High‐Performance Air‐Processed Polymer–Fullerene Bulk Heterojunction Solar Cells

High photovoltaic device performance is demonstrated in ambient‐air‐processed bulk heterojunction solar cells having an active blend layer of organic poly(3‐hexylthiophene) (P3HT): [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM), with power conversion efficiencies as high as 4.1%, which is comparable to state‐of‐the‐art bulk heterojunction devices fabricated in air‐free environments. High‐resolution transmission electron microscopy is combined with detailed analysis of electronic carrier transport in order to quantitatively understand the effects of oxygen exposure and different thermal treatments on electronic conduction through the highly nanostructured active blend network. Improvement in photovoltaic device performance by suitable post‐fabrication thermal processing results from the reduced oxygen charge trap density in the active blend layer and is consistent with a corresponding slight increase in thickness of an ～4 nm aluminum oxide hole‐blocking layer present at the electron‐collecting contact interface.     

Consideration of temperature and current stress testing on flip chip solder interconnects

This work discusses the experimental set-up and data interpretation for high temperature and current stress tests of flip chip solder joints using the four-point Kelvin measurement technique. The single solder joint resistance responses are measured at four different four-point Kelvin structure locations in a flip chip package. Various temperatures (i.e., 125–165 °C) and electric current (i.e., 0.6–1.0 A) test conditions are applied to investigate the solder joint resistance degradation behavior and its failure processes. Failure criterion of 20% and 50% joint resistance increases, corresponding to solder and interfacial voiding, are employed to evaluate the solder joint electromigration reliability. The absolute resistance value is substantially affected by the geometrical layout of the metal lines in the four-point Kelvin structure, and this is confirmed by finite element simulation.Different current flow directions and strengths yielded different joint resistance responses. The anode joint, where electrons flow from the die to the substrate, usually measured an earlier resistance increase than the cathode joint, where electrons flow in the opposite direction. The change in measured joint resistances can be related to solder and interfacial voiding in the solder joint except for ±1 A current load, where resistance drop mainly attributed to the broken substrate Cu metallization as a result of “hot-spot” phenomenon. The solder joint temperature increases above the oven ambient temperature by ～25 °C, ～40 °C and ～65 °C for 0.6 A, 0.8 A and 1.0 A stress current, respectively. It is found that two-parameter log-normal distribution gives a better lifetime data fitting than the two-parameter Weibull distribution. Regardless of failure criterion used, the anode joint test cells usually calculated a shorter solder joint mean life with a lower standard variation of 0.3–0.6, as compared to the cathode joint test cells with a higher standard variation of 0.8–1.2. For a typical flip chip solder joint construction, electromigration reliability is mainly determined by the under bump metallization consumption and dissolution, with intermetallic compound formation near the die side of an anode joint.     

平顶光束激光冲击2024铝合金诱导残余应力场的模拟与实验

对平顶光束激光冲击2024铝合金诱导的残余应力情况进行了有限元模拟与实验研究。改进了平顶光束诱导冲击波的压力分布模型,并将该模型用于残余应力场的有限元模拟。在实验室环境下获得了适合用于激光冲击的高质量平顶光束,并使用该光束进行激光冲击2024铝合金的实验,实验结果和模拟结果基本一致。研究发现平顶光束冲击2024铝合金有如下特点:存在一个阈值,当激光冲击波压力小于该阈值时,影响区内残余应力场近似均匀分布;当冲击波压力大于该阈值时会引起"残余应力洞",但该"残余应力洞"内部近似均匀分布。在深度方向上,塑性影响深度和最大残余应力深度随激光冲击波压力的增加而增加。     

Cryptographic Hash Functions from Expander Graphs

We propose constructing provable collision resistant hash functions from expander graphs in which finding cycles is hard. As examples, we investigate two specific families of optimal expander graphs for provable collision resistant hash function constructions: the families of Ramanujan graphs constructed by Lubotzky-Phillips-Sarnak and Pizer respectively. When the hash function is constructed from one of Pizer’s Ramanujan graphs, (the set of supersingular elliptic curves over with ℓ-isogenies, ℓ a prime different from p), then collision resistance follows from hardness of computing isogenies between supersingular elliptic curves. For the LPS graphs, the underlying hard problem is a representation problem in group theory. Constructing our hash functions from optimal expander graphs implies that the outputs closely approximate the uniform distribution. This property is useful for arguing that the output is indistinguishable from random sequences of bits. We estimate the cost per bit to compute these hash functions, and we implement our hash function for several members of the Pizer and LPS graph families and give actual timings.     

Evaluation of Corn/Soybeans Separability Using Thematic Mapper And Thematic Mapper Simulator Data

Multitemporal Thematic Mapper, Thematic Mapper Simulator, and detailed ground truth data were collected for a 9-by 1-km sample segment in Webster County, Iowa, in the summer of 1982. Three dates were acquired each with Thematic Mapper Simulator (June 7, June 23, and July 31) and Thematic Mapper (August 2, September 3, and October 21). The Thematic Mapper Simulator data were converted to equivalent TM count values using TM and TMS calibration data and model based estimates of atmospheric effects. The July 31, TMS image was compared to the August 2, TM image to verify the conversion process. A quantitative measure of proportion estimation variance (Fisher information) was used to evaluate the corn/soybeans separability for each TM band as a function of time during the growing season. The additional bands in the middle infrared allowed corn and soybeans to be separated much earlier than was possible with the visible and near-infrared bands alone. Using the TM and TMS data, temporal profiles of the TM principal components were developed. The greenness and brightness exhibited behavior similar to MSS greenness and brightness for corn and soybeans.     

