Unbalanced Mobilities Research Articles

High Thickness Tolerance in All-Polymer-Based Organic Photovoltaics Enables Efficient and Stable In-Door Operation.

Organic photovoltaics (OPVs) have great potential to drive low-power consumption electronic devices under indoor light due to their highly tunable optoelectronic properties. Thick devices (>300nm photo-active junctions) are desirable to maximize photocurrent and to manufacture large-scale modules via solution-processing. However, thick devices usually suffer from severe charge recombination, deteriorating device performances. Herein, the study demonstrates excellent thickness tolerance of all-polymer-based PVs for efficient and stable indoor applications. Under indoor light, device performance is less dependent on photoactive layer thickness, exhibiting the best maximum power output in thick devices (34.7 µW cm-2 in 320-475nm devices). Thick devices also exhibit much better photostability compared with thin devices. Such high thickness tolerance of all-polymer-based PV devices under indoor operation is attributed to strongly suppressed space-charge effects, leading to reduced bimolecular recombination losses in thick devices. The unbalanced charge carrier mobilities are identified as the main cause for significant space-charge effects, which is confirmed by drift-diffusion simulations. This work suggests that all-polymer-based PVs, even with unbalanced mobilities, are highly desirable for thick, efficient, and stable devices for indoor applications.

Why does Y6 with bulk charge photogeneration and bipolar charge transport properties still fail in non-heterojunction organic photovoltaics?

The JSC loss in non-heterojunction organic solar cells is discussed. The 0.65 eV energy barrier hinders the spontaneous charge photogeneration, while unbalanced mobility further decays JSC to nearly one percent of its theoretical limit.

Mobility and Energy Management in Electric Vehicle Based Mobility-on-Demand Systems: Models and Solutions

An electric vehicle based mobility-on-demand (EMoD) system provides shared transportation (e.g., car-sharing or ride-sharing) to satisfy customers’ individual mobility demands. It has been recognized as a vital alternative form of transportation between public and private transportations in future sustainable cities. Constrained by the long charging time and limited driving range of EVs, an operator of an EMoD system demands for decision-making models and algorithms to manage the mobility and energy of EVs to best serve customers with least costs. In this paper, we propose a stochastic dynamic program (DP) to model three operational decisions of the EMoD system: i) dispatching EVs to serve mobility demand from customers, ii) repositioning EVs to accommodate the unbalanced mobility demands between service regions, and iii) recharging EVs to maintain their sufficient state-of-charge levels. To handle this large-scale DP problem, we first observe and prove that it has a coordinate-wise concave value function. Based on this structural property, we propose to use a separable piecewise linear function to approximate the value function and design an approximation-based algorithm to efficiently derive the decision policy. Numerical tests show that our proposed algorithm significantly outperforms the existing model-free approaches (e.g., greedy heuristic and Q-learning) that fail to take into account the structural properties of the DP problem.

Internationalization initiatives of Taiwan's higher education: a stepping stone to regional talent circulation or reproduction of unbalanced mobility scheme?

PurposeAs an emerging market of international education, Asian countries ambitiously launched internationalization initiatives and strategies to attract international talent. Since the 1990s, Taiwan's government has implemented various internationalization policies. Partly affected by the political forces of neighboring China, Taiwan's government launched the New Southbound Policy (NSP) as the main regional strategy in 2016. One of the aims of this strategy was to promote mutual talent mobility between Taiwan and New Southbound Policy countries (NSPC). The purpose of this study is to explore how the NSP influences the student mobility scheme in Taiwan.Design/methodology/approachThis study adopted the qualitative document analysis to investigate and compare the major Asian countries' internationalization focus and summarize Taiwan's internationalization development process and policy priorities. Moreover, a qualitative approach was adopted in order to collect data from 2005 to 2018 to examine Taiwan's student mobility scheme under the policy change.FindingsUnder the influence of the NSP after 2016, the student mobility scheme between Taiwan and NSPC could be categorized into five categories in accordance with the mobility rate. Although the nation-driven policy was considered powerful, the unbalanced flow between Taiwan and NSPC became severe.Research limitations/implicationsThe study lacked statistics on the degree level of outbound Taiwanese students going to NSPC. It could not compare the student mobility scheme between Taiwan and NSPC by degree level.Originality/valueThe research looked at the initiatives Asian countries have developed in order to raise higher education internationalization and regional status, which shed light on the national/regional approaches under the global change.

