Igor Lubomirsky

Hybrid Organic–Inorganic Perovskites (HOIPs): Opportunities and Challenges

The conclusions reached by a diverse group of scientists who attended an intense 2-day workshop on hybrid organic-inorganic perovskites are presented, including their thoughts on the most burning fundamental and practical questions regarding this unique class of materials, and their suggestions on various approaches to resolve these issues.

Water Freezes Differently on Positively and Negatively Charged Surfaces of Pyroelectric Materials

Freezing Supercool Water Under equilibrium conditions, water will freeze at 0°C, but, under certain conditions, it can be kept in a supercooled liquid form below this temperature. Ehre et al. (p. 672) present a careful and detailed study of the freezing of water drops on both positively and negatively charged pyroelectric surfaces using a combination of optical microscopy and x-ray diffraction: Supercooled water froze at different temperatures, depending on the charge of the substrate with the initial freezing occurring at the liquid-substrate interface on a positively charged substrate and at the air-water interface on a negatively charged substrate. Thus, freezing could be induced upon heating when the substrate charge also changed from negative to positive. Supercooled water on a surface can freeze upon heating in response to surface charge switching from negative to positive. Although ice melts and water freezes under equilibrium conditions at 0°C, water can be supercooled under homogeneous conditions in a clean environment down to –40°C without freezing. The influence of the electric field on the freezing temperature of supercooled water (electrofreezing) is of topical importance in the living and inanimate worlds. We report that positively charged surfaces of pyroelectric LiTaO3 crystals and SrTiO3 thin films promote ice nucleation, whereas the same surfaces when negatively charged reduce the freezing temperature. Accordingly, droplets of water cooled down on a negatively charged LiTaO3 surface and remaining liquid at –11°C freeze immediately when this surface is heated to –8°C, as a result of the replacement of the negative surface charge by a positive one. Furthermore, powder x-ray diffraction studies demonstrated that the freezing on the positively charged surface starts at the solid/water interface, whereas on a negatively charged surface, ice nucleation starts at the air/water interface.

Conversion of CO2 to CO by electrolysis of molten lithium carbonate

The conversion of CO 2 to CO by electrolysis of molten Li 2 CO 3 was investigated. Using a cell comprising a Ti cathode, a graphite anode and a source of CO 2 allows the continuous electrolysis of the melt at 900°C with current densities at the electrodes higher than 100 mA/cm 2 . The faradaic efficiency of the process is close to 100%, and the thermodynamic efficiency at 100 mA/cm 2 is > 85%. The proposed method has several advantages: (i) No precious metal is required, (ii) no hazardous or toxic by-products are produced, and (iii) the method may operate continuously, producing pure CO rather than a mixture of CO and CO 2 . Therefore, the process described here has a potential application for converting electrical energy into fuel.

Invited review article: practical guide for pyroelectric measurements.

The characterization of pyroelectric materials is a necessary stage in the design of a large variety of pyroelectric-based devices ranging from intrusion alarms to IR cameras. The sample configurations and measurement techniques currently in use vary widely and require careful attention in order to avoid artifacts. In this review, we provide a practical guide to the measurement of the pyroelectric coefficient, paying particular attention to the new instrumental possibilities (fast sinusoidally modulated light sources, low impedance broad band current meters, and fast averaging oscilloscopes) that have become available during the last decade. Techniques applicable to bulk specimens, substrate-supported films, and self-supported films are described in detail. The most commonly used procedures are classified according to the type of thermal excitation: continuous ramping, heat pulse, and continuous oscillation. In the appendices, we describe the practical realization of these measurement schemes and provide mathematical descriptions for the extraction of the pyroelectric coefficient from the measured data.

