Visualizing time: how linguistic metaphors are incorporated into displaying instruments in the process of interpreting time-varying signals

Spatial visualization is a well-established topic of education research that has allowed improving science and engineering students’ skills on spatial relations. Connections have been established between visualization as a comprehension tool and instruction in several scientific fields. Learning about dynamic processes mainly relies upon static spatial representations or images. Visualization of time is inherently problematic because time can be conceptualized in terms of two opposite conceptual metaphors based on spatial relations as inferred from conventional linguistic patterns. The situation is particularly demanding when time-varying signals are recorded using displaying electronic instruments, and the image should be properly interpreted. This work deals with the interplay between linguistic metaphors, visual thinking and scientific instrument mediation in the process of interpreting time-varying signals displayed by electronic instruments. The analysis draws on a simplified version of a communication system as example of practical signal recording and image visualization in a physics and engineering laboratory experience. Instrumentation delivers meaningful signal representations because it is designed to incorporate a specific and culturally favored time view. It is suggested that difficulties in interpreting time-varying signals are linked with the existing dual perception of conflicting time metaphors. The activation of specific space–time conceptual mapping might allow for a proper signal interpretation. Instruments play then a central role as visualization mediators by yielding an image that matches specific perception abilities and practical purposes. Here I have identified two ways of understanding time as used in different trajectories through which students are located. Interestingly specific displaying instruments belonging to different cultural traditions incorporate contrasting time views. One of them sees time in terms of a dynamic metaphor consisting of a static observer looking at passing events. This is a general and widespread practice common in the contemporary mass culture, which lies behind the process of making sense to moving images usually visualized by means of movie shots. In contrast scientific culture favored another way of time conceptualization (static time metaphor) that historically fostered the construction of graphs and the incorporation of time-dependent functions, as represented on the Cartesian plane, into displaying instruments. Both types of cultures, scientific and mass, are considered highly technological in the sense that complex instruments, apparatus or machines participate in their visual practices

Carrier recombination flux in bulk heterojunction polymer: fullerene solar cells: Effect of energy disorder on ideality factor

Energy disorder reduces the achievable open-circuit voltage in organic bulk-heterojunction solar cells. Here the effect of disorder on charge carrier recombination flux is numerically modeled. The recombination current follows an exponential dependence on voltage (Fermi level splitting) parameterized by β (inverse of the diode ideality factor), which reduces the power conversion efficiency through lower fill factors. β-Parameter approaches unity (Boltzmann approximation) at room temperature only in the case of weak disorder (σ ≈ 50 meV). For larger disorder values (σ ⩾ 100 meV) usually encountered in real devices, a huge reduction in β (open-circuit voltage, and fill factor) is predicted following a relationship as β ∝ ln σ−1

Methylammonium Lead Bromide Perovskite Battery Anodes Reversibly Host High Li-Ion Concentrations

Ions migrate through the hybrid halide perovskite lattice, allowing for a variety of electrochemical applications as perovskite-based electrodes for batteries. It is still unknown how extrinsic defects such as lithium ions interact with the hybrid perovskite structure during the charging process. It is shown here that Li+ intake/release proceeds by topotactic insertion into the hybrid perovskite host, without drastic structural alterations or rearrangement. Even the perovskite electronic band structure remains basically unaltered upon cycling. The occurrence of conversion or alloying reactions producing metallic lead is discarded. Stable specific capacity 200 mA h g-1 is delivered, which entails outstanding Li-ion molar concentration, x in LixCH3NH3PbBr3, approaching 3. Slight distortions of the perovskite lattice upon cycling explain the highly reversible Li+ intercalation reaction that also exhibits an excellent rate capability

Recent advances to understand morphology stability of organic photovoltaics

Organic photovoltaic devices are on the verge of commercialization with power conversion efficiencies exceeding 10 % in laboratory cells and above 8.5 % in modules. However, one of the main limitations hindering their mass scale production is the debatable inferior stability of organic photovoltaic devices in comparison to other technologies. Adequate donor/acceptor morphology of the active layer is required to provide carrier separation and transport to the electrodes. Unfortunately, the beneficial morphology for device performance is usually a kinetically frozen state which has not reached thermodynamic equilibrium. During the last 5 years, special efforts have been dedicated to isolate the effects related to morphology changes taking place within the active layer and compare to those affecting the interfaces with the external electrodes. The current review discusses some of the factors affecting the donor/acceptor morphology evolution as one of the major intrinsic degradation pathways. Special attention is paid to factors in the nano- and microscale domain. For example, phase segregation of the polymer and fullerene domains due to Ostwald ripening is a major factor in the microscale domain and is affected by the presence of additives, glass transition temperature of the polymers or use of crosslinkers in the active layer. Alternatively, the role of vertical segregation profile toward the external electrodes is key for device operation, being a clear case of nanoscale morphology evolution. For example, donor and acceptor molecules actually present at the external interfaces will determine the leakage current of the device, energy-level alignment, and interfacial recombination processes. Different techniques have been developed over the last few years to understand its relationship with the device efficiency. Of special interest are those techniques which enable in situ analysis being nondestructive as they can be used to study accelerated degradation experiments and some will be discussed here.This work was partially supported by FP7 European collaborative project SUNFLOWER (FP7-ICT-2011-7- contract No. 287594), the Spanish Ministerio de Economı´a y Competitividad (project MAT2013-47192-C3-1-R), and Generalitat Valenciana (project ISIC/2012/008 Institute of Nanotechnologies for Clean Energies). A.G. would like to thank the Spanish Ministerio de Economı´a y Competitividad for a Ramo´n y Cajal Fellowship (RYC- 2014-16809)

