Mechanism of thermal transport in dilute nanocolloids

Thermal conduction modes in a nanocolloid (nanofluid) are quantitatively assessed by combining linear response theory with molecular dynamics simulations. The microscopic heat flux is decomposed into three additive fluctuation modes, namely, kinetic, potential, and collision. For low volume fractions (<1%) of nanosized platinum clusters which interact strongly with xenon host liquid, a significant thermal conductivity enhancement results from the self correlation in the potential flux. Our findings reveal a molecular-level mechanism for enhanced thermal conductivity in nanocolloids with short-ranged attraction and offer predictions that can be experimentally tested.     

Cucurbit[7]uril Disrupts Aggregate Formation Between Rhodamine B Dyes Covalently Attached to Glass Substrates

Dye aggregation is detrimental to the performance of high optical density dye-doped photonic materials. To overcome this challenge, the ability of cucurbit[7]uril (CB7) as a molecular host to disrupt aggregate formation on glass substrates was examined. Rhodamine B was covalently attached to glass slides by initially coating the surface with azidohexylsiloxane followed by copper-catalyzed “click” triazole formation with rhodamine B propargyl ester. The absorption and emission spectra of rhodamine B coated slides in water indicated diverse heterogeneous properties as surface dye density varied. Fluorescence quenching due to dye aggregation was evident at high surface dye density. Addition of aqueous cucurbit[7]uril (CB7) to the surface-tethered dyes perturbed the spectra to reveal a considerable reduction in heterogeneity, which suggested that the presence of a surface in close proximity does not significantly impair CB7’s ability to complex with tethered rhodamine B.     

Halogen-free solvent processing for sustainable development of high efficiency organic solar cells

Eliminating processing with halogenated solvents is desirable to achieve sustainable large-scale fabrication of organic solar cells. This work demonstrates a device processing approach completely free of halogenated solvents to yield high-performance (power conversion efficiency, η_P > 6%) polymer:fullerene bulk-heterojunction solar cells comprising a conjugated polymer PIDT-phanQ and [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM). Introducing 2% 1-methylnaphthalene (Me-naph) as a processing additive to toluene alleviates PC₇₁BM solubility problems, reduces phase domain size by two orders of magnitude, and boosts efficiency from η_P = 0.02% to 6.10%. Both AFM and TEM imaging show that the Me-naph additive promotes a more finely phase-separated morphology in spin-coated films, while photoluminescence quenching and photoinduced absorption spectroscopy confirm that this finer morphology results in both better exciton quenching and more efficient charge separation.     

A bidirectional AC-DC power converter with power factor correction

This paper presents new operation and performance of a thyristor-based AC-DC current-controlled boost-type converter that allows bidirectional power handling capability and provides input power factor correction and a near-sinusoidal input current waveform. The new converter can reduce harmonic pollution and disturbance on the supply mains. The feature of bidirectional power flow allows the stored energy in loads, such as motors, to regenerate back to the supply source, leading to an increase in overall energy efficiency and possibly a reduction in the size of the DC link capacitor. The operation is confirmed with the successful implementation of an experimental prototype     

Estimation of direction-of-arrivals based on the array manifold space

In this paper, a high resolution technique for estimating DOAs of spatially close source signals is presented. It is observed that the array manifold over a sector of interest is rank deficient and the dimension of the array manifold space, which is the range space of the array manifold, is less than the number of sensors in the array. The true signal subspace is a subspace in the array manifold space. A novel technique is provided that searches for the signal subspace in this array manifold space. The resulting estimated signal subspace has minimum principal angles with the data signal subspace generated by eigen-decomposing the covariance matrix of the array data vector. It is proved that the proposed estimator is asymptotically consistent and the estimated signal subspace is closer to the true signal subspace than the data signal subspace formed by MUSIC. The proposed novel technique has better performance than the MUSIC algorithm. Its performance is comparable to MLE and MD-MUSIC yet it requires only one-dimensional searches and is computationally much less intense. Simulation results are presented to show the effectiveness of the proposed technique, and comparisons with MUSIC, MLE, and MD-MUSIC algorithms are also included.This research was supported by TRIO and NSERC.     

Estimating the number of errors in a system using a martingaleapproach

A new, efficient procedure estimates the number of errors in a system. A known number of seeded errors are inserted into a system. The failure intensities of the seeded and real errors are allowed to be different and time dependent. When an error is detected during the test, it is removed from the system. The testing process is observed for a fixed amount of time τ. Martingale theory is used to derive a class of estimators for the number of seeded errors in a continuous time setting. Some of the estimators and their associated standard deviations have explicit expressions. An optimal estimator among the class of estimators is obtained. A simulation study assesses the performance of the proposed estimators     

Composition Engineering of All‐Inorganic Perovskite Film for Efficient and Operationally Stable Solar Cells

