Showing papers by "Baskar Ganapathysubramanian published in 2012"

Enhanced charge separation in organic photovoltaic films doped with ferroelectric dipoles

Abstract: A key requirement for realizing efficient organic photovoltaic (OPV) cells is the dissociation of photogenerated electron-hole pairs (singlet-excitons) in the donor polymer, and charge-transfer-excitons at the donor–acceptor interface. However, in modern OPVs, these excitons are typically not sufficiently harnessed due to their high binding energy. Here, we show that doping the OPV active-layers with a ferroelectric polymer leads to localized enhancements of electric field, which in turn leads to more efficient dissociation of singlet-excitons and charge-transfer-excitons. Bulk-heterojunction OPVs based on poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester are fabricated. Upon incorporating a ferroelectric polymer as additive in the active-layer, power conversion efficiencies increase by nearly 50%, and internal quantum efficiencies approach 100% – indicating complete exciton dissociation at certain photon energies. Similar enhancements in bilayer-heterojunctions, and direct influence of ferroelectric poling on device behavior show that improved dissociation is due to ferroelectric dipoles rather than any morphological change. Enhanced singlet-exciton dissociation is also revealed by photoluminescence lifetime measurements, and predicted by simulations using a numerical device model.

Computer simulation of heterogeneous polymer photovoltaic devices

Abstract: Polymer-based photovoltaic devices have the potential for widespread usage due to their low cost per watt and mechanical flexibility. Efficiencies close to 9.0% have been achieved recently in conjugated polymer based organic solar cells (OSCs). These devices were fabricated using solvent-based processing of electron-donating and electron-accepting materials into the so-called bulk heterojunction (BHJ) architecture. Experimental evidence suggests that a key property determining the power-conversion efficiency of such devices is the final morphological distribution of the donor and acceptor constituents. In order to understand the role of morphology on device performance, we develop a scalable computational framework that efficiently interrogates OSCs to investigate relationships between the morphology at the nano-scale with the device performance.In this work, we extend the Buxton and Clarke model (2007 Modelling Simul. Mater. Sci. Eng. 15 13–26) to simulate realistic devices with complex active layer morphologies using a dimensionally independent, scalable, finite-element method. We incorporate all stages involved in current generation, namely (1) exciton generation and diffusion, (2) charge generation and (3) charge transport in a modular fashion. The numerical challenges encountered during interrogation of realistic microstructures are detailed. We compare each stage of the photovoltaic process for two microstructures: a BHJ morphology and an idealized sawtooth morphology. The results are presented for both two- and three-dimensional structures.

Computational characterization of bulk heterojunction nanomorphology

Abstract: The bulk heterojunction (BHJ) nanomorphology in organic solar cells strongly affects the final efficiency of the device. Progress in experimental techniques now allows visualization of the complex 3D BHJ morphology. It is, therefore, important to characterize the topological properties of the morphology in order to quantify the link between morphology features and performance. Here, we introduce a suite of morphology descriptors which encode the complex nature of the multi-stage photovoltaic process in the BHJ. These morphology descriptors are easily determined using an approach based on converting the morphology into an equivalent weighted, labeled, undirected graph. We show how these descriptors can be used to interrogate BHJ morphologies, allow identification of bottlenecks in the photovoltaic process, and conduct quantitative comparison between morphologies with respect to each sub-process in the photovoltaic phenomena. This framework provides a simple and easy-to-use characterization tool that can be used to unravel the impact of morphology on complex transport phenomena.

Nanoscale surface roughness affects low Reynolds number flow: Experiments and modeling

Abstract: Most micro-channel fabrication strategies generate nano-to-micro-scale, stochastic surface roughness. This inherent stochasticity can potentially be harnessed to direct microfluidic operations such as self-cleaning behavior and localized mixing. This work investigates the effect of stochastic nanoscale roughness on low to moderate Reynolds number Newtonian flow using concurrent modeling and experiments. We fabricate a microscopic channel with tailored hydrofluoric-acid-etched rough surfaces. Optical profilometry and micro-particle-image-velocimetry (micro-PIV) are used to characterize the surface roughness and flow field and is integrated with direct numerical simulation that resolves effects of nanoscale roughness. Results indicate that nanoscale roughness causes flow perturbations that extend up to the mid-plane and is insensitive to flow-rates.

