Electroluminescence in polymer-fullerene photovoltaic cells

We report electroluminescence (EL) in photovoltaic (PV) cells based on semiconducting polymer-fullerene composites. By applying a forward bias to the PV cells, the devices exhibited a clear EL action with a peak around 1.5 eV. We ascribe this peak to an "electric field-assisted exciplex" formed between the electrons in the fullerenes and the holes in the polymers, thereby resulting in radiative recombination in the composites. This finding is totally unexpected because of a strong photoluminescence quenching in the same materials. Since the same devices also showed typical photovoltaic effects under illumination, our results demonstrate a dual functionality in one device; polymer photovoltaic cells and polymer light-emitting diodes.open464

Promoting Sustainability in University Classrooms Using a STEM Project with Mathematical Modeling

The purpose of this study was to explore how a science, technology, engineering, and mathematics (STEM) project with mathematical modeling influenced student competencies regarding sustainability in a university classroom. We used mixed methods with a convergent parallel design. Forty-two students participated in a STEM project during six consecutive sessions of an algebra class. Before and after the STEM project, the students completed surveys about their perceptions of the mathematical modeling approach. Semi-structured interviews and daily reflection sheets from 10 volunteers were used in qualitative analyses. According to the results of the quantitative analysis, after the completion of the STEM project students perceived that mathematical modeling is a useful tool for recognizing problem situations in the present, for predicting future societal changes, and identifying possible solutions to balance the needs of present and future generations. Our qualitative examination revealed that students’ modeling processes did not necessarily follow processes suggested by prior studies. In fact, students perceived more opportunities to practice modeling processes than we predicted. In addition, students indicated that their increased awareness of STEM projects provided opportunities to practice an interdisciplinary approach and to consider current and future real-world situations. We discuss the implications of our results for teaching sustainability using STEM projects and offer suggestions for future research

Synthesis and properties of polyfluorene, containing oxadiazole and carbazole units as pendants for white light-emitting diodes

Polyfluorene-based conjugated polymer, containing oxadiazole and carbazole units as pendants, was prepared as the emitting layer in light-emitting diodes (LEDs) to show that it has higher maximum brightness and electroluminescence (EL) efficiency as compared to poly(9,9-bis(2-ethylhexyl)fluorene-2,7-diyl). The prepared polymer, poly[9-(6-(N-carbazolyl)-hexyl)-9-(6-(4-(5-phenyl-1,3,4-oxadiazolyl)-phenoxy)-hexyl)-fluorene-2,7-diyl] was used as the emitting layer in LEDs. Oxadiazole and carbazole units as pendants caused aggregation to generate broad peak around 547 nm. The Commission internationale de l'eclairage coordinate of the LEDs was (0.28, 0.30) which is quite close to that of the standard white of National Television Systems Committee (0.31, 0.33). The maximum brightness and EL efficiency of the device with aluminum cathode were 6400 cd/m2 at 10V and 0.50 cd/A at 290mA/cm2, respectively.close10

Organic photovoltaic cells based on conjugated polymer/fullerene composites

We fabricated organic photovoltaic cells using poly[2-methoxy,5-(2'-ethylhexyloxy)-1,4-phenylene-vinylene] (MEH-PPV) and fullerene derivative, [6,6]-phenylen C-61-butyric acid methyl ester (PCBM), composites with various concentrations of the PCBM. The devices exhibit photoluminescence quenching and enhancement in photovoltaic response with increasing PCBM concentrations, both of which are associated with the photoinduced charge transfer characteristics of these materials. We also investigate the PCBM concentration dependence on the device performance near the percolation threshold for fullerene derivative charge transport channel, and discuss the role of fullerene interpenetrating networks in these organic photo voltaic cells.close343