Cesium‐based inorganic perovskites have recently attracted great research focus due to their excellent optoelectronic properties and thermal stability. However, the operational instability of all‐inorganic perovskites is still a main hindrance for the commercialization. Herein, a facile approach is reported to simultaneously enhance both the efficiency and long‐term stability for all‐inorganic CsPbI_2.5Br_0.5 perovskite solar cells via inducing excess lead iodide (PbI₂) into the precursors. Comprehensive film and device characterizations are conducted to study the influences of excess PbI₂ on the crystal quality, passivation effect, charge dynamics, and photovoltaic performance. It is found that excess PbI₂ improves the crystallization process, producing high‐quality CsPbI_2.5Br_0.5 films with enlarged grain sizes, enhanced crystal orientation, and unchanged phase composition. The residual PbI₂ at the grain boundaries also provides a passivation effect, which improves the optoelectronic properties and charge collection property in optimized devices, leading to a power conversion efficiency up to 17.1% with a high open‐circuit voltage of 1.25 V. More importantly, a remarkable long‐term operational stability is also achieved for the optimized CsPbI_2.5Br_0.5 solar cells, with less than 24% degradation drop at the maximum power point under continuous illumination for 420 h.     

Indium tin oxide-free semi-transparent inverted polymer solar cells using conducting polymer as both bottom and top electrodes

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is investigated as a transparent cathode to replace indium tin oxide (ITO) in inverted polymer solar cells. Increasing the thickness of the PEDOT:PSS electrode leads to a reduction in transparency and sheet resistance which lowers the photocurrent but increases the fill factor of the solar cells. The offset of photocurrent and fill factor as the thickness is increased leads to a saturation of the power conversion efficiency to 3%. These electrodes were applied to flexible substrates showing similar device performance to glass based devices. Cyclic bending test of these flexible polymer electrodes show improved conversion efficiency retention (92%) when compared to flexible ITO based electrodes (50%) after 300 bend cycles. In addition to using PEDOT:PSS as a cathode replacement for ITO in inverted solar cells, its use as a semi-transparent anode replacement to Ag is also examined. Semi-transparent inverted solar cells fabricated with ITO as the cathode and PEDOT:PSS as the top anode electrode were demonstrated showing efficiencies of 2.51% while replacement of both ITO and Ag with PEDOT:PSS as both the cathode and anode show efficiencies of 0.47%.     

Time and temperature-dependent mechanical behavior of underfill materials in electronic packaging application

The thermo-mechanical testing of HYSOL FP4549 polymer-filled underfill materials was conducted under different strain rate and temperature environment. A new specimen preparation procedure and further test methodology are developed to characterize the time–temperature mechanical behaviors of underfill materials. The stress–strain behavior of materials is simulated with constitutive framework, and the dependence of Young’s modulus on temperature and strain rate was evaluated. In addition, the specimens were tested with microforce testing system to evaluate the creep curve of underfill materials as a function of temperature and stress level. In view of the uncertainty of the Young’s modulus determination, the specimens were tested with unloading–reloading technique to verify the test results and investigate its cyclic mechanical behaviors. On the other hand, the adhesion strength of underfill materials are tested between different adhesion surface by different deformation rate after some isothermal and hygro-thermal environments attack, which is to simulate the environment that the electronic components may be encountered. The results reveal that the rise of the temperature and moisture cause the apparent reduction of the surface adhesion strength, due to the microstructure transition of materials and the diffusion and concentration of moisture. For all conditions of the experiment after environmental preconditioning, the specimen fracture surfaces occur between solder mask and FR4 substrates, which means the measured strength is the adhesion strength between solder mask and FR4. Comparing different adhesion surface, the adhesion strength of underfill/FR4 is higher than solder mask/FR4. The interface of solder mask/FR4 is more sensitive to the temperature and moisture. In all of the cases, increasing the moisture level has a varying but significant effect on both fracture strength and absorption energy Ψ. The failure mode transfer and the strength degradation are attributed to the moisture uptake between the FR4/solder mask and solder mask/underfill interface.     

Co‐Interlayer Engineering toward Efficient Green Quasi‐Two‐Dimensional Perovskite Light‐Emitting Diodes

With respect to three‐dimensional (3D) perovskites, quasi‐two‐dimensional (quasi‐2D) perovskites have unique advantages in light‐emitting devices (LEDs), such as strong exciton binding energy and good phase stability. Interlayer ligand engineering is a key issue to endow them with these properties. Rational design principles for interlayer materials and their processing techniques remain open to investigation. A co‐interlayer engineering strategy is developed to give efficient quasi‐2D perovskites by employing phenylbutylammonium bromide (PBABr) and propylammonium bromide (PABr) as the ligand materials. Preparation of these co‐interlayer quasi‐2D perovskite films is simple and highly controllable without using antisolvent treatment. Crystallization and morphology are readily manipulated by tuning the ratio of co‐interlayer components. Various optical techniques, including steady and ultrafast transient absorption and photoluminescence spectroscopies, are used to investigate their excitonic properties. Photoluminescence quantum yield (PLQY) of the perovskite film is dramatically improved to 89% due to the combined optimization of exciton binding energy and suppression of trap state formation. Accordingly, a high current efficiency of 66.1 cd A^?1 and an external quantum efficiency of 15.1% are achieved for green co‐interlayer quasi‐2D perovskite LEDs without using any light out‐coupling techniques, indicating that co‐interlayer engineering is a simple and effective approach to develop high‐performance perovskite electroluminescence devices.