Yang Yang - Academia.edu (original) (raw)

Papers by Yang Yang

Journal of Applied Physics, 2005

Regioregular poly͑3-hexylthiophene͒ ͑RR-P3HT͒ is a promising candidate for polymer photovoltaic r... more Regioregular poly͑3-hexylthiophene͒ ͑RR-P3HT͒ is a promising candidate for polymer photovoltaic research due to its stability and absorption in the red region. In this manuscript, we report polymer photovoltaic devices based on RR-P3HT:methanofullerene ͓6,6͔-phenyl-C 61 -butyric acid methyl ester ͑PCBM͒ 1:1 weight-ratio blend. We studied the effects of annealing temperature and time on the device performance for devices annealed before and after cathode deposition. Thermal annealing shows significant improvement in the performance for both types of annealing conditions, with postproduction annealing being slightly better. For devices with a 43-nm-thick active layer, maximum power conversion efficiency ͑PCE͒ of 3.2% and fill factor up to 67% is achieved under Air Mass 1.5, 100-mW/ cm 2 illumination. We performed atomic force microscopy and ultraviolet-visible absorption spectroscopy on the P3HT:PCBM films to explain the effect of thermal annealing. By keeping the optimized thermal annealing condition and by varying the active layer thickness, we fabricated devices with PCE up to 4.0%, which is the highest efficiency reported so far for this system.

Nature Materials, 2004

Building on the success of organic electronic devices, such as light-emitting diodes and fi eld-e... more Building on the success of organic electronic devices, such as light-emitting diodes and fi eld-effect transistors, procedures for fabricating non-volatile organic memory devices are now being explored. Here, we demonstrate a novel organic memory device fabricated by solution processing. Programmable electrical bistability was observed in a device made from a polystyrene fi lm containing gold nanoparticles and 8-hydroxyquinoline sandwiched between two metal electrodes. The asprepared device, which is in a low-conductivity state, displays an abrupt transition to a high-conductivity state under an external bias of 2.8 V. These two states differ in conductivity by about four orders of magnitude.

IEEE Transactions on Components and Packaging Technologies, 1997

The triple-dielectric polysilicon-blocking oxide-silicon nitride-tunnel oxide-silicon (SONOS) str... more The triple-dielectric polysilicon-blocking oxide-silicon nitride-tunnel oxide-silicon (SONOS) structure is an attractive candidate for high density E2PROMs suitable for semiconductor disks and as a replacement for high-density dynamic random access memories (DRAMs). Low programming voltages (5 V) and high endurance (greater than 107 cycles) are possible in this multidielectric technology as the intermediate Si3N4 layer is scaled to thicknesses of 50 Å. The thin gate insulator and low programming voltage enable the scaling of the basic memory cell and associated complementary metal-oxide-semiconductor (CMOS) peripheral circuitry on the memory chip. A SONOS 1TC memory cell is proposed in a NOR architecture with a cell area of 6F2 where F is the technology feature size. A 0.20 μm feature size permits a 1TC area of 0.24 μm2 for advanced 1-Gb nonvolatile semiconductor memory chips. A physical model is presented to characterize the erase/write, retention and endurance properties of the nonvolatile semiconductor memory (NVSM) SONOS device

Nature Photonics, 2009

Following the development of the bulk heterojunction 1 structure, recent years have seen a dramat... more Following the development of the bulk heterojunction 1 structure, recent years have seen a dramatic improvement in the efficiency of polymer solar cells. Maximizing the open-circuit voltage in a low-bandgap polymer is one of the critical factors towards enabling high-efficiency solar cells. Study of the relation between open-circuit voltage and the energy levels of the donor/acceptor 2 in bulk heterojunction polymer solar cells has stimulated interest in modifying the open-circuit voltage by tuning the energy levels of polymers 3 . Here, we show that the open-circuit voltage of polymer solar cells constructed based on the structure of a low-bandgap polymer, PBDTTT 4 , can be tuned, step by step, using different functional groups, to achieve values as high as 0.76 V. This increased open-circuit voltage combined with a high short-circuit current density results in a polymer solar cell with a power conversion efficiency as high as 6.77%, as certified by the National Renewable Energy Laboratory.

