Solar cells that operate efficiently under indoor lighting are of great practical interest as they can serve as electric power sources for portable electronics and devices for wireless sensor networks or the Internet of Things. Here, we demonstrate a dye-sensitized solar cell (DSC) that achieves very high power-conversion efficiencies (PCEs) under ambient light conditions. Our photosystem combines two judiciously designed sensitizers, coded D35 and XY1, with the copper complex Cu(II/I)(tmby) as a redox shuttle (tmby, 4,4′,6,6′-tetramethyl-2,2′-bipyridine), and features a high open-circuit photovoltage of 1.1 V. The DSC achieves an external quantum efficiency for photocurrent generation that exceeds 90% across the whole visible domain from 400 to 650 nm, and achieves power outputs of 15.6 and 88.5 μW cm–2 at 200 and 1,000 lux, respectively, under illumination from a model Osram 930 warm-white fluorescent light tube. This translates into a PCE of 28.9%. A dye-sensitized solar cell that has been designed for efficient operation under indoor lighting could offer a convenient means for powering the Internet of Things.

/pdf/dye-sensitized-solar-cells-for-efficient-power-generation-3uu4lit13i.pdf

Dye-sensitized solar cells for efficient power generation under ambient lighting

高等学校計算数学学報 = Numerical mathematics

Today's heterogeneous networks comprised of mostly macrocells and indoor small cells will not be able to meet the upcoming traffic demands. Indeed, it is forecasted that at least a $100\times$ network capacity increase will be required to meet the traffic demands in 2020. As a result, vendors and operators are now looking at using every tool at hand to improve network capacity. In this epic campaign, three paradigms are noteworthy, i.e., network densification, the use of higher frequency bands and spectral efficiency enhancement techniques. This paper aims at bringing further common understanding and analysing the potential gains and limitations of these three paradigms, together with the impact of idle mode capabilities at the small cells as well as the user equipment density and distribution in outdoor scenarios. Special attention is paid to network densification and its implications when transiting to ultra-dense small cell deployments. Simulation results show that comparing to the baseline case with an average inter site distance of 200 m and a 100 MHz bandwidth, network densification with an average inter site distance of 35 m can increase the average UE throughput by $7.56\times$ , while the use of the 10 GHz band with a 500 MHz bandwidth can further increase the network capacity up to $5\times$ , resulting in an average of 1.27 Gbps per UE. The use of beamforming with up to 4 antennas per small cell BS lacks behind with average throughput gains around 30% and cell-edge throughput gains of up to $2\times$ . Considering an extreme densification, an average inter site distance of 5 m can increase the average and cell-edge UE throughput by $18\times$ and $48\times$ , respectively. Our study also shows how network densification reduces multi-user diversity, and thus proportional fair alike schedulers start losing their advantages with respect to round robin ones. The energy efficiency of these ultra-dense small cell deployments is also analysed, indicating the benefits of energy harvesting approaches to make these deployments more energy-efficient. Finally, the top ten challenges to be addressed to bring ultra-dense small cell deployments to reality are also discussed.

Towards 1 Gbps/UE in Cellular Systems: Understanding Ultra-Dense Small Cell Deployments

Technology and market perspective for indoor photovoltaic cells

Photovoltaic cells are attracting significant interest for harvesting indoor light for low power consumption wireless electronics such as those required for smart homes and offices, and the rapidly-growing Internet of Things. Here, we explore the potential of solution processable, small molecule photovoltaic cells as indoor power sources. By optimizing the solvent vapour annealing (SVA) time for the photovoltaic layer, a balance between its crystallization and phase separation is obtained, resulting in a record power conversion efficiency (PCE) of over 28% under fluorescent lamps of 1000 lux, generating a maximum power density of 78.2 μW cm−2 (>10% PCE under AM1.5G). This high indoor performance surpasses that of silicon based photovoltaic cells, and is similar to that of gallium arsenide photovoltaic cells. Besides, the ratios of the voltage at the maximum power point (MPP) to the open circuit voltage are similar from indoor lighting to one sun conditions, which is unique and allows a less power consuming method to track the MPP for a broad range of light intensities (potentially attractive for wearable photovoltaics). New insight into the effect of SVA on the indoor and one sun performance is provided using advanced optoelectronic characterization techniques, which show that the mobility-lifetime products as a function of charge carrier density can be correlated well with the performance at different light levels. Our results suggest that organic photovoltaic cells could be promising as indoor power sources for self-sustainable electronics.

/pdf/organic-photovoltaic-cells-promising-indoor-light-harvesters-rk9skgfw3i.pdf

Organic photovoltaic cells – promising indoor light harvesters for self-sustainable electronics

GaAs solar cells are the highest efficiency single-junction photovoltaic technology. Their wide bandgap lends them to efficient conversion of indoor light. In this paper GaAs solar cells are experimentally compared to Dye Sensitised solar cells, traditionally used for indoor light harvesting. The power density of GaAs solar cells was found to be over 3× greater than DSSC modules under indoor light levels. A credit card sized GaAs cell can provide 0.67 mW or 4 mW to a wireless sensor node in a dimly lit corridor (∼200 lux) or well-lit office space (∼1000 lux) respectively. A near linear relationship is measured between the open-circuit and maximum power voltages of GaAs solar cells at low light levels allowing their use with power conditioning circuits (MPPT tracking) based on the fractional-voltage method.

