Session E7: Undergraduate Research/SPS IV

Capacitance of thin film solar cells: violating the depletion approximation

Capacitance measurements of solar cells are able to detect minute changes in charge in the material. For that reason, capacitance is used in many methods to electrically characterize the solar cell. Standard interpretations of capacitance relies on many assumptions, which, if wrong can skew the results. In some solar cells where a back contact barrier is suspected, measurements at high forward bias can be used. We have seen that apparent signatures of a back contact barrier in Cu(In,Ga)Se$_{\mathrm{2}}$ may actually be the first signs of a negative contribution to capacitance. We will discuss the implications of negative capacitance, and its relationship to other electronic characteristics of the device.   

Time-dependent efficiency measurements of donor-acceptor, dye-sensitized polymer solar cells.

The fullerene/polymer active layer pairing of PCBM/P3HT has become the model system within the field of polymer solar cell research. A large body of work concerned with reporting improved efficiencies for this system exists, but truly quantitative studies of device lifetime and long-term degradation tendencies are much rarer. Here, we report the effects of two donor-acceptor diazo dye sensitizers on efficiency and lifetime upon addition into the PCBM/P3HT active layer at varied concentrations. The electrical and efficiency measurements were supplemented by time-dependent UV-visible spectroscopy studies and morphology investigations via atomic-force microscopy (AFM). This pairing with spectroscopy offers an internal check on the data as the rate of change in absorbance of the active layer correlates almost exactly to the rate of power conversion efficiency decrease. Additionally, AFM imaging reveals different morphology patterns when dye concentrations and functionalities change. Such observations suggest that such small-molecule sensitizers exert yet undetermined effects on the organization of components within the active layer at the molecular level.   

Quantitative study of stress levels in AT and BT cut quartz crystal microbalances associated with surface laser irradiation.

The frequency response of an AT cut Quartz Crystal Microbalance (QCM) to laser irradiation has been increasingly studied in recent years, as the combination of photons with materials on a QCM's electrodes enables fundamental studies of topics that span biophysics to photovoltaics. In order for such studies to advance, however, the impact of heating effects associated with laser irradiation of the QCM must be accounted for. Prior studies reached qualitative conclusions that laser irradiation induces stress QCM's arising from non-uniform thermal expansion, but did not quantitatively measure the degree of stress. Secondary effects such as surface film desorption and/or changes in temperature were also reported to be present. We report here a study of the frequency response of AT and BT cut QCM's to laser irradiation. AT and BT cut QCM's have similar response to mass adsorption, but opposite frequency response to stress levels, allowing the stress levels induced by the laser light to be quantitatively measured when the results are compared. Studies were performed in both vacuum and air, to control for the presence of adsorbed films. As expected, system designs that minimize temperature gradients result in less of an effect.   

Photovoltaic performance parameters at the nanoscale from in situ I-V curve measurements

Monocrystalline photovoltaic (PV) devices exhibit higher efficiencies than polycrystalline devices, but the high manufacturing costs associated with single crystal solar cells pose a hindrance to their wide implementation. Polycrystalline CuIn$_{\mathrm{x}}$Ga$_{\mathrm{(1-x)}}$Se$_{\mathrm{2}}$ (CIGS) material with high optical absorbance and low cost/Watt, is a promising alternative. Yet, the efficiency of this low-cost technology is still substantially lower than the theoretical values estimated by the Shockley-Queisser limit. This is likely due to microstructural non-uniformities, which cannot be accessed by macroscopic light I-V measurements. Therefore, we spatially resolve the electrical response of these devices by `local' I-Vs. For that, we utilize a 100x objective as a local excitation source and LabVIEW to map the PV performance with sub-micronscale resolution through extrapolation of key parameters from pixel by pixel I-V curves. Extraction of performance parameters such as short-circuit current, opencircuit voltage fill factor, and maximum power point can provide useful information regarding optimal microstructural characteristics. This information is not only valuable for CIGS-based devices, but also will be an essential tool for maximizing performance across all PV technologies.   

Optimizing Hydrogen Storage by Doping the LiBH4$+$MgH2 Reaction with Various Niobium Based Oxides

In this study, the effects of doping the dehydrogenation reaction of MgH2 $+$ 2LiBH4 was combined with 5 mole{\%} of three different Niobium based oxides (Nb2O5, NbO2, and LiNbO3). The compounds were mixed using high energy ball milling, and then heated using an air tight heating stage. We looked for changes in the Raman spectra as temperature increased (up to 350C) as an indication of hydrogen desorption reaction. We found that milled LiBH4 undergoes significant changes in Raman spectra during heating to 130C. MgH2 undergoes significant changes when comparing before and after milling---but in each case, the spectral peaks remain unchanged during heating to 350C. The sample with LiNbO3 exhibited a concrete change in Raman spectrum at 300 C while the sample doped with Nb2O5 underwent a change in spectra at 170C. The sample doped with NbO2 showed little change in spectra when the samples were heated up to 350C. Further studies are underway to examine the nature of the changes in the Raman spectra using X-ray diffraction and residual gas analysis.   

