Copper Outdiffusion from Copper-Plated Solar Cell Contacts during Damp Heat Exposure.

Plated copper (Cu) contacts for silicon (Si) solar cells are an attractive alternative material to conventional screenprinted silver, but there are unresolved questions on the long-term integrity of plated contact structures. In this work, we perform characterization on plated Cu contacts from encapsulated cells that were degraded during extended exposure to damp heat (DH) stress. First, using energy-dispersive X-ray spectroscopy, we find evidence of Cu outdiffusion upward through capping layers made of both tin and silver applied with light-induced plating, resulting in a layer of Cu on the outer contact surface. We hypothesize that if Cu is mobile in the module, it may eventually find some route by which to enter the Si cells where it can degrade performance. Subsequently, in several types of Cu-plated, DH-degraded cells, secondary ion mass spectrometry detects elevated levels of Cu at the Si surface and in the Si cell bulk, which suggests that Cu can indeed migrate from contacts into Si over the course of DH stress.

Atomistic Insights into Lithium Storage Mechanisms in Anatase, Rutile, and Amorphous TiO2 Electrodes.

Density functional theory calculations were used to investigate the phase transformations of LixTiO2 (at 0 ≤ x ≤ 1), solid-state Li+ diffusion, and interfacial charge-transfer reactions in both crystalline and amorphous forms of TiO2. It is shown that in contrast to crystalline TiO2 polymorphs, the energy barrier to Li+ diffusion in amorphous TiO2 decreases with increasing mole fraction of Li+ due to the changes of chemical species pair interactions following the progressive filling of low-energy Li+ trapping sites. Sites with longer Li-Ti and Li-O interactions exhibit lower Li+ insertion energies and higher migration energy barriers. Due to its disordered atomic arrangement and increasing Li+ diffusivity at higher mole fractions, amorphous TiO2 exhibits both surface and bulk storage mechanisms. The results suggest that nanostructuring of crystalline TiO2 can increase both the rate and capacity because the capacity dependence on the bulk storage mechanism is minimized and replaced with the surface storage mechanism. These insights into Li+ storage mechanisms in different forms of TiO2 can guide the fabrication of TiO2 electrodes to maximize the capacity and rate performance in the future.

Early and Long Term Outcomes Following Long Posterior Flap vs. Skew Flap for Below Knee Amputations.

OBJECTIVE: To compare outcomes between long posterior flap (LPF) and skew flap (SF) amputation over a 13 year period. METHODS: This was a retrospective observational cohort study. Consecutive patients undergoing a LPF or SF below knee amputation (BKA) over a 13 year period at one hospital were identified. Both techniques were performed regularly, depending on tissue loss and surgeon preference. The primary outcome was surgical revision of any kind. Secondary outcomes included revision to above knee amputation (AKA), length of hospital stay (LOS), and mortality. A smaller cohort of patients who were alive and unilateral below knee amputees were contacted to ascertain prosthetic use and functional status. RESULTS: In total, 242 BKAs were performed in 212 patients (125 LPF and 117 SF; median follow up 25.8 months). Outcomes for the two groups were equivalent for surgical revision of any kind (27 LPF vs. 31 SF; p = .37), revision to an AKA (18 LPF vs. 14 SF; p = .58), LOS (29 days for LPF vs. 28 days for SF; p = .83), and median survival (23.9 months for LPF vs. 28.8 months for SF; p = .89). Multivariable analysis found amputation type had no effect on any outcome. Functional scores from a smaller cohort of 40 unilateral amputees who were contactable demonstrated improved outcomes with the LPF vs. the SF (p = .038). CONCLUSION: Both techniques appear equivalent for rates of surgical residual limb failure. Functional outcomes may be better with the LPF.

Evidence for Fast Lithium-Ion Diffusion and Charge-Transfer Reactions in Amorphous TiO x Nanotubes: Insights for High-Rate Electrochemical Energy Storage.

The charge-storage kinetics of amorphous TiO x nanotube electrodes formed by anodizing three-dimensional porous Ti scaffolds are reported. The resultant electrodes demonstrated not only superior storage capacities and rate capability to anatase TiO x nanotube electrodes but also improved areal capacities (324 µAh cm-2 at 50 µA cm-2 and 182 µAh cm-2 at 5 mA cm-2) and cycling stability (over 2000 cycles) over previously reported TiO x nanotube electrodes using planar current collectors. Amorphous TiO x exhibits very different electrochemical storage behavior from its anatase counterpart as the majority of its storage capacity can be attributed to capacitive-like processes with more than 74 and 95% relative contributions being attained at 0.05 and 1 mV s-1, respectively. The kinetic analysis revealed that the insertion/extraction process of Li+ in amorphous TiO x is significantly faster than in anatase structure and controlled by both solid-state diffusion and interfacial charge-transfer kinetics. It is concluded that the large capacitive contribution in amorphous TiO x originates from its highly defective and loosely packed structure and lack of long-range ordering, which facilitate not only a significantly faster Li+ diffusion process (diffusion coefficients of 2 × 10-14 to 3 × 10-13 cm2 s-1) but also more facile interfacial charge-transfer kinetics than anatase TiO x.

Angular matrix framework for light trapping analysis of solar cells.

Complex light trapping structures make it challenging to simulate the optical properties of solar cells accurately. In this paper, a framework is proposed where matrices are used to describe the transition of the angular distribution of the light when it is reflected, transmitted or absorbed. The matrices can be computed using a range of different simulation methods and when parts of a complex structure are to be optimized, or the incident light is altered, the pre-computed matrices can be used with the potential benefit of saving computational time. The optical properties of silicon wafers with different texturing, surface coatings and light incident angles were simulated and compared with measurements to demonstrate the accuracy of the proposed framework. It is shown that different simulation methods can be effectively integrated to model different parts of the solar cell and structures with multiple coherent and incoherent layers. These features enable efficient and rapid evaluation of the optical properties of the device as a function of its physical properties.