Bulletin of the American Physical Society
Annual Meeting of the Four Corners Section of the APS
Volume 58, Number 12
Friday–Saturday, October 18–19, 2013; Denver, Colorado
Session D6: Renewable Energy and Materials |
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Chair: Michael Petras, Freescale Semiconductor, Inc Room: 253 |
Friday, October 18, 2013 2:00PM - 2:24PM |
D6.00001: Silicon Quantum Dot Mesomaterials for Solar Energy Harvesting Invited Speaker: Mark Lusk Recent progress in understanding electronic wave functions in condensed matter nanostructures has led to an ability to synthesize isolated, quantum confined building blocks with a variety of tailored optical properties. No matter what optical gap is engineered and how cleverly exciton energy is redistributed, though, novel materials composed of such nanostructures need to also exhibit efficient carrier dynamics. Transport of energy and charge is now the central issue in harnessing the true power of quantum dot materials for solar and many other uses. This is a critical bottleneck in the science because charge and exciton transport tend to proceed via low mobility, incoherent hopping associated with weak electronic coupling and high reorganization energies in these nanostructures. A number of promising strategies seek to improve energy and charge transport between quantum dots by focusing on important properties such as translational symmetry, electronic overlap, matrix encapsulation, and crystalline orientation. Our approach, though, is to consider the entire assembly as a quantum dot mesomaterial (QDM), wherein entirely new transport physics may emerge from the complex interactions between components. For instance, the superb exciton harvesting efficiency of photosynthetic complexes is at least partly due to conditions that support an element of coherent character for exciton transport. Here proteins and pigments are exquisitely structured and combined so that they perform a number of integrated functions—e.g. proteins serve to correlate electronic excitations on neighboring pigments, supporting coherence and allowing exciton transport with a degree of wave-like character. We seek to design materials composed of quantum dots in which components may carry out integrated tasks that optimize dynamics ranging from incoherent random walks to coherent transport. An emphasis is placed on the robustness of such transport in the face of geometric uncertainties intrinsic to synthesized systems. The computational facet of our investigation, emphasized in this talk, utilizes an open dissipative system approach, wherein a cumulant expansion strategy is used to approximate the quantum Liouville equation via a hierarchy of density operators. This has been successfully employed to scrutinize partially coherent transport in protein/pigment complexes, but here we focus on silicon quantum dot mesomaterials and use excited state many-body calculations to populate the associated meta-Hamiltonian. After an overview of the mesomaterial perspective, this talk will focus on our computational assessment of the prospects for partially coherent exciton transport through these silicon quantum dot mesomaterials. [Preview Abstract] |
Friday, October 18, 2013 2:24PM - 2:36PM |
D6.00002: The Effect of Morphology on Charge Transport Properties in OPVs Alex Dixon, Nikos Kopidakis, Sean Shaheen The Organic Photovoltaic (OPV) field is rapidly advancing, however several important issues regarding the device physics in these systems remain~unresolved. We investigated the relationship between morphology and charge transport properties in OPV materials. To do this, we created devices with poly 3-hexothyophene (P3HT) of a range of molecular weights (from 13kDa to 331kDa). These various molecular weights cause the films to have different morphologies. The low molecular weight P3HT forms semi-crystalline domains, as the molecular weight increases, amorphous regions appear connecting and surrounding the semi-crystalline domains. The devices were measured using the Charge Extraction by Linearly Increasing Voltage (CELIV) technique in order to determine the charge carrier mobility and recombination rate of the various P3HTs. We found that while the recombination rate decreases with increasing molecular weight, the mobility peaked at around 47kDa. We believe that the decrease in recombination is due to charge dissociation of electrons and holes into separate regions of the film. While the lower recombination rate from the separate amorphous and semi-crystalline domains raises the mobility, the increasing amount of amorphous material at larger molecular weights causes a decrease in mobility, resulting in the observed mobility peak. [Preview Abstract] |
Friday, October 18, 2013 2:36PM - 2:48PM |
D6.00003: Mott-Schottky Analysis of Normal and Inverted Organic Photovoltaic Devices Xin Jiang, Alexandre Nardes, Alexander Dixon, Nikos Kopidakis, Sean Shaheen We use impedance spectroscopy to examine the electronic structure and energy band diagrams of Organic Photovoltaic (OPV) devices based on the standard donor-acceptor combination of P3HT-PCBM. Mott-Schottky analysis is performed to characterize the dark carrier densities, built-in voltages of Schottky junctions, and overall energy band diagrams in standard and inverted geometry devices with aluminum and silver top electrodes, respectively. Evidence for the decrease of dark carrier densities upon post-production thermal annealing is seen for devices in both geometries, illustrating the impact of thermal processing on the energetic band diagram. Furthermore, we find evidence for p-n junction formation at the ZnO/active layer interface in inverted devices, owing to the existence of dark carriers on both sides of the interface. We suggest that the resulting band bending at this interface helps explains the enhanced photocurrents often seen in inverted devices when compared to normal geometry devices for nominally identical active layer structures. [Preview Abstract] |
Friday, October 18, 2013 2:48PM - 3:00PM |
D6.00004: J-V Distortion of CIGS Solar Cells with Sputtered Zn(O,S) Buffer Layer Tao Song, J. Tyler McGoffin, Russell Geisthardt, Kannan Ramanathan, James Sites Sputtered-deposited Zn(O,S) is an attractive alternative to CdS for Cu(In,Ga)Se2 (CIGS) thin-film solar cells' buffer layer. It has a wider band gap and thus allows greater blue photon collection to achieve higher photon current. A key parameter for the sputtering deposition of Zn(O,S) has been the oxygen fraction in the argon sputtering beam. Current-Voltage (J-V) distortion, observed in some cases, varied with oxygen fraction in Zn(O,S). The details are in good agreement with predictions of a photodiode model, in which a conduction-band offset (CBO)-induced barrier at the buffer-absorber interface is responsible for the distortions (both red kink and crossover). Varying oxygen fraction in Zn(O,S) may play a role in adjusting the CBO at the interface and thus modulating the J-V distortion. [Preview Abstract] |
Friday, October 18, 2013 3:00PM - 3:12PM |
D6.00005: Optimization of Layer Properties for High Efficiency CdTe solar cell with Higher Band Gap CdMgTe Layer as Back Electron Reflector Jinming Zhang, James Sites Thin film CdTe solar cell has been an option for PV energy solution with a potential of massive terrestrial installation. The open circuit voltage of traditional CdTe cell (up to 0.85V) has been the biggest limit to higher cell efficiency. Adding an additional layer of CdMgTe as back electron reflector with fitted band alignment to CdTe has been proposed to increase Voc by as high as the conduction band energy offset (CBO). Further increase from the same approach may cause tradeoff of fill factor and Jsc. Study of band structure, dark and light J-V curves, quantum efficiency spectrum has been carried out by applying a1-D numerical simulation program. Layer properties are optimized considering experimental facts for charge generation, separation and collection. Primary experimental results have demonstrated the prediction on cell degradation caused by valence band barrier between CdTe and CdMgTe. Solutions have been proposed for further improvements. [Preview Abstract] |
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