Chemical Degradation of OLED Host Materials: The Role of Non-Local Interaction in Electronic Excited States

Department of PhysicsOrganic light-emitting devices (OLEDs) have been attracted to both academia and industry, because of their wide applications and remarkable advances in display. Despite the conspicuous advances in OLEDs and even their successful implementation in commercial displays, device degradation issues still remain as one of the most important problems. Because degradation occurs mostly in operating conditions, the underlying mechanism is considered to be linked to carriers in excited states. While several theoretical and experimental studies have focused on the mechanisms of OLED degradation, the microscopic role of the excited carrier remains elusive. In this work, I study how chemical degradation is driven by the excited carrier in OLED materials, using occupation-constrained density functional theory calculations. The results show that the C-N bond is a weak link of OLED molecules both in the electronic ground and excited states, and the rupture of the bond is the main cause of the chemical degradation in short lifetime. While the excited carrier generally weakens the bond, the effect can be mitigated by the non-local interaction with the other bonding and anti-bonding states. The results suggest that the presence of such a non-local interaction can contribute to an enhancement of the chemical stability of the materials in operating environments.ope

Coverage and Economy of Cellular Networks with Many Base Stations

The performance of a cellular network can be significantly improved by employing many base stations (BSs), which shortens transmission distances. However, there exist no known results on quantifying the performance gains from deploying many BSs. To address this issue, we adopt a stochastic-geometry model of the downlink cellular network and analyze the mobile outage probability. Specifically, given Poisson distributed BSs, the outage probability is shown to diminish inversely with the increasing ratio between the BS and mobile densities. Furthermore, we analyze the optimal tradeoff between the performance gain from increasing the BS density and the resultant network cost accounting for energy consumption, BS hardware and backhaul cables. The optimal BS density is proved to be proportional to the square root of the mobile density and the inverse of the square root of the cost factors considered.Comment: 3 pages, 3 figures, to appear in IEEE Communications Letter

Comparative Studies of Detecting Abusive Language on Twitter

The context-dependent nature of online aggression makes annotating large collections of data extremely difficult. Previously studied datasets in abusive language detection have been insufficient in size to efficiently train deep learning models. Recently, Hate and Abusive Speech on Twitter, a dataset much greater in size and reliability, has been released. However, this dataset has not been comprehensively studied to its potential. In this paper, we conduct the first comparative study of various learning models on Hate and Abusive Speech on Twitter, and discuss the possibility of using additional features and context data for improvements. Experimental results show that bidirectional GRU networks trained on word-level features, with Latent Topic Clustering modules, is the most accurate model scoring 0.805 F1.Comment: ALW2: 2nd Workshop on Abusive Language Online to be held at EMNLP 2018 (Brussels, Belgium), October 31st, 201

Random Access Scheduling without Message Passing: A Collision-based AIMD Approach

Department of Computer EngineeringWireless scheduling has been extensively studied in the literature. Since Maximum Weighted Scheduling has been developed and shown to achieve the optimal performance, there have been many efforts to overcome its complexity issue. Random access has attracted much attention due to its potential for low complexity and distributed control, which are desirable for scheduling in multi-hop wireless networks. Although several interesting random access scheduling schemes have been shown to be provably efficient, they suffer in practice from high packet delays or severe performance degradation due to the control overhead to exchange information between neighboring links. In this paper, we develop a novel random access scheduling scheme that does not need message passing. We pay attention to the interplay between the links and control their access probabilities targeting at a certain collision rate. We employ the Additive Increase Multiplicative Decrease (AIMD) algorithm for convergence, and show that our proposed scheme can achieve the same performance bound as the previous random access schemes with high control overhead. We verify our results through simulations and show that our proposed scheme achieves the performance close to that of the centralized greedy algorithm.ope

Impact of universal design ballot interfaces on voting performance and satisfaction of people with and without vision loss

Since the Help America Vote Act (HAVA) in 2002 that addressed improvements to voting systems and voter access through the use of electronic technologies, electronic voting systems have improved in U.S. elections. However, voters with disabilities have been disappointed and frustrated, because they have not been able to vote privately and independently (Runyan, 2007). Voting accessibility for individuals with disabilities has generally been accomplished through specialized designs, providing the addition of alternative inputs (e.g., headphones with tactile keypad for audio output, sip-and-puff) and outputs (e.g., audio output) to existing hardware and/or software architecture. However, while the add-on features may technically be accessible, they are often complex and difficult for poll workers to set up and require more time for targeted voters with disabilities to use compared to the direct touch that enable voters without disabilities to select any candidate in a particular contest at any time. To address the complexities and inequities with the accessible alternatives, a universal design (UD) approach was used to design two experimental ballot interfaces, namely EZ Ballot and QUICK Ballot, that seamlessly integrate accessible features (e.g., audio output) based on the goal of designing one voting system for all. EZ Ballot presents information linearly (i.e., one candidate’s name at a time) and voters can choose Yes or No inputs that does not require search (i.e., finding a particular name). QUICK Ballot presents multiple names that allow users to choose a name using direct-touch or gesture-touch interactions (e.g., the drag and lift gesture). Despite the same goal of providing one type of voting system for all voters, each ballot has a unique selection and navigation process designed to facilitate access and participation in voting. Thus, my proposed research plan was to examine the effectiveness of the two UD ballots primarily with respect to their different ballot structures in facilitating voting performance and satisfaction for people with a range of visual abilities including those with blindness or vision loss. The findings from this work show that voters with a range of visual abilities were able to use both ballots independently. However, as expected, the voter performance and preferences of each ballot interface differed by voters through the range of visual abilities. While non-sighted voters made fewer errors on the linear ballot (EZ Ballot), partially-sighted and sighted voters completed the random access ballot (QUICK Ballot) in less time. In addition, a higher percentage of non-sighted participants preferred the linear ballot, and a higher percentage of sighted participants preferred the random ballot. The main contributions of this work are in: 1) utilizing UD principles to design ballot interfaces that can be differentially usable by voters with a range of abilities; 2) demonstrating the feasibility of two UD ballot interfaces by voters with a range of visual abilities; 3) providing an impact for people with a range of visual abilities on other applications. The study suggests that the two ballots, both designed according to UD principles but with different weighting of principles, can be differentially usable by individuals with a range of visual abilities. This approach clearly distinguishes this work from previous efforts, which have focused on developing one UD solution for everyone because UD does not dictate a single solution for everyone (e.g., a one-size-fits-all approach), but rather supports flexibility in use that provide a new perspective into human-computer interaction (Stephanidis, 2001).Ph.D