GaAs solar cells for Indoor Light Harvesting

This article presents a fundamental investigation in which velocity amplification is employed in non-resonant structures to enhance the power harvested from ambient vibrations. Velocity amplification is achieved utilising sequential collisions between free-moving masses, and the final velocity is proportional to the number of masses and the mass ratios selected. The governing theory is discussed, particularly how the final velocity scales with the number of masses. This article examines n-mass velocity-amplified vibration energy harvesters and examines their performance relative to single-mass harvesters. Electromagnetic energy conversion is chosen as it is fundamental in allowing the free movement of the masses. Experimental results from two- and three-mass prototypes are presented that demonstrate a wider frequency response and a gain in power of 33 times compared to single-mass configurations under wideband random excitation. The volume of the devices was constrained, which resulted in the two-mass sys...

Enhanced vibrational energy harvester based on velocity amplification

In this paper we extend dynamic controllability of temporally-flexible plans to temporally-flexible reactive programs. We consider three reactive programming language constructs whose behavior depends on runtime observations; conditional execution, iteration, and exception handling. Temporally-flexible reactive programs are distinguished from temporally-flexible plans in that program execution is conditioned on the runtime state of the world. In addition, exceptions are thrown and caught at runtime in response to violated timing constraints, and handled exceptions are considered successful program executions. Dynamic controllability corresponds to a guarantee that a program will execute to completion, despite runtime constraint violations and uncertainty in runtime state. An algorithm is developed which frames the dynamic controllability problem as an AND/OR search tree over possible program executions. A key advantage of this approach is the ability to enumerate only a subset of possible program executions that guarantees dynamic controllability, framed as an AND/OR solution subtree.

/pdf/dynamic-controllability-of-temporally-flexible-reactive-3z4451nm0f.pdf

Dynamic controllability of temporally-flexible reactive programs

The major focus of this work is to examine the dynamics of velocity amplification through pair-wise collisions between multiple masses in a chain, in order to develop useful machines. For instance low-cost machines based on this principle could be used for detailed, very-high acceleration shock-testing of MEMS devices. A theoretical basis for determining the number and mass of intermediate stages in such a velocity amplifier, based on simple rigid body mechanics, is proposed. The influence of mass ratios and the coefficient of restitution on the optimisation of the system is identified and investigated. In particular, two cases are examined: in the first, the velocity of the final mass in the chain (that would have the object under test mounted on it) is maximised by defining the ratio of adjacent masses according to a power law relationship; in the second, the energy transfer efficiency of the system is maximised by choosing the mass ratios such that all masses except the final mass come to rest following impact. Comparisons are drawn between both cases and the results are used in proposing design guidelines for optimal shock amplifiers. It is shown that for most practical systems, a shock amplifier with mass ratios based on a power law relationship is optimal and can easily yield velocity amplifications of a factor 5-8 times. A prototype shock testing machine that was made using above principles is briefly introduced.

/pdf/the-dynamics-of-multiple-pair-wise-collisions-in-a-chain-for-nt663uzjly.pdf

The Dynamics of Multiple Pair-Wise Collisions in a Chain for Designing Optimal Shock Amplifiers

Recent advances in micro-electro-mechanical system (MEMS) fabrication technology have resulted in proliferation of micro-scale mechanical devices, some of which are applied in environments with severe levels of shock. The objective of this paper was to investigate the use of experimental and numerical methods in quantifying the behaviour of representative MEMS devices subject to high-g impact stimuli. Micro-cantilevers were analysed under vibration and shock on a modified Hopkinson pressure bar and vibration table to determine the mechanical properties of single crystal silicon (SCS). The characteristic dimensions of the beams were of 100 μm in height/width with beam lengths ranging from 5 to 7 mm. The experimental approach allowed non-invasive in situ monitoring of the micro-cantilevers upon impact through accelerometry and high-speed imaging. An investigation of the shock response of micro-cantilever beams indicates that orientation plays a significant role in their sensitivity to shock because of the planarity of the fabrication technique. Scanning electron microscopy identified octahedral cleavage of SCS as the dominant failure mechanism of the micro-cantilevers. Finite element analysis in conjunction with in situ high-speed imaging proved to be a viable non-invasive inverse technique to determine the loci and amplitude of tensile stress within generic micro-scale devices.

Gerard Kelly

Papers

GaAs solar cells for Indoor Light Harvesting

Enhanced vibrational energy harvester based on velocity amplification

Dynamic controllability of temporally-flexible reactive programs

The Dynamics of Multiple Pair-Wise Collisions in a Chain for Designing Optimal Shock Amplifiers

The Failure Mechanisms of Micro‐Scale Cantilevers Under Shock and Vibration Stimuli