Cantilever deflection associated with hybridization of monomolecular DNA film

Abstract: Recent experiments show that specific binding between a ligand and surface immobilized receptor, such as hybridization of single stranded DNA immobilized on a microcantilever surface, leads to cantilever deflection. The binding-induced deflection may be used as a method for detection of biomolecules, such as pathogens and biohazards. Mechanical deformation induced due to hybridization of surface-immobilized DNA strands is a commonly used system to demonstrate the efficacy of microcantilever sensors. To understand the mechanism underlying the cantilever deflections, a theoretical model that incorporates the influence of ligand/receptor complex surface distribution and empirical interchain potential is developed to predict the binding-induced deflections. The cantilever bending induced due to hybridization of DNA strands is predicted for different receptor immobilization densities, hybridization efficiencies, and spatial arrangements. Predicted deflections are compared with experimental reports to validate the modeling assumptions and identify the influence of various components on mechanical deformation. Comparison of numerical predictions and experimental results suggest that, at high immobilization densities, hybridization-induced mechanical deformation is determined, primarily by immobilization density and hybridization efficiency, whereas, at lower immobilization densities, spatial arrangement of hybridized chains need to be considered in determining the cantilever deflection.

Exploring the Effect of Stick-Slip Friction Transition Across Tape-Roller Interface on the Transmission of Lateral Vibration

Abstract: In magnetic tape drives, lateral in-plane vibration of the tape leads to misalignment between data tracks and read/write head's position resulting in reduced storage capacity. To attenuate this lateral tape motion (LTM), surface guides-which include grooved, porous or roughened rollers-are used. The axial motion of the tape over the roller surface switches between two states: a sticking state when the axial force is smaller than the static frictional force; and a slipping state when the axial force is larger than the frictional force. A good understanding of the physical phenomena involved in this frictional interaction between the magnetic tape and surface friction guides will allow the appropriate and optimal choice of roller characteristics. We conduct a parametric study of frictional interaction between roller surface and the traveling magnetic tape by systematically varying the axial tension and transport velocity of the tape. An experimental setup is used to independently control these parameters and obtain lateral vibration measurements at two equidistant points-upstream and downstream-from the tape-roller interface. Techniques from spectral analysis are applied to the two signals to analyze and isolate the effect of stick-slip friction on LTM. It is noticed that the coherence function between the LTM signals provides valuable insight into the nature of stick-slip friction at the interface. We subsequently use it as a metric to construct and understand the “dynamic phase diagram,” i.e., to demarcate regions in the tension-velocity phase-space where predominance of stick or slip occurs.

WiME: a departmental effort to improve recruitment, retention and engagement of women students in Mechanical Engineering

