The phase behavior of a polymer‐fullerene bulk heterojunction system that contains bimolecular crystals

Polymer:fullerene blends have been widely studied as an inexpensive alternative to traditional silicon solar cells. Some polymer:fullerene blends, such as blends of poly(2,5‐bis(3‐tetradecylthiophen‐2‐yl)thieno[3,2‐b]thiophene (pBTTT) with phenyl‐c71‐butyric acid methyl ester (PC₇₁BM), form bimolecular crystals due to fullerene intercalation between the polymer side chains. Here we present the determination of the eutectic pBTTT:PC₇₁BM phase diagram using differential scanning calorimetry (DSC) and two‐dimensional grazing incidence X‐ray scattering (2D GIXS) with in‐situ thermal annealing. The phase diagram explains why the most efficient pBTTT:PC₇₁BM solar cells have 75–80 wt % PC₇₁BM since these blends lie in the center of the only room‐temperature phase region containing both electron‐conducting (PC₇₁BM) and hole‐conducting (bimolecular crystal) phases. We show that intercalation can be suppressed in 50:50 pBTTT:PC₇₁BM blends by using rapid thermal annealing to heat the blends above the eutectic temperature, which forces PC₇₁BM out of the bimolecular crystal, followed by quick cooling to kinetically trap the pure PC₇₁BM phase. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Energy and hole transfer between dyes attached to titania in cosensitized dye-sensitized solar cells

Cosensitization of broadly absorbing ruthenium metal complex dyes with highly absorptive near-infrared (NIR) organic dyes is a clear pathway to increase near-infrared light harvesting in liquid-based dye-sensitized solar cells (DSCs). In cosensitized DSCs, dyes are intimately mixed, and intermolecular charge and energy transfer processes play an important role in device performance. Here, we demonstrate that an organic NIR dye incapable of hole regeneration is able to produce photocurrent via intermolecular energy transfer with an average excitation transfer efficiency of over 25% when cosensitized with a metal complex sensitizing dye (SD). We also show that intermolecular hole transfer from the SD to NIR dye is a competitive process with dye regeneration, reducing the internal quantum efficiency and the electron lifetime of the DSC. This work demonstrates the general feasibility of using energy transfer to boost light harvesting from 700 to 800 nm and also highlights a key challenge for developing highly efficient cosensitized dye-sensitized solar cells.     

Multifrequency acoustical volume backscattering patterns in the Arabian Sea--265 kHz to 3 MHz

High-frequency acoustical volume backscattering was examined during three periods of the annual cycle of winds in the Arabian Sea-the Winter Northeast (NE) Monsoon, the Summer Southwest (SW) Monsoon, and the Fall Intermonsoon-using a multifrequency acoustic profiling system deployed on an undulating towed body. This system measured acoustical volume backscattering strengths at six frequencies ranging from 265 kHz to 3.0 MHz. Measurements were made from near the surface to 250 m depth along a ship track that paralleled the coast of Oman, then proceeded away from the coast towards the middle of the Arabian Sea. At all frequencies, volume backscattering strength was highest near the surface, decreasing with increasing depth. Contrary to expectation, backscattering was generally lower during the Summer SW Monsoon than during the Winter NE Monsoon. Also contrary to expectation, backscattering was not appreciably higher near the coast than it was offshore during the Summer SW Monsoon, although it was higher near the coast during the Winter NE Monsoon. Generally speaking, regional and seasonal differences were smaller than expected, and much smaller than the fine-scale spatial and temporal variability, particularly at frequencies below 1 MHz. There was a daily pattern of increased nighttime backscattering in the upper 100 m at 265 and 420 kHz. This pattern was less evident at 1.1 MHz and above.     

Conley decomposition for closed relations

This paper presents a theory of dynamics of closed relations on compact Hausdorff spaces. It contains an investigation of set valued maps and establishes generalizations for some topological aspects of dynamical systems theory, including recurrence, attractor–repeller structure and the Conley decomposition theorem.     