Extreme unbalanced mobility network in bike sharing system

Bike sharing, such as Mobike, are thriving throughout the past three years. Mobike allows users to rent bikes left by previous users. This mobility mode generates self-organized bike distribution over the city. We analyze real data from Mobike bike sharing system in Beijing and Citibike bike sharing system in New York. In this paper, we study the spatial pattern of sharing activity from this new business model. We construct an Origin–Destination spatial network of bike sharing traffic flow based on trip data and study the characteristic of unbalance in the systems. The unbalance of nodes in Mobike bike mobility OD network is revealed to follows a power law distribution with different exponents on weekdays and weekends. We also found that there exists a scaling between maximum unbalance and network degree in both systems. These features of bike sharing can provide a better understanding of sharing mobility behaviors and help to develop more efficient management methods.

Organic bulk heterojunction photodetectors based on polymer blends with unbalanced mobilities

We investigate bulk heterojunction organic photodetectors with unbalanced charge carrier mobilities in the active layer. We present an experimental example of an organic photodiode with very unbalanced mobilities but achieving state-of-the-art parameters in terms of external quantum efficiency exceeding 75% (at illumination 638 nm), and of bandwidth exceeding 1 MHz. The active layer of the photodiode is a 50 nm thick bulk heterojunction made of a blend of PCBM and copolymer of 5,6-difluoro2,1,3-benzothiadiazole, with mobility of slower charge carriers of order of 5 × 10−7 , estimated using space-charge-limited photocurrent technique. To understand why such a low mobility does not have deteriorating effects on the photodetecting performance of the photodiode, we performed drift-diffusion simulations of bulk heterojunction photodiodes with Langevin recombination. According to the simulation, the bandwidth of the photodiode is approximately independent of the mobility of slower charge carriers in the blend, and the negative effect of low mobility on the responsivity can be compensated by increasing the reverse bias. Our study shows that well-performing organic photodetectors can be fabricated using organic semiconductors having too low mobility for photovoltaic applications.

New wine in old bottles? Examining institutional hierarchy in laureate mobility networks, 1900–2017

One of the major trends in academia today is the constant growth of the mobility of scientists. However, the unbalanced mobility of scientific elites can not only lead to institutional hierarchy in research sectors but also result in an “elite circulation” phenomenon. In this study, we collected the information on the institutional mobility of 10,918 laureates of 308 international academic awards, and constructed laureate mobility networks. Furthermore, we examined the structure of these networks by using several statistical approaches. We find that there are hierarchical structures in laureate mobility networks, both in different scientific fields and at different historical periods, indicating that only a few institutions link a higher number of laureates and most institutions at a lower number. In addition, the Gini coefficient shows a higher level of institutional inequality in laureate mobility networks over time.

Controlling Ambipolar Transport and Voltage Inversion in Solution-Processed Thin-Film Devices through Polymer Blending

Ambipolar semiconductors are attracting a great interest as building blocks for photovoltaics and logic applications. Field-effect transistors built on solution-processable ambipolar materials hold strong promise for the engineering of large-area low-cost logic circuits with a reduced number of devices components. Such devices still suffer from a number of obstacles including the challenging processing, the low Ion/Ioff, the unbalanced mobility, and the low gain in complementary metal–oxide–semiconductor (CMOS)-like circuits. Here, we demonstrate that the simple approach of blending commercially available n- and p-type polymers such as P(NDI2OD-T2), P3HT, PCD-TPT, PDVT-8, and IIDDT-C3 can yield high-performing ambipolar field-effect transistors with balanced mobilities and Ion/Ioff > 107. Each single component was studied separately and upon blending by means of electrical characterization, ambient ultraviolet photoelectron spectroscopy, atomic force microscopy, and grazing incidence wide angle X-ray scattering to unravel the correlation between the morphology/structure of the semiconducting films and their functions. Blends of n- and p-type semiconductors were used to fabricate CMOS-like inverter circuits with state-of-the-art gains over 160 in the case of P(NDI2OD-T2) blended with PDVT-8. Significantly, our blending approach was successful in producing semiconducting films with balanced mobilities for each of the four tested semiconductor blends, although the films displayed different structural and morphological features. Our strategy, which relies on establishing a correlation between ambipolar performances, film morphology, molecular structure, and blending ratio, is extremely efficient and versatile; thus it could be applied to a wide range of polymers or solution processable small molecules.