Fingerprint indexing based on minutiae-dependent statistical codes

Methods based on minutiae matching have been extensively used in fingerprint recognition because minutiae can be reliably extracted from poor quality and noisy fingerprints. However, structures have to be defined due to minutiae displacements and irreproducibility. Some of the structures, though very efficient, incur large computational complexities. In this article, a feature vector of statistically based values derived from the minutiae pattern in a fingerprint is proposed for indexing fingerprints using the incremental search retrieval method. The proposed indexing technique in combination with the incremental retrieval method proves to have an added advantage over certain minutiae-based structures, especially when the minutiae points are numerous in a fingerprint. The feature vector for a fingerprint requires negligible storage resources and, consequently, the computational time in the retrieval of a candidate list for a query fingerprint is very little.     

Tracing the route of China’s Internet censorship: An empirical study

Faced with the world’s largest Internet population, the Chinese government is torn between its massive drive for marketization and the need to curb cyber dissent. This paper investigates how the Chinese state censors the Internet by tracing the trajectory of mechanisms to block websites non grata. Results show that Chinese government’s Internet control methods are diverse with systematic collaborations from local authorities at various administrative levels. We also found evidence that the government has customized blocking strategies for what it considers to be important websites. The efficacy and implications of China’s Internet censorship system were also discussed.     

Interfacial Connections between Organic Perovskite/n+ Silicon/Catalyst that Allow Integration of Solar Cell and Catalyst for Hydrogen Evolution from Water

The rapidly increasing solar conversion efficiency (PCE) of hybrid organic–inorganic perovskite (HOIP) thin-film semiconductors has triggered interest in their use for direct solar-driven water splitting to produce hydrogen. However, application of these low-cost, electronic-structure-tunable HOIP tandem photoabsorbers has been hindered by the instability of the photovoltaic-catalyst-electrolyte (PV+E) interfaces. Here, photolytic water splitting is demonstrated using an integrated configuration consisting of an HOIP/n⁺silicon single junction photoabsorber and a platinum (Pt) thin film catalyst. An extended electrochemical (EC) lifetime in alkaline media is achieved using titanium nitride on both sides of the Si support to eliminate formation of insulating silicon oxide, and as an effective diffusion barrier to allow high-temperature annealing of the catalyst/TiO₂-protected-n⁺silicon interface necessary to retard electrolytic corrosion. Halide composition is examined in the (FA_1-xCs_x)PbI₃ system with a bandgap suitable for tandem operation. A fill factor of 72.5% is achieved using a Spiro-OMeTAD-hole-transport-layer (HTL)-based HOIP/n⁺Si solar cell, and a high photocurrent density of −15.9 mA cm⁻² (at 0 V vs reversible hydrogen electrode) is attained for the HOIP/n⁺Si/Pt photocathode in 1 m NaOH under simulated 1-sun illumination. While this thin-film design creates stable interfaces, the intrinsic photo- and electro-degradation of the HOIP photoabsorber remains the main obstacle for future HOIP/Si tandem PEC devices.     

The hypervolume under the ROC hypersurface of "near-guessing" and "near-perfect" observers in N-class classification tasks

We express the performance of the N-class "guessing" observer in terms of the N2-N conditional probabilities which make up an N-class receiver operating characteristic (ROC) space, in a formulation in which sensitivities are eliminated in constructing the ROC space (equivalent to using false-negative fraction and false-positive fraction in a two-class task). We then show that the "guessing" observer's performance in terms of these conditional probabilities is completely described by a degenerate hypersurface with only N-1 degrees of freedom (as opposed to the N2-N-1 required, in general, to achieve a true hypersurface in such a ROC space). It readily follows that the hypervolume under such a degenerate hypersurface must be zero when N > 2. We then consider a "near-guessing" task; that is, a task in which the N underlying data probability density functions (pdfs) are nearly identical, controlled by N-1 parameters which may vary continuously to zero (at which point the pdfs become identical). With this approach, we show that the hypervolume under the ROC hypersurface of an observer in an N-class classification task tends continuously to zero as the underlying data pdfs converge continuously to identity (a "guessing" task). The hypervolume under the ROC hypersurface of a "perfect" ideal observer (in a task in which the N data pdfs never overlap) is also found to be zero in the ROC space formulation under consideration. This suggests that hypervolume may not be a useful performance metric in N-class classification tasks for N > 2, despite the utility of the area under the ROC curve for two-class tasks.     

MnO2-Based Electrochemical Supercapacitors on Flexible Carbon Substrates

Manganese dioxide films were grown on large area flexible carbon aerogel substrates. Characterization by x-ray diffraction confirmed α-MnO₂ growth. Three types of films were compared as a function of hexamethylenetetramine (HMTA) concentration during growth. The highest concentration of HM TA produced MnO₂ flower-like films, as observed by scanning electron microscopy, whose thickness and surface coverage lead to both a higher specific capacitance and higher series resistance. Specific capacitance was measured to be 64 F/g using a galvanostatic setup, compared to the 47 F/g-specific capacitance of the carbon aerogel substrate. Such supercapacitor devices can be fabricated on large area sheets of carbon aerogel to achieve high total capacitance.