Si Radial p-i-n Junction Photovoltaic Arrays with Built-In Light Concentrators.

High-performance photovoltaic (PV) devices require strong light absorption, low reflection and efficient photogenerated carrier collection for high quantum efficiency. Previous optical studies of vertical wires arrays have revealed that extremely efficient light absorption in the visible wavelengths is achievable. Photovoltaic studies have further advanced the wire approach by employing radial p-n junction architectures to achieve more efficient carrier collection. While radial p-n junction formation and optimized light absorption have independently been considered, PV efficiencies have further opportunities for enhancement by exploiting the radial p-n junction fabrication procedures to form arrays that simultaneously enhance both light absorption and carrier collection efficiency. Here we report a concept of morphology control to improve PV performance, light absorption and quantum efficiency of silicon radial p-i-n junction arrays. Surface energy minimization during vapor phase epitaxy is exploited to form match-head structures at the tips of the wires. The match-head structure acts as a built-in light concentrator and enhances optical absorptance and external quantum efficiencies by 30 to 40%, and PV efficiency under AM 1.5G illumination by 20% compared to cylindrical structures without match-heads. The design rules for these improvements with match-head arrays are systematically studied. This approach of process-enhanced control of three-dimensional Si morphologies provides a fab-compatible way to enhance the PV performance of Si radial p-n junction wire arrays.

Evidence against a chondritic Earth.

The (142)Nd/(144)Nd ratio of the Earth is greater than the solar ratio as inferred from chondritic meteorites, which challenges a fundamental assumption of modern geochemistry--that the composition of the silicate Earth is 'chondritic', meaning that it has refractory element ratios identical to those found in chondrites. The popular explanation for this and other paradoxes of mantle geochemistry, a hidden layer deep in the mantle enriched in incompatible elements, is inconsistent with the heat flux carried by mantle plumes. Either the matter from which the Earth formed was not chondritic, or the Earth has lost matter by collisional erosion in the later stages of planet formation.

Effects of peripheral and axial substitutions on electronic transitions of tin naphthalocyanines.

Tin naphthalocyanine molecules display strong absorption in the infrared region (IR), making them ideal as components of organic photodiodes and solar cells. We use density functional theory and time-dependent density functional theory (TD-DFT) at the B3LYP level to study the influence of axial and peripheral ligands on the absorption wavelength of tin naphthalocyanines. We find that TD-DFT is successful at reproducing the experimental absorption spectra of free base naphthalocyanine and tin naphthalocyanine molecules and can be used as a reliable tool to predict absorption spectra of substituted naphthalocyanines. Functional groups attached axially to tin (-F, -Cl, -Br, -I) and peripherally to the inner ring (-F, -Cl, -Br, -Ph, -OH, -COCH(3), -O(CH(2))(3)CH(3)) of the tin naphthalocyanine molecule tune the excitation wavelength in the near-infrared region between 770 and 940 nm. While substituents to the outer naphthalocyanine ring (-Cl, -Br) affect the intensity of the absorption peaks in the NIR region, they do not influence their absorption wavelength. Asymmetric substitution of naphthalocyanine pendant arms can be employed to decrease intensity of the absorption peaks in the visible region with respect to the intensity of the peaks in the NIR.

Measuring changes in activity patterns during a norovirus epidemic at a retirement community.

Ubiquitous and unobtrusive in-home monitoring has the potential to detect physical and mental decline earlier and with higher precision than current clinical methods. However, given that this field is in its infancy, the specific metrics through which these changes are detected are not well defined. The work presented here offers room-transitions, the act of physically moving from one area of a home to another, as a quantifiable measure for total daily activity that can be inferred from a network of passive infrared sensors. We describe a method to calculate this value from raw sensor data and validate this method on an acute health event: an 18-day quarantine at a retirement community that was initiated in the midst of a norovirus outbreak. The results from this case study show that room-transition values increased significantly as subjects remained in their homes during the quarantine, demonstrating a mean increase of 12 transitions per day. Furthermore, a time-adjusted measure of room-transitions is examined that did not significantly change across the group. Finally, the healthy subjects and those that fell ill were analyzed separately, and significant differences were found between them for both the raw and time-adjusted metrics. As detection algorithms improve, these types of measures may be useful in the early detection of a change in health status.

Asteroids and andesites.

The production of terrestrial andesites in subduction zones is well established. Day et al. describe two examples of meteorites (GRA 06128 and GRA 06129) that they claim to represent "an entirely new mode of generation of andesite crust compositions" on asteroids; this suggestion has wide implications for the generation of andesitic planetary crusts in general. However, here we show that compositional data, particularly for the rare-earth elements (REEs) and other lithophile elements, presented in their paper do not substantiate this claim. We conclude that existing mechanisms for andesite generation do not need revision.