Nature Materials, 2005

Converting solar energy into electricity provides a much-needed solution to the energy crisis the... more Converting solar energy into electricity provides a much-needed solution to the energy crisis the world is facing today. Polymer solar cells have shown potential to harness solar energy in a cost-effective way. Significant efforts are underway to improve their efficiency to the level of practical applications. Here, we report highly efficient polymer solar cells based on a bulk heterojunction of polymer poly(3-hexylthiophene) and methanofullerene. Controlling the active layer growth rate results in an increased hole mobility and balanced charge transport. Together with increased absorption in the active layer, this results in much-improved device performance, particularly in external quantum efficiency. The power-conversion efficiency of 4.4% achieved here is the highest published so far for polymer-based solar cells. The solution process involved ensures that the fabrication cost remains low and the processing is simple. The high efficiency achieved in this work brings these devices one step closer to commercialization.

IEEE INFOCOM, 2002

Mobile ad hoc networking has been an active research area for several years. How to stimulate coo... more Mobile ad hoc networking has been an active research area for several years. How to stimulate cooperation among selfish mobile nodes, however, is not well addressed yet. In this paper, we propose Sprite, a simple, cheat-proof, creditbased system for stimulating cooperation among selfish nodes in mobile ad hoc networks. Our system provides incentive for mobile nodes to cooperate and report actions honestly. Compared with previous approaches, our system does not require any tamperproof hardware at any node. Furthermore, we present a formal model of our system and prove its properties. Evaluations of a prototype implementation show that the overhead of our system is small. Simulations and analysis show that mobile nodes can cooperate and forward each other's messages, unless the resource of each node is extremely low.

Algorithmic Aspects of Wireless Sensor Networks, 2004

Determining the positions of the sensor nodes in a network is essential to many network functiona... more Determining the positions of the sensor nodes in a network is essential to many network functionalities such as routing, coverage and tracking, and event detection. The localization problem for sensor networks is to reconstruct the positions of all of the sensors in a network, given the distances between all pairs of sensors that are within some radius r of each other. In the past few years, many algorithms for solving the localization problem were proposed, without knowing the computational complexity of the problem. In this paper, we show that no polynomial-time algorithm can solve this problem in the worst case, even for sets of distance pairs for which a unique solution exists, unless RP = NP. We also discuss the consequences of our result and present open problems.

ACM SIGCOMM Conference, 2001

ABSTRACT In secure group communications, users of a group share a common group key. A key server ... more ABSTRACT In secure group communications, users of a group share a common group key. A key server sends the group key to authorized new users as well as performs group rekeying for group users whenever the key changes. In this paper, we investigate scalability issues ...

International Conference on Network Protocols, 2000

Instead of the increase-by-one decrease-to-half strategy used in TCP for congestion window adjust... more Instead of the increase-by-one decrease-to-half strategy used in TCP for congestion window adjustment, we consider the general case such that the increase value and decrease ratio are parameters. That is, in the congestion avoidance state, the window size is increased by α per window of packets acknowledged and it is decreased to β of the current value when there is congestion indication. We refer to this window adjustment strategy as general additive increase multiplicative decrease (GAIMD). We present the (mean) sending rate of a GAIMD flow as a function of α, β, loss rate, mean round-trip time, mean timeout value, and the number of packets acknowledged by each ACK. We conducted extensive experiments to validate this sending rate formula. We found the formula to be quite accurate for a loss rate of up to 20%. We also present a simple relationship between α and β for a GAIMD flow to be TCP-friendly, that is, for the GAIMD flow to have approximately the same sending rate as a TCP flow under the same path conditions. We present results from simulations in which TCP-friendly GAIMD flows (α=0.31, β=7/8) compete for bandwidth with TCP Reno flows and with TCP SACK flows, on a DropTail link as well as on a RED link. We found that the GAIMD flows were highly, TCP-friendly. Furthermore, with β at 7/8 instead of 1/2, these GAIMD flows have reduced rate fluctuations compared to TCP flows

IEEE INFOCOM, 2004

We provide a theoretical foundation for the problem of network localization in which some nodes k... more We provide a theoretical foundation for the problem of network localization in which some nodes know their locations and other nodes determine their locations by measuring the distances to their neighbors. We construct grounded graphs to model network localization and apply graph rigidity theory to test the conditions for unique localizability and to construct uniquely localizable networks. We further study the computational complexity of network localization and investigate a subclass of grounded graphs where localization can be computed efficiently. We conclude with a discussion of localization in sensor networks where the sensors are placed randomly.