Determination of the intrinsic and the injection dependent charge carrier density in organic solar cells using the Suns-VOC method

A method is presented to calculate the intrinsic and injection dependent average charge carrier density from the transport resistance of an organic solar cell. The latter is determined using a combination of a Suns-VOC and a current-voltage measurement under illumination which allows to split the total series resistance of the solar cell into an (ohmic) contribution of the circuitry and an injection dependent part which is caused by the transport of the charge carriers through the photoactive layer. In the derivation of the formula for the average charge carrier density, spatially homogeneous generation rate and gradients of the quasi Fermi levels as well as balanced mobilities had to be assumed. However, numerical simulations revealed that even for strongly inhomogeneous generation and unbalanced mobilities the results are reasonably accurate proving the practical applicability of the presented method. Using an inverted ITO-free P3HT:PCBM solar cell, we determined a value for the intrinsic charge carrier density of ni = 2.88 × 1011 cm−3 at T = 300 K and for the average density of mobile charge carriers at “1 sun” under open circuit conditions we obtained nav = 3.2 × 1016 cm−3.

Effect of doping on performance of organic solar cells

Conventional models of planar and bulk heterojunction organic solar cells have been extended by introducing doping in the active layer. We have studied the performance of organic solar cells as a function of dopant concentration. For bulk heterojunction cells, the modeling shows that for the most studied material pair (poly-3-hexylthiophene, P3HT, and phenyl-C61-butyric acid methyl ester, PCBM) doping decreases the short-circuit current density (JSC), fill factor (FF) and efficiency. However, if bulk heterojunction cells are not optimized, namely, at low charge carrier mobilities, unbalanced mobilities or non-ohmic contacts, the efficiency can be increased by doping. For planar heterojunction cells, the modeling shows that if the acceptor layer is n doped, and the donor layer is p doped, the open-circuit voltage, JSC, FF and hence the efficiency can be increased by doping. Inversely, when the acceptor is p doped, and the donor is n doped; FF decreases rapidly with increasing dopant concentrations so that the current-voltage curve becomes S shaped. We also show that the detrimental effect of nonohmic contacts on the performance of the planar heterojunction cell can be strongly weakened by doping.

Mobility and fill factor correlation in geminate recombination limited solar cells

Empirical data for the fill factor as a function of charge carrier mobility for two different polymer:fullerene systems is presented and analyzed. The results indicate that charge extraction depth limitations and space charge effects are inconsistent with the observed behavior, and the decrease in the fill factor is, instead, attributed to the field-dependent charge separation and geminate recombination. A solar cell photocurrent limited by the Onsager-Braun charge transfer exciton dissociation is shown to be able to accommodate the experimental observations. Charge dissociation limited solar cells always benefit from increased mobilities, and the negative contribution from the reduced charge separation is shown to be much more important for the fill factor in these material systems than any adverse effects from charge carrier extraction depth limitations or space charge effects due to unbalanced mobilities. The logarithmic dependence of the fill factor on the mobility for such a process is also shown to imply that simply increasing the mobilities is an impractical way to reach very high fill factors under these conditions since unrealistically high mobilities are required. A more controlled morphology is, instead, argued to be necessary for high performance.

Effects of Intervertebral Stabilization and Fusion to the Cases with Ossification of the Posterior Longitudinal Ligament of the Cervical Spine

50 operative cases treated by interbody fusion without excising the ossified mass and 30 non-operative cases treated conservatively were analyzed neurologically and radiologically to make clear the effects of intervertebral stabilization and fusion on the myelopathic symptoms of this condition, comparing with the corelation between the intervertebral mobility and immobility in the narrow canal.It was stressed that the cervical cord compressed and strangulated by the static factor of the narrow canal, increasing by the dynamic factor of the movements of the cervical spine would be mechanically and ischemically damaged and subsequently produce symptoms and functional disabilities.In conclusion, spontaneous and operative stabilization of the cervical disk space with unbalanced mobility acts effectively to the improvement of the cervical cord function.