Developing a Student Learning Strategy to Bridge Virtual Learning and Hands-On Activity

This paper addresses the effectiveness of combined virtual and physical hands-on activities in students’ learning which was infused in the capstone senior design project. Senior design projects are open-ended and are similar to the research that scientists perform toward a more comprehensive understanding of nature or new scientific knowledge. As a reinforced learning methodology to greatly assist students’ reasoning and problem-solving skills, virtual learning was first integrated at the planning stage of their projects. This approach is in contrast with the typical senior design courses where only limited resources are available for planning experiments. Using virtual learning, students are able to revisit or learn new background theories and principles and identify and test a hypothesis before they actually engage in physical hands-on activities. This reinforced learning strategy efficiently guided students in preparing, confronting, and tackling the open-ended, inquiry-based problem with solid theoretical knowledge and principles. As a result it provided better planning for the physical hands-on activities. When engaged with physical hands-on activities, virtual laboratories were also be used to identify the disparity between theoretical and experimental results and additional activities designed to interpret the differences. This practice truly allowed students to experience the entire scientific process from solid theoretical reasoning obtained from virtual laboratories, to designing their own activities, to initial observations, and to follow-on activities based on the results of earlier activities. Our evaluation indicates that pedagogical modules of team based investigation using virtual and physical hands-on activities were very effective for students\u27 learning

Efficient Removal of Lead Ions from Aqueous Media Using Sustainable Sources on Marine Algae

The goal of this project is to explore a new method to efficiently remove Pb(II) ions from water by processing Undaria pinnatifida into immobilized beads using sodium alginate and calcium chloride. The resulting biosorbent was characterized by Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS). Using immobilized U. pinnatifida, we investigated the effect of various factors on Pb(II) ion removal efficiency such as temperature, pH, ionic strength, time, and underlying biosorption mechanisms. For Pb(II) ion biosorption studies, Pb(II) ion biosorption data were obtained and analyzed using Langmuir and Freundlich adsorption models. It is found that the maximum Pb(II) ion adsorption capacity (Xm) of U. pinnatifida was estimated to be ∼5 times greater than that of activated carbon, indicating the superior Pb(II) ion removal capability of U. pinnatifida compared to activated carbon. In addition, a thermodynamic study indicates that biosorption processes are found to be endothermic and an increase in the Pb(II) ion solution temperature provides a more preferential reaction toward Pb(II) ion biosorption

Design of Organic Tandem Solar Cells Using PCPDTBT: PC61 BM and P3HT: PC71BM

We conducted optical and electrical simulations with the goal of determining the optimal design for conjugated polymer-fullerene tandem solar cells using poly[2,6-(4,4-bis-(2-ethylhexyl)- 4H-cyclopenta[2,1- b;3,4- b′] dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT): [6,6]-phenyl C61 butyric acid methyl ester (PC61 BM) as a bottom cell and poly(3-hexylthiophene) (P3HT): [6,6]-phenyl C71 butyric acid methyl ester (PC71BM) as a top cell. The effects of photon density, absorption, balanced and unbalanced charge carrier transport, and bimolecular recombination in the two subcells were incorporated into the simulations. We found that the maximum energy conversion efficiency (η) is 9% when charge carrier mobilities in both top and bottom cells are balanced. However, the efficiency drops significantly if the carrier mobilities are unbalanced in either the top or bottom cell. In addition, we found that unbalanced carrier mobilities in the top cell require a reduction in the thickness of the bottom cell whereas unbalanced bottom cell mobilities require an increase in the thickness of the bottom cell to compensate for the reduced current. © 2010 American Institute of Physics

A Review: Thermal Stability of Methylammonium Lead Halide Based Perovskite Solar Cells

Perovskite solar cells have achieved photo-conversion efficiencies greater than 20%, making them a promising candidate as an emerging solar cell technology. While perovskite solar cells are expected to eventually compete with existing silicon-based solar cells on the market, their long-term stability has become a major bottleneck. In particular, perovskite films are found to be very sensitive to external factors such as air, UV light, light soaking, thermal stress and others. Among these stressors, light, oxygen and moisture-induced degradation can be slowed by integrating barrier or interface layers within the device architecture. However, the most representative perovskite absorber material, CH3NH3PbI3 (MAPbI3), appears to be thermally unstable even in an inert environment. This poses a substantial challenge for solar cell applications because device temperatures can be over 45°C higher than ambient temperatures when operating under direct sunlight. Herein, recent advances in resolving thermal stability problems are highlighted through literature review. Moreover, the most recent and promising strategies for overcoming thermal degradation are also summarized

Reproducible Increased Mg Incorporation and Large Hole Concentration in GaN Using Metal Modulated Epitaxy

