time, we analyze the issue of power extraction from solar cells exhibiting hysteresis. We show that a standard tracking algorithm such as perturb and observe fails to converge to the maximum power point of the solar cell if it exhibits j(V) hysteresis, and we present an effective strategy to stabilize the algorithm. We show that enforcing oscillations in forward bias can boost the mean power output of some perovskite solar cells by more than 10%, in contrast to a reference crystalline silicon solar cell. Copyright (c) 2017 John Wiley & Sons, Ltd

Li14Ln5[Si11N19O5]O2F2 with Ln = Ce, Nd-Representatives of a Family of Potential Lithium Ion Conductors

The isotypic layered oxonitridosilicates Li14Ln5[Si11N19O5]O2F2 (Ln = Ce, Nd) have been synthesized using Li as fluxing agent and crystallize in the orthorhombic space group Pmmn (Z = 2, Li14Ce5[Si11N19O5]O2F2: a = 17.178(3), b = 7.6500(15), c = 10.116(2) Å, R1 = 0.0409, wR2 = 0.0896; Li14Nd5 Si11N19O5]O2F2: a = 17.126(2), b = 7.6155 15), c = 10.123(2) Å, R1 = 0.0419, wR2 = 0.0929). The silicate layers consist of dreier and sechser rings interconnected via common corners, yielding an unprecedented silicate substructure. A topostructural analysis indicates possible 1D ion migration pathways between five crystallographic independent Li positions. The specific Li-ionic conductivity and its temperature dependence were determined by impedance spectroscopy as well as DC polarization/depolarization measurements. The ionic conductivity is on the order of 5 × 10−5 S/cm at 300°C, while the activation energy is 0.69 eV. Further adjustments of the defect chemistry (e.g., through doping)can make these compounds interesting candidates for novel oxonitridosilicate based ion conductors

Association of kidney disease measures with risk of renal function worsening in patients with type 1 diabetes

Background: Albuminuria has been classically considered a marker of kidney damage progression in diabetic patients and it is routinely assessed to monitor kidney function. However, the role of a mild GFR reduction on the development of stage 653 CKD has been less explored in type 1 diabetes mellitus (T1DM) patients. Aim of the present study was to evaluate the prognostic role of kidney disease measures, namely albuminuria and reduced GFR, on the development of stage 653 CKD in a large cohort of patients affected by T1DM. Methods: A total of 4284 patients affected by T1DM followed-up at 76 diabetes centers participating to the Italian Association of Clinical Diabetologists (Associazione Medici Diabetologi, AMD) initiative constitutes the study population. Urinary albumin excretion (ACR) and estimated GFR (eGFR) were retrieved and analyzed. The incidence of stage 653 CKD (eGFR < 60 mL/min/1.73 m2) or eGFR reduction > 30% from baseline was evaluated. Results: The mean estimated GFR was 98 \ub1 17 mL/min/1.73m2 and the proportion of patients with albuminuria was 15.3% (n = 654) at baseline. About 8% (n = 337) of patients developed one of the two renal endpoints during the 4-year follow-up period. Age, albuminuria (micro or macro) and baseline eGFR < 90 ml/min/m2 were independent risk factors for stage 653 CKD and renal function worsening. When compared to patients with eGFR > 90 ml/min/1.73m2 and normoalbuminuria, those with albuminuria at baseline had a 1.69 greater risk of reaching stage 3 CKD, while patients with mild eGFR reduction (i.e. eGFR between 90 and 60 mL/min/1.73 m2) show a 3.81 greater risk that rose to 8.24 for those patients with albuminuria and mild eGFR reduction at baseline. Conclusions: Albuminuria and eGFR reduction represent independent risk factors for incident stage 653 CKD in T1DM patients. The simultaneous occurrence of reduced eGFR and albuminuria have a synergistic effect on renal function worsening

X-ray Absorption under Operating Conditions for Solid-Oxide Fuel Cells Electrocatalysts: The Case of LSCF/YSZ

We describe a novel electrochemical cell for X-ray absorption spectroscopy (XAS) experiments during electrical polarization suitable for high-temperature materials such as those used in solid oxide fuel cells. A half-cell LSCF/YSZ was then investigated under cathodic and anodic conditions (850 °C and applied electrical bias ranging from +1 V to −1 V in air). The in situ XAS measurements allowed us to follow the LSCF degradation into simple oxides. The rapid deterioration of LSCF is ascribed to the formation of excess of oxygen vacancies leading to the collapse of the mixed perovskite structure

The Significance of Ion Conduction in a Hybrid Organic-Inorganic Lead-Iodide-Based Perovskite Photosensitizer

The success of perovskite solar cells has sparked enormous excitement in the photovoltaic community not only because of unexpectedly high efficiencies but also because of the future potential ascribed to such crystalline absorber materials. Far from being exhaustively studied in terms of solid-state properties, these materials surprised by anomalies such as a huge apparent low-frequency dielectric constant and pronounced hysteretic current-voltage behavior. Here we show that methylammonium (but also formamidinium) iodoplumbates are mixed conductors with a large fraction of ion conduction because of iodine ions. In particular, we measure and model the stoichiometric polarization caused by the mixed conduction and demonstrate that the above anomalies can be explained by the build-up of stoichiometric gradients as a consequence of ion blocking interfaces. These findings provide insight into electrical charge transport in the hybrid organic-inorganic lead halide solar cells as well as into new possibilities of improving the photovoltaic performance by controlling the ionic disorder