Local Structure and Strain‐Induced Distortion in Ce0.8Gd0.2O1.9

Cerium oxide, in both pure and doped forms, is one of the most important and extensively studied oxygen ion conductors. It exhibits a number of interesting properties including ionic conductivity due to the high mobility of oxygen vacancies, a series of different phases formed upon reduction, dependence of the lattice parameter and electrical properties on grain size, and non-linear elastic effects, which have been named ‘‘chemical stress’’ and ‘‘chemical strain’’. Recent structural studies, both theoretical and experimental, have been aimed at understanding the mechanism of interaction between the cations and the oxygen vacancies. These interactions are thought to be directly responsible for a number of effects such as resistance to radiation damage, vacancy ordering leading to phase transformations, dependence of ionic conductivity on the ionic radius of the dopant, and the non-linear elastic effects. Furthermore, in addition to the fluorites, cation-vacancy interactions are observed in a number of other solids with a large concentration of oxygen vacancies, for instance, perovskites. Therefore, characterizing the details of cation-vacancy interactions in Ce0.8Gd0.2O1.9 may have implications for a wider range of materials. One property in which the cation-vacancy interaction is thought to be directly implicated is the chemical strain effect, which is the ability of thin films of Ce0.8Gd0.2O1.9 to exhibit two different elastic moduli at temperatures below 200 8C. This effect is accompanied by an absolute change in volume of 0.2% even when the external stress is homogeneous, which distinguishes it from the Gorsky, Snoek or Zener effects. The chemical strain effect has been tentatively attributed to a change in specific volume due to the interaction of the Gd3þ ions with oxygen vacancies (5% of all oxygen sites in Ce0.8Gd0.2O1.9 ). However, no evidence has been presented that the rearrangement of vacancies in Ce0.8Gd0.2O1.9 can be stress-induced, or even takes place at all. The present study uses extended X-ray absorption fine-structure (EXAFS) spectroscopy to evaluate the local structure of strain-free nanocrystalline films of Ce0.8Gd0.2O1.9 and to compare it with that of Ce0.8Gd0.2O1.9 films with in-plane compressive strain of 0.3% 0.1%. Since the lattice parameter of Ce0.8Gd0.2O1.9 varies by a few tenths of a percent, depending on the preparation route and sample history, the X-ray diffraction (XRD) and EXAFS measurements were performed on the same samples, thus permitting direct comparison of the local ion arrangement with the long-range structure. We report that even in strain-free Ce0.8Gd0.2O1.9, interaction of oxygen vacancies with Ce4þ ion neighbors is favored, rather than interaction with Gd3þ ion neighbors. As a result, Ce4þ ions are shifted away from the oxygen vacancies. Furthermore, compressive strain of 0.3% 0.1% causes the Ce O bond to contract by 1.0% 0.5%, whereas other bonds remain much less affected. This anomalous Ce O bond contraction potentially offers a microscopic explanation for the chemical strain effect observed in Ce0.8Gd0.2O1.9. Films of Ce0.8Gd0.2O1.9 (200–450 nm) were deposited on a (001) Si substrate by RF sputtering and then annealed as described in refs. . The films were kept at room temperature for > 4 months to achieve a constant value of the lattice parameter, indicating that the low-temperature equilibrium of the point defects had been reached. After annealing, the in-plane stress in the Ce0.8Gd0.2O1.9 films was found to be less than 30MPa. The sputtering conditions were adjusted so that the annealed films comprised two groups: (1) those films that were strain free after annealing and (2) those that had a residual compressive strain of 0.3% 0.1%. The presence of strain was evident from the anisotropy of the d-spacings as measured by XRD in the iso-inclination mode. According to XRD data, acquired in the Q–2Q mode (with Q offset of 38 for thin films in order to suppress Si single-crystal reflections from substrate), all films and the reference CeO2 and Ce0.8Gd0.2O1.9 powders were in a single fluorite phase (Fig. 2 in this work and Fig. 2 in Ref. [6]). The lattice parameters of the powders (milled parts of the sputtering targets, grain size > 1mm) determined from highangle (2Q> 808) measurements were 5.411 A and 5.425 A for CeO2 and Ce0.8Gd0.2O1.9, respectively. [6,20]

A two junction, four terminal photovoltaic device for enhanced light to electric power conversion using a low-cost dichroic mirror

A low-cost dichroic mirror can be used successfully for solar spectrum splitting to enhance solar to electrical energy conversion. The mirror is optimized for use with a polycrystalline silicon photovoltaic cell pc-Si. With the dichroic mirror simultaneous excitation of a medium-efficient 11.1% commercial pc-Si and a custommade high band gap GaInP cell 12.3%, yields 16.8% efficiency, with both cells operating at maximum power. Our results clearly show that what is missing for this simple low-cost enhancement of Si solar cell efficiency are low-cost high band gap cells.

Tetragonal CH3NH3PbI3 is ferroelectric

Significance Halide perovskite (HaP) semiconductors are revolutionizing the field of photovoltaic (PV) solar energy conversion by showing remarkable performance of solar cells made with HaPs. “Ferroelectrics” is one frequently suggested reason because it may allow the spatial separation of the flow of electrons from where they were generated (holes). Unlike common, electrically insulating, ferroelectric materials, HaPs [especially tetragonal methylammonium lead triiodide (MAPbI3)] are semiconducting, and to find out whether they are ferroelectric requires an approach that is different from what is done customarily. Using such an approach, we prove that tetragonal MAPbI3 is definitely ferroelectric. What still remains to be seen is whether this ferroelectric nature is important for how MAPbI3-based solar cells operate around room temperature. Halide perovskite (HaP) semiconductors are revolutionizing photovoltaic (PV) solar energy conversion by showing remarkable performance of solar cells made with HaPs, especially tetragonal methylammonium lead triiodide (MAPbI3). In particular, the low voltage loss of these cells implies a remarkably low recombination rate of photogenerated carriers. It was suggested that low recombination can be due to the spatial separation of electrons and holes, a possibility if MAPbI3 is a semiconducting ferroelectric, which, however, requires clear experimental evidence. As a first step, we show that, in operando, MAPbI3 (unlike MAPbBr3) is pyroelectric, which implies it can be ferroelectric. The next step, proving it is (not) ferroelectric, is challenging, because of the material’s relatively high electrical conductance (a consequence of an optical band gap suitable for PV conversion) and low stability under high applied bias voltage. This excludes normal measurements of a ferroelectric hysteresis loop, to prove ferroelectricity’s hallmark switchable polarization. By adopting an approach suitable for electrically leaky materials as MAPbI3, we show here ferroelectric hysteresis from well-characterized single crystals at low temperature (still within the tetragonal phase, which is stable at room temperature). By chemical etching, we also can image the structural fingerprint for ferroelectricity, polar domains, periodically stacked along the polar axis of the crystal, which, as predicted by theory, scale with the overall crystal size. We also succeeded in detecting clear second harmonic generation, direct evidence for the material’s noncentrosymmetry. We note that the material’s ferroelectric nature, can, but need not be important in a PV cell at room temperature.