Electromotive force and current in a superconducting solenoid with limited length induced by a bar magnet and a monopole

The magnetic flux $\Phi_{\mathrm{B}}$, electromotive force, EMF, and current $I_{\mathrm{in}}$, induced by a moving magnetic bar and an imaginary magnetic monopole in a superconducting solenoid of multiple turns and length $L$, are numerically calculated. The magnetic field of the bar magnet is approximated with the magnetic field along $z$ axis of a solenoid with length $l$ and radius $a$ and current $I$, while the magnetic field of the monopole is supposed to be inversely proportional to $r^{\mathrm{2}}$. Calculations show that, for a bar magnet, $\Phi_{\mathrm{B}}$ and $I_{\mathrm{in}}$ essentially saturate when the bar moves inside superconducting solenoid, so EMF is zero while $I_{\mathrm{in}}$ is constant. EMF is only induced when the bar enters and exits the solenoid and $I_{\mathrm{in}}$ is zero after the bar leaves the solenoid. For a magnetic monopole, $\Phi_{\mathrm{B}}$ is discontinuous (from positive maximum to negative maximum) when the it moves through each turn of the superconducting solenoid, but EMF caused by $d\Phi_{\mathrm{B}}$/\textit{dt} is continuous while the EMF induced by the a moving monopole is a delta function (moving monopole produces a ring-shaped $E$ field). The total EMF$_{\mathrm{Tot}}$ in solenoid is the superposition of EMF of each turn of coil and the plateau appears. The current $I_{\mathrm{in}}$ continues to grow while the monopole leaves the solenoid.   

Optimizing Production of Superconducting Bulk YBCO Crystals: Effects of Initial Temperature and Growth Time

We present our work on increasing both pinning force and critical temperature of bulk YBCO superconducting crystals. We optimized temperature profiles for growing bulk YBaCuO superconductors by varying (a) temperature and (b) growth time. We synthesize the precursor materials, superconducting Y$_1$Ba$_2$Cu$_3$O$_7$ (Y-123) and non-superconducting Y$_2$Ba$_1$Cu$_1$O$_5$ (Y-211), and mix the two in a 1:0.4 molar ratio before beginning melt textured growth. We heat the mixture to a temperature above the melting point of Y-123 but below the melting point of Y-211, where it is held until the Y-123 melts and the Y-211 impurities disperse in the mixture. The mixture is then quickly cooled to an initial temperature (a) near the peritectic point, and is then slowly cooled for 30-100 hours (b) to allow steady growth of large domain Y-123 crystals. We will discuss the effects of different initial temperatures near the peritectic point and growth times on the pinning force and critical temperature of these samples.   

The Effect of Impurities on the Superconductivity of BSCCO-2212

BSCCO-2212 is a high-temperature cuprate superconductor whose electronic structure is currently poorly understood. In particular, it is unclear whether its order parameter is consistent with s-wave or d-wave behavior. Leggett has suggested that its order parameter might take a certain form that is consistent with d-wave behavior. While some experiments on the surface of BSCCO seem to support this conclusion, other experiments have suggested that its order parameter is instead s-wave in the bulk. We present some quantum field theoretic calculations in the spirit of Abrikosov and Gorkov's approach to the theory of superconductivity that suggest that such an order parameter cannot be correct. We will demonstrate that having such an order parameter would mean that BSCCO's critical temperature would go to zero if it is sufficiently impure, contradicting experimental evidence otherwise. Because this would not happen with a d-wave order parameter, these calculations lend support to the hypothesis that BSCCO is s-wave.   

Superconductivity and anti-ferromagnetism through Bose-Fermi mixture on optical lattices.

Motivated by the recent experimental progress with ultra-cold atoms, we investigate the physics of a Bose-Fermi mixture on a two dimensional optical lattice. We treat the system parameters such that 2-component fermions are in a deep external trap and weakly interacting bosons are in a shallow external trap, however both of these atoms are subjected to the same optical lattice. In this parameter regime, the bosons form a Bose-Einstein condensate and mediate an attractive interaction between fermions through low energy Bose excitations. As a result, the dynamics of the fermions can be described by the single band Hubbard model that involves on-site repulsive interaction and elementary excitation mediated attractive interactions. Using a mean field theory, we derive an effective action up to the quartic order in both d-wave superconducting and anti-ferromagnetic order parameters. Using this Landau energy functional, we then discuss the phase transition and study the competition and/or cooperation of anti-ferromagnetism and d-wave superconductivity in the system.   

Coexistence of anti-ferromagnetism and anisotropic superconductivity in iron pnictides.