IEEE INFOCOM, 2001

We investigate the fairness, smoothness, responsiveness, and aggressiveness of TCP and three repr... more We investigate the fairness, smoothness, responsiveness, and aggressiveness of TCP and three representative TCP-friendly congestion control protocols: GAIMD, TFRC, and TEAR. The properties are evaluated both analytically and via simulation by studying protocol responses to three network environment changes. The first environment change is the inherent fluctuations in a stationary network environment. Under this scenario, we consider three types of sending rate variations: smoothness, short-term fairness, and long-term fairness. For a stationary environment, we observe that smoothness and fairness are positively correlated. We derive an analytical expression for the sending rate coefficient of variation for each of the four protocols. These analytical results match well with experimental results. The other two environment changes we study are a step increase of network congestion and a step increase of available bandwidth. Protocol responses to these changes reflect their responsiveness and aggressiveness, respectively

World Wide Web Conference Series, 2001

Learning is a socially embedded design process. But most of today's hypermedia systems fail to pr... more Learning is a socially embedded design process. But most of today's hypermedia systems fail to properly support both the design-related and the social aspects of learning. Authoring and web-publishing systems aim to support the authors' design process. Consequently, learners' activities are confined to selecting and reading. Based on some fundamental reflections on the role of technology in learning processes, we conclude that top priority must be given to the construction of infrastructures that support cooperative learning processes if we are to properly harness technology's potential.

Computer Communication Review, 2004

Multihoming is often used by large enterprises and stub ISPs to connect to the Internet. In this ... more Multihoming is often used by large enterprises and stub ISPs to connect to the Internet. In this paper, we design a series of novel smart routing algorithms to optimize cost and performance for multihomed users. We evaluate our algorithms through both analysis and extensive simulations based on realistic charging models, traffic demands, performance data, and network topologies. Our results suggest that these algorithms are very effective in minimizing cost and at the same time improving performance. We further examine the equilibrium performance of smart routing in a global setting and show that a smart routing user can improve its performance without adversely affecting other users.

Mobile Ad Hoc Networking and Computing, 2004

In the near future, the advent of large-scale networks of mobile agents autonomously performing l... more In the near future, the advent of large-scale networks of mobile agents autonomously performing long-term sensing and communication tasks will be upon us. However, using controlled node mobility to improve communication performance is a capability that the mobile networking community has not yet investigated. In this paper, we study mobility as a network control primitive. More specifically, we present the first mobility control scheme for improving communication performance in such networks. Our scheme is completely distributed, requiring each node to possess only local information. Our scheme is self-adaptive, being able to transparently encompass several modes of operation, each respectively improving power efficiency for one unicast flow, multiple unicast flows, and many-to-one concast flows. We provide extensive evaluations on the feasibility of mobility control, showing that controlled mobility can improve network performance in many scenarios. This work constitutes a novel application of distributed control to networking in which underlying network communication serves as input to local control rules that guide the system toward a global objective.

ACM SIGCOMM Conference, 2008

The emergence of peer-to-peer (P2P) is posing significant new challenges to achieving efficient a... more The emergence of peer-to-peer (P2P) is posing significant new challenges to achieving efficient and fair utilization of network resources. In particular, without the ability to explicitly communicate with network providers, P2P applications depend mainly on inefficient network inference and network-oblivious peering, leading to potential inefficiencies for both P2P applications and network providers. To address the issues, we propose a simple, light-weight framework called P4P to allow more effective cooperative traffic control between applications and network providers. Our evaluations show clear performance benefits of the framework. The formation of the P4P working group consisting of major ISPs and P2P developers will further develop the framework.

IEEE INFOCOM, 2003

ACM SIGCOMM Conference, 2003

ABSTRACT A recent trend in routing research is to avoid inefficiencies in network-level routing b... more ABSTRACT A recent trend in routing research is to avoid inefficiencies in network-level routing by allowing hosts to either choose routes themselves (eg, source routing) or use overlay routing networks (eg, Detour or RON). Such approaches result in selfish routing, because routing ...

IEEE Transactions on Mobile Computing, 2006

In this paper, we provide a theoretical foundation for the problem of network localization in whi... more In this paper, we provide a theoretical foundation for the problem of network localization in which some nodes know their locations and other nodes determine their locations by measuring the distances to their neighbors. We construct grounded graphs to model network localization and apply graph rigidity theory to test the conditions for unique localizability and to construct uniquely localizable networks. We further study the computational complexity of network localization and investigate a subclass of grounded graphs where localization can be computed efficiently. We conclude with a discussion of localization in sensor networks where the sensors are placed randomly