Stephanie V. Chasteen

Comparison of blended versus layered structures for poly"p-phenylene vinylene…-based polymer photovoltaics

electron- and hole-transporting polymers, respectively. We find that both blended and bilayered structures have substantially improved current densities 3m A/cm 2 and power efficiencies 1% under white light over neat films. Improved exciton dissociation at multiple interfaces and reduced recombination due to energy and charge transfers increases the charge-carrier collection in both types of heterojunction devices, but low electron mobilities in the polymers lead to low fill factors and reduced quantum efficiency 20% that limit the power efficiency. Time-resolved photoluminescence reveals that for blended structures both the hole and electron-transporting polymers undergo efficient quenching with the exciton decay being dominated by the existence of two fast decay channels of 0.12 and 0.78 ns that are assigned to interspecies charge transfer and account for the increased short-circuit current observed. For layers, these components are not as prevalent. This result indicates that greater exciton generation at the dissociating interface and more efficient charge collection in the thin layers is primarily responsible for the improved short-circuit current, a conclusion that is further supported by numerical simulations of the exciton generation rate and charge collection. We also report evidence for an intermediate exciplex state in both types of structures with the greatest yield for blends with 50 wt % of CN-ether-PPV. Overall, the improved performance is due to different processes in the two structures; efficient bulk exciton quenching and charge transfer in blends and enhanced exciton generation and charge collection in layers. The optimization of each photovoltaic heterostructured device relies on this understanding of the mechanisms by which each material architecture achieves high power efficiencies.

Numerical simulations of layered and blended organic photovoltaic cells

We present results obtained from numerical simulations of organic photovaltaic cells as the donor–acceptor morphology evolves from sharply defined layers, to partial blends and finally homogeneous blends. As the mixing percentage increases, the exciton dissociation increases and the diffusion counter-current decreases, resulting in substantially greater short circuit currents but reduced open circuit voltages. Blended structures are more sensitive to mobility than layers due to recombination throughout the bulk. Our model indicates that solar power efficiencies greater than 10% can be achieved when the zero-field charge mobilities approach 10−3cm2∕Vs for partially blended structures.

Thinking like a physicist: A multi-semester case study of junior-level electricity and magnetism

Physics faculty agree on many of the skills and habits of mind they expect physics students to acquire by the end of their degree, including mathematical sophistication, problem-solving expertise, and an ability to work independently and become expert learners. What is less clear is how these outcomes are best achieved within the context of upper-division courses. Focusing on one key course in the career of an undergraduate major—junior-level Electricity & Magnetism (E&M)—we have investigated this critical question over the course of 4 years and across multiple universities and instructors. With the aim of educating our majors based on a more complete understanding of the cognitive and conceptual challenges of upper-division courses, we transformed junior-level E&M using results and theory from education research. We present the process and content of the transformation and several measures of its success. Students and instructors enjoyed the new course materials. Students in courses using the new materia...

Tapping into Juniors' Understanding of E&M: The Colorado Upper-Division Electrostatics (CUE) Diagnostic

As part of an effort to systematically improve our junior‐level E&M I course, we are developing a tool to assess student learning of E&M concepts at the upper‐division. Along with a faculty working group, we established a list of learning goals for the course that, with student observations and interviews, served as a guide in creating the Colorado Upper‐Division Electrostatics (CUE) assessment. The result is a 17‐question open‐ended post‐test (with an optional 7‐question pre‐test) diagnostic, and accompanying grading rubric. We present the preliminary validation of the instrument and rubric, plus results from 226 students in 4 semesters at the University of Colorado, and 4 additional universities.

But does it last? Sustaining a research-based curriculum in upper-division electricity & magnetism

We report on the process and outcomes from a four-year, eight-semester project to develop, establish, and maintain a new course approach in junior-level electricity and magnetism (E&M). Almost all developed materials (i.e., clicker questions, tutorials, homework, and student difficulties) were used successfully by several subsequent instructors, indicating a high rate of sustainability over time and between instructors. We describe the factors related to successful transfer and to decisions not to adopt the materials, based on observations, instructor interviews, and student data.