The metal modulated epitaxy (MME) growth technique is reported as a reliable approach to obtain reproducible large hole concentrations in Mg-doped GaN grown by plasma-assisted molecular-beam epitaxy on c-plane sapphire substrates. An extremely Ga-rich flux was used, and modulated with the Mg source according to the MME growth technique. The shutter modulation approach of the MME technique allows optimal Mg surface coverage to build between MME cycles and Mg to incorporate at efficient levels in GaN films. The maximum sustained concentration of Mg obtained in GaN films using the MME technique was above 7 × 1020 cm-3, leading to a hole concentration as high as 4.5 × 1018 cm-3 at room temperature, with a mobility of 1.1 cm2 V-1 s-1 and a resistivity of 1.3 Ω cm. At 580 K, the corresponding values were 2.6 × 1019 cm-3, 1.2 cm2 V-1 s-1, and 0.21 Ω cm, respectively. Even under strong white light, the sample remained p-type with little change in the electrical parameters. © 2008 American Institute of Physics

Precise Control of Highly Ordered Arrays of Nested Semiconductor/Metal Nanotubes

Lithographically defined microporous templates in conjunction with the atomic layer deposition (ALD) technique enable remarkable control of complex novel nested nanotube structures. So far three-dimensional control of physical process parameters has not been fully realized with high precision resolution, and requires optimization in order to achieve a wider range of potential applications. Furthermore, the combination of composite insulating oxide layers alternating with semiconducting layers and metals can provide various types of novel applications and eventually provide unique and advanced levels of multifunctional nanoscale devices. Semiconducting TiO2 nanotubes have potential applications in photovoltaic devices. The combination of nanostructured semiconducting materials with nested metal nanotubes has the potential to produce novel multifunctional vertically-ordered three-dimensional nanodevices. Platinum growth by ALD has been explored, covering the initial stages of the thin film nucleation process and the synthesis of high aspect ratio nanotube structures. The penetration depth of the Pt into porous templates having various pore sizes and aspect ratios has been investigated. Several multi-walled nested TiO2-Pt nanotubes in series have been successfully fabricated using microporous Si templates. These innovative nested nanostructures have the potential to produce novel multifunctional vertically-ordered three-dimensional nanodevices in photovoltaic and sensing technologies

Metal Modulation Epitaxy Growth for Extremely High Hole Concentrations Above 10(19) cm(-3) in GaN

The free hole carriers in GaN have been limited to concentrations in the low 1018 cm−3 range due to the deep activation energy, lower solubility, and compensation from defects, therefore, limiting doping efficiency to about 1%. Herein, we report an enhanced doping efficiency up to ~10% in GaN by a periodic doping, metal modulation epitaxy growth technique. The hole concentrations grown by periodically modulating Ga atoms and Mg dopants were over ~1.5 x 1019 cm−3. © 2008 American Institute of Physics

Stability of High Band Gap P3HT: PCBM Organic Solar Cells Using TiOx Interfacial Layer

We fabricated a poly [3-hexylthiophene] (P3HT) and [6,6] -phenyl-C61-butyric acid methyl ester (PC61BM with the TiOx layer. We found that a solution based TiOx coated at a spin speed of 3000 rpm improved the photon absorption of the active layer. An optimized TiOx layer was also used as the interfacial layer to investigate the stability of P3HT: PC61BM OPC. After 70 days of storage, we observed that the short-circuit current density (JSC) dropped by 16.2%, fill factor (FF) dropped by 10.6%, and power conversion efficiency (PCE) dropped approximately by 25%, while the open-circuit voltage (VOC) remained relatively stable. We found that a solution based TiOx layer synthesized using a sol-gel chemistry method was very effective in protecting the active layer from degradation

Investigation of Cation Exchange Behaviors of FA\u3csub\u3ex\u3c/sub\u3eMA\u3csub\u3e1-x\u3c/sub\u3ePBl\u3csub\u3e3\u3c/sub\u3e Films Using Dynamic Spin Coating

In this study, we fabricated and characterized uniform multi-cation perovskite FAxMA1−xPbI3 films. We used the dynamic spin-coating method to control the cation ratio of the film by gradually increasing the FA+, which replaced the MA+ in the films. When the FA+ concentration was lower than xFA ~0.415 in the films, the stability of the multi-cation perovskite improved. Above this concentration, the film exhibited δ-phase FAPbI3 in the FAxMA1−xPbI3 films. The formation of δ-phase FAPbI3 disturbed the homogeneity of the photoluminescence spatial distribution and suppressed the absorption spectral bandwidth with the increasing bandgap. The precise control of the cation ratio of multi-cation perovskite films is necessary to optimize the energy-harvesting performance