Light Capacitances in Silicon and Perovskite Solar Cells

The framework on which the physics of silicon solar cells (SiSCs) is based robustly predicts dependences of capacitance on light intensity and voltage, even when most recent innovations are considered as the incorporation of transition metal oxide/Si heterojunctions. However, perovskite solar cells (PSCs) challenge most of the established paradigms, claiming for rethinking of known theories and devising novel models. Here we tackle this scenario by probing and comparing light-induced capacitance responses yielded by these two major exponents in the field of photovoltaic research. SiSCs light capacitances can be easily interpreted in the framework of the so-called chemical capacitance. Current approaches addressing the intriguing low-frequency capacitive features of PSCs are outlined and compared. Here, apparent similarities and differences between both photovoltaic technologies are highlighted, concerning the observation of light capacitances of chemical origin. It is concluded that, contrary to that occurring in SiSCs, bulk electronic chemical capacitances are not straightforwardly observed in PSCs. As capacitive features exhibited by PSCs are believed to be critically connected to performance degradation and device instability, future research and explanation directions are proposed here for advancing in the understanding of this challenging photovoltaic technology

Light-Induced Space-Charge Accumulation Zone as Photovoltaic Mechanism in Perovskite Solar Cells

We fabricated formamidinium lead iodide perovskite solar cell for analysis of the photovoltaic mechanism based on the interpretation of the capacitance variation under illumination. It was shown that the low-frequency capacitance increases proportional to incident light intensity, and in addition it increases proportional to absorber thickness. Furthermore, the voltage dependence of capacitance is exponential with slope 1/2 (thermal energy). We conclude that the large photovoltage and capacitance are associated with electronic accumulation zone at the interface with the metal oxide contact. While this type of accumulation capacitance is common in many devices as transistors, the perovskite solar cell shows a singular behavior in that under light the electronic carrier accumulation grows unlimited by another series capacitance, reaching values as large as 10 mF cm–2 at one sun illumination.The work was supported by Generalitat Valenciana project PROMETEO/2014/020 and MINECO of Spain under project MAT2013-47192-C3-1-R. I.Z. thanks CONACYT for the international fellowship

Elucidating Operating Modes of Bulk-Heterojunction Solar Cells from Impedance Spectroscopy Analysis

We discuss the progress and challenges in the application of impedance spectroscopy analysis to determine key processes and parameters in organic bulk-heterojunction solar cells. When carrier transport or outer interface extraction do not severely influence the solar cell performance, a simple method to quantify the open-circuit voltage loss caused by the kinetics of charge carrier recombination is provided, based on the determination of chemical capacitance and recombination resistance. This easily allows distinguishing between energetic and kinetic effects on photovoltage, and establishes a benchmark for the performance comparison of a set of different cells. A brief discussion of impedance analysis in the much less studied case of collection-limited solar cells is introduced.We acknowledge the financial support from Ministerio de Educacion y Ciencia (Spain) under Project HOPE CSD2007-00007 (Consolider-Ingenio 2010), and Generalitat Valenciana (Prometeo/2009/058, and ISIC/2012/008 Institute of Nanotechnologies for Clean Energies)

Role of Vertical Segregation in Semitransparent Organic Photovoltaics

In this work, the efficiency of semitransparent organic photovoltaic (OPV) devices for low intensity applications is investigated as a function of the processing conditions. It is observed that a thermal treatment of the organic layer induces fullerene migration toward the active layer/air interface. This physical process gives rise to different vertical segregation profiles of donor and acceptor molecules. Once the back contact is deposited, the amount of fullerene covering the surface will determine the contact selectivity and leakage current of the device. Control of this leakage current may not be essential for devices fabricated for high illumination condition applications. However, devices to be used under low illumination conditions may be highly influenced by the presence of this parasitic dark current which flows in the opposite direction to photogenerated current. At the proximity of the contacts, the vertical segregation profile is inferred from optical and electrical measurements. In particular, external quantum efficiency (EQE) measurements carried out from a relatively opaque back contact provide local information on the materials spatially close to the light source. Alternatively, capacitance–voltage measurements enable calculation of the percentage of fullerene molecules covering the cathode contact. Overall, a versatile method that can be used in regular and inverted configuration is presented that explains the different behavior observed for devices to be used under low illumination conditions.1) FP7 European collaborative project SUNFLOWER (FP7-ICT-2011-7-contract num. 287594). 2) Generalitat Valenciana (project ISIC/2012/008 Institute of Nanotechnologies for Clean Energies). 3) Brno University of Technology for financial support (CZ.1.07/2.3.00/30.0039

Consistent formulation of the crossover from density to velocity dependent recombination in organic solar cells

Carrier recombination is a central process in bulk heterojunction organic solar cells. Based on the competition of hopping rates that either implies escape in a broad density of states or recombination across the interface, we formulate a general theory of recombination flux that distinguishes reaction or transport limited recombination according to charge density. The Langevin picture is valid only in the low charge density limit, and a crossover to the reaction controlled regime occurs at higher densities. We present results from impedance spectroscopy of poly(3-hexylthiophene):methanofullerene solar cell that exhibit this crossoverThe work was supported by Generalitat Valenciana (Project No. ISIC/2012/008)