Abstract: The Mechanical Engineering department at Iowa State University started the ‘Women in Mechanical Engineering’ (WiME) program 3 years ago. WiME is a student run, faculty moderated, and department funded program with a three pronged approach to enhance women participation in mechanical engineering – retention, outreach, and recruitment. WiME organizes various social and professional development activities that are intended to • Provide a forum for women to socially interact with faculty and their peers; • Provide students with women role models in engineering and academic professionals thereby helping to develop and sustain a network of professional colleagues and; • Leverage the interests of the WiME group to undertake outreach to local K-12 schools, thus creating a sustainable group. Several specific events and activities include (a) the “Chat-with-ME” series where students are provided an opportunity to interact with a successful women mechanical engineer in a very informal setting; (b) social events like picnics, bowling evenings and ‘Ice-cream socials’ where the women students can interact with each other and ME faculty; (c) professional development events like mock interview and interview walk-up sessions before career fairs and more recently; (d) “efficient energy use” outreach sessions to local middle schools in collaboration with industrial partners. Students have pointed to the social interaction opportunities with their peers and with faculty as a major attraction of the program. The department has also set up WiME scholarships to prospective high school students to enhance recruitment and also employs personalized phone calls from the department chair to all women applicants to highlight the program. Since the launch of the WiME program the women enrollment in the ME program has increased from 76 to 128 women (7.0% to 10%). Disciplines Engineering Education | Higher Education | Mechanical Engineering | Science and Mathematics Education This conference proceeding is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/me_conf/49 AC 2012-5501: WOMEN IN MECHANICAL ENGINEERING: A DEPARTMENTAL EFFORT TO IMPROVE RECRUITMENT, RETENTION, AND ENGAGEMENT OF WOMEN STUDENTS Prof. Sriram Sundararajan, Iowa State University Sriram Sundararajan is an Associate Professor of mechanical engineering at Iowa State University. He is currently the Associate Chair for Undergraduate Programs, and he oversees curricular and program matters, including assessment and continuous improvement efforts. His research areas encompass scanning probe microscopy, multiscale tribology (friction, lubrication and wear), and surface engineering. More recently, he has focused on atom scale mapping of thin film material systems using 3D-atom probe microscopy. He has authored more than 50 articles in peer-reviewed journals and conference proceedings and two invited book chapters. He serves on the conference committee for the International Conference on Wear of Materials and has been recognized for his accomplishments with the Young Engineering Faculty Research Award and Early Achievement in Teaching Award at Iowa State University. He received his B.E. degree in mechanical engineering from the Birla Institute of Technology and Science, Pilani (India), followed by M.S. and Ph.D. degrees in mechanical engineering from the Ohio State University, Columbus, Ohio. He is a member of ASEE, ASME, and ASM. Dr. Theodore J. Heindel, Iowa State University Dr. Baskar Ganapathysubramanian Shankar Subramaniam, Iowa State University Shankar Subramaniam is an Associate Professor in the Department of Mechanical Engineering at Iowa State University. He received his B.Tech. in aeronautical engineering from the Indian Institute of Technology, Bombay (Mumbai) in 1988 and is a recipient of the President’s Silver Medal. He earned his Ph.D. at Cornell University, subsequent to an M.S. in aerospace engineering at the University of Notre Dame, USA. After his Ph.D., he spent two years as a postdoctoral researcher at Los Alamos National Laboratory in the Theoretical Division’s Fluid Dynamics Group. Prior to joining the ISU faculty in 2002, Subramaniam was an Assistant Professor at Rutgers University. He is a recipient of the U.S. Department of Energy’s Early Career Principal Investigator award. His areas of expertise are in theory, modeling and simulation of multiphase flows (including sprays, particle-laden flows, colloids, and granular mixtures), turbulence, mixing, and reacting flows. His current research concerns hierarchical coarse-graining approaches, mesoscale models of colloidal aggregation, and direct numerical simulation of gas-solid flows. c ©American Society for Engineering Education, 2012 WiME: a departmental effort to improve recruitment, retention and engagement of women students in Mechanical Engineering Abstract The Mechanical Engineering department at Iowa State University started the ‘Women in Mechanical Engineering’ (WiME) program 3 years ago. WiME is a student run, faculty moderated, and department funded program with a three pronged approach to enhance women participation in mechanical engineering – retention, outreach, and recruitment. WiME organizes various social and professional development activities that are intended to • Provide a forum for women to socially interact with faculty and their peers; • Provide students with women role models in engineering and academic professionals thereby helping to develop and sustain a network of professional colleagues and; • Leverage the interests of the WiME group to undertake outreach to local K-12 schools, thus creating a sustainable group. Several specific events and activities include (a) the “Chat-with-ME” series where students are provided an opportunity to interact with a successful women mechanical engineer in a very informal setting; (b) social events like picnics, bowling evenings and ‘Ice-cream socials’ where the women students can interact with each other and ME faculty; (c) professional development events like mock interview and interview walk-up sessions before career fairs and more recently; (d) “efficient energy use” outreach sessions to local middle schools in collaboration with industrial partners. Students have pointed to the social interaction opportunities with their peers and with faculty as a major attraction of the program. The department has also set up WiME scholarships to prospective high school students to enhance recruitment and also employs personalized phone calls from the department chair to all women applicants to highlight the program. Since the launch of the WiME program the women enrollment in the ME program has increased from 76 to 128 women (7.0% to 10%).The Mechanical Engineering department at Iowa State University started the ‘Women in Mechanical Engineering’ (WiME) program 3 years ago. WiME is a student run, faculty moderated, and department funded program with a three pronged approach to enhance women participation in mechanical engineering – retention, outreach, and recruitment. WiME organizes various social and professional development activities that are intended to • Provide a forum for women to socially interact with faculty and their peers; • Provide students with women role models in engineering and academic professionals thereby helping to develop and sustain a network of professional colleagues and; • Leverage the interests of the WiME group to undertake outreach to local K-12 schools, thus creating a sustainable group. Several specific events and activities include (a) the “Chat-with-ME” series where students are provided an opportunity to interact with a successful women mechanical engineer in a very informal setting; (b) social events like picnics, bowling evenings and ‘Ice-cream socials’ where the women students can interact with each other and ME faculty; (c) professional development events like mock interview and interview walk-up sessions before career fairs and more recently; (d) “efficient energy use” outreach sessions to local middle schools in collaboration with industrial partners. Students have pointed to the social interaction opportunities with their peers and with faculty as a major attraction of the program. The department has also set up WiME scholarships to prospective high school students to enhance recruitment and also employs personalized phone calls from the department chair to all women applicants to highlight the program. Since the launch of the WiME program the women enrollment in the ME program has increased from 76 to 128 women (7.0% to 10%). 1. Introduction With the changing demographics of the nation and state of the engineering workforce, the underrepresentation of women among engineering undergraduates and the subsequent lack of females in the workforce is a subject of national concern. Studies show that about 20% of engineering baccalaureate degrees are awarded to women, which is significantly less than corresponding numbers in mathematics, statistics and various science degrees. Consequently, studies focusing on identifying the factors contributing to the lack of women at the degree program level and in the engineering workforce 20 have been invaluable in suggesting best practices to address this critical issue. For example, peer-peer interactions and faculty-student interactions that promote respect and encouragement as well as learner-centered approaches to pedagogy have been shown to have a positive impact on the college experience of women students as well as on their decision to pursue an engineering career. These studies and others suggest that creating environments that emphasize care and respect for students as well as positive student interaction during group work can make a difference in students’ satisfaction in the engineering major and in interest in engineering as a career, particularly for women. Mechanical Engineering (ME) in particular is challenged with lower than average women en