Developing Interdisciplinary Units: A Strategy Based on Problem Solving

While the benefits of the interdisciplinary unit are well documented, it presents a complex challenge to teachers in the natural and social sciences, mathematics, and humanities. Teachers must become active curriculum designers who shape and edit the curriculum according to students' needs. This paper describes knowledge for teachers as curriculum designers and a framework for interdisciplinary unit development. The framework addresses a metacurricular process (problem solving) that will be the unit centerpiece, the development of this central process related to the learner, and the tasks that teach explicit learning and thinking skills attached to the central process. An example of the framework in action is also described. As the faculty and curriculum coordinators for an innovative summer academy for minority students in northern Arizona have used this framework, they have evolved from a group that created a good idea to interest students with parallel subject development in separate classrooms to humanities/mathematics/science teams united in one team/classroom, in which content is integrated through the actions of the problem solving process.     

Complementary C3-symmetric donor-acceptor components: cocrystal structure and control of mesophase stability

The overlap of pi-complementary planar organic frameworks is used to direct the assembly of extended columns of alternating donor and acceptor units. The electron-rich partner, hexaalkoxytriphenylene, is a familiar mesogen, while the electron-accepting complement is mellitic triimide, a new C(3)-symmetric building block that may be readily alkylated at its periphery without compromising its electron-accepting ability. A cocrystal of examples of the two components demonstrates pi-facial overlap of the complementary aromatic surfaces. Preparation of a series of alkylated derivatives of each component allowed the study of an array of 1:1 stoichiometry mixtures. For the optimum donor-acceptor organized mesophases within this grid, temperature stability ranges of well over 100 degrees C are observed, some of which extend below room temperature. X-ray analysis confirms the formation of hexagonally packed, alternating, donor-acceptor columns within each of the observed mesophases. The dramatic effect on mesophase formation and stability engendered via donor-acceptor organization within discrete columns is discussed in terms of the interplay of forces leading to mesophase formation, and the potential to tune mesophase characteristics via manipulation of these factors.     

3,4-Disubstituted polyalkylthiophenes for high-performance thin-film transistors and photovoltaics

We demonstrate that poly(3,4-dialkylterthiophenes) (P34ATs) have comparable transistor mobilities (0.17 cm(2) V(-1) s(-1)) and greater environmental stability (less degradation of on/off ratio) than regioregular poly(3-alkylthiophenes) (P3ATs). Unlike poly(3-hexylthiophene) (P3HT), P34ATs do not show a strong and distinct π-π stacking in X-ray diffraction. This suggests that a strong π-π stacking is not always necessary for high charge-carrier mobility and that other potential polymer packing motifs in addition to the edge-on structure (π-π stacking direction parallel to the substrate) can lead to a high carrier mobility. The high charge-carrier mobilities of the hexyl and octyl-substituted P34AT produce power conversion efficiencies of 4.2% in polymer:fullerene bulk heterojunction photovoltaic devices. An enhanced open-circuit voltage (0.716-0.771 eV) in P34AT solar cells relative to P3HT due to increased ionization potentials was observed.     

A Layered Hybrid Perovskite Solar‐Cell Absorber with Enhanced Moisture Stability

Two‐dimensional hybrid perovskites are used as absorbers in solar cells. Our first‐generation devices containing (PEA)₂(MA)₂[Pb₃I₁₀] ( 1 ; PEA=C₆H₅(CH₂)₂NH₃⁺, MA=CH₃NH₃⁺) show an open‐circuit voltage of 1.18 V and a power conversion efficiency of 4.73 %. The layered structure allows for high‐quality films to be deposited through spin coating and high‐temperature annealing is not required for device fabrication. The 3D perovskite (MA)[PbI₃] ( 2 ) has recently been identified as a promising absorber for solar cells. However, its instability to moisture requires anhydrous processing and operating conditions. Films of 1 are more moisture resistant than films of 2 and devices containing 1 can be fabricated under ambient humidity levels. The larger bandgap of the 2D structure is also suitable as the higher bandgap absorber in a dual‐absorber tandem device. Compared to 2 , the layered perovskite structure may offer greater tunability at the molecular level for material optimization.