The use of concept tests and peer instruction in upper‐division physics

Many upper‐division courses at the University of Colorado now regularly use peer instruction in the form of clicker questions during lectures. Particular attention has been paid to developing and implementing clicker questions in junior‐level E&M and Quantum mechanics. These transformed classes largely follow traditional local norms of syllabus and content coverage, but are designed to address broader learning goals (e.g developing math‐physics connections) that our faculty expect from physics majors in these courses. Concept‐tests are designed to align with these goals, and have altered the dynamic of our classes. Coupled with other course transformations, we find measurable improvement in student performance on targeted conceptual post‐tests. Here, we discuss classroom logistics of upper‐division clickers, purposes of clicker questions, aspects of student engagement facilitated by concept‐tests, and observations of and challenges to sustainability of this activity.

Transforming Upper‐Division Electricity and Magnetism

We transformed an upper‐division electricity and magnetism course for physics and engineering majors using principles of active engagement and learning theory. The teaching practices and new curricular materials were guided by observations and interviews to identify common student difficulties. We established explicit learning goals for the course, created homeworks that addressed key aspects of those learning goals, offered interactive help room sessions, created and ran small‐group tutorial sessions, and used interactive classroom techniques such as peer discussion and “clickers.” We find that students in the transformed course exhibit improved performance over the traditional course, as assessed by common exam questions and a newly developed conceptual post‐test. These results suggest that it is valuable to further investigate how physics is taught at the upper‐division, and how PER may be applied in this context.

Our best juniors still struggle with Gauss’s Law: Characterizing their difficulties

We discuss student conceptual difficulties with Gauss’s law observed in an upper‐division Electricity and Magnetism (EM we present further quantitative and qualitative evidence that upper‐division students still struggle with Gauss’s law. This evidence is drawn from analysis of upper‐division E&M conceptual post‐tests, traditional exams, and formal student interviews. Examples of student difficulties include difficulty with the inverse nature of the problem, difficulty articulating complete symmetry arguments, and trouble recognizing that in situations without sufficient symmetry it is impossible (rather than “difficult”) to calculate the electric field using Gauss’s law. One possible explanation for some of these conceptual difficulties is that even students at the upper level may struggle to connect mathematical expressions to physical meanings.

Facilitating faculty conversations: Development of consensus learning goals

Our upper-division course reform efforts at the University of Colorado start with expert input from non-PER faculty, and these conversations with faculty enrich and guide our course reforms. We have discovered additional benefits of these conversations, such as the fact that they serve as a forum for discussions of pedagogy and PER. However, it is not always obvious - to the faculty or to the PER researchers - what approach will lead to successful meetings. During the process of several course transformations we have met with diverse faculty to generate consensus learning goals and course assessments. We describe the general approach used to structure and facilitate these meetings, and include details on what these meetings entailed, how we achieved broad participation and productive conversations, as well as potential pitfalls to avoid.

Longer term impacts of transformed courses on student conceptual understanding of E&M

We have measured upper‐division physics majors’ performance using two research‐based conceptual instruments in E&M, the BEMA [1] and the CUE (Colorado Upper Division Electrostatics assessment[2].) The BEMA has been given pre/post in freshman E&M (Physics II) courses, and the BEMA and CUE have been given pre/post in several upper‐division E&M courses. Some of these data extend over 10 semesters. We used PER‐based techniques to transform the introductory and upper‐division courses starting in Fall 2004 and 2007, respectively [2, 3]. Our longitudinal data allow us to measure “fade” on BEMA performance between freshman and junior year. We investigate the effects of curricula on students by comparing juniors who were enrolled in traditional vs. transformed physics as freshmen, as well as those who were enrolled in traditional or transformed upper‐division E&M I, using both BEMA and CUE measures. We find that while freshman reforms significantly impact BEMA scores, junior‐level reforms affect CUE but not BEMA o...