Modeling of space-charge effects in nanocrystalline ceramics: The influence of geometry

The distribution of mobile charge carriers in the space-charge regions at grain boundaries of ceramic materials was modeled. Delocalization effects are neglected, i.e., we consider ionic defects or polarons. The calculations were performed for cubic-shaped grains of equal size. When considering the size dependence, the standard free chemical potentials of the defects rather than the specific grain-boundary charge density or the defects’ boundary concentration were set to be constant in accordance with the core space-charge model. Although specific edge and corner effects are neglected in the present analysis and hence the structural potentials are invariant along grain boundaries, the accumulation or depletion of charge carriers turns out to be inhomogeneous along the grain boundary and to be particularly pronounced near grain edges and grain corners if the grain size was smaller than four Debye lengths. Especially the accumulation near grain edges can have a strong influence on the effective conductivity...

Space charge effects on dopant diffusion coefficient measurements in semiconductors

Systematic errors are likely to affect the results of indirect methods used for measuring dopant diffusion in semiconductors, which, for this purpose should be considered as mixed electronic-ionic conductors. The highest contribution to these errors is introduced by the presence of an internal electric field, i.e., by space charge effects. The electric field can be the result either of a dopant concentration gradient or of external bias, applied during the measurement. We consider here three methods in detail, viz. measurement of p-n junction motion, of current or potential decay, and of the time dependence of capacitance (transient ion drift). We show that space charge effects can lead to overestimating diffusion coefficients by a few orders of magnitude. We use the results of our analyses to review and compare the experimental data obtained by different direct and indirect methods, for Cu diffusion in CuInSe2, an issue of considerable current interest for solar cells.

Tetragonal CH3NH3PbI3is ferroelectric

Significance Halide perovskite (HaP) semiconductors are revolutionizing the field of photovoltaic (PV) solar energy conversion by showing remarkable performance of solar cells made with HaPs. “Ferroelectrics” is one frequently suggested reason because it may allow the spatial separation of the flow of electrons from where they were generated (holes). Unlike common, electrically insulating, ferroelectric materials, HaPs [especially tetragonal methylammonium lead triiodide (MAPbI3)] are semiconducting, and to find out whether they are ferroelectric requires an approach that is different from what is done customarily. Using such an approach, we prove that tetragonal MAPbI3 is definitely ferroelectric. What still remains to be seen is whether this ferroelectric nature is important for how MAPbI3-based solar cells operate around room temperature. Halide perovskite (HaP) semiconductors are revolutionizing photovoltaic (PV) solar energy conversion by showing remarkable performance of solar cells made with HaPs, especially tetragonal methylammonium lead triiodide (MAPbI3). In particular, the low voltage loss of these cells implies a remarkably low recombination rate of photogenerated carriers. It was suggested that low recombination can be due to the spatial separation of electrons and holes, a possibility if MAPbI3 is a semiconducting ferroelectric, which, however, requires clear experimental evidence. As a first step, we show that, in operando, MAPbI3 (unlike MAPbBr3) is pyroelectric, which implies it can be ferroelectric. The next step, proving it is (not) ferroelectric, is challenging, because of the material’s relatively high electrical conductance (a consequence of an optical band gap suitable for PV conversion) and low stability under high applied bias voltage. This excludes normal measurements of a ferroelectric hysteresis loop, to prove ferroelectricity’s hallmark switchable polarization. By adopting an approach suitable for electrically leaky materials as MAPbI3, we show here ferroelectric hysteresis from well-characterized single crystals at low temperature (still within the tetragonal phase, which is stable at room temperature). By chemical etching, we also can image the structural fingerprint for ferroelectricity, polar domains, periodically stacked along the polar axis of the crystal, which, as predicted by theory, scale with the overall crystal size. We also succeeded in detecting clear second harmonic generation, direct evidence for the material’s noncentrosymmetry. We note that the material’s ferroelectric nature, can, but need not be important in a PV cell at room temperature.