By treating both anti-ferromagnetism (AFM) and superconductivity (SC) on an equal footing, we investigate the possible coexistence of AFM and SC of recently found high-temperature superconducting compounds. Assuming that the electron pairing is mediated by the spin fluctuations and using a mean-field theory, we derive a set of gap equations for both AFM and SC order parameters. In the spirit of the second order phase transition, we then linearize the gap equations using various base functions for superconducting order to include the different pairing symmetries. By analyzing the solution of our linearized equations, we then discuss the possible coexistence of AFM and anisotropic SC in these compounds.   

Optimization of the high frequency magneto-impedance effect in Co-based amorphous ribbons

The magnetic field dependence of the impedance, known as magneto-impedance (MI), was measured as a function of excitation frequency in Co-based amorphous ribbons. An optimization of the MI profile on the high frequency regime (100 MHz -- 1000 MHz) was attempted through annealing techniques. Current annealing was performed with different annealing amplitudes ranging from 200 mA up to 1 A. Field annealing was also performed by raising the temperature of the sample through Joule heating and applying an external magnetic field of 55 Oe transversal to the ribbon. It was found that annealing at low current improved the MI response at lower frequencies, between 100 MHz and 300 MHz. On the other hand annealing at higher amplitude, past the Curie temperature (Tc) favored higher frequencies. These findings provide good guidance toward the optimization of the MI response of Co-based amorphous ribbons for high-frequency sensor applications.   

Charge density wave phase transitions in Tantalum Disulfide

The discovery of transition metal dichalcogenides has reopened the interest in charge density wave physics. By looking at the phase transition properties, it is possible to create switches with properties exceeding the traditional FET’s. In this work, we fabricated 2D nanoscale switches made of 1T-TaS2 and graphene. We tested the transport properties between the Mott insulating state and the conductive state of the switch. Our work will provide insight on possibilities for the production of ideal switches and transistor logic.   

Dielectric and electro-optic measurements of nematic liquid crystals doped with carbon nanotubes

We studied the effects of carbon nanotubes (CNTs) on the dielectric and electro-optic properties of nematic 5CB liquid crystals (LCs). Samples containing 0.01{\%}, 0.10{\%} and 1.00{\%} CNTs by weight were prepared. Anti- parallel rubbed cells with a nominal thickness of 10 $\mu $m were prepared using indium tin oxide coated glass cells and a polyimide alignment layer. The capacitance and dissipation factor were measured using an Agilent 4284A precision LCR meter. From these measurements, the complex dielectric permittivity was determined as a function of frequency. Analysis of the low frequency regime (f \textless 1000 Hz) indicates that 5CB samples containing CNTs have a higher conductance than neat samples. The Fr\'{e}edericksz transition critical voltage was noted by a sharp increase in capacitance after an initial plateau. Numerical simulations of CNT-facilitated switching show that polarization induced on the nanotubes from capacitive effects can significantly reduce the critical voltage in DC electric fields, in agreement with experimental results. Measurements of the critical voltage over a range of frequencies will also be presented.   

NaAlH4 Mixed with Carbon Nanotubes, Fullerene, and Titanium to Yield the Lowest Temperature for Hydrogen Desorption.

Recent research in hydrogen storage has improved dehydrogenation methods with solid-state compounds. NaAlH4 is a complex hydrides which release hydrogen at a lower temperature making the compound a great candidate for hydrogen storage. However, a catalyst should be combined with NaAlH4 to release the lowest desorption temperature. Prior research showed that interaction of NaAlH4 with nanotube or fullerene effectively weakens the Al-H bonds causing hydrogen desorption at lower temperatures. In the present study, NaAlH4 is ball milled with three of these catalysts: titanium, carbon nanotubes and fullerene and the milling time is varied from 10 to 30 minutes to compare the dehydrogenation rates in each setup. The phase structures were identified using the X-ray diffraction. Of these catalysts, the fullerene yielded the most interesting result showing nanostructuring of the hydride during ball milling. The possibility of ``shot peening'' of the NaAlH4 by the fullerenes is explored.   

Simulation of the Effects of Cooling Techniques on Turbine Blade Heat Transfer

Increases in the performance demands of turbo machinery has stimulated the development many new technologies over the last half century. With applications that spread beyond marine, aviation, and power generation, improvements in gas turbine technologies provide a vast impact. High temperatures within the combustion chamber of the gas turbine engine are known to cause an increase in thermal efficiency and power produced by the engine. However, since operating temperatures of these engines reach above 1000 K within the turbine section, the need for advances in material science and cooling techniques to produce functioning engines under these high thermal and dynamic stresses is crucial. As with all research and development, costs related to the production of prototypes can be reduced through the use of computational simulations. By making use of Ansys Simulation Software, the effects of turbine cooling techniques were analyzed.